Can We Actually Detect Gravitational Waves with Atoms? | Peter Graham

What happens when a psychiatrist sits down with a Stanford physics professor to talk about gravitational waves, dark matter, quantum mechanics, and atoms existing in two places at once?

In this episode of No Reason to Get Excited (NRTGE), Dr. Aaron Winkler talks with Stanford Physicist Peter Graham about the strange and fascinating world of modern physics. What starts as a conversation about gravitational wave detection quickly turns into a deep exploration of quantum mechanics, atom interferometry, atomic clocks, dark matter, and the bizarre reality of particles behaving like waves.

Peter explains how researchers are building tabletop experiments capable of measuring incredibly small distortions in space-time, why gravity is surprisingly weak compared to electromagnetism, and how a single atom can exist in two places at once. Along the way, Aaron asks the kinds of questions many listeners are probably thinking themselves, leading to a conversation that feels less like a formal interview and more like two curious minds trying to make sense of the universe together.

This episode is not a simplified science lecture. It’s an intellectually alive conversation about uncertainty, experimentation, physics, and the limits of human intuition.

About the Guest

Peter Graham is a professor of physics at Stanford University whose research focuses on fundamental physics, dark matter, gravitational waves, and precision measurement techniques using atomic systems. His work often bridges theoretical physics and experimental collaboration, helping develop new ways to probe some of the deepest unanswered questions in modern science.

Connect with Peter:

Website: https://physics.stanford.edu/people/peter-graham

Chapters 

00:00 – Introduction to Peter Graham and Stanford Physics
03:20 – Why Collaboration Matters in Modern Physics
05:10 – The Problem with Dark Matter and Fundamental Physics
06:00 – Building New Experiments Instead of Bigger Colliders
07:00 – How LIGO Detects Gravitational Waves
09:30 – Why Gravity Is Surprisingly Weak
11:20 – Gravitons, Dark Matter, and Unanswered Questions
15:15 – Atom Interferometry Explained
18:00 – Quantum Mechanics and Probability Waves
24:40 – Using Lasers to Manipulate Atoms
29:20 – The History of Particle Physics and Scientific Discovery
33:00 – What Quantum Waves Actually Mean
41:00 – Vacuum Chambers, Cooling Atoms, and Laser Physics
47:00 – How Laser Cooling Works
55:00 – Creating an Atomic Interferometer
1:00:30 – Measuring Time with Atomic Clocks
1:08:00 – Using Atoms to Detect Gravitational Waves
1:15:00 – Earth’s Gravity, Potential Energy, and Quantum States
1:20:00 – Why Vertical Mine Shafts Matter
1:24:00 – Measuring Acceleration with Atomic Systems
1:28:00 – Building the Future of Gravitational Wave Detection

If you enjoyed this episode of No Reason to Get Excited, make sure to follow the show, leave a rating or review, and share this episode with someone who loves deep conversations about science, physics, and the mysteries of the universe.

Connect with Dr. Aaron Winkler

• Website: www.aaronwinklermd.com
• LinkedIn: @NRTGEPOD
• Instagram @NRTGEPOD

Transcript

SPEAKER_13 0:00

That's right.

SPEAKER_08 0:02

So you're you're not actually like pushing it further and further and further. It's like it's you're just going, boom, and you're back.

SPEAKER_13 0:06

Well, no, you're right. You're right. I lied. We are actually pushing it many times. So in fact, we shoot many, many, many of these pulses. But the easiest way to let me put it this way.

SPEAKER_19 0:15

You didn't lie, you oversimplified.

SPEAKER_13 0:16

I oversimplified, yes, yes. Okay. The first place.

SPEAKER_19 0:19

When will you believe that I don't want to I may not be able to do the differential?

SPEAKER_13 0:26

You guys are all the right questions. These are these are the these are the steps we had to go through. The simplest iteration is where one pulse starts it and one pulse stops it. But to get more sensitivity, we didn't want to just hang out for 10 minus five seconds. We wanted more. So we developed these other pulse sequences to do exactly what you're saying. So this was this was non-trivial work that we did to figure out how to make this thing optimally sensitive, maximally sensitive. What'd you do?

SPEAKER_09 0:54

So Peter Graham, full professor of physics. Just like just physics. Professor of physics, the same thing. That's right. That's right. Exactly.

SPEAKER_10 1:02

That's cool, yeah. That's fun. It was uh it's definitely a dream of mine. Yeah, I bet that was a a loadoff when you when you got it. Yeah, yeah, definitely.

SPEAKER_14 1:12

It's a fantastic place. So yeah.

SPEAKER_10 1:15

It really is.

SPEAKER_14 1:16

Yeah, yeah. It's a great group, a great department.

SPEAKER_10 1:19

Yeah. Yeah. You have a lot of like cool colleagues doing other stuff. Oh yeah. Yeah, lots of collaboration.

SPEAKER_14 1:26

Absolutely. It is one of the one of the best in the world for sure.

SPEAKER_10 1:30

Yeah, that's one of the things I'm getting a lot is the is the amount of like I've been like Chem H.

SPEAKER_08 1:35

Um, I've I've talked to a couple of of like brand new, like newbies, like you know, knew this cycle, like in the last six months, associate professant professors over there.

SPEAKER_10 1:45

Um and the amount of collaboration is like a big part of what makes them yeah, yeah, that was cool.

SPEAKER_14 1:53

For me, that was definitely one of the reasons I wanted to stay here. Uh yeah. So I was here as a grad student and then wanted to stay because the collaboration.

SPEAKER_08 2:01

It was You were here as a grad student? Did you so did you do your PhD? Is that as a PhD? PhD. Did your PhD here? Where did you do your postdoc?

SPEAKER_13 2:08

Uh postdoc actually sort of a mix of here and just Slack. So basically here, still at Stanford.

SPEAKER_08 2:13

I'm so surprised. I thought they didn't internally hire. I thought that was like a thing that you want people to like go to other places.

SPEAKER_04 2:19

Uh often, yeah, but you know, I I hung on.

SPEAKER_13 2:22

I really didn't want to leave. It's a great place. Um Wow, man. That's impressive. It was good. Thank you. It struck up some really great collaborations here. There were some people here, even, you know, my my field is really, I guess you'd call it theoretical high-energy physics or particle physics. But we started working with some like atomic experimentalists and other people who are sort of variety of areas, and it's a rare place that has kind of fantastic people in this wide a variety of areas, also who are, you know, willing to jump really outside their area and talk. Yeah. Like for an atomic experimentalist to talk with kind of like a crazy particle theorist about whatever dark matter and gravitational waves and general relativity is uh, you know, you have to find the right person who's willing to do that. So this was a this was a really valuable place for that.

SPEAKER_08 3:09

Okay, that maybe that's a great place to start. Is like, is there a project you've done recently or just a project that is something that like really landed was like, wow, that was cool. Um that that was like very like required collaboration.

SPEAKER_13 3:25

Yeah, definitely. Actually, it's really a lot of how I work is um I kind of starting from PhD, I got really excited about collaborating with uh experimentalists with people who are in very different fields. Um uh kind of that seemed like a really good way to make progress. Um, you know, you find what other people aren't doing, right?

SPEAKER_08 3:45

But what kind of like what an odd, like because aren't most theoretical physicists like allergic to experimental, like right?

SPEAKER_13 3:51

It's certainly there is um uh a generally a pretty big separation, I would say. That's right. Um theorists tend to stay with doing more kind of traditional conventional theoretical things. Yeah. Uh yeah, no, it was that was sort of the thing. Yeah, exactly. For me, I actually I felt that was a a real missing piece of what you know, what we were missing in in particle physics uh was a real crossover between theorists and experimentalists. And it's something I've spent my career on. Is a lot of spent I've done a lot of projects.

SPEAKER_08 4:20

I would like have an immediate right, like to achieve what? Like what why was that like it was the missing piece to achieving what?

SPEAKER_13 4:26

Yeah, so I would say, you know, we have we have a fantastic understanding of many parts of the fundamental laws of nature.

SPEAKER_10 4:32

Yeah.

SPEAKER_13 4:33

We have a fantastic understanding of a lot of the history of the universe, like where it all came from.

SPEAKER_10 4:36

Yeah.

SPEAKER_13 4:37

Which is amazing.

SPEAKER_10 4:38

Just follow the math.

SPEAKER_13 4:38

Uh yeah. We have some really good known known laws that work really well, but there are some clear, outstanding things we just do not know. Like we do not know the nature of dark matter. We, you know, there's several other mysteries. We do not know where, you know, what is the origin of the various masses of the fundamental particles that we see, sort of what produce these fundamental laws that we see. We have a lot of good ideas.

SPEAKER_22 5:04

Yeah.

SPEAKER_13 5:05

So it's not that we're lacking, you know, theoretical possibilities. Yeah. What we're lacking is the means to test them. Yeah. And that's been really hard.

SPEAKER_08 5:13

Yeah. So and you know it is actually what make it's what lands Einstein, right?

SPEAKER_10 5:17

It's when, it's when they can finally do the eclipse experiment. And it's like, oh, we get some actual evidence that you're right. That's right. Yeah, that's right. It's not just theoretical.

SPEAKER_13 5:27

Exactly. Predictable. And that's what's key. That's what's key, right? We've we've got we've got a bunch of great ideas about dark matter. They're wildly different from each other, and they're just all possible. Yeah. The only way to choose is experiment. So, and and you know, it's why we build these ever bigger colliders. Yeah like so Slack up the up the hill is you know already outdated several decades ago. It's already too small, and we have to have to go to CERN and and uh we can build any bigger ones, are we? Like Slack. Well, the people are hoping to, but it's really expensive, takes decades.

SPEAKER_10 6:00

Yeah, and a lot of like a lot of political commitment.

SPEAKER_13 6:02

Exactly.

SPEAKER_10 6:02

Yeah, it takes a lot of commitment.

SPEAKER_13 6:04

So this is this is the this is essentially essentially the motivating factor for a lot of my career is how can we do other kinds of experiments faster with new technologies, yeah, you know, cheap, like tabletop type experiments that will actually probe fundamental physics.

SPEAKER_17 6:18

Yeah.

SPEAKER_13 6:19

Instead of I need to build the next$20 billion collider or something.

SPEAKER_08 6:26

One of the one of the other, one of the rare other podcasts that I have found that does science in the depth that I am interested in. I forget her name, but it's a woman um and she does it's a very high production value. And she gets like sometimes, I mean she gets interviews with like the CEO of NVIDIA, like, right? But she did one that was um the dark, the the Graviton Wave, uh-huh.

SPEAKER_10 6:53

Gravitational waves, yeah. The gravitational wave um like sensing facilities.

SPEAKER_08 6:56

Yes. That are the the like the two right angles that were bouncing the laser. Yeah, yeah, exactly. Um and she it was like a 20-minute thing of exactly how all it works, and they like let her in and she's got it's like it's it's cool. Oh, that's awesome. That's awesome.

SPEAKER_09 7:07

And about how they like, I guess they they turned up, they they turned up the sensitivity by right some in some fashion. All of a sudden, there they are. They've got them. Yeah, exactly. And we're actually we can sense gravitational waves now. Yeah, yeah.

SPEAKER_13 7:17

Yeah, they worked for decades slowly improving things and then finally got there.

SPEAKER_24 7:21

Yeah.

SPEAKER_13 7:22

That that's actually so so this project that I was working on as a grad student uh with my atomic colleagues, that's what it was actually, was detect gravitational waves uh in a different way than LIGO. And actually, that's been one of the my biggest, most long-running projects sort of on my whole career. It's something I'm still working on. Uh, is we have this new way to look for gravitational waves. Uh, and we're it takes time, it takes a lot of effort, but we're making it happen. I'm in. Um I'm in, I'm in.

SPEAKER_10 7:49

What do you got?

SPEAKER_13 7:49

Yeah, so this is you know, this was sort of very exciting to us. So, you know, the way LIGA works is you have these crossed uh laser beams, and you're essentially using, if you like the the light travel time, how long it takes the laser to go down and come back uh to look for the gravitational wave, which is the stretching and squeezing of space-time.

SPEAKER_08 8:09

Right. Yeah, so you see you can you del you one one of the two beams gets they the beams become slightly off sync with the with the stretch of space-time, right? Exactly. The gravitational wave going through. And it's like, and and it's so it's this thing where where the like because it's actually a wave, like in like in the ocean. So and if a wave is coming this, like coming this direction, yeah, then then a surfer going directly at the wave is gonna like something, and a surfer going across the wave, very different. So you've got these two light waves, and the and the gravitational wave passes through and it and it bends them differently.

SPEAKER_13 8:42

Exactly. Yeah, exactly. If you have this gravitational wave coming down, let's say through through down through the surface of the earth, and you have your your two ligos at you know, right angles on the surface of the earth. One of them, one of the one of the laser baselines will be, say, squeezing in while the other one is stretching out. Yeah, yeah.

SPEAKER_09 8:59

And then you stop getting turp perfect perfect cancellation.

SPEAKER_13 9:01

Exactly, exactly. And then they alternate, stretch, squeeze, stretch, squeeze, back and forth, and you, and you see that wave form. You see that pattern. Yeah. Um, and that's so so they do it with what's called laser interferometry, which is essentially this very careful comparison of of how many laser wavelengths each beam has gone, if you like.

SPEAKER_10 9:18

Yeah.

SPEAKER_13 9:18

Um, and you you beat those two uh laser beams against each other, you interfere them. Yeah, yeah.

SPEAKER_10 9:23

And there's all this stuff about having to like account for tectonic play moving.

SPEAKER_08 9:28

Everything like all these, right? It's wild.

SPEAKER_13 9:30

It's uh incredibly challenging because the signal is so small.

SPEAKER_05 9:33

It's so small. How is gravity like, how is something that is like the central force, right?

SPEAKER_08 9:40

That that in in the universe it would seem. Like, how is it so weak?

SPEAKER_13 9:44

Yeah, it is. It's a weird, it's a weird fact. The, you know, we actually, it's one of the deep uh puzzles of higher uh high energy physics is uh gravity turns out to be the weakest force. Yeah. Now that's weird from our point of view because when we walk around in the world, you know, gravity is kind of the force that we're most aware of. Yeah, but we're but but it's because of the size scale. It's yeah, it's because we're being attracted, if you like, by the entire Earth. One way we say it is, you know, electromagnetism is actually much stronger. In fact, it's so strong that it immediately attracted, you know, electrons to protons and stuck them together and stuck atoms together and made chemistry and all that. Everything's been neutralized. So you don't ever see just like a ball of charge attracting another ball of charge.

SPEAKER_08 10:25

Because all the because all the electrons and the protons are are stuck so precisely together.

SPEAKER_13 10:30

Wow. So, you know, usually the way we that we say it actually, it's actually one of the deepest problems in in modern physics, we call it the hierarchy problem, which is essentially the hierarchy between why is gravity so much weaker, say, than electromagnetism. Yeah. And and one of the kind of ways we say it, say the problem is, well, you know, the whole earth is pulling on you right now, right? But like this floor is holding you up. Just just a little piece of the floor is strong enough to hold you up with electromagnetic forces. Those are you know, electromagnetic propulsions between your atoms. Yeah, yeah. Can hold you against the combined gravity of all the atoms in the earth. Yeah. That's how much stronger electromagnetism is. Yeah.

SPEAKER_08 11:02

So essentially all we need to do is like put down a brick.

SPEAKER_13 11:05

Right, exactly, right. Yeah. And you know, you don't you don't crease the brick. Exactly, exactly.

SPEAKER_05 11:11

Yeah.

SPEAKER_13 11:12

So and it it's we that's a deep problem. We do not know the answer. Um, it's actually something uh a bunch of us have thought about for a long time. We have we have again, we have speculations, we have theories.

SPEAKER_08 11:22

Um but uh what it's it's what's the fluid that transmits the gravitational waves. It's gonna come down to like what's the they could like forces don't act at a distance without something to transmit the force. Like what are what are the gravitational waves like flowing through?

SPEAKER_13 11:37

Yeah, we know essentially they are uh transmitted by, we would call them by gravitons, the particle of gravity. And that is the particle which makes up the gravitational wave. Right, okay, okay.

SPEAKER_08 11:47

But but like, have we found gravitons? I mean, I know we've sent we've got finally got the Higgs boson, right?

SPEAKER_13 11:53

Yeah, you can't see individual gravitons, or in principle, maybe you could, but but we are, you know, centuries or millennia away from being able to do that. Why? They're just too weak. They interact so much more weakly than every other particle. We can see all these other fundamental particles. Is that just what dark matter is? It's all gravitons? We don't think so, because gravitons are massless. In fact, we really know, we've measured very precisely that they're they must be very, very small mass because we see the gravitational waves. It's one of the cool things we directly see these gravitational waves now, thanks to LIGO, and we see that they're basically moving at the speed of light. So we know these gravitons don't have any mass. So we know they cannot make up the dark matter. It's got to be something else new that we don't know about. That's kind of one of the exciting things about dark matter.

SPEAKER_08 12:34

Why do we know they're gravitons? Uh good. We we And why couldn't it just be a way, I mean, right? Like like the only other thing that moves at the speed of light is is EM radiation, is like, right? That's right. So why couldn't it just be gravit why why couldn't we why why couldn't gravitons be a kind of photon?

SPEAKER_13 12:50

They're related. They're related, but we know that the how the photon interacts. We know how those electromagnetic waves interact. They interact with our charged particles. You know, we use them every day in our cell phones and radios and all that. Yeah, yeah. Our eyes. Whereas uh the graviton has to basically mediate gravity. So it has to be the thing that you know attracts you to the earth and things like that. So it's got a very different interaction. It has to be a different kind of particle, or actually we use the word field, a different kind of field that would describe these uh these gravitational waves.

SPEAKER_08 13:19

That overlaps with other particles? Like we are surrounded by gravity.

SPEAKER_13 13:25

That's right. They're they're flying right through us. These gravitational waves exist. We know that for sure.

SPEAKER_08 13:29

And because the interactions are so weak, they go right through us.

SPEAKER_13 13:33

Exactly, exactly.

SPEAKER_08 13:33

And that's the thing where like when you look at a when you look at an atom, if you made an atom the size of a baseball field, it's like the nucleus is the is the size of a the what the head of a fly on the side. Right, right. Exactly. Um and so there's it's mostly empty space.

SPEAKER_13 13:46

It's mostly empty space. It's it's filled with an electron cloud around the nucleus, but even that, the graviton interacts with so weakly that it can just fly right through.

SPEAKER_08 13:54

Right. Whereas like a whereas like a NEM photon is gonna is gonna actually interact with it.

SPEAKER_13 13:58

Exactly, would excite the electron or ricochet off or whatever. Exactly. Yeah. Exactly. And you know, you know, if the graviton, if these gravitational waves were stronger, you could actually see that they're flying through you right now. You you would be stretching and squeezing yourself. Yeah. So it's there, they're they're flying right through you right now for sure, no question. Uh which is weird, weird to think about.

SPEAKER_08 14:20

If they were stronger, they would stretch space-time more. Exactly. And your your entire body would constantly, all the different pieces would constantly be like stretching in and out with time.

SPEAKER_13 14:28

Exactly. You could in principle you could see with your eyes if it was big enough. Of course, that would probably be very dangerous for us, but you could, exactly.

SPEAKER_08 14:35

Yeah, Chris Nolan has some great ways that we can see it with our eyes. Uh-huh.

SPEAKER_13 14:39

Yeah. Wow. So yeah, yeah, it's cool. Anyway, so so we started the project. I think they're they're fascinating things just to just to look for gravitational waves, period. And then they also tell you about the universe. They come from these big sources like black holes and neutron stars, white dwarfs, things like that. Yeah. So if you can observe them, it's like a different kind of telescope. Yeah. And you can learn about the universe. So this is this was our big motivation that we said, oh, let's let's come up with a different kind of telescope, a new, a new way to build a telescope.

SPEAKER_08 15:04

Uh that's going to be able to sense gravitational examples. Exactly. Exactly. Instead of sense um just visible light. That's right. I mean, as close as we get, which we get to we'll do you know infrared, but and you do color shift and stuff.

SPEAKER_13 15:16

Exactly. Yeah.

SPEAKER_10 15:17

So what do you got? What did you build?

SPEAKER_13 15:19

So the idea is to use this technique called uh atom interferometry or atomic interferometry. It's it's in a way similar to the lasers, the lasers going out, coming back, and interfering in LIGO. But you do this wacky thing, which is only possible because of quantum mechanics, which is you take individual atoms and then you uh uh split their wave function, it's called. So in in quantum mechanics, you know, things are never just a particle. It can't be that simple. They're both particle and wave at the same time. It's weird to picture. I want to talk about it. I want to talk about it. Yeah, but keep going. I want to talk about, yeah. So it's this is a it's a it's a cool, this is a really cool technology that that these atomic experimentalists have developed, um, which is essentially you should think of you, we often think of the atom as just being like a little dot, like a little bullet, like a little particle, right? Uh that's true, that works well in many regimes, but actually the world is quantum mechanical. So you should really think of the atom as being a wave. It has some um uh probability to be many places. Yeah. That's sort of a spread-out probability.

SPEAKER_08 16:20

Is that true of the nucleus too, or just that's true of the nucleus? Because I know that's true of the electron cloud.

SPEAKER_13 16:24

Yeah, the nucleus is just much more tightly constrained. The electron is is spread over a much wider region, but the nucleus same it's it's true of every single particle in the world has to obey quantum mechanics, so the nucleus is spread out too. Absolutely. So they you can take the whole atom. Whoa. It's weird. It's really weird. What they can do with this technology is they take the whole atom and they split it. And then they don't split it in the way you'd think when I first say that, which is like the way they did an atomic bomb where they literally made the nucleus, you know, half the nucleus went one way, you know, half the number of neutrons and protons went one way, half the number went the other. This is no, they've split the probability of the atom. So there's a 50% probability to find the atom here, and a 50% probability to find the atom here. Okay. But wherever you find it, it's gonna be a whole atom. So it the atom itself wasn't cut, but its probability was cut. It's its wave function, as we say, was was split in two. How do you do that? Uh it's weird. Uh, with pulses of laser light, it turns out. It's not obvious. In fact, this was uh you know, uh really difficult technology that's somebody won a Nobel Prize. The the atomic physicists won ultimately for this technology that could ultimately do this, they won multiple Nobel Prizes over multiple years to do all the it was ultimately actually just to cool and trap the atoms down to a level where you can just hold onto them and and mess with them, was already several Nobel Prizes. It was not trivial.

SPEAKER_08 17:46

Aaron Powell How did you So you can't do that with an electron.

SPEAKER_13 17:50

It's it's very hard. That's right.

SPEAKER_08 17:53

The the like even today, like we can't, it's still like I can tell you how fast it's going, but I can't tell you where it is. I can tell you where it is, but I can't tell you how fast it's going.

SPEAKER_13 18:01

Exactly. And the and the same thing with atoms, but the trick is you get them very cold. Aaron Powell And when you say atom, what you mean is nucleus. I mean it's just actually the whole atom, the nucleus and the electron cloud around it. Yeah, the whole atom, the whole atom together. Uh you take it and you put it in we call it a superposition of like this point and this point. Actually, they're not even at the you say they're not at that one point. They're actually a spread-out probability cloud around each point. So think of them as like two wave packets or two waves, you know, one here and one here.

SPEAKER_08 18:29

So you it could be in many other places, but you cool it and you get it to like one Kelvin and then to like decrease, yeah, to decrease the amount of things. Exactly.

SPEAKER_13 18:39

The amount of sort of random spread and spray. We don't want those things going all over the place. We want to control them. Exactly. But but they're still, they're never just in one point, exactly, as you said. Even if you get to zero Kelvin. Uh right, they're still spread, exactly. Exactly.

SPEAKER_05 18:51

What's the energy of the what's the at zero energy?

SPEAKER_13 18:55

Yeah, the weird thing is if you if you knew for sure their velocity, you would actually have, you would have no idea where they are in position. This is what's called the uncertainty principle. Yeah. So so if you knew for sure they weren't moving, so you you nailed their momentum down to zero, then actually their their position would have to be spread over the entire universe. So we never quite get to, you know, we never get there. They when they cool these things, you get them pretty cool. Uh well, let's say the math agrees, you know, what do you mean by true? The math agrees with every experiment we've ever done. So that's my meaning of true.

SPEAKER_08 19:31

So it does it, well, but it does, it's like I see where we get to multiple unit, where we get to like alternate realities, right? It's like they say they must be existing somewhere else, and we're only seeing an echo of them somehow.

SPEAKER_13 19:42

Right. Quantum mechanics makes you, leads you down some really weird paths. Yeah. That's for sure. That's for sure. Okay.

SPEAKER_08 19:49

So so the math works. So there we are.

SPEAKER_13 19:53

Yep. Yep, exactly.

SPEAKER_08 19:54

We take what kind of atom are we doing it with helium, hydrogen, what are we doing?

SPEAKER_13 19:57

Uh they're very often done with rubidium. Actually, now we've moved the donch for some technical details, we moved on and we think strontium is maybe the optimal atom for this.

SPEAKER_09 20:04

What do we like about strontium?

SPEAKER_13 20:06

It turns out it has a good level structure that um you can you can uh drive it into an excited state, but that excited state lives a long time. And that's uh very beneficial for us. We like to put these atoms in this superposition of these two states.

SPEAKER_08 20:19

Aaron Powell And when you say excited, is this the same thing that I'm gonna think of when I think of like electrons jumping up to higher orbital. Exactly.

SPEAKER_13 20:24

The same thing you learned in in high school chemistry. Exactly. Yep. And this one doesn't decay very fast. It spends a long time, relatively long time in that excited state. Aaron Powell, Jr.

SPEAKER_10 20:34

What is it what is it about strontium that the electrons don't because I thought I thought electrons they jump up, they jump back right back down.

SPEAKER_13 20:39

Yeah, generally they would. Shoot off a photon. Yeah, exactly. This one it's what's called a forbidden transition. Basically, it doesn't, it's not in a a state that that um likes to shoot off just one photon and decay to the ground state. It's gotta generally gotta shoot off more than one photon, and that uh that uh slows it down. It's less likely to do that. It'll do that eventually, but it's less likely too.

SPEAKER_08 21:00

Because of the because of the particular But then you have to excite it with two photons.

SPEAKER_13 21:06

Ah good, yeah. You have to that's right. We have to use in the lab some tricks, or they sometimes use magnetic fields to excite it, but that we've sort of learned how to do. That was again some, you know, I'm skipping there's a lot of non-trivial.

SPEAKER_08 21:15

No, yeah, but no, but this is where like this is where it gets really interesting to me, is is I was like, I was talking to somebody else. Um the overlap of of uh electricity and magnetism, right, is so profound. Yeah, and and what's really going on there, and what is like what is a magnetic field like really? And what is an electric field really? Like what is a chart like? Yeah, um it gets down to like an electron itself, right? How is an electron both a magnetic particle but also an electrical particle? Yeah, right. Like is it creating the wave of the two wave functions? Is it is its existence just the presence of the two wave functions, right? Right, um, it's like over. It's like it's interfere. Maybe it's just interference like with the other universe, right? Where right and that's like there's no way you have an electron or something, right?

SPEAKER_13 22:06

Right.

SPEAKER_08 22:07

It has these properties, it has electric charge, it has this magnetic interaction. Yeah. So you so you because if you could just excite it with a if you could excite the strontium atom with a single photon, then it would be of a of a quanta that it could just emit a single fraction.

SPEAKER_13 22:28

That's right. That's right, that's right. You do it with magnets. You can do it with magnets, you could do it with multiple photons too. It it sort of depends what is going to be the sort of uh, let's say, optimum engineering to make this uh, you know, whatever experiment you want to do with it work. How do you do it with magnets? Um the applying magnetic fields across it can change the uh the state, can change the wave function of the electrons if you like in a way that then lets it uh absorb a photon more easily. That's probably that's probably the best way I can explain it. Um you you would still use a laser to drive it. You're just also using a magnetic field. Um one way or another, you want you want a laser tuned with that, uh resonant with that transition that you're trying to drive. Yeah so it has just the right energy to drive it to just the right state because you've got to control that state very carefully. You know, you want to know for sure I put this atom in this excited state, and actually, uh I didn't say this yet, but you want to you want to um split this two halves of the atom's wave function by a very controlled uh position in space. So we actually take this, the two halves of the atom, we bring them out, we separate them, and then bring them back together and interfere them. And that's kind of the analog of LIGO bringing these laser beams out and then bringing them back together and interfering them.

SPEAKER_08 23:38

But the two positions of the atom are not two real positions of the atom, it's not like two halves of the atom, it's two whole atoms and it's bouncing back and forth between the two positions.

SPEAKER_13 23:46

Or you should think about it being in both places at the same time. That's what's really weird. Exactly. It's in both places at the same time, and when you bring it back together and and re-interfere them, you then measure it in a sense, collapse it. You make you force it to choose and you measure which way it went, and that tells you that's how you do this interferometry. That's how you measure ultimately uh, you know, how much longer did one half take than the other, or something like that, sort of analogous to LIGO.

SPEAKER_07 24:11

How the hell do you measure that?

SPEAKER_08 24:14

The how do you how do you know that's what you're doing?

SPEAKER_13 24:17

Yeah, yeah. Well, that I mean, that took a lot of work to know for sure that you were driving the atom exactly the way you wanted it to. This was this was multiple Nobel prizes to get the atom even, you know, cold and just to sit there, and then you could drive it with these resonantly with these lasers at just the right frequency.

SPEAKER_08 24:32

But what are even the instruments that that that are the they're reading this because what you're because everything you're shooting in is also going to interact with those instruments, right? Like what are you even reading? Great question.

SPEAKER_13 24:43

So what you do again, you you uh shoot more lasers, you shine light on it very more lasers.

SPEAKER_04 24:50

That should be like there should be like there should be a t-shirt. It's like it's like physics.

SPEAKER_13 24:54

More lasers. Well, you have to basically, you're not doing a physics experiment until you've got a laser.

SPEAKER_17 24:58

Yeah.

SPEAKER_13 24:59

You know, to see these atoms, and actually, so if you go to my friend Jason's lab, you can see them with your eye. You can see this cold ball of atoms, substrontian atoms, with your eye, uh, which is which is really cool to see just a few atoms, but it's because you're you shine a lot of laser light on it, and uh the laser light kind of drives uh transitions the atoms, which which ref which essentially reflect a lot of that laser light off. And then you can see that reflected light with your eye, or what we use to measure it is do you just put a camera there. You just put a good like a basically like a good cell phone camera.

SPEAKER_16 25:30

Yeah.

SPEAKER_13 25:30

And um, by once you drive it with the exactly the right frequency, that's the tricky part, then the camera can just pick up those reflect those scattered laser photons and tell you, oh yeah, the atom is here, or the atom is here. And that's how we kind of measure how many atoms went one way and how many atoms went the other. It's a cool, it's a cool thing to see.

SPEAKER_08 25:50

Yeah, okay. So, okay. So you got a strontium atom, but you don't have a strontium atom. You have a strontium atom.

SPEAKER_04 25:58

Right, split into two places.

SPEAKER_08 25:59

But you don't have one strontium atom in one place. It's split, it's like two places.

SPEAKER_13 26:03

It might be here, it might be there. You don't know exactly. You can't know.

SPEAKER_08 26:06

And then you're shooting a light through it. You're shooting photons through it. That's and you're trying, and it how like how does the phone how does it scatter the photons?

SPEAKER_13 26:14

So basically, um uh once they uh are tuned carefully to be just resonant with this with a particular transition, they uh absorb a photon and they from the laser and then they re-emit it, and when they re-emit it, they can just shoot it off in any random direction. And that's what basically what we call scattering. That's how light scatters.

SPEAKER_08 26:33

So the the laser becomes it's it's not reflection or refraction, it's absorption and and and then re-emission.

SPEAKER_13 26:40

With an atom, that's exactly basically how atoms reflect light, is is one way to say it, that they're basically absorbing and re-emitting. Exactly. That that kind of is reflection for an atom, if you like. Okay. That's what a reflection means for one atom.

SPEAKER_25 26:53

Okay.

SPEAKER_13 26:54

Um and so when it does that, it scatters this light off in around directions. We have a camera off to one side or or 360, I would think, right? If you can scatter anywhere, you'd learn in a sphere. Usually you try to enclose as much solid angle as possible. You might use a lens to bring as many of those photons back towards your camera as possible. Well, very nice.

SPEAKER_09 27:10

Yes, you can like to capture and redirect around through the lots of lenses.

SPEAKER_13 27:14

Usually you try to drive it with a with a strong laser too, so it scatters a lot of photons. So even if you lose 90% of them, that's okay because you can still picture the atom, you still get enough that you know where it is.

SPEAKER_08 27:22

Um is it the is it the actual wavelength of the of the photon coming in, is the distance between them?

SPEAKER_13 27:29

Uh good. So you you actually you make the distance between the two halves of the atom uh much bigger than a photon wavelength so that you can see it easily. So you you let the two halves separate more, then you shine this laser light, and then you can And that's by turning up the heat from like one Kelvin to 1.1 Kelvin or something.

SPEAKER_08 27:47

That's how you let them separate more?

SPEAKER_13 27:49

Actually, we don't want to do it with heat because that's uncontrolled. We want to we want to like have precise control of these atoms. So again, we do it with laser pulses. You you uh different laser. Yeah, you you the more laser pulses you hit them with, the more you can separate them. Uh essentially, if you like, imagine imagine I have this atom and I start hitting the the magic is I can hit just half of its probability, half of its wave function with some laser. But if you can buy that for a second, when I hit the half of the laser, it gets kicked by the photon, right? The the light actually has some momentum. Like you don't feel it when you're when you're getting hit by this light, but the light that we're getting hit by is actually pushing us.

SPEAKER_08 28:25

The particle component of the of the photon kicking its mass.

SPEAKER_13 28:29

Exactly. Or it has energy. We would it has momentum, we would say. It has momentum without even having rest mass, which is weird, which is relativity for you. Um it's kind of like you know, in in the sci-fi novels or even in real world, people have talked about making solar sails where you put out a satellite and you have big sails essentially on a tension. That's the solar wind. They get exactly they get the sunlight from the just from the sun could actually push you. It's not much force. We don't feel it, right? When we're walking around the earth, but it's enough that it will push you. So we use that to push the atoms around.

SPEAKER_07 28:59

Does the light photon have no mass?

SPEAKER_13 29:01

It has no mass, no rest mass, we would say. Exactly. It's massless, so it moves at the speed of light. Um, but it has momentum. That's the weird part about relativity. You can have no mass, but you can have momentum and energy. You can't.

SPEAKER_08 29:12

So what has the momentum if there's no mass?

SPEAKER_13 29:15

That's the weird thing, right? It's hard for us to picture. It's really hard for us to picture because we live at we live at non-relativistic speeds. We don't we don't see relativity every day, right? That's what's so weird about that.

SPEAKER_08 29:25

There's a little bit of a sidetrack, but we're gonna get back into it. Um something that I have you read Too Big for a Single Mind?

SPEAKER_13 29:31

No, no.

SPEAKER_08 29:32

It's wonderful. Oh, cool. It's wonderful. Uh-huh. And you might actually really enjoy it. Cool. This is the kind of thing that like real scientists like um, so I I I bought this at the bookshop at a car wash.

SPEAKER_10 29:47

It's cool.

SPEAKER_08 29:48

And it's and it's the story of um the the discovery of particle physics, essentially. Oh, cool. It starts with Marie and Pierre Curie, uh-huh, right? And the discovery of radiation, uh-huh. And it goes all the way through um like the post-war like Heisenberg. Oh, that's neat, right? That's awesome. And it and I had this question, which was much less a scientific question, more a human question, which is um when you learn about this stuff, you learn um, you know, Einstein is like, you know, talking to Bohr, right, and then Heisenberg's working with Bohr, right? And then and then um I'm forgetting the name of the like of the American, you know, but like, you know, and Schrödinger is like disagreeing and like all this, right? Or arguing. And it sounds like a bunch of people who are in the same building, right? Like going to faculty dinners. Right, right, right, right. But the reality was they were people that had to interact either in person, yeah, because like and a lot of times it was that. It was like, you know, it's like, and that's what like it's get together at a conference. Right. Or but they'd say or they'd send letters or they'd like you know, they'd have publications that everybody would write. Yeah. Um but very different than the publications we have now, where it's where, you know, where there's like you know, a whole team of editors and and all these peer reviews and all this stuff. I mean, this was like who could possibly peer review Einstein? Well like only Bohr, right? So like what's right. Um so they're just it's basically just them writing what they're thinking, right? Yeah, and then after the fact they're like working through the math with each other, right? But while all this is happening, there's two world wars. Yeah, that's the thing that blew my mind. It's like it's like Einstein's German, yeah, and this all happens from like 1905 to like it to in the 1930s, yeah. And it's like the what? Yeah, like the all of Europe exploded, yeah, and and all these countries hated Germany, right? Right, and then and then Germany hated its Jews, right? Like how on earth how is Einzel, right? And he ends up like he ends up like at Princeton, yeah, you know, yeah, but like, but like how are how did they stay focused on these things while the world around them was just like collapsing in on itself, right? Well that's what the book's about. It's about and it's like all the different, it's like it's literally, and then at the specific time, like Heisenberg is like on a hike with Bohr. And it turns out that like they went on hike, like part of what made that work is like he, you know, went for a year to work with him, they became friends, but it's like his, it's his, you know, 20 or 30 years older than him. It's like it's his advisor, it's his mentor, his teacher, right? But they would go on hikes together. And then they're having these conversations just like this, like trying to wrap, trying to like trying to speak to each other in in colloquial language about the what the math is showing them.

SPEAKER_09 32:44

To really understand it, right?

SPEAKER_08 32:45

And trying to like, and they're trying, it's all metaphor and it's all like how do we right anyway. Trying to get intuition is astounding. Um the thing that came from for me from that is somehow something like popped where I was like, oh, they did he the writer did such a good job of writing about it. Um like, oh, I get it. A particle and a wave, which I've like, you know, for 25 years, right? Whatever. It's confusing. But but all of a sudden something occurred to me. Like I I can see a wave right in the ocean. Right. Like I can see a wave, that's really easy. But the wave is not the particles. Right. The wave is transmitted through the the like the water particles, like, and it's the movement of the water particles and mass that is the wave, right? So if if a light particle, if a photon has both mass and excel, and and it's like a particle and a wave. Yeah, yeah. So yeah, it's a particle and a wave. Like what's transmitting wave?

SPEAKER_13 33:44

That's what's weird about quantum mechanics, that you know, you because we can understand macroscopic waves like water waves, because uh, they're really made up of little atoms. But but the the individual, these quantum mechanical waves, like the wave that is an electron or the wave that is a photon is not made up of things. Yeah. That's what's really weird.

SPEAKER_08 34:01

It is the thing. It is the thing. And we only call them waves because the way they scatter has the shows the same interference exactly. Same sorts of mass, same sorts of equations. Exactly.

SPEAKER_13 34:10

They act a lot like it, but they really are different than a water wave.

SPEAKER_08 34:13

Yeah, like through the two-slit experiment, we see that we see the interference.

SPEAKER_13 34:16

We see the you know the additive and subtractive interference, right? But but that's not Yeah, it's it's weird. It's quantum mechanics is really weird, you know?

SPEAKER_08 34:23

So okay, okay, okay. So you got two strontium.

SPEAKER_13 34:25

So you got so you have one strontium. One strontium atom, exactly. In two places. In two places.

SPEAKER_08 34:29

That you have excited by by putting it in a magnetic, in a magnetic like environment that excites it a little bit so that you can hit it with a photon that will excite it in a very precise way. So that an electron will jump to a different orbital that can only decay if it emits two photons. Right, basically, right. And so now it's gonna hang out there for a little while. Right. Um and that energy is what is what is what gives the sled.

SPEAKER_13 34:55

What you get basically is you can um uh push just one half of this atom by shining laser light on it. And then and then that one uh that it won't absorb.

SPEAKER_08 35:07

It's not gonna, you're gonna shine laser light on it, but in it, but in a way that it like with the quantas wrong for its electrons to absorb it. Uh so only the momentum is gonna affect it.

SPEAKER_13 35:15

You can uh that's right, or actually you can you can, if you like, you can say you make it um absorb and then re-emit uh in in just the right way that it gets uh net kicks forward, let's say. So I keep pushing this half of the atom by having it absorb and then re-emit photons. You know, like if I absorb a photon, I I if I you know a photon's coming from the right and hits me, I get kicked to the left. Yeah. Or, you know, if I uh if I shoot a photon uh back out to the right, I recoil to the left, right? Either way. Yeah, yeah. So you can you can arrange these absorptions and emissions. But I thought the emissions were random. Yeah. They if if you just let it decay spontaneously. You thought you were gonna get off easy. Yeah, that's good. No, that's well, you're getting it, you're getting it kind of what is the the key heart of the technology and the and the and the critical det you know, the devil is in the details. Yeah. Making this work, right? I mean, it took people years and decades.

SPEAKER_08 36:11

Theory is great, but like really making this function is astounding.

SPEAKER_13 36:14

Right, right, exactly. So so getting it so that these you would, we would say, coherently transfer these momentum kicks from the laser to the photon was is highly non-trivial. And I'm, you know, uh luckily that's the work that all my all my hardworking, brilliant experimental colleagues have have done over the years. Uh let's say, you know, the seem complicated, but you know, roughly, roughly what happens is Don't be rough, man.

SPEAKER_04 36:41

Just give it to me hard, just give it to me all the way, and and if I don't understand it, then I'll ask.

SPEAKER_13 36:46

You can believe that when I absorb it from the laser, right, I get this one coherent kick just in whatever direction the laser was going. Yeah. Right. Well, there's a weird, there's a weird thing that happens with with quantum mechanics, let's say, or with these rules of how the atoms interact with light, which is if I'm if I'm bathing an atom in this whole big laser beam, not only might it like to absorb from that beam, but actually, if it's in an excited state, it has a strong preference to emit a photon back into that beam in the same direction. In the same direction.

SPEAKER_08 37:14

Yeah, because it, because it, because it would, it would neutralize the momentum change. Uh yeah, right? Because then it would, it would, it would, it would, it would just like be like it was gonna flow through.

SPEAKER_13 37:23

It kind of, yeah, exactly. Like it would like to, we we might call it a a Bose enhancement, or it's sometimes called stimulated emission instead of random spontaneous emission. Oh man, does it?

SPEAKER_08 37:33

Is it like is it like and that's gonna now we were talking about the position of the atoms. Yeah. But inside that we have the positions of the electrons inside their clouds. And like I'm now wondering, is like the as the electron absorbs the momentum, does its does its position shift some? It does, it jumps up to this higher level. Yeah, but the higher, but like the higher level. It it should pick up this momentum kick. Is that like shares at the level have like a position also?

SPEAKER_13 38:01

It would. It shares it all with the nucleus, so the whole atom recoils together. So the electron and the nucleus both get pushed together.

SPEAKER_08 38:07

As the electron absorbs the photon.

SPEAKER_13 38:09

Exactly.

SPEAKER_08 38:10

The nucleus also moves.

SPEAKER_13 38:13

It was it was what they sort of never taught you in chemistry, right? They just said, oh, it just absorbs and jumps up, but it's also getting pushed. The whole atom is getting pushed.

SPEAKER_08 38:21

And as that is that because of the electrical, like is it actually just the electron is getting pushed and then the magnetism between or like never I still don't know what weak and strong force are.

SPEAKER_13 38:31

Oh yeah. This case is just electromagnetism between the the electron and the nucleus is all held together by electromagnetism. Right. Okay.

SPEAKER_08 38:36

So exactly it's just again the it's so the electron is getting pushed by the photon, and then the and but then the attachment, the like strength, but the electron has to pull the nucleus with it. Exactly or leave the atom.

SPEAKER_13 38:46

That's right. If you hit it hard enough, if you hit the electron hard enough, you can ionize it. You can you can kick it out. Yeah, yeah. But if you don't, and we don't want to do that, then it must pull the nucleus with it. Exactly.

SPEAKER_08 38:55

Okay. So now we're so so now we're doing that to only one of the two superpositions.

SPEAKER_13 39:00

One is two halves of this wave function, exactly.

SPEAKER_08 39:02

Am I using the word superposition correctly?

SPEAKER_13 39:03

We would say it's a the atom itself is in a superposition state. It's both here and here.

SPEAKER_08 39:07

Okay.

SPEAKER_13 39:08

It's both in you know, position.

SPEAKER_08 39:09

So what do you call one half of the superposition?

SPEAKER_13 39:11

Um half the wave function. Half the wave function. Yes. It's weird. Quantum mechanics is weird.

SPEAKER_07 39:19

It's so weird, like because it's like a thing that isn't a thing.

SPEAKER_13 39:22

It's very hard to understand. It's very, you know, I I think uh I think it was paraphrasing Feynman here who said, and you know, he he, if anyone understood quantum mechanics, it was Feynman. And he said, anyone who tells you they understand quantum mechanics doesn't. Yeah. Something along those lines. Yeah, yeah. Which I think is very apt. It's it's you can't it's hard ultimately to really get intuition for it. Yeah.

SPEAKER_08 39:42

So you don't call it anything.

SPEAKER_13 39:44

So we'd say half the wave function. You know, you can describe it in math and you can say, oh, well, half its probability is over here, and it's getting kicked more, more to the left. But you're not kicking the other half. I'm not kicking the other half. How is that possible? Uh there's a few ways.

SPEAKER_08 39:58

Um maybe You have to tune that laser so precisely.

SPEAKER_13 40:02

Right.

SPEAKER_08 40:02

What's the what's the distance we're talking about? Like what's the distance between the two halves of the wave function?

SPEAKER_13 40:06

Uh good. Well, they start right on top of each other, but no, they can't start right on top of each other because they they have to exist in the whole universe. Well, well, that that's true. That's true. The the wave function. The wave function. You're right, you're right. The wave function starts in a small ball. You know, you can think of it being maybe millimeter big. The atom is spread over maybe about a millimeter or so. It can vary, but a millimeter.

SPEAKER_08 40:26

Or it could be even smaller, but we we they make these things so cool that they're A millimeter is is like that contains multiple Avogadros numbers of atoms.

SPEAKER_13 40:35

Right, right. It would normally, if you were in solid, it would contain a lot of atoms. So we do this in high vacuum, hard vacuum, and you have just a few atoms.

SPEAKER_07 40:42

How do you even create a vacuum that that's that's that?

SPEAKER_13 40:45

All of this is tricky. All of this is tricky technology. Exactly. You gotta pump down to hard vacuum.

SPEAKER_21 40:50

Uh but what are you even pumping? Like dude can't pump air.

SPEAKER_13 40:53

There's there's that's what the other lasers?

SPEAKER_21 40:55

There's there's it to like push things out?

SPEAKER_13 40:58

Uh they they you you have a variety of pumps, but some of them, actually, some of them are are in a sense really old school. You just use fans, basically, as a pump. Like literally air. It just blows it out, and then when you get down to very few molecules left, you don't really think of it as air. You think of it as individual like nitrogen molecules bouncing around your tank, and occasionally one of them hits the fan and gets shot out. Think of it like billiard balls. Well, occasionally one of them reaches the exit and leaves. That's how a vacuum pump works. Yeah.

SPEAKER_08 41:22

Yes. You just have to like pump it for a while.

SPEAKER_13 41:24

A long time. That's right.

SPEAKER_05 41:25

But where are you pumping it from that nothing's coming in? Uh that's what is it blowing?

SPEAKER_13 41:31

You gotta push hard, right? So nothing goes back the other way. You gotta make this one-way valve that just shoves it out. Exactly. Exactly. Oh, I see. Okay, okay. So you're like pumping it out into the air, right? Right. It's like an outfan. Yeah, exactly. You have you have a very sealed container. It's very hard to make this thing very sealed, so there's no leaks, but you gotta make it really sealed. Uh you know, the the people who do this have to worry about getting pull getting this kind of hard vacuum, it's now old technology, but when it was to make it happen, now we know how to do it, but to make it happen was not easy. Yeah. You know, it's still it's the sort of thing that when a when a person is first learning it for the first time, it takes a lot of work. You will very often would have leaks and you gotta go find what do you even use to okay. Right. You know, metal will hold a good vacuum.

SPEAKER_05 42:11

And gravitons are going in and out at no matter what there is no vacuum. There's gravitons doing them, so they just don't care about it.

SPEAKER_08 42:18

Yeah, that's right. Okay, so you got vacuum. Then how do you get a single strontium atom into the vacuum?

SPEAKER_13 42:23

So you build in your in your system a little ball of strontium somewhere. And then what you do is but it would be like uh it could be in solid form. Yeah, like stuck in the wall, for example.

SPEAKER_08 42:33

And then you like shift the electrics somehow that it like releases the strontium ball and for that actually the field.

SPEAKER_13 42:39

You don't even want, you don't want to release a macroscopic bunch of strontium. You want to boil off individual atoms. So you tend to boil off. You tend to heat it. Come on. So you you would you would rip off individual little atoms boiling out of the strontium.

SPEAKER_08 42:52

Aren't we at like near absolute zero?

SPEAKER_13 42:54

Well, that's the weird thing. First you make them hot, you boil them all out of the macroscopic sample, then you have this gas of strontium, and then you cool it way down. So you have to do this multiple stage process. It's it's highly non-trivial.

SPEAKER_07 43:06

Okay.

SPEAKER_13 43:07

Okay.

SPEAKER_08 43:08

How hot do you have to get a ball of strontium to volatilize single atoms?

SPEAKER_13 43:13

Yeah, it's an So you don't boil the whole thing. So you don't get it above like a uh even you know, a boiling point of the actual metal strontium. Yeah. You know, what like like uh water, right? Water would boil at 100 Celsius. Oh yeah, but it volatilizes like little bits. But that's right, even at room temperature. Exactly, right? So so same sort of idea with anything. Yeah, yeah. Even well below the boiling point, you get some atoms coming off. But it's a metal. Even I know it's weird, but metal, but metal, metal will melt and metal will ultimately vaporize like other things. Vaporize it like, you know, like you know, a thousand. Well that's why they need to use a heater to get enough strontium. But but as you said, you know, we have Avogadra's numbers of strontiums. We have 10 to the 23. And we only want uh ultimately about a million atoms.

SPEAKER_08 43:56

Okay, but now we now how do we even know that it's there like in the first place? Okay. Right. Actually, even seeing that you've got it is a little non-trivial. Right. But because your vacuum space, like it's bouncing around. Exactly. Exactly.

SPEAKER_05 44:09

How do you even tune the laser to push one hat?

SPEAKER_13 44:12

Ah, so so exactly. So then grabbing it and holding it in place where you want it, this is what we call this cooling and trapping of atoms. This was like three Nobel prizes. Yeah. It was non-trivial. It was highly non-trivial. And it's done in multiple stages. You first cool in 2D and it becomes a single beam that's very hot, like in the X direction. It's cooled in the Y and Z axis and it's hot in the X-axis. So it sort of shoots along this way.

SPEAKER_08 44:36

Wait, wait, wait, wait, wait, wait, wait. Wait, wait, wait, wait, wait. You're cooling X and Y?

SPEAKER_13 44:41

You cool like two of the dimensions, the X and Y. But heating Z? But you leave it hot in Z and it travels and then you funnel it. How do you do that? Let's say, let me let me describe in general one of the ways to there's there's actually it seems like it's gotta be a laser. Yeah, yeah, exactly.

SPEAKER_08 44:59

But what you do is you cool the whole space and then you shoot a laser along one plane.

SPEAKER_13 45:02

And exactly, we in fact the experiment is not cryogenic at all. So the walls of the experiment are room temperature. You can touch them. They're they're completely room temperature. It's just the tiny amount of strontium atoms that is super cold, because that's all we care about.

SPEAKER_22 45:13

How do you do that?

SPEAKER_13 45:15

So so uh one of the dysentropic that uh is this gonna be like sucking the entropy out? There is some of that. There is actually so one of the ways to cool, in fact, one of the last stages they use to get it as cold as possible is if you if you have a a cloud of strontium atoms that are contained in some in a what we call a trap, which is basically electromagnetic fields that are holding them in place, then they're held in place up to a certain, like if you could you think of like an activation energy in chemistry, right? There's some potential barrier. If you have enough energy, you'll climb out of it, but if you're if you're not energetic enough, you stay in this well. Okay. Well, then we we use essentially evaporation in reverse. You just slowly turn down the height of the barriers and the hottest particles leave first. That's evaporation, right? The hottest water molecules in your in your water cup leave first. Yeah. And that's why evaporative cooling works. That's how you cool things, right? A water cup as it's evaporating, actually continually cooling because you've selected the hottest molecules.

SPEAKER_08 46:11

If they leave, how do you keep them from re-entering?

SPEAKER_13 46:13

Uh actually, they're cool. They're fine. They can just bounce around wherever they want. We just ignore them because there's so few of them. Yeah. And then you take this ultra-cold cloud you've got left, and you can work with those. You shine your lasers on them, you do all this fancy stuff that's so you're not actually cooling them. The last stage is only selecting. The last stage is only selecting. However, there's earlier stages that are actually cooling all of them because that selecting is very lossy. We lose a lot of atoms and we don't want to lose too many because we need them for statistics for our experiment. Yeah, yeah. So the the first, some of the earlier stages for which there were Nobel Prizes are are um what's called laser cooling, which is uh essentially two opposite laser beams shot at each other. The atoms are in the middle, so they're they're bathed in this laser light, which is hitting them from both sides. Okay. But they're they're uh just slightly what's called red detuned, which means um the laser's a little bit lower frequency than it would need to interact with the atoms splitting with the atom. And what that means is when the atom moves toward the laser beam, it gets a Doppler shift. If you if you remember a Doppler shift. Right? So the if you think in the atoms rest frame, if you're riding along with the atom, when the atom is moving toward the laser beam, it sees a slightly higher frequency laser beam, right? It's it's blue shifted. Oh yes, yes. And then it can interact. So the atom, if it moves toward the right laser beam, it's more likely to get kicked from the right. If it moves toward the left laser beam, it's more likely to kick from the left. This is cooling. You slow it down this way. If it wants to move to the right, it gets kicked backwards. If it wants to move to the left, it gets kicked backwards. This was a, this was a, you know, this was a very impressive piece of work when it was first done several decades ago. And you can actually, you can have this counterpropagating.

SPEAKER_08 47:51

With a kind of energy, with a, with a, with a, with a fo a laser photon that is just the wrong quanta to actually pop an electron up.

SPEAKER_13 47:58

To be absorbed, that's right. And and in fact, you have to control the frequencies very precisely. It's only right, you know, you make you you you tune it carefully so it will cause cooling, for example, so it will kick anything backwards away from the direction it's going.

SPEAKER_08 48:11

What's the like um the nanometer frequency shift?

SPEAKER_13 48:16

Oh, you know, I don't know. This is not my work, because I'm not I'm not an atomic experimentalist, right? So I couldn't tell you the numbers off the top of my head.

SPEAKER_08 48:23

So I'm thinking like, you know, visible light, like 300 to 800 or whatever. And like, is it is are we talking like, are we talking like if one lay like the lasers at like would have to be at 280, but it's at 245?

SPEAKER_13 48:34

Oh, it's even like or is it like 280 to like 268 or something like that? Even much smaller than that. So an atom at room temperature is moving at a few hundred meters. I can approximate it for it. I don't know the exact numbers, but I can approximate it for you. Atom at room temperature is moving at a few hundred meters per second, which is about uh uh 10 to the minus six of the speed of light. Okay. And the Doppler shift is controlled by that, the ratio of the speed to the speed of light. So the amount at Doppler shifts by its even room temperature motion would only be a part in a million, one a part in 10 to the minus six. So that that the your your frequency shift should be about that or smaller in order to um, you know, you want to walk it on resonance by this Doppler shift from the speed. Yeah. So it's a really tiny freak. We have to control those frequencies very well. You know, this is really precision technology. Yeah, like what the what the So be less than a part in a million or something. I like I said, I don't know the number exactly. We can look at that.

SPEAKER_08 49:23

And are we talking about less than a part in a million of like nanometer wavelength? Or are we talking like exactly? Yep. And is it but but there, but like nanometer is already like I'm now really, but anyway, we can consider a few hundred nissometers. No, no, no, let's know, it's not a big so okay, so now we've got um strontium atoms that we boiled off, that we that we that we that we like allowed to volatilize off. Right, right. Not really boiled, but it's just like a like get a couple of airborne, get some airborne, right? Right, right. Um, in this vacuum that we've created by turning on a really high power fan that that like eventually the nitrogen atoms bounce into. Right. Exactly, exactly. And so then we've got some of these, okay. Now we now, and these are still moving at what you said 10 percent meters per second. Um bound just so like constant motion in this. How big's the how big's the vacuum?

SPEAKER_13 50:12

Let's say 10 centimeters or something.

SPEAKER_08 50:13

Okay, so it's like it's on your desk.

SPEAKER_13 50:15

Two centimeters. Yeah, yeah, yeah.

SPEAKER_08 50:16

It's like yeah, it's like a uh so like 10 centimeters.

SPEAKER_13 50:18

Yeah, yep, yeah. They're souncing around all the time.

SPEAKER_08 50:20

Now you've got, now you've now you've got two lasers that are shooting at that are shooting like right at each other. Right. And any and any uh individual atoms that get like just happen to get stuck in between those two, or do the lasers fill the entire space?

SPEAKER_13 50:35

You would tend to fill as or a good chunk of the space so that you could slow a lot of the atoms down.

SPEAKER_08 50:39

And slowing them down cools them, which is weird because like no, Cassandra, that's weird, because you'd think that it would actually be potential energy, that you'd be heating them up.

SPEAKER_13 50:49

You'd think lasers should eat them, right? That's your gut. Only very carefully tuned frequencies and stuff will do this, right? This was uh this is why this took Nobel Prizes. Yeah, you know, this is why it took a lot of work that this is not just what happens if you randomly shine lasers on atoms. Yeah, yeah.

SPEAKER_17 51:02

Yeah, yeah.

SPEAKER_13 51:03

Um exactly. So you you cool them, uh, you get them in this one spot, and then you you use sort of similar techniques to to excite them with these lasers to split this atom's wave function to start driving one half away from another half. And then that's basically the experiment. You you separate these two halves of the atom's wave function of its probability.

SPEAKER_07 51:20

But then like, how do you only hit one half?

SPEAKER_13 51:22

Ah the you use some of the weirdness of quantum mechanics. If so, so you so if I shine a laser on an atom, it can cause it to jump up a level. Right. Right? And when it does that, it's also like it absorbed a photon, so it'll it'll recoil. The atom will recoil, right? It got killed. Okay. Okay. Here's the weird thing. There is a there is a time scale for that to happen. So it won't happen instantaneously, right? If I shine just a tiny bit of light, of course the atom won't jump. It takes some amount of time. We would call that like a Robbie flopping time, it turns out. The amount of time to flop up from this state to this state, that's that's the way we call it. If you do it, so if you do it for one times that time. Yeah.

SPEAKER_08 52:04

So so if you do it for that amount of time, he was a server, just hanging out. Robbie. He's like, Sup, Robbie. How you doing? How you doing? Yeah, man.

SPEAKER_19 52:16

You figured it out. I don't know.

SPEAKER_04 52:20

But uh, this guy was definitely at UC Davis or Sunabar. Probably not. Probably not. This was a while ago. Yeah. But uh I should not assume it was a male. It was not necessarily male. I probably think he was.

SPEAKER_13 52:33

But um uh so but so it moves from one state to the other. If you shine it for this amount of time, it'll do that precisely. The whole atom will move from one state to the other. If you shine it for half that time, only half the atom moves. Half the atom's probability moves. And the the math of quantum mechanics describes this very clearly.

SPEAKER_08 52:48

And again, the excitement is the same thing. It's an electron just jumping up a just jumping up a quant uh a lot.

SPEAKER_13 52:54

The atom's been changed its state and it's gotten a kick. Right? Exactly. That's the that's the process. And it's and if it takes a certain amount of time to do that, and you wait only half that time, you'll move only half the atom. But only half the atom's probability. So the atom will be half 50% chance it's in this state. 50% chance it's in this state.

SPEAKER_08 53:14

You're threatening quantum theory, right? That's what's the weird about quantum. That's just so weird about it. Like I'm saying, like this isn't quantized. Uh yeah, quantum is sort of a misnomer. We could you, could you, could you shine it for a quarter of the time? And move it a quarter of the distance. One quarter here, three quarters of the distance. Is this now continuous? I thought nothing in physics was continuous.

SPEAKER_04 53:31

Ah, yeah, no, it's a weirdness of the way we use it.

SPEAKER_13 53:33

The fundamental math underlying quantum mechanics is continuous differential equations based on you know the real number line. So everything we do in every every time we're solving something in quantum mechanics, we're solving basically continuous differential equations. So it is continuous in that sense. We the word quantum is used because these energy levels are quantized. But you don't have to be all in one or all in the other. That's the weird thing. It's really weird.

SPEAKER_08 53:59

I thought that was the whole point, was that you could only like, but is that just a macro uh they they teach it that way in high school to simplify this the matter.

SPEAKER_13 54:08

But quantum mechanics is actually you can be 59% in one state, or you know, 58.5% in what then there wouldn't be a sodium spectrum. The thing is, that's what's so weird. The the half that's in the ground state can't emit any photons at all. And the half that's an excited state can only emit photons of the right energy. So the spectrum is still there. You only get discrete sets of photons. You still, you know, you see those lines that that of course people have known about for over a hundred years. Yeah. But that's but it there's a continuous amount of probability that could be in that state. That's what's so weird about quantities. It is quantized. The the levels are quantized, the energy splittings are quantized, but how much can be in that state? Continuous. It's continuous. Quantum mechanics is weird. It's weird. It's funny to think about.

SPEAKER_08 54:54

So now that we've got, okay, so now we've now we've used now we've used a laser.

SPEAKER_13 55:01

Yeah, we lose the laser, we put half the atom in one in one of the states, half the other end, and the half that that absorbed the photon gets the kick. Yeah. Then you just wait and the two separate, right? The half that didn't get a kick sits here. The half that got the kick moves, separates out.

SPEAKER_08 55:16

As it absorbs more and more and more in control.

SPEAKER_13 55:18

And then you can do it again, right? You can keep doing it.

SPEAKER_08 55:20

But how do you but like wouldn't the other half isn't the other half as likely to get kicked?

SPEAKER_13 55:24

Yeah, so here's one way to do it. This is this is maybe the best way to explain, and one of the ways they do it is the half that's moving is Doppler detuned, right? Now it's moving, so it sees all the frequencies shift a little bit, and these things are so precise, such narrow atomic line widths, that you apply a laser of the right frequency to interact with this guy, the one that's moving, the half that's moving, and not with the half that's stationary, use that Doppler shift to do it for you. That's one way to describe it.

SPEAKER_15 55:49

Yeah.

SPEAKER_13 55:49

So that's one of the many cool tricks that goes into this technology. So now you've done this, now what did we just achieve? That's right. Okay, good. So what do we got? So now we can separate two halves of the atom. We actually we use lasers to separate them and then to bring them back together and re-interfere them. And that's sort of analogous to LIGO shooting two photons in different directions, shooting them, bringing them back together with mirrors, and then reinterfering them. Now we've made what's called an interferometer. Only we've made it with atoms, with matter instead of with light. We've done matter wave. Sometimes we call it a matter wave interferometer.

SPEAKER_08 56:19

What's the inner? So what we're set so when they come back together, you're now pushing it back? With light, you should use the other wave. Same thing. Like and catch the ones that now you're trying to figure out how how much have they like detuned from each other?

SPEAKER_18 56:35

Yeah.

SPEAKER_08 56:35

And that's how much have the two halves.

SPEAKER_13 56:38

Basically you're asking gone out of phase. Exactly, exactly. How much have they gone out of phase? What has happened to one that didn't happen to the other? Right. With with LIGO, it's the gravity wave like squeezed one and stretched the other. Yeah. Right? Well, kind of the same thing here. We're gonna be looking for the gravitational wave came through and it it affected one half of the atom somewhat differently than the other half, is one way to say it. Uh so it's it can't be.

SPEAKER_08 57:06

What are the other ways to say it? Like, what do you what does that mean, right? Like because these grab but gravitons don't do anything. Like so the the forces are so weak. Each one is the shit, like with because it makes sense that you could read this incredibly weak thing with lasers that like using mirrors are traveling hundreds of thousands of miles, right? Like, so you've got this huge, so there's like a largeness to what you're what you're doing to my understanding is that what made LIGO really work is when they they just made them like a little bit long, they made them longer. Oh that you can't do it.

SPEAKER_02 57:38

That's right, that's part of it for sure. That's part of it.

SPEAKER_08 57:40

So that the late so that the light travels to sharp so you get more shift.

SPEAKER_13 57:44

So the same read. Yep. So we will want to build a really big one. Exactly. We want to build a really big one. Uh no, no. We will want ultimately kilometers. Kilometers. Yeah.

SPEAKER_08 57:54

You're gonna have to push a strontium atom into two, into a super, into a two, but they're like where one half of the atom, one half of the wave function of the atom is a kilometer away from.

SPEAKER_13 58:06

Uh no, this is the trick. This is the trick. That would probably be a little too hard. So we came up instead with a different trick. However, I will tell you what's really cool. There, there, the more you can separate these two halves, basically, the more sensitivity your device has. Because the further apart they get, the more whatever hits them is gonna interact with them differently. Definitely, exactly. Exactly, right? So we definitely we do want to separate them as much as possible. When we started this whole project, which was like 20 years ago, when we started thinking about this, it was just you know ideas on a blackboard when we were just sketching it out. The most they could separate it was like millimeters. But we knew we needed money.

SPEAKER_08 58:38

I'm amazed you're measuring in millimeters.

SPEAKER_13 58:40

Even that is a lot. Even this is impressive. I'm talking about that.

SPEAKER_05 58:43

I was expecting, I was expecting angstroms. Yeah, but I'm expecting nanometers in the most. Exactly. No, millimeters is already really impressive. That's crazy. I can see a millimeter on a ruler. Yes, yes. Now here's the really cool thing.

SPEAKER_13 58:55

Since that time, my colleagues here at Stanford and other places have been working to improve and improve this technology. Now they've gotten something amazing. Now they can do almost a meter. So they can have one atom in a superposition here and here, literally a meter apart. Like you can just, you could walk through it, right? You can stick your hand through it. It's insane. A macroscopic superposition. It's crazy. It's a really cool demonstration.

SPEAKER_08 59:15

Except for the environment in which it which it exists. You can't do that.

SPEAKER_13 59:20

But it's it's that far apart, right? It's a meter, two one atom split in two places. It's both here and here, a meter apart. It's insane. It's quantum mechanics on a macro scale, it's really insane. Makes a great demonstration for like a quantum class, undertime quantum class. And you can and you can do it on a desktop. Uh yep, they've got it. They've got the lab downstairs. Exactly. Exactly.

SPEAKER_08 59:39

You can collect all this machinery, like tuned lasers and all this stuff. You can just do it on it on a table.

SPEAKER_13 59:44

Yep. Yep. And be like, here we go. It's a big table. But yeah, exactly. Exactly. It's really cool. And you can you can see that you've separated them, you've controlled them, uh, and and this uh is not just a really cool demonstration with quantum mechanics, but it's also was crucial for getting the sensitivity we needed to actually see gravitational waves.

SPEAKER_07 1:00:02

Well, so what so then what's the actual shift that you're measuring?

SPEAKER_13 1:00:06

Ah, good. Okay, so it's a it's a little complicated to explain. There's a few different ways to say it which sound different but are all the same thing in physics, which is happens a lot. Let me let me try one way to explain it. I love how it becomes poetry. It kind of does. It kind of does.

SPEAKER_08 1:00:22

It's like, how do you explain? Well, there's a couple of different ways to explain joy.

SPEAKER_04 1:00:25

Yes, right?

SPEAKER_08 1:00:26

That's right. But they all kind of come down to the same thing.

SPEAKER_04 1:00:28

Yes, yes. Oh boy, okay. Exactly. Okay.

SPEAKER_13 1:00:31

Um one way to explain it would be the um uh the phase that an atom picks up as it's kind of existing, right? We're gonna we're gonna measure the phase difference of these two halves that went out and come back together. That's what we're gonna see, right? How much did one's phase evolve relative to the other? Phase. Yeah, it's weird. If you think of the atom as a wave now, so now I'm gonna jump and think of it as a wave. The phase is essentially how many cycles it's gone through.

SPEAKER_08 1:01:04

So with uh cycles of like the going from high amplitude to like peak to trot. Yeah, peak to trot.

SPEAKER_13 1:01:10

Exactly. So if you think of a laser beam, you you might think of it as like a sinusoid, right? Yeah, yeah. And and la what does LIGO do? It it essentially shoots this laser out and back and it checks. Did this laser do, you know, 1,700,000 blah blah blah thousand point one radians?

SPEAKER_08 1:01:26

Yeah, and and you and you want it to be less than one full 2 pi radians. Yeah, once it's gone 2 pi, then you're back to in sync again. So it's no longer valid.

SPEAKER_13 1:01:35

Exactly. Our signals are so incredibly tiny that our problem is we're dealing with like thousandths of a radian. So we're we don't have a problem, you know. I would love if our problem was that they were they were a whole cycle pig, you know, like these tiny signals. So my go compares one laser beam to another and asks if they're slightly off. Right, right. Right, right. But but how does it work in strontium atoms? An atom is also a wave. So picture it again as a sine curve. It's a wave in its probability space, it's a wave function, so it's really hard to picture. It's a wave in this probability space. But anyway, it's a wave. And um, and if the two halves come back a little bit off, we can see that in the interferometer.

SPEAKER_08 1:02:10

What is it you're actually measuring to see that? Uh because when because when you do it with the lasers, yeah, you you you like then direct them at the same target and you can clearly see.

SPEAKER_11 1:02:20

Right. You can clearly see constructively interfere.

SPEAKER_08 1:02:23

Or destructively interferes. It's basically the same thing. If the atoms come back where they're a photometer, like that's it's really easy to just write, like how do we measure the like the photon?

SPEAKER_13 1:02:31

Yeah, that's measuring like how many photons hit you, right? Yeah. What we do is we measure how many atoms. So we just count atoms with this camera again that I was telling you about. So so if the atoms come back maximally interfering, there'll be a lot of atoms going that direction. And if they come back destructively interfering, there'll be no atoms, it'll be dark. Our camera will show nothing. So we just count atoms with the camera, and that's how we measure this.

SPEAKER_08 1:02:51

Wait, wait, wait, wait, wait, wait, wait. So it's they they emit light or reflect light?

SPEAKER_13 1:02:56

Uh they they we shine this laser on them to make them reflect light so we can see them with a camera. Exactly.

SPEAKER_08 1:03:00

And if they are constructively interfering, there'll be a lot of them. Then they are absorbing and re-emitting photons.

SPEAKER_13 1:03:07

Yeah, or you say there's a bunch of atoms there, and then they can absorb and re-emit these photons.

SPEAKER_08 1:03:11

And if they are destructively interfering, then there's no atoms there.

SPEAKER_13 1:03:14

They were they're gone. They went a different direction, really, is what happened to them. But they didn't show up in this camera, for example. It's weird. It's weird to think about.

SPEAKER_08 1:03:25

How do you know there's just no atoms there? Like, how do you know that what happened is they destructively interfered?

SPEAKER_13 1:03:30

As opposed to we messed up and we just somewhere else now. Very good, very good. We we there's actually in LIGO, there's two output ports. When the lasers come back together, they hit a final beam splitter, and each laser beam can go one of two directions. So the the one that's coming in, you know, from from the from the west goes axis two ways. Exactly. X-axis. The one that's coming in on the y-axis goes two ways. And then you actually have two, you can have two photometers measuring those two output ports. And if one of them is now they're overlapping, right? Yeah, now they're yeah, so right? Now now the the one and overlap the way. Exactly, and you overlap each each quarter, right? The two quarters overlap one way, two quarters overlap the other. Yeah. If one of the output ports goes dark in LIGO, the other goes maximally bright, and vice versa. So you never lose the photons. They just go one way or the other way. And measuring which way they went tells you the phase splitting in LIGO. And it's kind of the same thing.

SPEAKER_08 1:04:21

No, they did go both ways. They just destructively interfered.

SPEAKER_13 1:04:24

That's right. So they destructively interfered. So actually, one of the output ports might be dark if there's destructive interference, right? So that meant there were really no photons hitting that camera. And all the photons went the other way and hit it.

SPEAKER_05 1:04:32

No, there were photons hitting that camera. It was destructive interferes. The two beams destructively interfered.

SPEAKER_13 1:04:36

This is the weirdness of quantum mechanics. So it doesn't, so it doesn't register. You could say both things. Both things are true. And they both describe the same ultimate output, which is this camera saw nothing, this camera saw a lot of photons. And we could say the same thing with the atoms. There's atoms destructively interfered in this camera, and they constructively interfered in this other camera, so this camera saw them all, and this camera didn't see any.

SPEAKER_08 1:04:55

But what is it? But what are you actually seeing? Like I'm still in this place, like what is it, what is because like with the with the light waves, I get like I get it. It's like two wave functions overlapping. Right, right. Right?

SPEAKER_13 1:05:07

And see these two waves over the water. Right.

SPEAKER_08 1:05:09

And the but the atoms themselves don't emit a wave function. They have to, right? Like they are a wave function.

SPEAKER_13 1:05:14

So you don't emit a wave. Well, you have to picture them as a wave. Right. So they could either constructively or destructively interfere. Right. And then we hit that wave with photons. But it's not right.

SPEAKER_08 1:05:22

Well, but that's what I'm saying is like then you hit that wave.

SPEAKER_13 1:05:24

That's the extra step. So then it's gotta like it's gotta emit We gotta drive it. And it's being, but it's being driven the same because if we're not gonna do it.

SPEAKER_08 1:05:30

Wait, wait, wait, wait, wait, wait, wait, wait, wait, wait, wait. Okay, so so what you're saying is if there if there's maximal destructive interference. Right.

SPEAKER_13 1:05:39

You should really say there's no atoms there. Because there were two wave functions, but they added to give zero. That's the problem. Now there's nothing.

SPEAKER_08 1:05:47

So there's nothing there to absorb wave.

SPEAKER_13 1:05:49

Which is really weird. Exactly. Exactly. Oh my god. Exactly. It's the weirdest of quantum mechanics.

SPEAKER_08 1:05:55

Oh my god.

SPEAKER_13 1:05:56

It is. It's weird.

unknown 1:05:58

Yeah.

SPEAKER_08 1:05:59

How does that how does that That measure a graviton?

SPEAKER_13 1:06:02

Ah, good question. Exactly. So so far we've just described what an anomaly pherometer is. Yeah. Now the trick was we had to figure out all right, how do we use this tool to measure gravitational waves, right?

SPEAKER_08 1:06:12

What is it about the gravit the gravitational wave that shifts that shifts the interference of the right? Because with the with the with the again, with the laser beams, it makes sense. It's this, it's it's the the actual beam itself goes faster or slower.

SPEAKER_04 1:06:26

Yeah.

SPEAKER_08 1:06:26

Yeah. Right? And that's what causes, that's what creates the interference pattern.

SPEAKER_13 1:06:29

That's right. That's right.

SPEAKER_07 1:06:30

What is it in atoms?

SPEAKER_13 1:06:31

So good. Here's one way to say how how what it is. The we have this atom interferometer. The atom's been split in these two halves, and then I'm gonna make sure that this is still recording. Yes, yes. All right, nice. So so we have we have this atom, right? It's been split in these two parts. Okay. Imagine that what you've done is half the atom is actually in this uh excited atomic level, right?

SPEAKER_20 1:07:01

Okay.

SPEAKER_13 1:07:01

Well, here's an interesting fact about quantum mechanics. The phase with which this wave function ticks or or wiggles, you know, the how much it's waving, it's its frequency, how much it's wiggling, is set by its energy. So what happens is when we've when we've kicked the atom into a higher level, its phase is ticking faster. Think of it like a clock. In fact, this is actually literally how you make an atomic clock. The technology I just described to you is also an atomic clock. It's another word for atomic clock, basically. So what happens is you put the atom in this superposition. Half the atom is in this higher state, half in the lower state. Well, the higher state's phase ticks faster. So it's a faster clock than this lower state. And when you bring it back together, you can see that. The phase has evolved.

SPEAKER_08 1:07:47

How's that different from just so okay, so so then you're gonna see a slightly higher frequency. But there's not gonna be any change in amplitude.

SPEAKER_13 1:07:55

Well, right, no change in amplitude, but it's ticked a little more. So its phase has advanced a little more, right? That's what you see with the interferometer.

SPEAKER_08 1:08:01

Because if it was just energy coming in, it would change the frequency and the amplitude.

SPEAKER_13 1:08:05

Uh right. So it's exactly. So it's it's by being in this higher state, it's it's ticks faster. And then essentially you can measure time with that.

SPEAKER_08 1:08:12

Yeah.

SPEAKER_13 1:08:12

That's how they make an atomic clock.

SPEAKER_08 1:08:14

You're shifting one, only one component of the wave function.

SPEAKER_13 1:08:18

Right, exactly. Exactly. That's how an atomic clock works. And that makes, in fact, the world's most precise clocks are atomic clocks, because they're determined just in terms of this strontium.

SPEAKER_08 1:08:26

Wait, wait, wait, wait. Okay. If you increase the energy, and again, we're in the same place of like we're only increasing the energy in one half. Yeah. Um Whoa. Uh it's like there's an XY thing here where frequency is X and amplitude is Y, right? We're like, we're like the energy of movement of like how how far we what is like how many atoms didn't change.

SPEAKER_13 1:08:51

It's still still 50%.

SPEAKER_08 1:08:53

But then but then the speed with which the wave passes through, right?

SPEAKER_13 1:08:56

Right is changing.

SPEAKER_08 1:08:57

How on earth are you hitting? How is a graviton only hitting?

SPEAKER_13 1:09:00

Ah, well, we're not at the Graviton yet. Okay. So first imagine I can make a clock, an atomic clock with this. So the trick is, right? Um uh when I bring it back together, the amount by which the phase is shifted is proportional to how much time they spent apart. Because the phase ticks at a rate.

SPEAKER_07 1:09:16

Yeah, because once they come back together, they have to collapse.

SPEAKER_13 1:09:18

That's right. Then it's over. Measurement's over, you've made it's you've experiments over, you've made your measurement. But the phase difference that you read out is proportional to T, the time elapsed while they were in different states, right? Because this one was ticking faster while it was in a different state.

SPEAKER_08 1:09:29

And we're talking thousands of the radian.

SPEAKER_13 1:09:31

So exactly tiny fractions of radian, and that we can see. We can see, and so we can measure time really precisely. It turns out it can be measured to, you know, tiny, tiny, tiny fractions of a second, right? Femtoseconds, atomosec, whatever. So once you've made this basically makes an atomic clock. Once I've made an atomic clock, I actually make two of these things and I space them out really far, like kilometers apart, and then I shoot a laser beam across the baseline.

SPEAKER_08 1:09:58

I if you like, in fact, the baseline, you mean between the two, between the two clocks. Like in the straight, in the straight line.

SPEAKER_13 1:10:04

That's right. Between the two. So I shoot it from one to the other. Uh-huh. And actually, I usually I shoot it and shoot it back.

SPEAKER_23 1:10:09

Uh-huh.

SPEAKER_13 1:10:09

And I the imagine the laser beam is the thing that starts the the stopwatch. It starts the clock, and then when it comes back, it stops the clock. That measures how long it took light to travel. Why?

SPEAKER_08 1:10:21

Like what do you because because like what's actually happening when it hits the second one that it's now absorbing or changing in some fashion based on how far that atom, like that half of the atom's wave function, has shifted in time.

SPEAKER_13 1:10:33

We're actually using those lasers to drive the atom interferometer. They are the lasers that are doing this splitting that I talked about before. Yeah, yeah. So that when they hit it, boom, you split the atom and the clock starts ticking. And when they come back and hit it again, that's the final recollapse where they reinterfere. And so between those two pulses is when the atom interferometer was happening.

SPEAKER_08 1:10:52

And that measures- Wait, so you've only got the two strontium, though the two halves of the single strontium atoms wave function separated for like the smallest, I mean, the amount of time it takes light to travel.

SPEAKER_13 1:11:03

Uh let's say, let me let me describe in general one of the ways to that there's there's actually it seems like it's gotta be a laser. Yeah, yeah, exactly.

SPEAKER_08 1:11:13

But what you do is you cool the whole space and then you shoot a laser along one plane.

SPEAKER_13 1:11:16

And and exactly, we in fact, the experiment is not cryogenic at all. So the walls of the experiment are room temperature. You can touch them. They're they're completely the room temperature. It's just the tiny amount of strontium atoms that is super cold, because that's all we care about.

SPEAKER_22 1:11:28

How do you do that?

SPEAKER_13 1:11:29

So, so uh one of the dysentropic the is this gonna be like sucking the entropy out? There is some of that. There's actually, so one of the ways to cool, in fact, one of the last stages they use to get it as cold as possible is if you if you have a cloud of strontium atoms that are contained in some in a what we call a trap, which is basically electromagnetic fields that are holding them in place, then they're held in place up to a certain, like if you could you think of like an activation energy in chemistry, right? There's some potential barrier. If you have enough energy, you'll climb out of it, but if you're if you're not energetic enough, you stay in this well. Okay. Well, then we we use essentially evaporation in reverse. You just slowly turn down the height of the barriers and the hottest particles leave first. That's evaporation, right? The hottest water molecules in your in your water cup leave first. Yeah. And that's why evaporative cooling works. That's how you cool things, right? A water cup as it's evaporating, actually continually cooling because you've selected the hottest molecules.

SPEAKER_08 1:12:25

So if they leave, how do you keep them from re-entering?

SPEAKER_13 1:12:27

Uh actually, they're cool. They're fine. They can just bounce around wherever they want. We just ignore them because there's so few of them. Yeah. And then you take this ultra cold cloud you've got left, and you can work with those. You shine your lasers on them, you do all this fancy stuff that's.

SPEAKER_23 1:12:41

So you're not actually cooling them.

SPEAKER_13 1:12:43

The last stage is only selecting. The last stage is only selecting. However, there's earlier stages that are actually cooling all of them because that selecting is very lossy. We lose a lot of atoms, and we don't want to lose too many because we need them for statistics for our experiment. Yeah, yeah. So the the first, some of the earlier stages for which there were Nobel Prizes are are um what's called laser cooling, which is uh essentially two opposite laser beams shot at each other. The atoms are in the middle, so they're they're bathed in this laser light, which is hitting them from both sides. Okay. But they're they're um just slightly what's called red detuned, which means um the laser is a little bit lower frequency than it would need to interact with the atoms splitting with the atom. And what that means is when the atom moves toward the laser beam, it gets a Doppler shift. If you if you remember a Doppler shift. I'm with that, man. Right? So the if you think in the atoms rest frame, if you're riding along with the atom, when the atom is moving toward the laser beam, it sees a slightly higher frequency laser beam, right? It's it's blue shifted. Oh yes, yes. And then it can interact. So the atom, if it moves toward the right laser beam, it's more likely to get kicked from the right. If it moves toward the left laser beam, it's more likely to kick from the left. This is cooling. You slow it down this way. If it wants to move to the right, it gets kicked backwards. If it wants to move to the left, it gets kicked backwards. This was a, this was a, you know, this was a very impressive piece of work when it was first done several decades ago. And you can actually, you can have this counterpropagating.

SPEAKER_08 1:14:05

With a kind of energy, with a, with a, with a, with a a fo a laser photon that is just the wrong quanta to actually pop an electron out.

SPEAKER_13 1:14:12

To be absorbed, that's right. And and in fact, you yeah, you have to control the frequencies very precisely. It's only right, you know, you make you you you tune it carefully so it will cause cooling, for example, so it will kick anything backwards away from the direction it's going.

SPEAKER_08 1:14:26

What's the like um the nanometer frequency shift?

SPEAKER_13 1:14:30

Oh, you know, I don't know. This is not my work, because I'm not I'm not an atomic experimentalist, right? So I couldn't tell you the numbers off the top of my head.

SPEAKER_08 1:14:37

Sound thing like, you know, visible light, like 300 to 800 or whatever. And like, is it is are we talking like are we talking like if one lay like the lasers at like would have to be at 280, but it's at 2 you know 45? Oh, it's a five. Or is it like 280 to like 268 or something like that?

SPEAKER_13 1:14:52

Even much smaller than that. So an atom at room temperature is moving at a few hundred meters. I can approximate it for it. I don't know the exact numbers, but I can approximate it for you. An atom at room temperature is moving at a few hundred meters per second, which is about uh uh 10 to the minus six of the speed of light. Okay. And the Doppler shift is controlled by that, the ratio of the speed to the speed of light. So the amount at Doppler shifts by its even room temperature motion would only be a part in a million, one a part in 10 to the minus six. So that that the your your frequency shift should be about that or smaller in order to um, you know, you want to walk it on resonance by this Doppler shift from the speed. Yeah. So it's a really tiny freak. We have to control those frequencies very well. You know, this is really curious like precision technology. Yeah, like what the what the So be less than a part in a million or something. Uh like I said, I don't know the number exactly. We can look at that.

SPEAKER_08 1:15:37

And are we talking about less than a part in a million of nanometer wavelengths or are we talking like exactly? But like nanometer is already like I'm now I'm really but anyway we can consider a few hundred nissometers. No, no, no, let's know it's not a big so okay, so now we've got um strontium atoms that we boiled off, that we that we that we that we like allowed to volatilize off.

SPEAKER_01 1:16:00

Right, right.

SPEAKER_08 1:16:01

Not really boiled, but it's just like a like get a couple of airborne, get some airborne, right? Right, right. Um, in this vacuum that we've created by turning on a really high power fan that that like eventually the nitrogen atoms bounce into. Right. Exactly, exactly. And so then we've got some of these, okay, now we now, and these are still moving at what you said 10 percent meter per second. Um bound just so like constant motion in this. How big's the how big's the vacuum?

SPEAKER_13 1:16:26

Let's say 10 centimeters or something like that.

SPEAKER_08 1:16:27

Okay, so it's like it's on your desk.

SPEAKER_13 1:16:29

Two centimeters. Yeah, yeah, yeah.

SPEAKER_08 1:16:30

It's like uh yeah, it's like a uh so like 10 centimeters, yeah.

SPEAKER_13 1:16:32

Yep, yeah.

SPEAKER_08 1:16:33

They're souncing around all the time. Now you've got now you've now you've got two lasers that are shooting at that are shooting like right at each other. Right. And any and any uh individual atoms that get like just happen to get stuck in between those two. Or do the lasers fill the entire space?

SPEAKER_13 1:16:49

You would tend to fill as or a good chunk of the space so that you could slow a lot of the atoms down.

SPEAKER_08 1:16:53

And slowing them down cools them, which is weird because like now, Cassina, that's weird, because you'd think that it would actually be potential energy, that you'd be heating them up.

SPEAKER_13 1:17:03

You'd think lasers should eat them, right? That's your gut. Only very carefully tuned frequencies and stuff will do this, right? This was uh this is why this took Nobel Prize. You know, this is why it took a lot of work that this is not just what happens if you randomly shine lasers on atoms. Yeah, yeah.

SPEAKER_17 1:17:17

Yeah, yeah.

SPEAKER_13 1:17:17

Um exactly. So you you cool them, uh, you get them in this one spot, and then you you use sort of similar techniques to to excite them with these lasers to split this atom's wave function, to start driving one half away from another half. And then that's basically the experiment. You you separate these two halves of the atom's wave function of its probability.

SPEAKER_07 1:17:34

But then, like, how do you only hit one half?

SPEAKER_13 1:17:36

Ah. The you use some of the weirdness of quantum mechanics. If so, so you so if I shine a laser on an atom, it can cause it to jump up a level. Right. Right? And when it does that, it's also like it absorbed a photon, so it'll it'll recoil. The atom will recoil, right? It got killed. Okay. Okay. Here's the weird thing. There is a there is a time scale for that to happen. So it won't happen instantaneously, right? If I shine just a tiny bit of light, of course the atom won't jump. It takes some amount of time. We would call that like a Robbie flopping time, it turns out. The amount of time to flop up from this state to this state, that's that's the way we call it. If you do it, so if you do it for one time Robbie is the guy who figured it out, or describing the length breath of this, right? Yeah.

SPEAKER_08 1:18:19

So so if you do it for that amount of time, he was a server, just hanging out. Robbie. He's like, Sup, Robbie. How you doing? How you doing? Yeah, man.

SPEAKER_19 1:18:29

Just flopping. He figured it out. I don't know.

SPEAKER_04 1:18:34

But uh this guy was definitely at UC Davis or Sonabar, bro. Probably not. Probably not. This was a while ago. Yeah. But uh I should not assume it was a male. It was not necessarily male. I probably think he was.

SPEAKER_13 1:18:47

But um uh so but so it moves from one state to the other. If you shine it for this amount of time, it'll do that precisely. The whole atom will move from one state to the other. If you shine it for half that time, only half the atom moves. Half the atom's probability moves. And the the math of quantum mechanics describes this very clearly.

SPEAKER_08 1:19:03

And again, the excitement is the same thing. It's an electron just jumping up a just jumping up a quant uh age.

SPEAKER_13 1:19:08

The atom's been changed its state and it's gotten a kick. Right? Exactly. That's the that's the process. And it's and if it takes a certain amount of time to do that, and you wait only half that time, you'll move only half the atom. But only half the atom's probability. So the atom will be half 50% chance it's in this state. 50% chance it's in this state.

SPEAKER_08 1:19:28

You're threatening quantum theory, right? That's what's the weird about quantum. That's just so weird. Also, but like I'm saying, like this isn't quantized. Uh yeah, quantum is sort of a misnomer. We could you could you could you shine it for a quarter of the time and move it a quarter of the distance? One quarter here, three quarters of distance. Is this now continuous? I thought nothing in physics was continuous.

SPEAKER_04 1:19:45

Ah, yeah, no, it's a weirdness of the way we use it.

SPEAKER_13 1:19:47

The fundamental math underlying quantum mechanics is is continuous differential equations based on you know the real number line. So everything we do in every every time we're solving something in quantum mechanics, we're solving basically continuous differential equations. So it is continuous in that sense. We the word quantum is used because these energy levels are quantized. But you don't have to be all in one or all in the other. That's the weird thing about quantum mechanics. It's probabilistic. It's really weird.

SPEAKER_08 1:20:14

I thought that was the whole point, was that you could only like, but is that just a macro uh they they teach it that way in high school to simplify this the matter.

SPEAKER_13 1:20:22

But quantum mechanics is actually you can be 59% in one state, or you know, 58.5% in what state of the U.S.

SPEAKER_08 1:20:30

Then then then there wouldn't be a sodium spectrum.

SPEAKER_13 1:20:34

The thing is, that's what's so weird. The the half that's in the ground state can't emit any photons at all. And the half that's in excited state can only emit photons of the right energy. So the spectrum is still there. You only get discrete sets of photons. You still, you know, you see those lines that that of course people have known about for over a hundred years. Yeah. But that's but it there's a continuous amount of probability that could be in that state. That's what's so weird about quantities. It is quantized. The the levels are quantized, the energy splittings are quantized, but how much can be in that state? Continuous. It's continuous. Quantum mechanics is weird. It's weird. It's funny to think about it.

SPEAKER_08 1:21:09

So now that we've got okay, so now we've now we've used now we've used a laser.

SPEAKER_13 1:21:15

Yeah, we lose the laser, we put half the atom in one in one of the states, half the other end, and the half that that absorbed the photon gets the kick. Yeah. Then you just wait and the two separate, right? The half that didn't get a kick sits here. The half that got the kick moves, separates out.

SPEAKER_08 1:21:30

As it absorbs more and more and more intentional.

SPEAKER_13 1:21:32

And then you can do it again, right? You can keep doing that.

SPEAKER_08 1:21:34

But how do you but like wouldn't the other half isn't the other half as likely to get kicked?

SPEAKER_13 1:21:38

Yeah, so here's one way to do it. This is this is maybe the best way to explain, and one of the ways they do it is the half that's moving is Doppler detuned, right? Now it's moving, so it sees all the frequencies shift a little bit, and these things are so precise, such narrow atomic line widths, that you apply a laser of the right frequency to interact with this guy, the one that's moving, the half that's moving, and not with the half that's stationary, use that Doppler shift to do it for you. That's one way to describe it.

SPEAKER_15 1:22:03

Yeah.

SPEAKER_13 1:22:04

So that's one of the many cool tricks that goes into this technology. So now you've done this, now what did we just achieve? That's right. Okay, good. So what do we got? So now we can separate two halves of the atom. We actually we use lasers to separate them and then to bring them back together and re-interfere them. And that's sort of analogous to LIGO shooting two photons in different directions, shooting them, bringing them back together with mirrors, and then reinterfering them. Now we've made what's called an interferometer. Only we've made it with atoms, with matter instead of with light. We've done matter wave. Sometimes we call it a matter wave interferometer.

SPEAKER_08 1:22:34

What's the inner? So what we're set, so when they come back together, you're now pushing it back? With light, you should use the other way. Same thing. Like and catch the ones that now you're trying to figure out how how much have they like detuned from each other?

SPEAKER_18 1:22:49

Yeah.

SPEAKER_08 1:22:50

And that's how much have the two has.

SPEAKER_13 1:22:53

Basically you're asking. Gone out of phase. Exactly, exactly. How much have they gone out of phase? What has happened to one that didn't happen to the other? Right. With with LIGO, it's the gravity wave like squeezed one and stretched the other. Yeah. Right? Well, kind of the same thing here. We're gonna be looking for the gravitational wave came through and it it affected one half of the atom somewhat differently than the other half, is one way to say it. Uh so it's one of the other ways to say it.

SPEAKER_08 1:23:22

Like, what do you what does that mean, right? Like, because these grab but gravitons don't do anything.

SPEAKER_05 1:23:26

Like, yeah, well, so there's so the forces are so weak.

SPEAKER_08 1:23:29

Each one is the shit, like with because it makes sense that you could read this incredibly weak thing with lasers that like using mirrors are traveling hundreds of thousands of miles, right? Like, so you've got this huge, so there's like a largeness to what you're what you're doing to my understanding is that what made LIGO really work is when they they just made them like a little bit long, they made them longer. Oh that you can't do it.

SPEAKER_02 1:23:52

That's right, that's part of it for sure. That's part of it.

SPEAKER_08 1:23:54

So that the late so that the light travels so you get more shift.

SPEAKER_13 1:23:58

So the same read. Yep. So we will want to build a really big one. Exactly. We want to build a table. No, no, we will want ultimately kilometers. Kilometers. Yeah.

SPEAKER_08 1:24:08

You're gonna have to push a strontium atom into two, into a super, into a two, but they're like where one half of the atom, one half of the wave function of the atom is a kilometer away from.

SPEAKER_13 1:24:20

Uh no, this is the trick. This is the trick. That would probably be a little too hard. So we came up instead with a different trick. However, I will tell you what's really cool. There, there, the more you can separate these two halves, basically, the more sensitivity your device has. Because the further apart they get, the more whatever hits them is gonna interact with them differently. Definitely, exactly. Exactly, right? So we definitely we do want to separate them as much as possible. When we started this whole project, which was like 20 years ago, when we started thinking about this, it was just you know, ideas on a blackboard when we were just sketching it out. The most they could separate it was like millimeters. But we knew we needed milk.

SPEAKER_08 1:24:52

I'm amazed you're measuring in millimeters.

SPEAKER_13 1:24:55

Even that is a lot. Even this is impressive. Yeah, I'm talking about it.

SPEAKER_05 1:24:58

I was expecting, I was expecting angstroms. Yeah, but I'm expecting nanometers at most. Exactly. No, millimeters is already really impressive. That's crazy. I can see a millimeter on a ruler. Yes, yes. Now here's the really cool thing.

SPEAKER_13 1:25:09

Since that time, my colleagues here at Stanford and other places have been working to improve and improve this technology. Now they've gotten something amazing. Now they can do almost a meter. So they can have one atom in a superposition here and here, literally a meter apart. Like you can just, you could walk through it, right? You can stick your hand through it. It's insane. A macroscopic superposition. It's crazy. It's a really cool device.

SPEAKER_08 1:25:29

Except for the environment in which it which it exists. You can't do that.

SPEAKER_13 1:25:34

But it's it's that far apart, right? It's a meter, two one atom split in two places. It's both here and here, a meter apart. It's insane. It's quantum mechanics on a macro scale, it's really insane. Makes a great demonstration for like a quantum class, under time quantum class. And you can and you can do it on a desktop. Uh yep, they've got it. They've got the lab downstairs. Exactly. Exactly.

SPEAKER_08 1:25:53

You can collect all this machinery, like tuned lasers and all this stuff. You can just do it on it on a table.

SPEAKER_13 1:25:59

Yep. Yep. And be like, here we go. It's a big table. But yeah, exactly. Exactly. It's really cool. And you can you can see that you've separated them, you've controlled them. Uh, and and this uh is not just a really cool demonstration of quantum mechanics, but it's also was crucial for getting the sensitivity we needed to actually see gravitational laser.

SPEAKER_07 1:26:16

Well, so what so then what's the actual shift that you're measuring?

SPEAKER_13 1:26:20

Ah, good. Okay, so it's a it's a little complicated to explain. There's a few different ways to say it which sound different but are all the same thing in physics, which is happens a lot. Let me let me try one way to explain it. I love how it becomes poetry. It kind of does. It kind of does.

SPEAKER_08 1:26:36

It's like, how do you explain? Well, there's a couple of different ways to explain joy.

SPEAKER_04 1:26:39

Yes, right?

SPEAKER_08 1:26:40

That's right. But they all kind of come down to the same thing.

SPEAKER_04 1:26:42

Yes, yes. Oh boy, okay. Exactly. Okay.

SPEAKER_13 1:26:45

Um one way to explain it would be the um uh the phase that an atom picks up as it's kind of existing, right? We're gonna we're gonna measure the phase difference of these two halves that went out and come back together. That's what we're gonna see, right? How much did one's phase evolve relative to the other? Phase. Yeah, it's weird. If you think of the atom as a wave now, so now I'm gonna jump and think of it as a wave. The phase is essentially how many cycles it's gone through.

SPEAKER_08 1:27:18

So with uh cycles of like the going from high amplitude to like peak to trap peak to trap.

SPEAKER_13 1:27:24

Exactly. So if you think of a laser beam, you you might think of it as as like a sinusoid, right? Yeah, yeah. And and li what does LIGO do? It it essentially shoots this laser out and back and it checks. Did this laser do, you know, 1,700,000, blah, blah, blah, thousand point one radians?

SPEAKER_08 1:27:40

Yeah, and and you and you want it to be less than one full two pi radians. Yeah, once it's gone two pi, then you're back to in sync again. So it's no longer valuable.

SPEAKER_13 1:27:49

Exactly. Our signals are so incredibly tiny that our problem is we're dealing with like thousandths of a radian. So we're not we don't have a problem. You know, I would love if our problem was that they were they were a whole cycle pig. These tiny signals. So LIGO compares one laser beam to another and asks if they're slightly off. Right, right. Right, right. But but how does it work in strontium atoms? An atom is also a wave. So picture it again as a sine curve. It's a wave in its probability space, it's a wave function, so it's really hard to picture. It's a wave in this probability space. But anyway, it's a wave. And um, and if the two halves come back a little bit off, we can see that in the interferometer.

SPEAKER_08 1:28:24

What is it you're actually measuring to see that? Uh because when because when you do it with the lasers, yeah, you you you like then direct them at the same target and you can clearly see you can clearly see constructively interfere or destructively interfering. It's basically the same thing. If the atoms come back where they're that's a but that's a photometer. Like that's it's really easy to just write, like how do we measure the like the photon?

SPEAKER_13 1:28:46

Yeah, that's measuring like how many photons hit you, right? Yeah. What we do is we measure how many atoms. So we just count atoms with this camera again that I was telling you about. So so if the atoms come back maximally interfering, there'll be a lot of atoms going that direction. And if they come back destructively interfering, there'll be no atoms, it'll be dark. Our camera will show nothing. So we just count atoms with a camera, and that's how we measure this.

SPEAKER_08 1:29:06

Wait, wait, wait, wait, wait, wait, wait. So it's they they emit light or reflect light?

SPEAKER_13 1:29:10

Uh they they we shine this laser on them to make them reflect light so we can see them with a camera. Exactly.

SPEAKER_08 1:29:15

And if they are constructively interfering, there'll be a lot of them. Then they are absorbing and re-emitting photons.

SPEAKER_13 1:29:22

Yeah, or you say there's a bunch of atoms there, and then they can absorb and re-emit these photons.

SPEAKER_08 1:29:25

And if they are destructively interfering, then there's no atoms there.

SPEAKER_13 1:29:29

They were they're gone. They went a different direction, really, is what happened to them. They didn't show up in this camera, for example. It's weird. It's weird to think about.

SPEAKER_08 1:29:40

How do you know there's just no atoms there? Like, how do you know that what happened is they destructively interfered?

SPEAKER_13 1:29:44

As opposed to we messed up and we're as opposed to like they're just somewhere else now. Very good, very good. We we there's actually in LIGO, there's two output ports. When the lasers come back together, they hit a final beam splitter, and each laser beam can go one of two directions. So the the one that's coming in, you know, from from the from the west goes ax two ways. Exactly. The one that's coming in on the y-axis goes two ways. And then you actually have two, you can have two photometers measuring those two output ports. And if one of them is now they're overlapping, right? Yeah, now they're yeah, so right? Now now the the one and overlap. Exactly, and you overlap each each quarter, right? The two quarters overlap one way, two quarters overlap the other. Yeah.