Your Future Might Already Exist According To Relativity

What would alien astronomers see if they pointed an impossible telescope at Earth from millions of light-years away?

This episode uses that thought experiment to explain light travel time, relativity, time dilation, the Andromeda paradox, and the strange idea that there may be no single universal “now.” We also get into the block universe, why GPS depends on relativity, and what all of this might mean for how we think about the future.

Hosted by Dr. Dakotah Tyler.

Lead with curiosity.

Transcript

SPEAKER_00 0:00

Welcome back, Dr. Starkid. I'm Dr. Dakota Tyler. Today I want to walk through a really interesting thought experiment. I made a short form video the other day that was about viewing the earth from an incredible distance that I thought was really cool. Is this alien astronomer thought experiment? And somebody asked a really good question that brings in relativity and how multiple times seem to be able to exist at the same time, which is a little bit of a mind-blowing result from relativity. And the implications of relativity, I think, on a personal level, can be philosophically meaningful in a way that for me, I think helps me with mindset, with framing, and just like getting through life. And I wonder if it'd be helpful for other people as well. So the first thing to understand is that just classically, like forgetting relativity, the universe, weirdly, when you look out, is a time machine in a way. Everything that you have ever seen, you have seen in the past. Right? Even even you watching your phone right now, the photons that your phone is generating, they're leaving the screen of your phone and they're going into your eye. And they're traveling at the speed of light through air, which is really, really fast. So it it seems instantaneous to us, but there is a delay there, right? Like it's it leaves your phone and then it reaches your eye at a later time. So you're not you're not seeing exactly what your phone is doing instantaneously, you're seeing what it did like a trillionth of a billionth of a second, or we, you know, whatever the number is. But it's a it's a very small number because light travels fast and your face is very close to your phone. But as you look at things that are farther away, that starts to matter. The moon is about one light second away. So sunlight comes, it hits the moon, and then that bounces and then it reflects and then reaches your eye about 1.3 seconds later. So you've never looked at the moon and saw a real-time view of it. You've always only ever seen it the way that it looked 1.3 seconds ago. Similar thing with the sun. You look at the sun, sunlight takes about eight minutes to leave the surface and then get to us. So, you know, any light that you've ever seen from the sun, I mean, you shouldn't be looking at the sun. Don't do that, it'd be bad for you. But if you did, I mean you got some solar uh eclipse classes or something like that, what you're seeing is the sunlight. You're seeing the surface of the sun as it was eight minutes ago. It's very, very important. And as you go further and further, the effect just continues to take place. Another way to think about this is that if the sun were to disappear right this second, it wouldn't go dark for me for about eight minutes. You wouldn't know until the last light that had left the sun had reached me, and then no new light was then being created. The nearest stars to us that aren't the sun are like light years, meaning that the closest star to us, Proxima Centauri, it's somewhere around four light years away. If it just magically disappeared today, we would not know that for four years. We would continue to see the light that has already left it and is traveling towards us, and we only would realize it was gone when there had been enough time for that last photon, that last beam of light to reach us. And this is where the unit of light year comes from. Light year is not a unit of time, it's a unit of distance. How far does light travel in one year? For reference, one light year is about 5.8 trillion miles or 9.5 trillion kilometers. So it's a very, very long distance. Let's apply this to the other galaxies that are out there. The Andromeda Galaxy, if you're in a dark sky part of the world, you can see the Andromeda Galaxy with your naked eye. It's about two and a half million light years away. That means that when you see it, whether you're using your eye or a telescope or a camera, it doesn't matter. What you're seeing is the light that left it two and a half million years ago. If the Andromeda Galaxy disappeared today, we would not know ever in our lives. In two and a half million years, future astronomers on Earth would realize that the Andromeda Galaxy just disappeared and be like, oh shit, two and a half million years ago, something happened to this galaxy, right? Like, that's not how it couldn't just disappear. But the idea there is that there's this huge time lag because it's so far away. Now, cool thing about astronomy is that means that we can look at very, very faint light that has been traveling a long time and we can understand what the universe looked like 10 million years ago, a hundred million years ago, five billion years ago. And indeed, this is how we string together the history of the universe, is a lot of what cosmology is about. How did the universe evolve over time? It's something that we can study because we can see it, because the constant light travel time through a vacuum results in the universe keeping a fine record of exactly what has happened. Okay, this is the this is the base rule that we need for this thought experiment. It's that light takes time to travel. So any light that you collect, you can only collect after it has had enough time to travel from wherever its source is to get to you. Let's say you're an alien astronomer on a planet orbiting a star in the Andromeda Galaxy. It's about two and a half million light years away. You start to construct this enormous telescope, an unrealistically enormous telescope. Well, what you see is light from the earth that from two and a half million years ago, because that's how long it takes that light to travel. And so they are unaware of the fact that uh humans on earth right now have TikTok and society and Love Island and all they they don't know any of this, they only know what was around two and a half million years ago, and that's if they can see on the surface of the planet. And the reason why I say that it's unrealistic to construct a telescope that has that ability is the following in astronomy, there's this concept called resolution, it's what you can resolve. Most of the time, when we are looking at stars, we're just seeing what's called a point source. You can't actually see on the surface of the star. It's as though everything is just filling one pixel. And this makes sense when you realize how far things are away. But there's this simple formula, it's called the Rayleigh criterion, and you can basically work out how large of a telescope that you need to be able to see something based on how far it is and how small you want to be able to see. And the reason you need a large telescope is because objects that give off light, like stars, planets, the light kind of spreads out over time. And this should make sense. You know, if you are, if you have your face like right in front of a light bulb, a lot of that light coming from the surface of the light bulb is just smacking your eyes. But as you get further and further and further away from that light bulb, and you can still see the light, you have to realize that basically everywhere in a sphere, your same distance from that light bulb, there is light. But it's not because the light bulb is producing more light, it's because the light is spreading out the further you go, which just basically means that your eye receives fewer photons the further you get away from something, because it's kind of fill spread out to fill that sphere. Now, what a telescope does is it collects more light. And if you collect a lot more light, you can focus it back down onto a point, and it's as though you were much closer. It's as though you were seeing that light before it had spread out as much. Because what you're doing is you're you're bringing all that spread out light back into focus. So it like literally the magnification that you experience from a telescope, it's as though you were closer to the object. And the larger the telescope, the closer it seems like you are to the object. Again, it's just an apparent effect, right? It's it's not like you're actually closer, so the light hasn't had to take as long to get to you. It's literally that you're just taking all this light that is spread out and for you makes it very hard to see, like with your eyeball, and you're refocusing all that light together. It's essentially it's like having a giant eyeball. That's kind of what a telescope is, kind of. Now, just how big of a telescope do you need if you want to be able to see onto a planet that is two and a half million light years away? I worked out these numbers for myself. And if you wanted to see like a meter onto a planet, so it's not like you could zoom in to the cellular level, you could zoom in to the point where you could see a meter at a time. So, you know, I think a meter is maybe close to a large animal or like a medium-sized animal. So you couldn't, you couldn't make out the features on a person, but you could tell that there was a blob that was that was at least, you know, a meter box. From 2.5 million light years, you would need a telescope that was over a light year in diameter, something like 14 trillion kilometers. So again, this is larger than a light year, meaning that if there was one person that was working on uh a mirror on one side of this telescope, and there was another person working on a mirror on the other side of the telescope, and they were trying to communicate back and forth with each other, it would take a year for them to send radio signals at the speed of light because they are more, it would take longer than a year because they're it's more than a light year. But this is crazy. It's like a couple light years in diameter. This is an enormous telescope, it's not possible to make something that large. There's not even enough stuff in the solar system to make a telescope that big, like there's not enough ingredients if you started like breaking down the components of like planets and stuff. You just there just isn't enough stuff to make a telescope that large. Also, something that big would have its own gravity that would kind of be pulling it in. It's like technologically, it's not feasible. Uh, and no, I don't know what future technology could exist in, you know, some faraway advanced species or something. I don't see any way that that anything like that is ever possible. But, you know, if you you know, if you want to like fudge future technology or whatever, okay. So let's just pretend, however, for this example that you have a telescope that big. Okay, so what can you see on Earth? Well, again, you're seeing the light from two and a half million years ago. Just because you have this big telescope, it doesn't mean that you can see light that hasn't had a chance to get to you. You can still only see the light that had time to hit the mirror and then get refocused. So you're still seeing the light from two and a half million years ago. There aren't any cities yet, you know, no city lights, no farm, there's no internet, there's not even any modern humans. There's no Homo sapiens. There are humans, our distant ancestors from two and a half million years ago, but there there are no modern humans. There are no, you know, if you if you went back to two and a half million years ago on Earth, you would not find anything that that resembled what any humans are today. You would find our ancestors in Africa. All human ancestors at the time, two and a half million years ago, lived in eastern Africa. Some of those species were walking upright, beginning to walk upright. Some of them had been walking upright for a while. The Australopithecenses are one group. You may be familiar with Lucy, the famous Australopithecus fossil that is a human ancestor from like somewhere around four million years ago. Tiny, you know, only a few feet tall, uh, very, very small brain, but clearly had already branched off from the other great apes and was on its way toward the lineages that we are more familiar with and was a human. Australopithecus is a human. Again, not a modern human like a Homo sapiens, but it is a human. And I guess the way that you think about it is um canine, maybe. Like what when you think of canine, what are you thinking of? You're thinking of uh dogs, wolves, foxes, coyotes. These are all canines, right? Not all of them are our dogs, our modern dogs, um, but they are all canines. When you look at every single animal that exists on earth, I think it it should raise this question in you, you know, what is the human equivalent of that? Is it is it like the different races? You know, race is a social construct, but is it this different ethnicities? Um, people for in Eastern Asia tend to look different than people in northern Europe tend to look different than people in Western Africa, for example. Is that is that like different species? No, that's not where everybody, you know, all humans are the same species. We're all very, very, very, very closely related. But there was a point in time when the humans in one part of Africa and the humans in another part of Africa, and the humans uh when you know, when they started to spread out, uh Neanderthals and Homo Floresienzis, all these different humans species were different species of human. That's a crazy thing to think about. You, as an alien astronomer, you're witnessing that there are these two-legged, hairy things, but they're not, hey, they're not that impressive. I guess some of them are probably using sticks, they're using rudimentary tools, but they're still prey. You know, uh the Australopithecines, um, paranthropis, they they are they are smart and they're getting smarter, but they're still prey. They're still getting regularly, they're regularly getting their skulls pierced by large saber-toothed cats. Um, you know, they're there's they're just prey. They're like mid-rung in the food chain. But they are interesting. And you know, you're you're a scientist, you're an alien scientist, but you are a scientist, and you probably know a lot about the evolutionary biology on your own planet. So you suspect that this planet that's about two and a half million light years away, maybe today this these humans have uh evolved to become more intelligent. And how how could you speed up time? Because we know that so far you're you're relegated to just wait for this light to reach you, but you want to see, you want to see what it looks like today. And the only way that you could really see what it looks like today is to basically be close enough that you could see the earth, right? You could minimize all that light travel time, but that's not an option. You're in the Andromeda Galaxy, you know? You see these giant short-faced bears that are probably eating anything around. Uh, you see the these huge woolly mammoths and mastodons, giant ground sloths, and you just you just wonder right now it seems like the humans, it just seems like they're outmatched, and physically they definitely are. But maybe, maybe this is a species that has a chance to you know build some sort of interesting technological civilization. So, what can you do? Well, you can start traveling towards the planet, right? You can take your unrealistically big multi-light year telescope, and you can right. This is this is a thought experiment, and you can start moving towards the earth, right? Because, well, that does a couple things. First of all, you are intercepting the light faster because you're moving towards it, right? You're like, you're literally a way that you could think about this is um in the rain. This is a famous physics problem that any physics student in like first year freshman physics has to calculate. If you're standing in the rain and letting the rain fall on you, how many raindrops hit you per second, let's say? Well, what about if you start running in the rain? Do you encounter more raindrops or do you encounter less raindrops, or do you encounter the same amount of raindrops? Well, it turns out you can you can work this problem out for yourself, or you can just take my word for it. If you run into the rain, you hit more raindrops because you are ex you are accelerating towards these raindrops that are falling at a constant rate, right? So instead of just the ones that are hitting you, ones are hitting you, and you're also running into them. This is just a classical metaphor for the light that you are encountering because you are the light is traveling to us. There are these frames, these image frames that are moving to you uh when your giant telescope and then drop it a galaxy, you start moving towards the earth and you're encountering them faster. And so what you see, the images that you're able to reconstruct, they appear as though time is running faster. So if you move at an extremely high speed of relativistic speed, let's say 99% the speed of light, the real answer requires relativity. And the relativistic answer that you get and the time dilation experienced here is about 14 times. So you are witnessing the history of Earth play out at 14 times the normal speed. Essentially, an entire year of activity is being compressed from your perspective into just a month because you are moving so fast towards this light source, the earth. And it's because in relativity, both space and time do not behave in the way that we experience and expect on Earth in normal circumstances. It's just one of the weird things about the universe. This is why this is so cool, and it's just gonna continue to get more and more extreme. That was 99% the speed of light, a 14x speed up in time. What about 99.99% the speed of light? Much closer, not the speed of light, but much closer to it. The effect goes from a 14x to 140x. So now instead of having a year compressed into a month, you have something like a decade compressed into a month. And the faster and faster you go, you see this exponential increase in the behavior of the humans on earth. You see them eventually start to use fire, you see them eventually start to engage in uh more sweating and like persistence hunting hunting when they run after these animals for a longer amount of time. But the important thing here is that you know, this is a this is a thought experiment, right? So this is all theoretical, but there's a problem because as you continue to go faster and faster, there is another effect. The light begins to first of all get blue shifted. So at first it just appears bluer, um, but eventually it'll get blue shifted into like the ultraviolet, which human eyes can't see, and then it'll get blue shifted into X-rays and gamma rays, which are dangerous, like these are high-energy photons that can kill you. So the light becomes invisible to you, and also there's this some there's something called a relativistic aberration. If you've ever seen a sci-fi show or movie, you're familiar with you know, when they hit the warp speed and they start going super fast in space, all the stars kind of stretch and you get this like zoomed uh aspect that's really weird looking. It looks like everything is being stretched. This is the relativistic aberration. And what's really happening is that as you move faster and faster and faster, it's like the universe compresses into a smaller beam in front of you. It's called relativistic aberration. So, you know, as you go faster and faster and faster, yes, you encounter the light more quickly. You can watch, you know, the history of life on Earth sped up. But it becomes invisible to you because it gets blue shifted into shorter and shorter wavelengths that you can't see and they become very damaging. And then also it gets compressed into a smaller and smaller beam in front of you, which you also can't see. So, you know, we're ignoring that, but I just thought that we should throw those out there. Depending on how fast you go and how long you travel, you know, eventually you see um these humans evolve to be less hairy and to migrate out of Africa, and then there is some ice age that happens that kind of fractures the human population, and then you get another round of speciation where you get the Neanderthals in like northern Eurasia, and then there's Homo erectus spread out, and there's some uh smaller humans that fall find themselves in like Southeast Asia and those islands. My favorite example is called Homo Floresiensis. This is a human that is only a couple feet tall, fully mature, they get stuck on an island. And this is one of my favorite examples of human evolution because it's a it's one of the key examples, and I mean all examples follow this trend, but it's one of the key examples that human evolution just follows the same sort of evolutionary rules and paths as the evolution of every other animal, and it's that on islands, larger animals evolve to be smaller, especially if they're predators. I mean, in most cases, they evolve to be smaller, but um smaller animals evolve to be larger, and this should make sense. If you find yourself on an island and you are a big uh predator cat, you are now on an island that doesn't have infinite resources. There can't be an like a countless amount of prey animals because you're stuck on an island with limited resources. So uh, you know, like a giant tiger saber-toothed cat that needs to eat a lot of food will go extinct on a small island unless it evolves over time to be smaller and require less resources, like less food. On the other hand, you take an animal like a rat, a rat which needs to be small and fast and be able to escape predators on mainland, on uh on an island, it actually helps a rat to be bigger. You know, they can eat more resources. The predators are kind of stuck getting smaller. Being larger may help you mate more, um, it may help you just you may just evolve to be larger because there's no um, there's like nothing that's keeping you quick and small and nimble. Uh, and being larger may also make you harder to eat for any predators that are there that are also getting smaller. And so on the Isle of Flores, this island in uh Southeastern Asia, we found these bones, these fossils of Homo Floresiensis, this couple foot tall human species. So the humans had gotten smaller on the island. There was also a tiny stegodon, like a little mini elephant, a small elephant that had evolved to get smaller for the same reason because you can't be as huge elephant, it needs to eat tons and tons and tons of food every day if you're stuck on an island because it's an island. There isn't enough, you you know, you would eat yourself out of existence. And so any animals, any of those stegadons that didn't evolve to get really small probably went extinct. This is one of the selection pressures in evolution on that same island. There were what I think are equivalent to Komodo dragons, big lizards, that were probably eating the people. There were also these giant storks, which honestly were also probably eating the people on that island. And it's just a cool example of the island rule that animals that are exceptionally small or large when trapped on an island will tend to reverse their body size. And it you know, it went with for humans as well. Very interesting. It seems like, and you know, a lot of this is still being discovered, right? Because you know, how how how do you find fossils of something? It has to get fossilized. Well, you know, it's it's unlikely that any given animal is going to get fossilized. Sometimes these human remains of different species of human are found in caves because caves they kind of protect, right? They protect from the elements, um, and they find these fossils that they found of homophresiens is in a cave. And that doesn't it it gives you an idea of when those fossils were left behind, but it doesn't tell you precisely when was the last time that that species was around, right? There could, you know, they could find they have found these fossils that were maybe 60,000 years old. So you know at least this species was around 60,000 years ago. It could have been a lot around much more recently and just not had those um bones get fossilized. But I say all that to say that it seems like the timelines of when the timelines of when modern humans start to show up around that area is very similar to the timeline that that species of human disappears. Doesn't mean that there were they ever met each other. It's just interesting that those timelines are close. Um, another point of interest is that you know, humans, modern Homo sapiens show up in North and South America sometime between like 15 to 20,000, the low 20,000s of years ago. Uh, lots of evidence of widespread human activity somewhere around 12,000, 13,000 years ago. So it kind of seems like um there are these white sands fossils of footprints of humans that seem like they were low 20s, 22,000, 23,000 years ago. So humans may have showed up first in North and South America around 20-ish,000 years ago, but they became very widespread and abundant around 12 to 15,000 years ago. And that is the timeline that the giant short-faced bears, the woolly mammoths, the woolly rhinoceroses, the cave lions, the saber-toothed cats, uh the giant ground sloths, the giant deer, that is the timeline that all of those animals started to disappear from the fossil record. And it's just very interesting to me. You know, I'm not a paleontologist or an archaeologist, and I think that they tend to debate these things, and it's like, oh, you know, the the climate, there's been periods of like glacial um overgrowth and and ice ages over the past few tens of thousands of years. And so you can never, it's hard to pin the disappearance of a given animal on anything in particular. However, I do think I find it personally, especially when I look at the behavior that we have today as humans, I find it very compelling that every time we're pretty sure humans are becoming widespread in an area, something about the area gets jacked up. Whether that's animals start disappearing, other species of humans start disappearing, you know, uh forests start disappearing, the the biodiversity starts to tank. And in today we're doing the same thing, but today we know, right? We know that what we're doing is is gonna have these impacts, and we still continue to do it. And I'm not so sure that you know early humans were doing the math on hey, if we you know, if we just wipe out you know one woolly mammoth a year in this region, we can actually deplete the entire population in just a few hundred years. Maybe they were able to to realize that, and they did it anyways. But, anyways, very, very interesting. So that's a little, you know, I'm an evolutionary biology guy. I like the history of life on earth because I think it's very important to get us to this today with us modern technology, you know, modern human intelligence, uh, our our current boundary of knowledge. You run this experiment, you're an alien astronomy, you get this giant telescope, you're moving super fast. Forget the fact that you can't have a telescope that big, forget the fact that you can't move that fast, forget the fact that the light is invisible, and forget the fact that the relativistic aberration makes it impossible for you to see it, anyways. Forget all of those things, and you could run this experiment in principle until you reached the earth and literally saw what was going on today. Now, there's this other this other factor, which is that as you're moving faster, it's not only that you're experiencing you know the images speeding up in time, but you're also having a clock that slows down relative to the one on earth because you're moving so fast. And this is what I think gets even more interesting. It's that time is relative, of course. This is this is what relativity means, but your motion relative to other places, and that affecting your clock has very deep implications about what it means for time to exist, deep implications about the future, the past, and the present, and how these things almost must, you know. The interpretation we're gonna get into is how if time is able to exist, right, the fact that you moving fast can change your clock relative to somewhere else, and then stopping, stopping your motion and returning those clocks back to the original cadence where they where they match up, where now to me is the same as now to them on earth. That means that past, present, and future maybe all exist all the time. And we're gonna dig into some really interesting uh examples, other thought experiments that that demonstrate this. Let's let's regroup because I know we went on a little tangent about uh the evolution of uh of humans, right? Forget the telescope, uh, forget the spaceship, forget light and how long it's taking to cross the distance. Just let's let's think about right now between you and I what's happening in Andromeda, right? You what will we see? Well, we would catch the light tonight from two and a half million years ago. So we're watching, we're watching ancient history. Let's move off of that. Let's move off of that for a second. The question is, what is actually going on right now over there in this instant, you know, as you're you're sitting and you're listening to me and you know, hoping that I don't get too derailed uh on whatever I'm about to say next. Right now, two and a half million light years away, something is happening in that galaxy, right? There's a planet, maybe that is very much like Earth, maybe with a civilization that is very much in the same technological phase that we are. It's very interesting to think about. Perhaps something like that is happening right now. Forget the time delay. Right now, something is happening here, and right now, something is happening there. We agree that things are happening here and things are happening there, but the the claim is that there is no one now there in Andromeda or in any distant galaxy. There is no set defined now. And the the example of the alien astronomer that's flying through at relativistic speeds demonstrates that, right? Because when they are moving at that relativistic speed, and they're and they say now, they're different things. The clocks are moving at a different pace on Earth. They're encountering light that appears to be moving at a very different speed. So what is now? Like, what is now? The the question doesn't have a fixed answer. This is relativity. Your now, my now, the now of someone sitting right next to you can reach out across vast distances and land in different moments if you go far enough away. And all of those different nows are still equally correct. And that sort of sounds impossible, but it definitely isn't. And I'm gonna give you a a uh a paradox. I remember learning about this in my special relativity class. Okay, so let's let's go back to the question what's happening in Andromeda right now? And let's let's make it concrete with an example. If you're sitting on a park bench and a friend, you're so you're you're motionless, sitting on a park bench, and a friend walks up to you. They're walking up to you. Let's say, for the sake of this thought experiment, that the Andromeda Galaxy is directly behind you, but 2.5 million, 2.5 million light years, but 2.5 million light light years directly behind you. So you have a you're sitting still, you have a friend that walks up to you and is walking towards, in a sense, directionally, towards the Andromeda Galaxy. Now, the moment that they reach you, you're both in the same spot in the same instant. You're in the exact same place, it's the exact same time, it's you're as close to the same now as two people can possibly be. And you both ask the same question. You say, out of all the events happening in the Andromeda Galaxy, two and a half million light years away, which one is happening right now, this very instant? The thing about that question is that when you say a far away, a faraway event is happening right now, it's almost like you're drawing a line. But it's not, it's not, it's not just a line in space, it's a line in time. Space-time has this geometry. This is very important to understanding space-time. It has a geometry, and it's it's space and time. And most people can imagine, they can visualize geometry in space, right? Like shapes and space. But time is a part, space-time, time is a part of that, and there is a geometry to time as well. And you can imagine drawing a line, like a slice, a time slice that makes sense in the geometry of space-time. And your slice, your now slice, a slice of now for you, it runs through you, it runs through the bench, it runs past the moon, it runs past the sun, it runs past the rest of the Milky Way galaxy, it slices through Andromeda, it continues the slice through to the end of the observable universe. And everyone carries one of those slices around with them. This now slice that slices through space-time. And your now slice, it changes angle. If you're moving, when you're in motion, as you move, your now slice changes. And again, it's it's what you're calling now. So as you start to as you go in motion, your now slice is hitting different nows in other places in the universe. You know, what is other nows? That is the past and the future. All right, stay with me. Your you who's sitting still, and your friend who's in motion both have now slices. And your slices are very close to each other. So you're experiencing almost essentially the exact same now. But because your friend is walking towards Andromeda, when you extend that now slice out to that galaxy, the nows are not aligned. Your now slice to Andromeda and your friend's now slice to Andromeda are misaligned. They're just walking a few miles an hour. That tiny amount of motion relative to you changes their now slice. So out in Andromeda, your friend's now slice that's slightly misaligned with yours, they're now off by a few days, about four days. So when you say now, the people in Andromeda, assuming that there's some peoples of Andromeda, agree with you, but there's a four-day difference between them and the friend that's walking and who agrees with them. That means two people, this is what this is like so weird. This is crazy. Because somebody's sitting still and somebody's walking, their now's are misaligned by four days in the Andromeda galaxy. Neither person is wrong. Both of those are right. You and your friend, you're in the same spot in the same instant. And and for y'all, when y'all say now, y'all are saying it at the exact same time. But the people in Andromeda, thought experiment, that say now at the same time you say now and the same time I say now, for them are split by four days. Crazy, crazy, crazy thing. But the crazier thing is that now if your friend turns around and walks in the opposite direction at the same speed, and you do the same thing, and we meet you and your friends say now at the same time, your now's match up damn near perfectly. But the now's of the people in the Andromeda Galaxy flip, instead of instead of one being four days before, that's the person who's moving now has a now that's four days after. This has a name. This example that we talked about is called the Andromeda Paradox. Um, uh Roger Penrose, who's a famous physicist, when you won a Nobel Prize, uh he he uh illuminated this, right? Imagine in Andromeda, something is unfolding on a human timescale, a decision being made, for example, uh say that there's a fleet that's preparing to launch toward Earth. And the person, for the person walking toward Andromeda, their slice of now has already swept past the launch. In their present, the fleet has already set off. For the person that's walking the other way, their launch has not happened yet. In their present, the decision hasn't been made yet. So you have to, I mean, that's to me, that is crazy. If you walk at some set speed away from Andromeda versus towards Andromeda, you are literally toggling a now, a real now in Andromeda that is oscillating from the future and the past. How could that be unless both the future and the past currently existed? How can you come to any other conclusion? Walk run jogging this way and you're in the future, jogging that way and you're in the past. Again, not your past, you're now, you're always in the now. No matter what you do, you're in the now. The clock runs the same for us no matter what, but jogging towards or away from a distant galaxy, you are in their relative past or future with respect to you standing still. This is, I mean, this is crazy. Now, this is something that is theoretical, right? We have countless, countless evidence for both general and special relativity being true. Uh, so far, not yet to be falsified, meaning there's no evidence that suggests that anything about them is is untrue. But we can't put a sensor, you know, on a planet a galaxy away. So that you know, this is like a thought experiment more than one that's uh could be practically done. And an important thing to point out here is that walking towards or away from a strong uh Andromeda, nobody can see the fleet, right? Because the light hasn't had time to reach us. You see what I'm saying? But this example, this thought experiment isn't about what can be observed, it's about which distant events um count as a part of the present. And on that, two people in the same place at the same time on earth can disagree with not. If you travel out farther far enough in the universe, it's so it's so cool and it's so weird. You know, put put another way, let's look at the other side of that coin. You know, I'm sitting here talking to a camera. Um, there is a person jogging towards me on a plant on a distant planet, and we are agreeing on now, right now. And that person right now is reaching a cul-de-sac on their planet. I'm assuming they have cul-de-sac, and they're now turning and jogging in the other direction. Now our now our nows no longer agree, and instead they are relative to me, either in the past or the future of the moment that we had just agreed on. I don't remember the directions that I said, but it it's it the nows no longer sync up. It's this is all real. This sounds crazy. It sounds like I'm just yapping. Like, this is special relativity. This is these are the consequences of special relativity. And I I talked about these now slices in this geometry. There's something called Minkowski diagrams, a space-time diagram, and you can actually use it to work on this for yourself. I'm not gonna dig into the Minkowski diagrams. Um, this video is already getting pretty long, but and there, you know, it's also like a little bit more technical, but the you you can look it up. I mean, there I'm sure there are tons of videos on Minkowski diagrams that you can check out on YouTube. And the idea is is is pretty simple. You know, instead of treating space and time like two separate things, you just put them on the same map. Space-time, a space-time diagram, you know, combine them together. This is this is this is why scientists say space-time. Instead of space and time is two separate things, treat them as though they are just two parts of the same coin. And you will see on a Minkowski space-time diagram that motion causes a different angle cutting through space-time, and distant observers will find themselves in different now slices for a source that is moving at a different speed. Motion literally changes the angle, which uh results in different nows as time is cut across the universe. So the idea that emerges from all this is called the block universe. And it's called the block universe because you imagine this four in it's hard to this four-dimensional chunk where there's not just three dimensions that you can that you can move to, which of course of course there is in our 3D universe, but there's also this fourth time dimension. And think of time spatially as a place that can be traveled. You sort of think of like a loaf, a giant loaf of bread, this big block, where time, you know, if we could step out, if we could pretend we were in the fifth dimension, then you could see that there is a time, there is a future, and there is a past. And that entire loaf, that universe loaf, that universe block exists, and the future and the past exist, all time exists in the same way that you know, the north and the south and the east and west exist. And in the same way that there's, you know, multiple points that you could point to on Earth. You think about multiple points that you could point to in Earth's time, in the history of time, and that all of these things have to be existing all of the time if simple motion allows you to align with a different now. I mean, it's so weird. You know, we we have the feeling, the experience that only the present is real, um, that the past is gone, that it's done, and that it's just like no longer exists. And the future is like not here yet. It doesn't exist yet, it still hasn't been written. And and that now is the present is the only real moment. And it's like it's like slowly inching forward, it's sliding forward, and there's like nothing behind us in the past, there's nothing yet in front of us in the future, and that is how it feels, and that is definitely, I think, how our minds have evolved to experience and deal with time in the block universe. The past is real, and also the present is real, but also the future is real, past, present, and future all equally real all at once. The the entire whole of the universe from the beginning to the end, whatever that means, you know, is all already laid out, uh sort of like a landscape, and there is no there is no special now, and that your sense of this instant being the real now, being the the the present moment is basically an illusion, which is I think it's such a it's such a deep thing to try and grapple with. It's and it's a very convincing illusion, but an illusion noneet. The reason that many physicists would take this illusory view of the block universe as serious, um, it just it comes, it comes straight out of that part bench example. You're now it reaches Andromeda and lands on one event. Your friends now walking past lands on a different event days later. An event that is still sitting in your future because you're sitting still. If you had a magic, you know, window, magic mirror, and you're seeing what's happening on the other side of your magic mirror, and your friend is doing the same thing that's walking by, you're seeing different events. The event that your friend is seeing is in the future and the of the event that you're seeing, which means that if you just sat there on the park bench for four days, you would eventually see the thing that your friend saw. How could that possibly be unless the future, the past, and the present were all existing always, all the time, always have been and always will be. You know, another way that you could think about it is a book, right? All the pages of a book already exist. Um, it's it's complete, right? Nothing is getting added to the book, nothing is getting taken away. You know, you can you read it one page at a time, and you can read through it fast, you can read through it slow, you could go back and you know, you could start over, you know, you could read it backwards if you wanted, but all you know, all the information is there, and that is the idea behind the block universe. And if we are right about special relativity in the way that it seems like we are, then that is very much, I think, a plausible interpretation of existence in of reality. And you know, it doesn't matter what page you're on in that book, all the other pages exist. You could hop around if you want, it doesn't matter. The pages are there and they exist. But again, you know, I gotta point out this is you know, this is not proven. I I use the example of of magic instantaneous mirrors. Of course, these don't exist. This is not something that you could, you know, do an experiment on. Uh, the relativity of simultaneity, this park bench uh thought experiment, the the tilting now of the Minkowski diagram, the Minkowski space-time diagram, all of it is rock solid, and it has been tested. Special relativity is like not in question, general relativity is not in question um at all. But the block universe is an interpretation that emerges from it, right? So this is, I don't want to say it is speculation, right? Um, it's it's a little bit more than speculation because it's rooted in solid physics, but it is an interpretation, and there are many interpretations that we have at any given time about any number of physical uh phenomena. And there are brilliant physicists and philosophers who argue both for and against this. I just think that it's a fascinating possibility. I mean, one of the more interesting ones. And whether or not you choose to believe in something like the block universe, what is not debatable is that there is no universal now. Um, there isn't like a single present moment ticking through the cosmos that everybody shares and everybody agrees upon. And the irrefutable evidence for that is uh GPS satellites, which exist in a less steep gravitational well. So from general relativity, we expect their clocks to move faster because they're further from the surface of the earth, and then special relativity because they're moving so fast, we expect their clocks to run slower because they're moving so fast as they orbit. And both of those have to be accounted for um when the a GPS, a satellite is giving you coordinates for your phone, otherwise you would get the wrong coordinates. So, I mean, that if there's if there's a you know a factual statement that all of this is anchored to, it's that general relativity and special relativity, the um differences in now's, the lack of a universal now is something that has to be accounted for every time you pull up Google Maps or Apple Maps, otherwise you would get the wrong directions. So everything, every time you know you put an address into your phone and drive somewhere, you are demonstrating that special relativity and general relativity are real effects that need to be accounted for. Now, in my opinion, Block Universe is a is a fine interpretation. I like it. Um, and you know, not just from I think obviously I think it's very interesting, but I think that there's very much of a sort of personal philosophy that I um appreciate from that interpretation. You know, you you have never doubted the existence of other countries, of land in other places. Antarctica exists, Paris, France exists, uh Lagos, Nigeria exists. Whether or not you've ever been to those places, whether or not you're in those places now, they currently exist. The block universe theory, it's just it it probes you to extend that same grace for time, that the future is real and exists. And for me, this is where there's a heavy weight, not as a physicist, but as a person. If the future is a real place, you know, it's already out there in landscape, um, then the the future that I'm reaching for, for example, in my life, that you're reaching for in your life, your future goals, your future objectives, where you want to see your life, it is not this esoteric abstract fantasy. It is a real thing that exists today, that exists now, because all now's there is no preferred now. All of this exists all the time. It is a real physical place in space-time. The future that you want is real. When you feel that thing that you want to get to, the place that you want to go is real. For me, it it it like it ignites this engine of making sure I'm doing the things to bridge the gap, to connect point A to point B. Like point B exists, it's out there, I know it's out there, I know that it's a physical destination. I just need to continue to do the things to get there. It's very real. You know, in psychology, they have studied uh quite a bit on hope. And I think that the that there's an element of hope and belief that um is inspired by the block universe, at least in my opinion. You know, hope isn't it's not just wishing for something, it's it's a couple things going at the same time. It's like it's the belief that the destination, the future is reachable, it's an actual place, um, but also it's the drive to be able to take the steps to get there. You know, it's you you having one without the having the drive, but not knowing where you're going, you know, what is what is that? What is the purpose? You know, you'll you'll burn out because there is no destination. But then having the a destination, but not having the drive to do what it takes to get there, you know, you have you just have the opposite problem. You know, you need you need both of these things. You need you need the motion, you need the drive, you need the legs, you need the power. You also need to believe that the destination is real. And that's what I carry out of out of all this relativity. You know, what what this means fascinating to think about. I've had a great time talking about this and working through these things, and hopefully, you know, I was clear and things weren't too confusing, even though I think this is an inherently confusing topic. Relativity is, it's a it's bizarre and it's sort of antithetical to the experience that we have day to day. But for me, the same physics that says that you can change your present um with a single step in one direction or another, somewhere else in the universe, it says something about the hope that you can have for your future. That it's out there and it's real, and the only road between here and there is motion. It's moving, it's moving towards it. So the next time you know you're sitting around, you're feeling like where, you know, the place that you want to go is way out there. Just you just gotta remember to stay in motion. You gotta remember to keep going. That uh it's it's real, the future is real, it exists. What you want, where you want to be, exists somewhere in space time, and it's up to you to connect those dots. That's it for this episode on relativity. As always, lead with curiosity.