How do we know that the universe doesn’t rotate? Why would it matter if it did? What does all this have to do with time travel? I discuss these questions and more in today’s Ask a Spaceman!

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EPISODE TRANSCRIPTION (AUTO-GENERATED)

I can say with an extreme amount of confidence that the Earth rotates, and I don't think anyone disagrees with that steam. In fact, I don't even think that the people who believe that the Earth is flat think that it's motionless, although I could be wrong about that. But I'm not willing to go out and find out for sure. So let's just take it as a given. But how can I be so confident if I point to the sky and say, Hey, everything repeats every 24 hours and it's doing that because we're spinning around, and so we get brought back to the same point of view every time. You could just counter that with this simple statement that we the Earth is perfectly stationary, and it's the rest of the universe that's doing the rotating, and this is exactly what many cultures throughout history believed that many cultures in time and space believed that the universe rotates but were not many cultures throughout history. They were just as smart as we are for sure, but we happen to have a tool for understanding the universe that they didn't physics, and with this tool we can say again, with an extreme amount of confidence that it's the earth that's rotating and not the universe.

There's there's evidence all over the place like there's the breaking of symmetry. If the Earth were stationary, then there would be nothing special about East versus West. Uh, storms could come from the East just as easily as they could come from the West. They could come from the north just as easily as they come from the South. There should be no preferred direction for wind patterns or storm patterns. And yet there is. Storms flow in one direction along the equator and then another direction along the Northern Hemisphere and then another direction along the Southern Hemisphere. If you're living in the United States, for example, or Europe, storms will tend to come from the West and move to the east and not the other way around. Yes, there are exceptions, but that's the general rule. That's the general pattern. There shouldn't be a pattern if the Earth isn't rotating, But the rotation of the earth sets up certain wind patterns that we see. We see an asymmetry appear from a rotating earth.

We have the centrifugal force that If the Earth is a solid ball of stuff and it's spinning, then it's spinning faster at the equator than it is at the poles. And so there'll be more centrifugal force at the equator than there is at the poles, and those will cause the equator to bulge out. And so you can measure the circumference of the earth around the equator and compare that to, say, the circumference of the earth around a prime meridian, the north south direction and you'll see that it's bulged out, and you can explain that through centrifugal force of a rotating earth. There's also the Coriolis forces that make hurricanes spin in certain directions. There's also the focal pendulum. If you take a pendulum and just let it swing, it will slowly trace out a circle, not because it's moving in a circle, but because the earth underneath it is moving in a circle. Uh, we also have the modern day observations of of satellites. You can watch satellites move from north to south from south to north and from west to east, but never east to West because satellites moving in that direction.

In order to do that, you have to expend an enormous amount of energy to counteract the natural rotation of the Earth. When you launch the thing, we have evidence all over the place that the Earth is rotating. We can perform local experiments that confirm that the Earth is the one doing the rotating and not the universe. And that's it. The earth spins and and the earth isn't the only spinning thing in the universe. It turns out all sorts of things in the universe spins. The solar system spins. We can observe the orbits of planets. What we call orbits is just a spinning solar system. Our sun itself is spinning. Galileo figured that out. Don't try this At home, he monitored sunspots, realized that sunspots were actually on the surface of the sun and not some atmospheric phenomenon. As many ancient thinkers and observers had theorized. He watched the sun spots move around and concluded that the sun is spinning, our galaxy is spinning, and we can measure this with electromagnetic radiation.

All sorts of stuff in the galaxy is glowing stars, big giant balls of dust and gas all the, you know, just junk floating around is glowing. And if that bit of junk or star is moving towards us. The radiation coming off of that will get blue shifted, and if it's moving away from us, the radiation from that junk will get red shifted. It will get shifted into redder parts of the spectrum. And so if you know the wavelength of light that you're supposed to be getting or what was emitted, you can compare that to what you actually get. You can measure a red shift or a blue shift. What's really handy here is the wavelength emitted by neutral hydrogen neutral hydrogen has this really cool property that it can emit radiation of a very specific wavelength 21 centimeters, to be precise. This radiation is caused by neutral hydrogen. It turns out there's a lot of neutral hydrogen all over the galaxy, and you look in one direction and it's 21 centimeters plus a little, and you look in the other direction and it's 21 centimeters minus a little.

It's getting blue shifted and red shifted. You can conclude that our galaxy rotates. We can use similar techniques to determine that other Galaxies are rotating. You observe one chunk of a distant galaxy. It's moving towards us and and the opposite side is moving away from us. It's spinning. Other stars are spinning. Other planets are spin. We're we're seeing all sorts of stuff spin across the universe, even filaments in the cosmic web. These are long strands of Galaxies up to hundreds of millions of light years, long containing tens of thousands, sometimes hundreds of thousands of Galaxies. They make these filaments make up these tendrils. In the cosmic web. They appear to be spinning as well. They're rotating on their long axis. Ancient people believed that the entire universe is rotating. Modern observations tell us that the Earth is rotating. The sun is rotating. The solar system is rotating, Galaxies are rotating. Filaments of Galaxies are rotating. So were the ancient people, right?

In a way, it's obviously the universe is obviously not rotating around once on its axis every 24 hours. That that's pretty much settled. But is the universe rotating in any sense at all? Even just a little, just a midge. Does our universe rotate? I'm going to warn you. This seems like an innocent question, and it is an innocent question. Does the universe rotate but innocent questions can have dangerous consequences. If we're gonna talk about rotating the entire universe, we have to shift our language. It's one thing to talk about rotating planets and stars and Galaxies and even filaments. This is all just stuff that's spinning. But to talk about a rotating universe to talk about a spinning universe, we need to bring in our dear friend, old Uncle Albert into the discussion because it's his rules, the general theory of relativity that govern the behavior of the entire universe. Because at the very, very largest scales, when you look at truly cosmological scales and you ask, Well, what forces are at play here, there's only four options.

There's only four forces of nature. Uh, and so you work down the list. Strong nuclear, too short range. That's not going to govern the behavior of the universe. Weak nuclear force. No one cares about weak nuclear. It's not going to govern the behavior of the universe. Electromagnetism has infinite range, but on the very largest scales, the universe is electrically neutral, so all of it balances out, so electromagnetic force isn't going to govern the behavior of the universe. If you want to understand the physics of the large scale universe. The only force left is gravity. And the only language we have to describe gravity at those scales is the general theory of relativity. But general relativity itself isn't going to tell you if the universe rotates or not. You can't look into Einstein's equations and have those equations tell you if the universe is rotating. It doesn't work that way because general relativity is a machine for making models.

General relativity tells us how gravity works, but it doesn't tell us what the situations are. For example, general relativity doesn't tell me that a solar system exists. But general relativity tells me how that solar system behaves. I have to plug into the machinery of general relativity. I say, Hey, Einstein, I got one for you. I want a large amount of mass in the center, and then I want some smaller masses moving in orbits around them. Tell me how this system evolves through gravity and you crank the handle on the side of the box label GR. And you put in the inputs and outcomes in evolution of the solar system. You say, Hey, Einstein, I got another one for you. I want to collapse a pocket of matter so much that no other force can compensate, and it just keeps collapsing. What happens? You put that into the machinery of GR. You put that situation and you crank the handle on the side and out pops the physics of a black hole.

When we're talking about the universe, we can't just say, Hey, Einstein, what is the universe like? It doesn't work that way. We have to put in a model. We have to put in assumptions about what we think the universe is like. Then the box of GR will spit out how that universe evolves and behaves. And then we go Compare that to observations to see if our model is right or not. When it comes to cosmology, our favorite model is called the FLRW Model, named for four people. Freedman lament Robertson and Walker, who developed this over the course of decades, and this model is relatively straightforward. It says the universe is homogeneous and isotropic, which means our universe is relatively the same at large scales from place to place. Obviously, the earth is different than Mars in the middle of the void between Galaxies, but at large scales When you smooth everything out, it's pretty much the same from place to place and looks pretty much the same in any random direction.

In this universe, This universe, this model universe that we use in cosmology, is filled with various substances like matter and radiation and dark energy. These substances experience an expanding universe. In this model, if you assume that the universe is homogeneous and isotropy, you get automatically get an expanding universe, the components of the universe alter how this expansion proceeds and that is modern cosmology. And you plug this into them all you say. Hey, Einstein, um, let's assume the universe is homogeneous. An isotropic filled with matter, radiation and dark energy. How does this behave? How does it evolve? You turn the crank and out pops a model for the history of the universe, Then you can compare it to observations to see if you got it right. This is the program of cosmology. And as far as we can tell, this universe is is right. These assumption assumptions are correct. The universe is homogeneous and isotropic and filled with matter and radiation and dark energy.

And it is expanding but this model didn't have to be right. There are many potential models, an infinite number of models, actually, that could have described the universe that could have been right, because GR by itself, general relativity by itself doesn't tell us which is the correct universe. We must rely on observations, and there are many, many potential models of the entire universe that general relativity allows. There are many potential models that could have been right, but it was a matter of observations to winnow them out. This show is sponsored by better help. One of the most awesome things about physics is that it's like a user manual for the universe. You can literally use it to predict the future Now. Now, humans are a little bit more complicated. Believe it or not, people are more complicated than quantum physics. I am not joking, and and life and dealing with people does not come with a user manual, and the next best thing is therapy.

I have been using therapy for years. It's such a powerful tool for me to to answer life's questions when those questions don't come in the form of of of physics problems and I think you will benefit a lot from it, too. And that's why I'm proud to have better help as a sponsor. Better help is the world's largest therapy service. Better Help has matched 3 million people with professionally licensed and vetted therapists available 100% online. Plus it's affordable. Just fill out a brief questionnaire to match with the therapist. If things aren't clicking, you can easily switch to a new therapist any time. It couldn't be simpler. No waiting rooms, no traffic, no endless searching for the right therapist to learn more and save 10% off your first month at better help dot com slash spaceman. That's better. Help HE LP dot com slash spaceman. And so it could be that our universe is rotating, even though our current best model says that the universe is homogenous and isotropic and expanding.

That is one model among many potential candidates that satisfy the equations of general relativity that are allowed by general relativity. And when I say allow, I mean that when you put these inputs into the machine of general relativity, you get a sensical answer, you get a you, you get an answer. That makes sense the machine doesn't break down. There are scenarios that you can concoct where you say, Hey, Einstein, I got another one for you. And and you put in some weird scenario and general relativity breaks down or tells you that Oh, yeah, yeah. In order for this to be possible, um, objects have to travel faster than the speed of light. And then, you know, like, OK, this, this isn't gonna work. This isn't a physically viable scenario, but even with that constraint that the machinery of GR of general relativity must allow, it must permit it and allow you to make predictions.

Even in that constraint, there are an infinite number of cosmological models that could have described our universe. So it could be that our universe is rotating. But in order to ask this question, is our universe rotating? We have to pass two tests. The first test is to see if we can put in a rotating universe into the math of general relativity and see if it allows it if make sure you actually get an answer. The second test is that if general relativity allows for the existence of a rotating universe, then we have to figure out what the implications are and see if we can go measure that the first person to realize that general relativity permits a rotating universe was Kurt Godel in 1949. And yes, it is the Kurt Godel of Godel's incompleteness theorem. Uh, ok for the two people in the audience who know who that it was. Uh, you're you're welcome. And if you're curious who Kirk goal is, uh, don't feel bad.

Just ask Kurt Godel developed a cosmological model of a rotating universe. He was the first person to do it. He was the first person to show that when we're speaking cosmological and we're using Einstein's language, which we have to do, we have no other choice to construct a physical model of a rotating universe. We must use general relativity. And it was an open question for decades. If relativity permitted the existence of a rotating universe, it turns out it does. And goal was the first one to figure that out. His model is a little artificial. The universe that Godel constructed, his model of the universe is rotating, and besides that, it contains only one other ingredient, which is a negative cosmological constant. Uh, that has to be in there, because if you have set up a rotating universe, there's gonna be a centrifugal force. He wanted his universe to be static and not a universe that bulges out on the side that that broke down. Like if you just put in Hey, Einstein.

I want a universe that rotates and it's full of matter and nothing else that breaks down that doesn't survive the test. So you have to add an extra ingredient with a negative cosmological constant, Uh, that is constantly pulling in words on the entire universe that counterbalances the centrifugal force to keep things nice and static. When you put that ingredient in, then it does survive the test. It is allowed by general relativity. Nothing breaks down. This model is artificial. Our universe, as far as we can tell, does not contain a negative cosmological constant contains a positive cosmological constant, which we call dark energy that is accelerating the expansion of the universe. Our universe is not static, it is expanding and it is full of stuff like matter and people and Hulu. But hey, it's interesting. Even if this model isn't very accurate to the universe that we observe in terms of contents. It's interesting that it's even allowed by general relativity.

Prior to 1949 it was an open question. If general relativity could even permit a rotating universe. And here Kurt Godel showed a scenario as artificial as it might be could permit a rotating universe. General relativity was just fine with a rotating universe under certain circumstances and check here we use artificial models all the time in physics that we know to fully capture a reality. We use these toy models to understand concepts or to poke at edges. And sometimes those toy models that don't look like reality. If you poke at it long enough, you find a way to make it look exactly like reality. So, yeah, Kirk Goel's model of the universe with rotation, no expansion, negative cosmological constant doesn't look like our universe. But maybe if we poked at it for a while and massaged it, we could get a version that did look like our universe.

But if general relativity didn't allow a rotating universe at all within its own consistent equations, then you knew the whole program would be useless and you could just say, Look, the universe isn't rotating, and we know because it violates general relativity. But general relativity permits rotating universes, at least model ones, which may or may not look a lot like the universe we live in. Kurt Gold used this rotating universe that he had discovered in the equations of general relativity to demonstrate that Einstein's theory was incomplete. You see, developed this model as a way to, um, honor his friend and Princeton neighbor Einstein. But as usual in academic circles, honoring can come in strange ways. And in goal's case, he wanted to show that Einstein was was wrong. That wasn't that he wasn't done, that General relativity wasn't all it was cracked up to be.

This is 1949. This is decades after Einstein developed it, generally accepted to be a correct revolution of our understanding of gravity. Goal came and wanted to poke at Einstein a little because this is what he enjoyed doing and why he was never invited to parties. The problem isn't a rotating universe itself. After all, the universe could be rotating even just a tiny bit, even a little bit slowly. It could be the first test was to see if general relativity permits a rotating universe, and it does so we can't rule it out on theoretical grounds. We can only rule it out on observational grounds. And there's this tiny, tiny little issue that a rotating universe allows for time travel into the past. We've done plenty of episodes on why time travel into the past is not a good idea. Most notably, it leads to violations of causality. All of our sense of cause and effect breaks down. It appears in our universe that we cannot travel back in time. We see no evidence for it, and if it were to happen, then our conception of reality would not make any sense anymore.

So the question isn't if we can travel into the past. For all intents and purposes, we can't. The question is why we can't travel into the past. The flow from past to future is so natural and so fundamental. We talked about this in the episode about causality and tachyons that it's not. It's not even mentioned in physics. It's not even brought up like Newtonian physics. Classical physics just assumes that time marches from past to future, and that's it that there's nothing else to it that you don't even need to think about the passage of time, because it just is. It's a feature of reality, but general relativity is supposed to be the complete be all and end all description of not only space but also time. General relativity is a theory of gravity, and it's a theory of gravity through the structure of space time. Remember, matter and energy tells spacetime how to bend, and the bending of spacetime tells matter how to move. And that's what we call gravity. General relativity should, at least, according to Kurt Godel, provide in its math a natural way to account for the flow of time from past to future.

Go to argued that because general relativity specifically deals with the subject of time that it should naturally include in its math a sense of the flow of time from past to future and gold found a way for general relativity to allow for backwards time travel. Now we Nowadays, we have other ways in general relativity to construct time travel, uh, infinitely long cylinders, uh, worm holes. You name it, there's there's a bunch of options. Kirk Gold was the first to realize that general relativity permitted backwards time travel and for Kurt goal, this was a huge deal. And for us it's a huge deal. Because if general relativity is supposed to describe the nature of time and apparently we live in a universe where time travel into the past is forbidden because it would violate causality, then general relativity should say something about that.

But instead KGO found a universe a model artificial universe. But that's not the point here, a model universe where general relativity was totally chill and totally cool and totally fine with backwards. Time travel. So is general relativity complete? Or is there more to say on the subject of time that general relativity is missing to explain what's going on and how this permits backwards time travel? Let me pause here or divert here and describe what it would mean to live in a rotating universe. First, we have to set up the right reference frame. How do you know that the universe is rotating? Is the earth is rotating? You could be rotating right now. How do you know? How would you go about making this measurement to confirm that you live in a rotating universe? Well, imagine building yourself a laboratory, a closed room in the middle of nowhere of space.

You you launch it in a rocket, you go off into space and you've got your little box. You've got your little lab. You've got beakers and spectrograph and a fridge full of sandwiches in this laboratory. If the laboratory were spinning, you could figure it out. All the stuff would go flying to the sides of the room from centrifugal forces. You would feel a weight of gravity from the centrifugal force. You would toss up into the air and follow weird curved path from the Corolla forces. You know all the experiments and observations that we can perform on the surface of the earth to determine that it is us, the earth that is rotating you could perform in that laboratory. You could perform local measurements to determine that you are indeed rotating. So in order to test if the universe itself is rotating, you first need to put a stop to your rotation. So you imagine you put your little rocket engines on the outside of your lab and you, you you fire them to counteract your rotation. They go in the opposite direction and you fire them just right so that all the centrifugal forces and Coriolis forces and all the other forces that go along with the rotation go away this way.

You know for sure that you're not rotating, you perform all your local measurements, and they all confirm that you are no longer rotating. Now you open a window. Hopefully there's glass on it. It's not a screen door. And you look at the distant stars and Galaxies if they're staying still on average. Of course, if you look at any one particular star or a galaxy, of course is gonna be moving. So you need to do a survey. You need to put some sweat into this. But if all the stars and Galaxies are staying still, and you know for sure that you are staying still, too, then you know that the universe isn't rotating. But if you know for sure that you are still and you measure a rotation to distant stars and Galaxies, then you know that the universe is rotating. And it looks like in your little laboratory that the universe is rotating around you. You're the fulcrum. You're the axis. You're the center. You're the patreon patreon dot com slash PM Sutter U really are the center of this podcast universe.

It is your contributions that keep the show going, and I can't thank you enough. That's patreon dot com slash P MS U TT ER You're the center of rotation of the universe according to your observations. But if you were to pick up your laboratory and blast off billions of light years away and repeat the exercise, make sure you had all the rockets just right and cancel out all your local rotation. You knew you were perfectly still. You would still look like you're the center of the rotating universe. In fact, in a rotating universe, the universe spins around every point. Every point is the center of rotation in a rotating universe. I know this is hard to picture. It's hard to picture for me, too. But one way to think about this just like an expanding universe, there is no center to an expanding universe. The Big Bang didn't happen over there. The big bang happened everywhere. Us here on earth, it looks like the universe is expanding away from us. We go over to the Andromeda Galaxy. It looks like the universe is expanding away from the Andromeda Galaxy.

You go pick some random galaxy out there in the universe, move there, make your observations. It looks like the universe is expanding away from you. The universe doesn't expand away from a fixed point. The universe expands away from itself. A rotating universe doesn't rotate around a fixed point or a fixed axis. It rotates around itself. And I admit that's hard to think about. And it's super weird. But that's not the weirdest part. The weirdest part is that a rotating universe allows for time travel into the past because we're not talking about rotating stuff. We're not talking about rotating planets, rotating gas clouds, rotating Galaxies, rotating that's rotating stuff. When we talk about the universe, we're talking about a rotation in space and time itself. That's different, and that makes things really freaky. For example, you know that light has to follow the curvature of space time like bends around a massive object like the sun or a cluster of Galaxies.

It has to go over these hills and valleys as it travels. If whatever the warping of space. Time is underneath it. Light is forced to follow it. If you flash a beam of light out into a rotating universe, then that light will race outwards. But eventually it gets tilted, it gets turned. It gets curved, as as it gets swept up in the rotation of the universe because the rotation of the universe is the rotation of space and time itself. Eventually, that beam of light will reach a point where it curves so much that it'll be forced to loop back around, eventually returning to where it started. That means in a rotating universe, there's a limit to how far you can see. And beyond that, you just see images of yourself from your own past or or the planet you're sitting on from billions of years ago, which is pretty wild. And it also means that you can travel back into your own past if you work hard enough. And the problem here is light cones. Light cones are this abstract concept that appear in discussions of space time. When you hear the word light cone, I want you to not focus on the word light, not focus on the word cone and not focus on the word light cone.

I want you to instead mentally swap out mentally replace the word lico with the set of places, I can go in a given amount of time, forced to travel slower than the speed of light. That is my light cone. So, for example, I would like to go to my neighbor's house tomorrow, and I I'm gonna start traveling there, and I want to reach at my neighbor's house by tomorrow. My neighbor's house is within my light cone because I know I can travel there by tomorrow in a fixed amount of time. By traveling slower than the speed of light, I would like to to make it to Mars within a month. Mars one month from now, is in my light cone. Because even though I'd have to work really hard and build a really, really powerful rocket, I could still travel slower than the speed of light and make it to Mars in less than a month.

I would like to go to the Andromeda Galaxy in a year that is outside my light cone. If I want to make it to the Andromeda Galaxy in less than a year. I have to travel faster than the speed of light. I'm not allowed to do that. That's outside my light cone. The word light cone comes from the shapes these limits make. When you draw diagrams involving both space and time, I like to imagine it like it's those cones that they put on dogs When, when they're healing from something, the light cone is the limit of the places they can go in a given amount of time. Dog sees a a treat right in front of it. It's in in in the light cone. It sees a tree far away. It can reach there, traveling slower than the speed of light. It can reach that tree, but if something's off to the back, it just can't see it. It's outside the light cone. Light cones are simply the set of destinations that you can reach in a given amount of time, being forced to travel slower than the speed of light, which you are.

The problem with a rotating universe is that it changes the shape of light cones. I mean, light cone shapes get changed all the time. This is what gravitational wells do This is how they deflect the path of light. They change light cone shapes, but what happens with a rotating universe is that a rotating universe bends and widens the shape of light cones and eventually allows them to turn back in on themselves. I don't have the perfect analogy here, but it's almost like the normal centrifugal force that you experience in any rotating situation. But this time the centrifugal force is getting applied to your own future. You can find special paths in a rotating universe where you always stay inside your light cone, never going faster than the speed of light. That's a no no. But because the rotation alters the direction of the light cone itself, you can end up in your own past. You you set off in some direction and you chart out your light cone like OK, I can reach the Andromeda galaxy in 3 million years.

That's within your light cone, and you go off and say You live in a rotating universe. You keep going off and your light cone changes directions. Then now you have a new chart of Galaxies that you're allowed to reach in a certain amount of time and a new chart of Galaxies and a new chart, and it starts curving around and around around, and eventually the Milky Way galaxy appears in front of you. In a rotating universe, you don't have to do the work of turning around. You just keep marching forward. You don't change the controls. You, you, you duct tape those controls to the console. You don't change direction of your rocket ship at all. And because the light cone itself your set of possible futures, your set of possible destinations that you can reach alters before your very eyes just through the fact that you're traveling through a rot rotating universe without changing any of your controls. Eventually the earth appears in front of you, and not just any Earth but the earth of your own past. Your light cones have become so twisted, so turned around, so distorted that the past becomes accessible all while traveling slower than the speed of light your own past.

Imagine, like just consider how wild that statement is. Your own past becomes a possible destination for you in a rotating universe. For all practical purposes, this is a nonstarter, as you would have to travel billions of light years to get this to work. But the fact that it works in principle is what got. Got so excited about it. It was the ultimate gotcha to Einstein. Time travel into the past violates causality. There ought to be a law against it, and that law ought to appear in general relativity because general relativity is supposed to be the ultimate arbiter of space and time. And right there, general relativity through the existence of a rotating universe, permits time travel. I told you this was an innocent sounding question, but it held something very, very dangerous. Is the universe rotating? Turns out the answer to that question involves time travel. Who would have guessed Kirk goal? I guess. Yeah, it only happens in a very special kind of universe, but there's nothing in GR itself that rules out the existence of a rotating universe as a possibility.

You need to go out and observe it to confirm or deny this idea. The fact that general relativity doesn't self censor against time travel is kind of a big deal. Other modes of time travel that we have concocted since Goel's time end of violating some fundamental knowledge of the universe. Oh, you need to use exotic matter or matter with negative mass. You know, you you need some crazy thing that violates something else. We know about physics, and that rules out the time travel but not go to a solution. We could have found ourselves in a rotating universe. Wait, wait, wait. I didn't answer the question. Do we find ourselves in a rotating universe? Well, the story goes that for the next couple of decades, gold would walk around asking cosmologists Well, well, is it rotating yet? And they would say No, Mr Gool, it's not. The universe is not rotating. Well, to be clear, as far as we can tell, the universe is not rotating slightly different statement. We have many, many different kinds of observations to point to, uh, you can use planetary orbits. If, if stars were orbiting around us, then their slight gravitational influence would be seen as slightly messing up to a certain level of precision that we can reach the orbits of the planets.

We don't see it. We have massive galaxy surveys spanning hundreds of millions of light years. We can look for red shift and blue shift in them. And if they and apparently as far as we can tell all the Galaxies are red shifting away from us, there's just expansion. There's no shift there where there's no sense of rotation there. And for the ultimate test, we can look to the cosmic microwave background itself. This light that was emitted billions of years ago. If the universe were rotating, then one side of that light would be blue shifted and the other side of the light on the other side of the sky would be red shifted. And we don't see it. We don't measure it. We can put an upper limit to the rotation of the universe to somewhere around 10 to the minus 17 degrees per century. And while that's not exactly zero, it's it's close enough for me. As far as we can tell, We do not live in a rotating universe. Stuff inside the universe rotates, but the universe as a whole apparently does not. But Godel's objection Gold's Little little poke, as it at his friend and neighbor Einstein, still stands.

We don't appear to live in a rotating universe, but we didn't have to nature could have just as easily picked that one to be our universe and not the Friedman lament Robertson Walker metric of standard cosmology. It turned out that model is more correct, better able to describe nature. But it didn't have to be. We could have lived in a rotating universe. There's nothing fundamental about this. It's ruled out by observations. Not by theory. It's not ruled out by anything anything self consistent in the theory of GR self general relativity is supposed to be the ultimate discussion of space and time. And yet general relativity enables and allows for backwards time travel, which appears to violate our notions of causality. So is general relativity the ultimate question about space and time? Or is there a more fundamental theory out there waiting to be discovered? Gold argued that this very, very simple question is the universe. Rotating is a piece of evidence, not for the rotation of the universe itself.

Who cares? The universe isn't rotating is done, but the fact that GR permits it that general relativity permits it demonstrates that there is more to be learned. But that's a different episode. Thank you to William T, Sam C and Tech all an email for the questions that led to today's episode. Hey, if you want to support the show, go to patreon dot com slash PM Sutter Thank you to my top Patreon contributors this month. Justin G, Chris Barbara K Duncan M, Corey D, Justin Z, Andrew Naia, Scott M, Rob H, Justin Lewis, M John W, Alexis Gilbert, M Joshua John S, Thomas DE, Simon G, Aaron J and the hundreds more that continually support this show Month after month, I couldn't be more grateful. That's patreon dot com slash PM Suter drop a review on iTunes or Spotify, or however you digest this podcast. Keep sending me questions. Ask me spaceman at gmail dot com. Ask a spaceman dot com or hit me up on social media. Hashtag ask a spaceman or at Paul Mad Sutter.

However you want to reach out to me, I'll take your question and I'll see you next time. For more complete knowledge of time and space

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