How a Rotating Universe Permits Time Travel - Ask a Spaceman!

foreign extreme amount of confidence that the Earth rotates and I don't think anyone disagrees with that statement 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 could I be so confident if I point to the sky and say hey everything repeats every 24 hours and is doing that because we're spinning around until 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 in many cultures in time and space believed that the Universe rotates but we're 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 a storms could come from the East to 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 called 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 we also have the modern day observation 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 expand 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 in in 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 Spain Galileo figured that out don't try this at home he monitored sun spots 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 sunspots 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 galaxies glowing stars big giant balls of dust and gas all the you know just junk floating around is glowing and if that bit of junker 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 chunk 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 could measure a redshift 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 redshifted 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 this and and the opposite side is moving away from us it's spinning other stars are spinning other planets or Spain we're seeing all sorts of stuff spaying 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 smidge 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 going to 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 of the two 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 large 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 this side of the Box label gr and you put in the inputs and out comes 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 in 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 Friedman lament Roberson 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 and 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 in 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 the mall you say hey Einstein um let's assume the universe is homogeneous and isotropic filled with matter radiation and dark energy how does this behave how does it evolve you turn the cranking 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 and so it could be that our universe is rotating even though our current best model says that the universe is homogeneous 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 it 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 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 okay 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 godal's incompleteness theorem uh okay 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 goedle is I don't feel bad just ask kurgodel developed a cosmological model of a rotating Universe use the first person to do it he was the first person to show that when we're speaking cosmologically 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 Godel was the first one to figure that out his model is a little artificial the universe that godo constructed his model of the university is rotating and besides that it contains only one other ingredient which is a negative cosmological constant that has to be in there because if you have set up a rotating Universe there's going to be a centrifugal force he wanted his Universe to be static and not a universe that bulges out on the size that that broke down like if you just put in hey Einstein I want a universe that rotates and is 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 that is constantly pulling inwards on the entire universe that counter balances 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's full of stuff like matter and people and hulumi cheese 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 hear 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 yeah we use artificial models all the time in physics that oh 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 Kurt godel'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 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 goldle used this rotating universe that he had discovered in the equations of general relativity to demonstrate that Einstein's theory was incomplete you see godal 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 goldil'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 godol 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 you know 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 is 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 in its a theory of gravity through the structure of space-time remember matter and energy tells space-time how to bend and the bending of space-time tells matter how to move and that's what we call gravity general relativity should at least according to Kirk Godel provide in its math a natural way to account for the flow of time from past to Future godo 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 all 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 wormholes 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 Godel 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 kirgodle 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 spectrographs 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 of into the air and follow weird curved paths from the Coriolis 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 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 Galaxy of course is going to be movies 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.com pmsutter you 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.com pm s-u-t-t-e-r 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 add 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 and 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 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'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 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 lycone I want you to not focus on the word light not focus on the word cone and not focus on the word lycone I want you to instead mentally swap out mentally replace the word lycone 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 it 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 treat right in front of it it's in in the light cone it sees a tree far away it can reach there traveling slower than the speed of light and 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 this 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 lichens well I mean lycone 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 okay I can reach the Andromeda galaxy in three million years that's within your light code and you go off and say you live in a rotating universe you keep going off and your light cone changes directions 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 the new Trident 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 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 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 non-starter 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 all 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 Kurt Godel I guess yeah it only happens in 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 God's time and of violating some fundamental knowledge of the universe oh you need to use exotic matter or matter with negative Mass 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 Universal 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 decades go to a would walk around asking cosmologist well is it rotating yet and they would say no Mr godal it's not the universe is not rotating well to be clear as far as we can tell the universe is not rotating a 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 redshift in blue shift in them and if in apparently as far as we can tell all the galaxies are redshifting 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 redshifted 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 godl'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 goldil 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 vojtek all an email for the questions that led to today's episode hey if you want to support the show go to patreon.com pmsutter thank you to my top patreon contributors this month Justin G Crystal Barbara K Duncan M Corey D Justin Z Andrew EV nalia Scott M Rob H Justin Lewis M John W Alexis Gilbert M Joshua John S Thomas D Simon G Aaron J and the hundreds more that continually support the show month after month I couldn't be more grateful that's patreon.com pmsetter drop a review on iTunes our Spotify or however you digest this podcast keep sending me questions ask a Spaceman gmail.com ask a spaceman.com or hit me up on social media ask a Spaceman or at paulmad 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 thank you [Music]

Channel: Dr. Paul M. Sutter

Views: 9,885

Keywords: space, cosmos, universe, astronomy, physics, astrophysics, cosmology, science, time travel, einstein, godel

Id: wqv5M-AuE5s

Length: 41min 46sec (2506 seconds)

Published: Thu Feb 23 2023