Where do parallel universes exist? Could we use wormholes to travel to them? What does time travel have to do with all this? I discuss these questions and more in today’s Ask a Spaceman!

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EPISODE TRANSCRIPTION (AUTO GENERATED)

You ever need a vacation? Like a real escape? And not just somewhere exotic, like a tropical paradise or a mountain retreat or even another planet. No. You don't want to float through the storms of Jupiter or bounce around on Pluto.

Okay. Maybe you do, but that's you wanna go even bigger. You don't even want to go to another galaxy. Now have you ever had that urge to just quit the universe entirely? Go to an alternate reality, a parallel timeline, experience another existence entirely?

Well, it's gonna be tough. But hold on. While my usual MO in this show is to rain on everybody's parade and say that all the cool stuff from sci fi is implausible or not well justified or just plain doesn't work, in this case, I'm going to leave open a tantalizing possibility. It's just a tiny one, but it's still there. Because when it comes to parallel universes and getting there, there's a slight.

And trust me that word is doing a lot of heavy lifting in this sentence, slight possibility, which makes it really, really fun to talk about. First off, if we've packed our bags and decided that we want to travel to a parallel universe, it's not like we can just go to our favorite airline and book a ticket. We need to know where the alternate universe is so that we can, you know, go there. Thankfully, physics has our back. And if parallel universes exist, then we know exactly where they are.

And they are really, really far away. This is the multiverse which does allow fair parallel universes, other universes existing right now at the same time as ours just somewhere out there. How far? Well, yeah, this is going to be tough. The multiverse comes to us from our theories of inflation of the extremely early universe where we believe, when the universe was less than a second old, it underwent a period of accelerated expansion, not just mild and quiet accelerated expansion like getting on the on ramp on a freeway.

I'm talking out of control berserk accelerated expansion where the universe expanded by a factor of, like, I don't know, 10 to the 60 in somewhere around 10 to the minus 30 seconds. And there are some extensions to this idea of inflation where it never stopped, where the universe just keeps inflating on and on and on and on. And what we call our universe is really one little branch, one little pocket of that overall universe that, settled down where the quantum fields kicked into a lower gear where inflation shut off, but inflation just shut off locally for us. Meanwhile, the rest of the universe just kept going. And so the way I like to think of this is that, it's like an expanding foam of bubbles where you have these little pockets where the universe, little pieces of the universe have slowed down where inflation stopped locally.

It's stopped here. It stops somewhere somewhere else. It stopped over there. There are all these places where the universe, where inflation calmed down, but it was only on a local sense. Meanwhile, inflation continues throughout the whole entire universe, so you end up with a whole bunch, a lot of these pocket little bubbles that all exist simultaneously like bubbles in a foam.

But the real challenge with the multiverse here is that these pocket universes, these bubbles, these parallel universes, which exist right now completely in parallel in in space, they're they're over there. They are located a finite distance away, but that finite distance is very, very large. They are incredibly far away. And because of the accelerated expansion that's happening between us, that those pocket universes, those other bubbles are moving away from us faster than the speed of light. So it's not like you can get in a rocket, punch through the edge of our universe, travel through light years and light years of absolutely nothing, and then arrive in another pocket bubble.

It's just too far away. So you're gonna need a lot of frequent flyer miles. But what about wormholes? You know, wormholes connect to distant points in space through a bridge or tunnel, and that's the whole point is that you can make the tunnel as short as you want depending on how cleverly you construct your wormhole so you don't have to take the long route. So in principle, you could enter a wormhole in our pocket universe, our bubble, and exit in another bubble universe.

You get to skip all those light years. And wormholes aren't ruled out. Right? I mean, well, no. Not technically.

We we just don't know how to make one. You know, wormholes are allowed in the mathematics of general relativity, but the real sticking point to trying to make a traversable wormhole, one that you can actually travel down without being atomized, is that you need to build it out of negative mass. You need to thread it with negative mass and, like, negative mass doesn't exist in the universe as far as we know, so we don't know how to build 1. But, hey, in the spirit of optimism, we have to remember that wormholes are not ruled out entirely. They're not outright forbidden by everything we know about physics.

They just seem incredibly implausible. And, ultimately, it's a question left to a theory of quantum gravity. We need to understand how gravity works at extremely strong small scales, and and means it may be possible to build a wormhole in a way that we're just not clever enough to figure out yet. You know, there's there's that tiny possibility that somehow you could construct a wormhole. But unfortunately, in this case, wormholes aren't going to do the trick.

That's because you have to build a wormhole, which means if you want to build a wormhole, you need access to both ends to start off with. This is a thing that a lot of people miss when building wormholes. You don't get to just create a wormhole and then step in and get zoomed off to a distant part of the universe. You need to build the thing. You need both ends.

And then once you build both ends, you can transport one of the other ends anywhere you want. You take the slow way. You're gonna have to, but once you plant the other side, then you can cross the wormhole, then you can bridge that gap and make it short. It's like building a literal bridge. It's hard at first to build the bridge, but then once you built it, it's easy.

You just cross it. So unless the other end of the wormhole is already anchored in one of the other bubble universes, you can't use a wormhole to cross that distance. You need to build it 1st. Maybe some wormholes existed in the quantum foam of the early universe and persisted to the present day and and naturally existed and got inflated to macroscopic scales and their ends are anchored in other opposite, you know, other bubble universes, other parallel universes? Maybe.

But so far, we see no evidence of macroscopic wormholes existing naturally in the universe. So as far as we can tell, if the multiverse exists, there's no way to get to any of them. We're stuck. But wait a minute. Wait a minute.

Now that we're on the subject of wormholes, let's talk about them. Wormholes don't just connect points in space. They also connect points in time. So maybe we're asking the wrong question. Even if the multiverse exists and there are other bubble universes out there, we can't build a wormhole to take us to one of them because we need to anchor both ends.

But maybe we're asking the wrong question. Maybe it's not where parallel universes exist, but when. Check this out. Wormholes are time machines. If you take one end of a wormhole say you build your wormhole.

However, we'll leave that to the engineering department. Let's say you build a wormhole and you take one of the ends and accelerate it close to the speed of light. Now once you do that, the clocks on either end of the wormhole are unsynchronized. One of the ends is moving slower through time than the other which means you're building a time machine. You can walk in one end of a wormhole and arrive in your own past.

That's a time machine. Right? And I'm assuming that if you're able to build a wormhole in the first place, then the last part of accelerating one of the ends to nearly the speed of light to unsynchronize the clocks is is no big deal. So, okay, we can turn a wormhole into a time machine. But what do time machines have to do with parallel universes?

Well, let's explore time travel a little bit more. I know I've done episodes on it before, but it's it's worth getting into. Let's take a little diversion on the nature of wormholes and time machines in general to see where this goes and what it might mean. So time machines in general lead to lots of nasty paradox is that upend our understanding of minor things like the flow of time, cause and effect, and general sensible orderings of the workings of the universe. For example, there's the standard stereotypical grandfather paradox.

Now you go back in time, you kill your own grandfather, which means you'd never exist, but then how were you able to exist to go back in time to kill your grandfather? That's a paradox. But there are other paradoxes that don't involve violence to grandparents. Let's say you enter a time machine and go back in time, then you destroy the time machine. The machine is now destroyed so you can never go back in time, but that means the machine isn't destroyed because the only way to do that was to go back in time, which means you did go back in time to destroy the machine, which means you can't go back in time.

Which is it? It's a paradox. Or let's say you're sitting, eating breakfast, you know, starting your day, and a wormhole appears in your kitchen. Future you steps out of the portal probably wearing some snazzy future outfit, and future you tells you how to make the time machine. So then you build the time machine and go back in time repeating the exercise with a past version of you.

So when did the time machine first appear? Events need to begin in the past. That's kind of the definition of the past. So we just sort of have a machine existing with no beginning, which seems a bit odd. Time travel leads to paradoxes.

Okay. How do we respond to this? One response to the appearance of paradoxes with time machines is to say that our understanding of cause and effect is flawed or incomplete or that, the time is an illusion, and and you're welcome to entertain those ideas. But then things like, you know, all of physics and material experience and known reality breakdown. So that's a rather big step, but, hey, I'm not gonna stop you.

On the other hand, another response, another valid response to the appearance of paradoxes with time machines is that maybe time machines are impossible because they lead to inconsistencies. This is Stephen Hawking's chronology protection conjecture, which is just, okay. You see these inconsistencies? That makes it impossible to build a time machine. Machine.

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Anyway, that's one possible explanation. And on the 3rd hand, maybe time machines are allowed, but only when they don't lead to inconsistencies or paradox. This is known as Igor Novikov's self consistency conjecture. In other words, you're never allowed to change the past. The past is locked.

What has happened in the past is always what happened in the past. Even if you have a time machine, even if you can travel back into the past, you're not allowed to change it. It's forever locked and forever fixed. But how? How can you travel into the past without changing it?

And the answer is alternate histories, aka a parallel universe, a mainstay of science fiction. Now, as far as I can tell, this idea of alternate histories emerged in physics circles somewhere in the mid 19 eighties, and I'm honestly not sure which came first, the physics version or the sci fi story version. Either way and it's it's either way, it sounds crazy. And it's crazy enough. It just might work.

The deal is here's how you do it. Here's how you can go back into the past without changing what has already happened. The deal is that when you enter a time machine and go into the past, you don't enter your own past. Instead, you enter another history separate from your own. Let's take the popular option when discussing altering the past of of killing Hitler.

Everyone wants to do it, and I understand why. I get it. Trust me. If I had the option, I I I'd do it too. Let's say you go back in time and you kill Hitler and you do it.

You pull it off, but it's not your Hitler. It's somebody else's Hitler. That history unfolds with its past locked in. In that timeline, Hitler was always killed by a time traveling assassin from another universe. Everybody remembers that.

That's what goes in the history books. That history never changes. So when you return to the future, you come back to your future where Hitler did his usual evil things. Your past remains unchanged. This is how you can have time travel while also protecting the past.

There's no paradox. There are no inconsistencies. Time travel is still allowed because the past remains locked. It's just different universes get different histories. This means that if you didn't come back into the present, say, you traveled back in time, you killed Hitler, and then you hung around.

If you stayed in that other timeline, that alternate history, you're in another universe, a parallel one, evolving in some sense alongside our own. I don't know where it is. It's not a location. Like, the multiverse idea, I know where the other bubble universes are. I can point to them.

They're really far away, but they're over there. I don't know where these other universes are located, but they exist. And in this scenario, if this scenario is true, if you can travel into the past and and and experience alternate histories, then you can travel to a parallel universe by going back in time and making one. Sounds great? And in sci fi, that's about as far as you need to go, but this is physics.

We can't just come up with ideas as cool as they are. We need to explain how they work using our established knowledge of physics, a k a the the science of how things work. So how does it work? How do you go back in time and split timelines and create parallel universes? One idea floated around years ago was that space physically split in 2.

Like, there's a duplication of the cosmos with all of its three dimensions and quantum fields at the moment of arrival in backwards time travel. Sounds cool. No one can get it to work. The math just doesn't make sense. It gets all wonky.

You can't actually physically separate the universe into 2 branches with 2 separate space times. It doesn't work. The math doesn't work out. So maybe that's a wrong approach. Well, what about quantum mechanics?

Is there anything in that branch of physics that it could explain how history split? Hey. Wait a minute. Isn't there that whole many worlds interpretation? Yeah.

Yeah. Yeah. Where you have say you run a quantum experiment and, the result of the quantum experiment is random and you can't predict the exact result. You can only assign probabilities to the various results, and and we have a really, difficult time, explaining how that process unfolds and what exactly is happening at the moment of measurement. So there's this many worlds interpretation which says, oh, yeah.

Every time there's a quantum process, a quantum interaction, a quantum experiment, a quantum this, a quantum that, there's a whole new branch of the wave function where there are different universes where universe a gets one result and universe b gets the other result, and so everyone is happy because now there are are observers measuring this result in this universe and there are observers in the other universe measuring the other result. So that's cool. There's no, like, weird collapse of the wave function or any of that nonsense. So it's just this giant wave function that's constantly evolving and branching off. Seems like a natural fit.

Here, we already have alternate universes popping off constantly like popcorn in a kettle with every single possible quantum process, which, which is a lot. So maybe traveling back in time doesn't create a timeline. It doesn't create an alternate history. It just shuffles you to one of those other realities that already exists. Another part of the global universal wave function that doesn't exist in another, point in space the way the bubble multiverse does, but exists in another portion of the quantum wave function that describes the universe.

You're still over there. If you travel back in time and and get to experience an alternate timeline, you're over there, but not in a spatial sense, but in a quantum sense. And, and, I know you're still wondering, well, where is it, Paul? Where is the other universe located? And, I mean, that's the best answer you're gonna get.

I'm sorry. So, this all sounds great, but, again, we're lacking a mechanism. We can't explain the physical process that unfolds when you go back in time and do the quantum shuffle to an alternate history. I mean, to be fair, we're also lacking a mechanism in general for how the many worlds interpretation works, but to be even more fair, we're also lacking mechanisms for just about every weird quantum process, so we can't necessarily let that hold us back. But leaving aside the question of the mechanism or physical process, can this even work?

If we were able to build a wormhole and going back in time was allowed because you're really entering another branch of the quantum many worlds tree, can we find any reason to see if this would break? Can we rule it out? Even if we can't explain how it could work, can we at least say, oh, this is definitely not allowed because of known physics. Oh, yeah. It breaks this.

We don't know how it works. But if you were to do this, then it would mess up then. So we so we can we can stop talking about it now. Well, we've tried. We've explored.

When we look at this through the lens of quantum mechanics, say, seeing how the behavior of quantum particles or quantum fields behave, say, you were able to construct a wormhole, construct a time machine, allow the quantum particles to travel back in time, split off into exploring alternate parallel branches of the many worlds, etcetera, etcetera. Quantum fields tend to go haywire. They set up these feedback loops that send their energies to infinity and they essentially blow up. However, we were able to discover that you you can be allowed, you can get rid of all this nastiness, you can get the quantum fields to stop going haywire if you move away from established quantum mechanics, if you relax some of the rules in quantum mechanics. Like you allow for violations of the correspondence principle.

This is the fundamental thing that connects quantum systems to classical systems. Or if you allow for violations of unitarity, which means that quantum systems can be reversed. Like, if if you break standard quantum mechanics, then everything kinda seems hunky dory. We still don't know how it would work. We don't know how the universe would actually split.

We don't know how the time travel wormhole would actually be constructed. But leaving that aside, it doesn't necessarily break anything in terms of quantum mechanics if you allow quantum mechanics to be a little, looser than we currently understand it. It's interesting, you know, we're not exactly ready to give up those tenets of quantum mechanics, so no one's no one's really willing to jump, you know, in that swimming pool. And, believing we can leave quantum mechanics aside, I mean, quantum mechanics doesn't always get the last word. I know I know it's kind of an important theory and all that, but maybe the issue is that we don't understand quantum mechanics.

You know, like, we know we're lacking a theory of quantum gravity, which has to come into the picture somehow in terms of either generating a physical mechanism or ruling this whole thing out. You know, the whole deal involves quantum gravity. We know we don't have quantum gravity, so maybe asking these kinds of questions within the realm of quantum mechanics is is barking up the wrong tree. So maybe there's a way to get this to work at the macroscopic level and and just assume that later down the line with the quantum theory of gravity, it will all sort itself out in the end. But there we run into problems too.

Like, let's say your time machine, is a wormhole, and your wormhole has a switch. You flick the switch. The wormhole appears. You can travel back in time. Flick the switch back and the wormhole goes away.

No more. Now you're doing something, you're modifying space itself. You know, this is pure geometry. The complete opposite end of quantum mechanics. This is as classical as classical it can be.

Like, you're just wondering, okay, take away my knowledge of quantum mechanics. Take it all away. I just want to look, pretend quantum mechanics doesn't exist, pretend quantum mechanics has nothing to say about this problem. I just want to study it in terms of classical physics and look to see if I can break anything, if I can violate anything. And if I have a flickable, switchable wormhole, I end up breaking stuff.

And the problem is, like, let's say you turn on the switch. You enter the wormhole. You go back in time. Then you turn the switch off. The wormhole goes away.

Now you have a problem. And the problem is, at a very macroscopic level, at a very, classical physics level ignoring anything quantum, now you have one history, one timeline where the wormhole exists, and another history where it doesn't. In other words, you have one history where space time itself has a certain structure and another history where space time has a different structure. How do you resolve this? How do you get different histories or branches or paths in the many worlds, you know, however you want to construct this?

How do you create different spacetimes? What's the mechanism here? What's the process that allows space time to have different manifestations in this branching history's parallel universe story? And you can't appeal to quantum voodoo here because we're talking about big old macroscopic structures in space time that are way too big for quantum mechanics to have any say. Quantum rules don't apply.

No matter what, we can't get this process to actually work in any physically meaningful sense of the word work. Like I said, it's one thing to say that history split during time travel and you enter a parallel universe, and you can even assign cool concepts to it like the many worlds interpretation of quantum mechanics as a as a as a a venue where this takes place, but it's quite another to describe the physical unfolding of that process, and it's that step b that we're having trouble with. So once again, the only way we're probably going to figure this out is with a more complete understanding of space time, gravity, and quantum mechanics. So just add it to the to do list once we've cracked quantum gravity, I guess. We won't know for sure if any of these mechanisms or stories work until we've solved that.

And we hope that such a theory of quantum gravity would just tell us if time travel is allowed, if wormholes are allowed, if we can travel back in time, and all this so we don't have to keep guessing. On the other hand, maybe we could skip the theory altogether. Let's go experimental. Let's go practical. Let's go engineering.

Follow me here. What if we just built a wormhole? You know, stopped arguing about whether it would be possible or quantum this or chronology protection that blah blah blah. All these theorists just just wasting time in chalkboard space. What if we just built it?

What if we just gave it a shot? Here's the cool thing. If we could construct a wormhole and go back in time, we could start directly testing things like the chronology protection conjecture or the many worlds interpretation or the self consistency principle. Once you build it, you can have direct experimental evidence access to these wild and crazy ideas. For example, all you have to do let's say here's an experiment.

An experiment. If you could build a wormhole, run this experiment right now. Here's what I want you to do. I want you to memorize a number. It can be any number.

It doesn't matter. Pi. Whenever someone asks me to pick a number between 1 and 10, I always pick pi by the way. You memorize the number, then you step into your wormhole. You step into your time machine, and you go back in time, and you meet your past self.

And you say, hello, me. And it's important you don't go too far back. I want you to go back just a little bit in time to a to a time when you've already memorized the number but before you stepped into that wormhole. So, you know, you you have a little cushion there. Say there's like a 10 minute window.

Okay. I've got my number. I'm gonna wait 10 minutes. Now I'm gonna go into the wormhole. I'm gonna go back in time, but I just can't go back in time 5 minutes.

So you say, hello, me. Nice to meet me. Say, I know you've memorized a number. I've got it in my head and I want you to tell me that number. You ask your past self what is that number that you both memorized.

Now, here's where the test comes in. If the past is locked and there's only a single timeline, then your past self will always and forever repeat the same number back to you. And, in fact, because of the way the past works, before you enter that time machine, let's say you've memorized your number and you're waiting your 10 minutes before you step into your time machine. In that time, your future self will come back to you and and you're gonna have a conversation. You're gonna spit out the number, then your future self goes away, and then you enter the warm the the time machine and you and you continue the loop forever because the past is always always locked.

So all you have to do is step into that time machine, ask yourself what that number is. And if if there is only a single timeline, if the past is truly locked and there's only one history, then your past self will always and forever repeat the same number back to you. And before you enter the time machine you will acquire a memory of your future you coming to talk to you and you telling them what that number is. But if there are multiple histories allowed and going back in time has slipped you into a parallel universe, then your past self has free choice. They know the number you're looking for.

They remember it too. It's still you, but they could refuse to tell you or tell a different number. They tell you something else, meaning that means that what's happening in the past is different than what you remember, a possibility allowed if there are multiple histories in a branching many worlds multiverse. Sounds like a fun experiment. We just need to figure out, a, how to build a wormhole, and, b, how to turn it into a time machine.

Then we can perform the experiment, verify what kind of reality we live in, and then let the physicists catch up with an explanation. But until then, we have to take the theory route. We have to keep pushing on quantum gravity. We have to search for these paradoxes and potential resolutions to the paradoxes. We have to try to just concoct a physical mechanism that allows for many worlds branches, allows you to to transfer from one to the other and all that and for everything to be hunky dory and consistent with what we know.

You know, it's a lot of work, but there is that tiny tiny tiny tantalizing possibility that this can all work out in the end. And then we can finally go on a vacation. Thank you to Sandra g, Shavam Sharma, and atdjdavis for the questions that led to today's episode. And thank you to all of my Patreon contributors. That's patreon.com/pmsutter.

But I would especially like to thank my top contributors this month. They are Justin g, Chris l, Huberto m, Duncan m, Corey, d, Robert, b, Nyla, Sam r, John s, Joshua Scott m, Rob, h Scott m, Lewis m, John w, Alexis, Gilbert m Rob w, Jessica m Jules r, Mike g, Jim l, David s Scott r, Heather, Mike s, Pete, H, Steve S, Watt, Waddworth, Lisa r, Koozie, Kevin b, Michael b, and Eileen g. That's patreon.com/pmsutter. Thank you so much for all of your contributions. Please keep those questions coming.

That's askaspaceman.com for the website. Askaspaceman@gmail.com. Please, keep leaving reviews on your favorite podcasting platform, and I will see you next time for more Complete Knowledge of Time Answers.

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