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47: Blast to the Past (Retrocausality)
471 Plays5 years ago
Time is something that everyone has an idea of, but is hard to describe. Roughly, the arrow of time is the same as the arrow of causality. However, what happens when that is not the case? It is so often the case in our experience that this possibility brings not only scientific and mathematic, but ontological difficulties. So what is retrocausality? What are closed timelike curves? And how does this all relate to entanglement?
This episode is distributed under a CC BY-SA 4.0 license. For more information, visit CreativeCommons.org.
[Featuring: Sofía Baca, Gabriel Hesch]
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Transcript
What is Retrocausality?
00:00:00
Speaker
Time is something that everyone has an idea of, but is hard to describe. Roughly, the arrow of time is the same as the arrow of causality. However, what happens when that is not the case? It is so often the case in our experience that this possibility brings not only scientific and mathematic, but ontological and linguistic difficulties. So what is retro-causality? What are closed timelike curves, and how does this all relate to entanglement? All this and more on this episode of Breaking Math, episode 47, Blast of the Past.
Meet the Hosts
00:00:35
Speaker
I'm Zavia. And I'm Gabriel. And you're listening to Breaking Math. Real quick, we have some plugs, like always. If you want to donate to us, the donations to Patreon really help a lot. If you want a poster from Patreon, you can pay, I think it's $22.46 a month. If you just want the poster itself, you can go to our Facebook poster store at facebook.com-breakingmathpodcast.
Support and Contact Info
00:00:58
Speaker
We also have a tier on Patreon where we deliver outlines. It's only five bucks a month.
00:01:02
Speaker
And you get our outlines and add free versions of the show. And by the way, the poster is an outline of the tensor mathematics used in Einstein's general theory of relativity. It's got lots of pretty pictures.
00:01:16
Speaker
Yeah, that's $22.46 plus $4.50 shipping and handling, and it's good quality and everything like that. You could go to Twitter, you can see us on Twitter at BreakingMathPod, and our website is breakingmathpodcast.com. However, that's down for right now due to some issues with our hosting. We'll try to get that back up as soon as possible.
00:01:35
Speaker
Yeah. So the best place to reach us probably is Facebook, uh, slash breaking math podcast or Twitter at breaking math pod, or you can email us at a breaking math podcast at gmail.com. We do read your emails. We enjoy them. We respond to them. They're great.
Retrocausality and Quantum Entanglement
00:01:49
Speaker
All right. So today we're going to talk about retro causality.
00:01:52
Speaker
fascinating topic. This is based on an article that I found about a month and a half ago and we have taken literally six weeks to carefully comb over this article. Make sure that we understand it and then look for some of the research behind it and prepare an episode for you guys. So we really hope you enjoy this one. This article is a proposed solution or perhaps a partial solution to the problem of quantum entanglement.
00:02:20
Speaker
Yeah, and specifically the question of an ontological framework for retrocausality.
Time Travel Paradoxes
00:02:28
Speaker
That is to say, stuff in the future affecting things in the past. And when we say stuff in the future affecting things in the past, we don't mean like something happened in the past.
00:02:37
Speaker
and then the future happens and then something happens to change the past. We're talking that the past always was influenced by the future. It's like a weird like almost you can think of it as like a weird cosmic connection kind of like entanglement. Correct. Now it's for this reason. It's for all of the difficulties of trying to conceptualize how the future at all could affect the past without some kind of a paradox. One such paradox is the grandfather paradox.
00:03:03
Speaker
There's other paradoxes as well that tend to make time travel impossible. So time travel proper, as you saw in, you know, say, Avengers Endgame or Back to the Future, that is impossible. That's not quite what we're talking about here, even though it's still a very murky topic. We have a lot that we're going to talk about in this episode. Should we go ahead and do a quick outline so the audience will know what's to come?
00:03:27
Speaker
Sure, we're
Basics of Quantum Entanglement
00:03:28
Speaker
going to start talking about just entanglement in general. Then we're talking about relativistic retrocausality, especially around really weird spacetime metrics, like when space is curved really a lot around an object in space.
00:03:41
Speaker
Correct. We will review a previous episode. We touched on this topic on episode 28, Bell's theorem, or what was the episode? It was called something with Bell's theorem. I can't remember. Episode 28. Yeah, episode 28. I can't remember right now. Yes, Bell's infamous theorem, I believe. Yeah, there we go. We will talk about what Einstein called spooky action and the distance and why he called it spooky.
00:04:04
Speaker
We'll talk about quantum measurements. We will also talk about specifically with the issue of the limit of the speed of light and information not being able to send faster than the speed of light. Now question, is it information that can't or is it communication that can't?
00:04:23
Speaker
Well, information is pretty much a communication, but information is not correlation. Okay. So in the case of like, let's just say, we will get to this topic more in more detail. In the case of quantum entanglement, if you have two entangled particles that are in a superposition, and if you measure one of them and you measure a down spin, would that be sending, would that be sending information in the past in order to have a measurement at the moment of inception?
00:04:52
Speaker
No, because information implies, uh, decision making. So it wouldn't be information being sent to the past. It would just be a correlation between the two. And okay. Okay. Yeah. This is, is anybody else as confused as me? Raise your hand. I saw that. I see all you out there with those hands raised.
00:05:08
Speaker
What else are we going to go here? We'll talk about relativity. Yeah, we'll have a whole lot to say. Closed time-like curves, something called the Godel metric. And then finally, we have a term that we're going to use on this podcast that we thought of after reading the paper. The term is called super time theory.
00:05:28
Speaker
Yeah, it's just pretty, it's fun, but I think it's an interesting way to look ontologically at the idea of retrocausality. It's maybe a way to kind of smooth some of this stuff. It works in the same way that the stretchy fabric metaphor works for quantum, not for quantum, for general relativity. Meaning that if you put a heavy object in the middle of the trampoline,
00:05:54
Speaker
Then you put like a small golf ball, you could toss it in such a way that it kind of is orbiting around the larger object. Correct. Yeah, so it's term we invented.
Quantum Entanglement and Communication Challenges
00:06:05
Speaker
Don't actually think that this term is used in published literature. To our knowledge, it's not. We invented it for this podcast or we believe we are the inventors of it. So without further ado, shall we do a review of what entanglement is?
00:06:23
Speaker
Sure, so entanglement is sometimes you could take a particle like a photon and split it into two particles that are also photons, or like these can be electrons, but let's say they're photons. You can have them so that one is polarized one way and the other is polarized in a completely perpendicular way to the other one. And the cool thing about it is that you're not sure which way they're polarized until you measure them. And once you measure one though,
00:06:52
Speaker
you know that the other one will measure the opposite if they're entangled.
00:06:58
Speaker
Now what you're about to mention here, this is what Einstein incorrectly described as the two gloves analogy, where he said just because you know if you buy two gloves and they're in a box and you don't know which glove is which, let's say you were to split the box somehow in a way where you don't know which glove is in which half of the box, you don't look into that giant hole that you just created,
00:07:22
Speaker
I'm trying to use this analogy here. You know, if you separate two gloves without looking at them, one of them is a right hand glove and the other is a left hand glove. And they've always been that way. You may not know what it is, but they've always been that way.
00:07:37
Speaker
That's not quite what's happening with entangled particles. It's more appropriate to say that both gloves, if they were quantum mechanically entangled, both gloves would be left and right until one of them is measured. And then the other one turns into the opposite one. And some people have thought that this is a completely philosophical problem.
00:08:00
Speaker
until Bell came up with his Bell's theorem. And like we said, it's a long thing. You can review it on episode 28, but the gist of it is that if you use statistics to analyze this, you basically see that the particles' nature changes at this last split second. Yes. This is taking the strangeness of the double slit experiment and it is, what should we say, spreading it over time, which is even stranger, I think.
00:08:29
Speaker
Yeah, and I mean, Leah, like, um, and what's interesting about this too is that it's okay. Let's say we have two electrons and they're entangled and I take my electron and I go to one place and then you take your electron and go somewhere else. Now let's say I'm traveling close to the speed of light, um, perpendicular to like a road that would connect the two people.
00:08:53
Speaker
And let's say they had a deal saying that in 10 minutes, one is going to measure their electron, and in 10 minutes in one second, the other one is going to measure. Basically, one of them measures their particle, and they get it. Let's say they get spin up. And so we know, if we're watching them, that the other one is going to go spin down. So it kind of looks to us like the particle in the past was affected by one in the future.
00:09:20
Speaker
However, if we were going the opposite direction, it would look like the opposite. It would look like the particle was entangled afterwards. And if you didn't move at all, it would look just like they measure it at nearly the same time. Now, I want to mention a couple of things here. The first one is entanglement itself. What does it even mean for two things to be entangled? Does this relate to the Pauli exclusion principle with electron orbitals?
00:09:44
Speaker
Um, I don't think it does. I think it's just a property that sometimes two particles have. Well, actually, wait, hold up. I think, yeah, I think, um, it's the entanglement.
00:09:54
Speaker
Yeah, actually it does relate to that because I know they can only have one spin up and one spin down in orbital and things like that. That's exactly why I bring it up. So my next question would be how, now this may not be a question that you have the answer to. I know it's probably as simple as a Google search. How does, how exactly does one entangle two particles? You can use a crystal, for example, for photons. Okay. You basically take a photon and you've sent it into a crystal that splits it into two photons.
00:10:21
Speaker
So funny. I read about how much crystals are used in pseudoscience now that we're using them in actual science here. So the crystals are awesome. Not just not for the reason that people sometimes think they are healing crystals. No, no. So, okay. So, so in terms of entanglement, two photons become entangled when they touch. Is that right? No, no. Two photons are entangled if they're created from one photon. Okay. Okay. You split a photon into two, into an entangled pair.
00:10:49
Speaker
Okay. Okay. So what they were touching. Well, no, they were, they were, they weren't two things. They're one thing. Okay. So I guess if they're one, okay. But what about that moment when they're not split yet? Well, they don't exist as two. They're just one. Okay. Okay. Yeah. Maybe this is okay. Okay. So, so it's not like one. And then you pull it apart, like how you pull out part two conjoined twins or something. No, it literally takes the nature of it and splits into two things. Okay. All right. All right. All right. Each lower energy than the original photon, but
00:11:17
Speaker
Okay, fair enough, fair enough. So yes, so that is entanglement in the case of photons. And then in the case of electrons, it could be that you just have them both in the same energy level where they, you know, following the power of the exclusion principle, one of them has to be spin one way and the other has to be spin the other way. Although if you haven't observed it yet, they are both both.
00:11:37
Speaker
So now that we've talked about exactly what entanglement is, a few other notes here, Einstein, as we said earlier, talked about entanglement as being spooky action at a distance because you shouldn't be able to have. Now he said, you know, you can't send anything faster than the speed of light. And we have said on this podcast that one cannot send information faster than the speed of light.
00:12:03
Speaker
And you look at, let's say we're trying to communicate using entanglement. You might be able to think like, okay, this person is in space, but let's say they measure it and let's say they can guarantee a spin up or a spin down at the far location. In that case, then you'd be able to just have a constant stream of reading these entailed photons and you would have faster than light communication.
Black Holes and Light Cones
00:12:26
Speaker
However, anything that you do to guarantee spin up or spin down will unentangle the photons because it interacts with one of the particles. So there's no way to send that information in the past. And if you try to do it statistically, there's different schemes that people have thought about. But if you do it statistically, basically you went into the problem that you can't copy an entangled photon like to do multiple experiments on it. Once you measure it, it's gone forever.
00:12:53
Speaker
I think this also by the way isn't it funny isn't it cool the web of connection that one topic has with others? Entangled photons are a major major part of talking about black hole information theory actually there's a lot of thought experiments involving what happens when you take two or three entangled photons and one of them goes into the black hole and one of them still entangled goes outside of the black hole so that's a
00:13:16
Speaker
That's just a fun aside if you guys ever want to go to YouTube or Google and type in black holes and entangled photons. You can learn all about the firewall principle. Fascinating. In the process of trying to actually communicate using quantum entanglement, you will unentangle the photons. Yeah, basically.
00:13:41
Speaker
This is a really, really cool topic that I was only introduced to with the research for this episode. A light cone. What cones do we think about that we already know? There's ice cream cones. There is the cone that goes on your dog's head after a surgery. The cone of shame. The light cone is just like that, right?
00:13:59
Speaker
Pretty much, I mean, those cones, if you look at them, it's a circle getting bigger over space. And it starts from a single point, right? Yeah, and what is a 3D circle? It's a sphere. So if you think of a sphere getting bigger over time, that is like a cone that is pointed in the direction of time.
00:14:16
Speaker
And if you make sure that this grows at the speed of light, like the radius grows at the speed of light, then we get what's called the time cone. And the time cone, I mean the time cone, the light cone, and the light cone is everything that that event can affect. It's all the things that can be influenced by that event are in this time cone.
00:14:39
Speaker
Cool, so basically it shows the life cycle of that particle of light from its birth to any given point at the end of the cone. And I'm not really sure what would be the end of the light cone, like it would go infinity, wouldn't it? Oh yeah, and what's interesting though is because space is curved, the light cones can be tilted sometimes,
00:15:01
Speaker
So like if we're near a black hole, for example, the light cone would tilt into the black hole and could even tilt so that it's looking back at itself. If the light cone's on the event horizon, then you can see the back of your head on the event horizon. That is insane. Oh man, seeing the back of your own head. Ooh. So it'd be a twisty cone. I love black holes. I love black holes. Did I say that earlier? I love them.
00:15:22
Speaker
Yeah, and what's cool about that is that let's say if it's tilted beyond 45 degrees, meaning what that means is there are future positions from the object's frame of reference. So let's say we're this object and we're in this really twisty space. Something can happen in the future. And so that's something that happens after me. So let's say that I'm eating an orange and my orange is here and then it's eaten.
00:15:49
Speaker
However, if we're from outside of the light cone, it could look like the orange was there and then instantaneously faster than the speed of light move somewhere else. This is trippy. And observing things faster than the speed of light is actually pretty easy. Just go out at night, look at the moon and spin around. If it takes a second to spin around, the moon's going something like four times the speed of light or something like that. Wow. I never thought about that. And again, that's just an observation frame of reference thing.
00:16:18
Speaker
Yeah. So, and like, if, if we had a big flashlight that could illuminate the moon and we put our hands in front of it and somehow it didn't fry our hands, we could, the shadow could be going faster than the speed of light. If we moved our hand across the, uh, the, this, uh, light source. Now it doesn't actually mean that the, wow. Yeah, this is crazy. And can you explain for all of our listeners out there, how, how it doesn't mean that the shadow went into the future or.
00:16:40
Speaker
Yeah, because I mean, you could kind of think of it as like light as being like this thing that carries information, but like just because it carries information, that doesn't mean that you can't have illusions making things move faster than light. You can also imagine a situation where you had, um,
00:16:56
Speaker
a thousand light, I don't even know how many light bulbs, from here to Alpha Centauri four light years away. Now let's say all these light bulbs decided to blink at almost the same time. We could observe a trail of lights going from here to Alpha Centauri in a split second, and it would almost look like it's animated. Even though there's no signal being sent to fashion than the speed of light, because there's coordination it can be.
00:17:22
Speaker
In the same way, if you had, if you were building a road in space, if you placed everybody a few feet away from each other, you could theoretically build a road faster than the speed of light, but it would require prior communication. So you couldn't just build a road faster than the speed of light starting from one point, but, um, yeah, you definitely could have been like, um, if you had a coordinated, you could have the build look like it's going faster than the speed of light.
00:17:45
Speaker
Interesting. Yeah, wow. And again, this is all how we're defining things. So isn't it crazy how you can describe things happening where it's not actually technically true? Yeah, it's all about frames of reference. And that's the thing about these retrocausality things is that the frame of reference is so important.
00:18:03
Speaker
Oh, and one thing real quick before we move on to the paper, the object, if it's in this really curved space, it could even look like it's moving backward in time to an external observer because of the way that light is, not that light, but space is bent.
Time Symmetry in Quantum Theory
00:18:23
Speaker
All right, so now we're going to talk a little bit about a paper from Chapman University Institute for Quantum Studies called, Does Time Symmetry in Quantum Theory Imply Retrocausality? by Matthew Leifer. Basically what the paper talks about is, okay, we're going to talk real quick about quantum, about quantum measurement and preparations, stuff like that. So you can do things to, like the way that a quantum computer works is you have all these entangled
00:18:51
Speaker
photons or electrons, and you do these operations on them to change the probability of certain outcomes, and then you measure them and then you basically look at that outcome and you hopefully get a good calculation from that. Now, I think one of the cool things about the paper is the principle that they come up with about two-thirds of the way through, saying that if we watch the video of a process and cannot tell whether it's plane forwards or in reverse, that the means of the process is operationally times symmetric.
00:19:20
Speaker
Wow. I think you said one more time for the people in the back. So basically let's say we, let's say we throw a ball, for example, and there's no air resistance. It's just a simple ball and it flies through the air and somebody catches it. If, if it's don't carefully, it like no friction, nothing like that, especially no friction, you can basically reverse it. And, uh, you wouldn't be able to tell who's throwing the ball and who's catching it, uh, because every action has equal and opposite reactions and all that.
00:19:49
Speaker
Wow, almost like a pendulum kind of thing. Again, assuming no friction. Yeah, it's like this isolated system that can go backwards or forwards. And what they talk about in the paper is that if it's operationally time symmetric, it is also ontologically time symmetric. It is a term they define, meaning that the process itself exists both forward and backwards in time. This reminds me of our talk on time crystals a while back.
00:20:12
Speaker
Yeah, I mean, I would like to know more research about time crystals, because they're pretty cool. Yeah, of course. That's where the lattice structure exists in four dimensions, one being time. Ooh, fun stuff, man. Light cones and time crystals. We sound like a hippie story of some kind. There actually was a paper in the process of doing this called, like, how hippies saved physics. Oh, yeah, I've heard about that. Yeah, it's kind of the whole thing.
00:20:39
Speaker
In this paper it talks about, if we have a quantum system, we start with a certain quantum system, we do a preparation to modify the variables of the quantum system, and then we collapse it by measuring the particles, and we get a certain outcome from our experiment. They say that the time symmetry exists when you can do the same experiment in reverse, starting from the results you get all the way to the input, and get the same answer that you started with.
00:21:06
Speaker
So there are certain processes in quantum physics like that. And you could almost like one that you could kind of think of is an entangled particle that is just flying through space. If you reverse time, it's just gonna, it's gonna exist the same way. Until it interacts with something, it kind of exists almost outside of time, except for, you know, yeah. Okay.
00:21:27
Speaker
And one thing to note about this paper is that it ends on kind of a cliffhanger, basically saying that if we do introduce retrocausality to the ontic framework of just quantum physics, you do end up with fine tuning. And that leads maybe to the idea that there's some physical theory that we need to, or ontological framework that we need to better come up with.
Closed Timelike Curves
00:21:50
Speaker
It's a very interesting paper. I would take a moment to read it if you're into quantum physics.
00:21:58
Speaker
So in all of this fun talk about light cones and about time crystals, we have something just as trippy to serve up right now and it's called closed timelike curves and something specifically called the Godel metric. And the Godel metric is so weird. It's basically like, let's imagine all of space is filled with this uniform dust and that's a term in general relativity. Basically, let's say we have all these dust particles, they have a little bit of weight each, there's a total density to them.
00:22:26
Speaker
And let's say each particle is spinning around, let's say the y-axis or like up, like just spinning around the way that. Now, wait, are these particles uniformly distributed? Yes. Okay. So the length between each particle is the same all throughout.
00:22:40
Speaker
Oh yeah, and really with dust solutions, you're assuming that these particles are infinitely small and smooth. And we're imagining that every particle is spinning. And the way that that warped spacetime is really interesting, and you get what are called closed time-like curves,
00:22:56
Speaker
meaning that there are events that affect themselves. They cause loops in space and time. So basically any solution within Godel has to account for these closed timelike curves. And one of the interesting things about this is that if you look in one of the directions, you'll see yourself in the past because those light cone loops are background to itself, but not through space, through time.
00:23:21
Speaker
Oh man, that is just absolutely trippy. That is just wild. I don't even know what to say about this. Aside from, wow, for all of you who are lost right there, raise your hand. My little hand goes up. Yeah, the Godel metric, I've thought a lot about this and it's still trippy. It's one of those things that it's better. It drives you nuts and it's fun for that reason. Wow. Yeah, it is wild.
Super Time Theory and Algorithmic Universe
00:23:48
Speaker
It's wild.
00:23:51
Speaker
Okay, so there's something else that we want to talk about. In Superglick too. Yes, Superglick. Something called...
00:23:59
Speaker
super time theory from breaking math, just a term that we came up with as we were talking about this. Now, I'm going to say this came from me trying to talk about the past, the present, and the future with respect to quantum entanglement. Now, here is what I'm talking about. There was a time in the past, let's pretend that in this scenario that I'm about to talk about, somebody has two particles that are entangled.
00:24:29
Speaker
And there is the part at the beginning of this thought experiment when they have them together. There is a part where they release one of the particles into outer space. Say they give them to a scientist who's on the International Space Station. Keeps them non-interactive with.
00:24:45
Speaker
correct. Then there is the next phase of this where the person who's not in the space station, the person on the ground, measures his particle or her particle. And because he makes a measurement, according to retro causality, that this measurement affects the past and changes both his or her particle and the particle in the space station, right?
00:25:10
Speaker
Yeah, basically they happen, one causes the other no matter how far away it is. And I remember talking to you about an idea that I had a long time ago, basically talking about particles like they were pool balls, saying that if a particle splits into two entangled particles and one hits a third pool ball, you can almost imagine that pool ball returning to the original split, just kind of going backwards in time.
00:25:37
Speaker
and then telling the original split, hey, I got here first, this is what the particle's gonna be. And then you can think of a signal being sent along the other one, like basically like the super time. And it's something that I think helps think about all this kind of stuff, because as long as you have another frame of time that only involves ironing things out, I can almost call this algorithmic time, because you could treat the universe as almost like this algorithm that takes these particles and does these very simple things to them.
00:26:05
Speaker
Yeah, we debated the term imaginary time. And again, what I'm trying to refer to when I say super time or this imaginary time or something like that, the issue is that once a measurement has happened, the past, well, that particle was always either spin up or spin down. It is no longer, it is and was no longer
00:26:28
Speaker
in superposition. It is and was now always what you measured it as at that time of measurement. So the super time is, well, when asterisk, I'll say it again, asterisk, when was it in superposition? It no longer ever was. So we use super time or this imaginary time to talk about actually it was at one point in superposition, but it changed now and in the past.
00:26:54
Speaker
So it's a term that we're using in order to describe when it was a superposition, even if that reality no longer exists. Yeah. And if you listen, imagine people are particles and let's say interacting with somebody is only like patting them on the shoulder. You can imagine walking around town and you're not really interacting with anything. You're just going along with some kind of like bizarre current and like you patting all these people on the shoulders between paths, you can almost exist.
00:27:20
Speaker
like think of that little bit of walk in time as something that you go forward and backwards in. And the same thing with particles. And I think it also is nice for explaining the double slit phenomenon because it allows the particle to go through both and explore and go back and forward and back and forward until it comes up with a probability distribution.
00:27:39
Speaker
Yes. So true to our name of breaking math, I think we've sort of broken logic and all reality here. And I'm anxiously waiting somebody to tell me that this has already been discovered. Yes. Yes, exactly. And if it is, tell us because we'd like to know. Yeah. And you know what I want to do? I want to insert a sound clip from the Big Lebowski of the guy saying, that's just like your opinion, man. Roll it. Yeah. Well, you know, that's just like, uh,
00:28:13
Speaker
math breaking of episode this one more and this of all entanglement to relate all this does how and curves time like closed or what retro causality is what's so difficulties ontological but mathematic and scientific only not brings possibility this that experience our encased the often so is the case is it case
00:28:34
Speaker
The nod is that when happens, what however causality of arrow the as same as is the time of arrow the roughly describe too hard is but of idea and has everyone that someone is time. I'm Sophia. And I'm Gabrielle. And this is math breaking. Man, after that, I wanted to add a sound of Barney Gumbo going, whoa! It's just like trippy as hell.