We all know that quantum mechanics is plenty confusing already, particles can weirdly maintain multiple possible states at the same time, there's instantaneous collapse of entangled particle states across various space time distances.

And on top of all of that, there are these articles you see in popular science venues claiming that quantum mechanics proves you can change the past.

Well, spoiler alert, these are wrong.

They're wrong when they were written, which means that they can't use quantum mechanics to go back in time to fix them later.

But there are some fascinating issues about how time works in quantum mechanics, and there are some amazing and crazy ideas for retro causality where the past can depend on the future.

These ideas might actually help unravel some confusion about instantaneous collapse and and also avoid creating signaling paradoxes.

So we can't send you back in time to kill your grandfather.

But today we are going to dig into all of that and hope that you understand time in quantum mechanics better after you hear this episode than you did before.

Welcome to Daniel and Kelly's extraordinarily timely universe.

Hello.

I'm Kelly Waitersmith.

I study parasites and space, and after looking at Daniel's outline for today, I wonder why physicists are always trying to confuse us.

Hi.

I'm Daniel.

I'm a particle physicist, and I'd like to go back in time and tell young Daniel that instead he should work on quantum foundations.

But I can't because time flows forwards.

All right, Well, so Daniel, here's my question for you.

So if you could go back in time and meddle with one historical moment, but not Hitler because that's too obvious, what would you meddle with.

I would love to somehow preserve more knowledge from ancient societies.

So I would love to, for example, keep the Spaniards from burning all of the Mayan books, or stop the library of Alexandria from burning down, or just like you know, go and scan a bunch of scrolls and a bunch of libraries in the ancient Greek world, because we have like a tiny fraction of ancient writing.

And not that I think that they like sold quantum gravity back then and we've lost it or anything, But it's just an amazing window into what people were thinking about, and the tiny fraction that we do have is fascinating and insightful.

So I'm always frustrated by lost knowledge.

That is a great answer.

About you, Kelly.

What would you go back and change?

Would you go back and become a particle physicist?

No, definitely, not that anything, but that No, Yeah, I don't know.

I kind of want to take your answer because I was thinking that maybe maybe I would go back and like thwart Lenin's revolution, because then you could avoid the Hulladamoor and the Stalins, Gulogs and you know, all of all of that other you know, Lisenko and how he you know, slowed Soviet science for so long, all of that stuff.

But I don't know your answer.

Although your answer was not just like one moment in history and everything that followed from it.

You were you were like hopping through multiple moments in history.

That's kind of cheating, But that would be a good use for a time machine, Yeah, I.

Think it would.

Or you could go back and like redesign the butterfly ballot in Florida in the two thousands and really changed the course of history.

Whoa, whoa, All right, but today you're gonna let us know what our options are here.

That's right.

Today we are digging into a fascinating question in quantum foundations, what quantum mechanics really says about the nature of reality and what it says about the nature of time.

We had an episode recently about quantum entanglement and instantaneous collapse across space, which never really sits well with people because of special relativity, and today we're not going to avoid that issue.

We're going to dig deep into what it means and whether all the popular science that says quantum mechanics has proved you could change the past is right or wrong.

Wait a minute, headlines and popside articles could be wrong.

I don't that doesn't seem like a good premise for a podcast.

Impossible.

I know it's crazy, but there is one famous experiment in quantum mechanics which is consistently always completely misrepresented, not misinterpreted, but the actual results of the experiment are misrepresented in dramatic fashion, which leads people astray.

So there's really interesting questions about time and order of events in quantum mechanics, but it's often cartoonized and misrepresented.

So we're going to clear all that up today and hopefully it penetrates back in time to clarify things for you before you even heard this episode.

Amazing, But first, I was wondering what people thought about the question of quantum mechanics in time.

So I went out there and I asked our amazing, good looking, intelligent, cat loving dog parenting listeners whether quantum mechanics can change the past.

Here's what folks had to say.

I love you, professor, but good heavens, not even a little googling.

I assume you mean more than make them more precise.

So perhaps it's a Heisenberg thing.

So weird.

I have absolutely no idea.

I know, I love this fantastic question.

It can be overwhelming food for thought.

I cannot wait for the podcast.

No, I don't think so.

I think quantum measurements can change the state of the quantum object when measured, but not the past.

Well it depends.

Oh sorry, this wasn't a biology question.

Right, Making quantum measurements is there's a possibility that it can affect the past.

I don't currently know how to tell the difference between spo action at a distance and an action that propagates into the past, but maybe when future me listens to the podcast, I can find a way to send the answer back to present me using quantum entanglement.

Changing the past sounds like science fiction to me, but on the smallest quantum scale things are weird.

Maybe it's possible.

I don't know.

No, that would be insane, but I know how this show goes, so maybe.

I think even if entanglement somehow violates causality and allows things to happen simultaneously, I don't know how that would allow it to be going backwards in time.

So no, I don't think it can change the past.

I'm gonna say no, but perhaps they can change our interpretation of the past.

Before meation goes from one particle to the past and the retransmitted to the other particles, so they are correlated in real time.

If the past changes with a quantum measurement, then all recordings and on memory will change as well.

So there's no way to know for sure.

The unpublished sci fi writer and meshauts yes enthusiastically, but the logical curious child to me says no, because that would then require any causality after the event, but before the observation the change as well well.

Our extraordinaries are clever in the past, present, and future.

And these were great answers and yes.

Not even a little bit of googling, and thank you for trying.

My goal here is really just to reveal what people already know, not what they could look up in a few minutes online, to get a sense for where people's brains are at, because that's where we want to meet you.

We want to clarify all this stuff, and so we want to know what you already know.

And you know, these are supposed to be our person on the street question, so it's like we just stopped you on the street, but actually we stopped you at your computer.

But Daniel, I think the place we should start probably is what causality means.

And we had a really great entire conversation with Sean Carroll about this.

But in case folks don't have a whole hour to catch up on that conversation, how about you give us the short version.

Yeah, causality really is at the heart of this and how we think about physics as a way to explain the universe, and we impose causality when we say that the past determines the future, which sounds like pretty obvious, right, because the choices you make now determine the future.

Causes happen before effects.

I shoot an arrow, and an arrow has to fly and hit the target after I shoot the arrow.

There's like a natural order to events, and that seems really crisp and clear immediately, But there's a whole philosophical swamp here of thinking about, like, well, which causes exactly influence those effects, Like if Kelly shouts at me just before I release the arrow and it changes the direction of the arrow, then Kelly is also a cause of the cause, And what she had for breakfast this morning is a cause for whether or not She's going to shout at me while I'm shooting the arrow, and you know how well she slept last night or whatever.

You could go back in time and basically then the whole universe is a cause.

So you could think about light cones, you can think about dominant causes.

It turns out to be a big philosophical mess.

And yes, please do dig into our episode with Sean Carroll about causality and locality.

So the philosophical swamp is identifying what counts as a cause.

Is that what you were saying, So like we're not talking about temporal ordering yet, we're just talking about how many things get to be included as a cause when you were trying to understand something.

Yeah, it gets a little bit messy because there are like things in the past which don't affect your decision, but they did happen in the past, and so like now you're deciding which things are important, which things are central to that decision having been made.

So it's not just as crisp and clean as saying like the whole past the terms the future.

It's like some elements of the past.

And then of course there's the fact that information doesn't propagate faster than light, So some things in the past can't affect your decisions, like things that are happening and Andromeda right now can't affect the decisions I'm making, And they can't affect the decisions I'm making tomorrow or the next day.

They can't affect decisions I'm making for millions of years, because that information will take millions of years to get here.

So that's sort of the beginning of the structure of the question of causality and locality.

Okay, And I always expect physics to get confusing, But we haven't gotten the confusing part yet.

Right now, ok that all makes sense?

Am I missing something?

And if you want to be like a philosopher and go back to the ancient Greeks, then you can ask, well, that all sounds natural, But why is it this way?

You know, why does it go in one direction?

It seems to flow from the past to the future.

Why is the past different from the future?

Essentially, why does time flow forwards?

Right?

You have this fundamental asymmetry in time, and you know, relativity tells us time is related to space.

But we know that in space you can go like backwards and forwards all you like, but in time it only flows forwards.

And that's fascinating and weird and not something that we understand.

I mean, there are ideas there about entropy that I think are over sold.

And it's not like nobody's worked on this question, but it's still a fundamentally important question about why the universe seems to flow forwards in time and whether it actually does.

Okay, so let's just take it as a given time flows forward, and we.

Think it's important for time to flow forward because we want the universe to make sense, right, We want the universe to be coherent.

At least you can.

Imagine another scenario where like you have time machines where you can send information back in time very quickly.

You get a universe that's incoherent, right, you can get things that are like paradoxes.

If you can send signals backwards in time, then I could like fire a death ray into my own past and kill myself yesterday, and then like, Okay, then I'm dead yesterday.

Who's firing the death ray today?

Nobody?

So I'm alive yesterday.

So now I'm firing the death rays, So now I'm dead yesterday.

So you see how very quickly sending any information backwards in time doesn't have to be murderous, can create paradoxes.

And so having a flow of causality where things only move forwards solves that problem.

Right, So any theory you have about how time flows and information can't have paradoxes because they contradict themselves, and the universe is one way, so it can't be a paradox.

But so what I've learned from science fiction is that you can go back in time.

You just have to make sure that you duck behind a wall when you from the past is in the area, then you're okay, yeah.

That's right.

And if you wear a dark cap and look the other way, then.

The universe is saved fromsolved.

And if Daniel goes back in time and prevents the Library of Alexandri from being burned down, then dot dot dot, he's not born.

He doesn't prevent the library from being burned down, so it does.

I don't know.

It's a big mess, all right, all right, I see your point.

And this way of thinking about physics that time flows from the past to the future.

You have this like chain of events, this causality and then locality that link the past to the future.

This is like treating the universe as if it was a computer.

You know, think about how we model things in simulations.

We talked about this in our weather episode.

For example, you have knowledge of what the universe is now, and then you have the laws of physics that determine what happens next.

So we think of the universe as sort of flowing forwards the way a computer might simulate a universe.

And this is you know, one of these sort of dorm room epiphanies that makes people think, oh, the universe is a computer because we model our physics after Isaac Newton, who sort of began this trend.

You said, like, let's think about the initial conditions and then the laws moving forward, which happened to be differential equations, and so at any moment, the current conditions and the laws predict the future.

Think about the universes like this is how Newton thought about it.

There's a universal clock, and there's a now, and then the now determines the next slice of the universe, which determines the next slice of the universe, which determines the next slice of the universe.

And if that sort of seems like obvious to you, you know, like, Okay, that's the way the universe does work.

That the universe calculates the future from the past using its laws.

Then you're already thinking about the universe as a computer.

But it's not necessarily the only way to think about the universe and to think about time.

Okay, But I feel like if you think about time as not just marching forward, but as being able to go forward and back, then everything is absolute chaos.

So what would the alternative be.

Yeah, So there's a really important subtlety here between depending on future events and being able to signal back in time.

Of course, being able to signal back in time or have information flow explicitly from the future to the past would create chaos.

Just like you said, that's not allowed.

It doesn't happen full stop.

But you can have the present depend on a non controllable detail of the future.

That sounds like it's the same thing, just sort of rewritten in physics loyally talk, but it's not.

It's crucially different, and we're going to dig into exactly what it means right now.

And so before we get into quant mechanics, I want to bring up a classical physics example of this, which is pretty widespread and I think not deeply enough understood.

And it's also going to set the stage for a future episode about the principles of least action.

So you know, Newton's way to solve problems is one way to solve problems, but sometimes it's a mess.

Sometimes it's complicated.

Like it works fine for throwing a ball through the air because it's pretty simple.

But now you add like a tornado, and you have like a squirrel dangling from a string, and the squirrel's wearing a jet pack and whatever make it complicated.

Any student physics will know that those problems are really hard to solve.

It's just a lot of terms in there.

And Newton's method isn't flawed, it's just not really very practical.

And then it comes along another way to solve these problems.

There are different ways to approach these problems that use different strategies and end up with the same answer, but they're much simpler, and these use minimal principles.

So let's start with simple example, Fermat's principle.

Vermot found a way to calculate how light is going to flow.

So if you want to know, for example, like where a beam of light is going to go you're calculating through optics or whatever.

Then one way to do it is to think about the light moving and it's going to hit the lens and how is that going to change its angle?

So to think about it from a computer point of view, flowing forwards in time.

But Vermont discovered that if you know the beginning and you know the end point, you can solve for the path of light by finding the shortest path from A to B.

So it's called a minimal principle, right, and that's cool and it's beautiful in some sense, but it's also a bit of a brain bender because you have to know B in order to predict the path.

You have to know where it ends to find the path from A to B.

So, like Newton's method says, start from A, move the light forwards, predict that it lands on B, Fermat's principle says, well, if you tell me A and B, I can tell you where the light had to have gone, cause I'm going to find the shortest path from A to B.

Okay, but that's not going backwards in time.

That's letting an event play out and then extrapolating in between, right sort of.

But it's saying that to know where the photon is between A and B, you have to know what B is.

Right, So you have to have this information about the future to calculate the now.

If you like, set the moment of nows the photon is halfway between A and B, how does it know where it's going if you need to know B to determine the path.

So this is an example of how those alternatives to Newton's computer can solve the problem of the photon's path.

But the path depends on something in the future, the photon's endpoint.

You have to know where it starts and where it lands later to know where it goes now in this alternative to Newton's computer.

Okay, so when I first read about this, this is weird because like, yes, you know where A and B are.

But if you want to calculate what the path of the photon is and use Fermat's principle, then like you have to know B already has A.

Plug it into your equation, and it's like a beautiful simple way to calculate the path.

But you can't use that to predict the path, right.

You can't use that to say, my photon started at A, tell me where it's going to go.

For Matt Fromat can't tell you until you know B, and you don't know B until it's already done.

So even if you just knew A and B, Like even if you just knew where the photon left and where it landed, Firmat could tell you what the path was without having seen it, right, just knowing where it started and where it ended.

But you can't use Fermat's principle to derive your universe as a computer, right, You can't use it to predict where it's going to land before it does.

Just after the fact, you could just look back at the whole process and derive what its path must have been.

Okay, okay, I see what you're saying now, But that feels like you've you understand a process that happened in the past.

But that still feels different than changing the past.

Yeah, it's not change the past exactly.

It's not retrocausal, and you can't use this to send information backwards in time.

Right.

The solution to the problem of where's the photon in the middle depends on where the photon lands, But as you say, you can't control it.

It's not like I'm changing where the photon landed to change where the photon went.

As we were saying before, the photon's path in Fermont's minimal approach depends on the detail of the future.

But since you can't control that detail in the future that it depends on, you can't use that to change the past.

There's no retro signaling or true retrocausality here.

And this is actually the fundamental misunderstanding at the heart of the movie Arrival, which comes from tit Channing's short story The Story of Your Life, which is a really fun story and really well written, and like, I'm a big fan of his writing.

But this idea that you could like look at the process as sort of a block universe, you know, and think about the future and the past and use that to solve these problems doesn't mean that the future controls the past, right, Because in these minimal principle approaches, like Format's approach, the solution depends on the future.

But that doesn't mean that you could control the past, right.

These things are not controllable, right.

You can't change be it's like a boundary condition in your mathematical approach to solving this problem.

But you can't use it to send messages back in time.

You can't use it to like change where the photon went right.

So, like, that's a fundamental misunderstanding about the heart of that science fiction, which frustrates me because I feel like, well, it's a beautiful story, but it also gives people the wrong impression that like part of physics is retrocausal.

So therefore there could be aliens out there that like you know, exists all through time and can change the past and all sorts of silly stuff.

And so the crucial thing to understand here is that the universe as a computer flowing forwards in time is not the only way to think about physics.

These other approaches to sort of block universe, minimal principle approaches also work, and they're fascinating, but they don't send messages back in time, they don't create paradoxes, and in many ways they're much more powerful.

So this same approach, generalized beyond light, can be used to solve all sorts of problems in physics.

And it's the principle of least action that like if you said some crazy system, agog it will go and it will follow the rules of physics by minimizing this quantity we call action.

So this principle of least action tells you what happened in a system over time, and it lets you solve problems that like Newton would really struggle with.

And so this is the foundation of Lagrange and mechanics and Hamiltonian mechanics, which is also the foundation of all of quantum field theory.

So this is actually the way we solve problems in the universe.

We don't use Newton as a computer we use these minimal principles because they're much more powerful.

So just to make sure I understand, So we were talking about what are the alternatives to the past determining the future, And what we've just decided is that if you know the past and the future, you can figure out what happened in between by knowing what happened at.

The extremes exactly.

And that's much more powerful way to solve physics problems.

It is the foundation of all of modern physics, but does not imply retrocausality.

So important thing to take away there is that you can have physics which depends on future conditions which are not controllable, and so you won't get paradoxes.

Got it all, right, So that's the crucial thing.

And so what does that mean about the universe?

Right?

Like, is the universe a computer or does the universe like use minimal principles to figure out after the fact what happened, right?

And so the answer is we don't really know.

And things get really sticky once you move to the quantum version of Newton's computer.

Okay, yeah, so we haven't broken my brain yet, which must mean we're not done.

So let's go ahead and take a break, and when we get back we'll work on break in the brain.

All right, we are back, and we're moving to the quantum version of our explanations, So put on your quantum hats and gear up.

Yeah.

So, now let's think about whether this idea of like Newton's computer really works to describe the universe once we start to have the rules of quantum mechanics.

Because we know that, you know Newton's classical theory of physics, where you have all the information about the universe and you can predict it perfectly into the future.

If you have the laws of physics, it doesn't really work because we don't have the information.

So what happens when we try to make a quantum version of Newton's computer?

And here I have to give a shout out to Ken Wharton, a listener who's also a professor in foundations at San Jose State, who send me an email after the Entanglements episode and a really fun paper to read which outlines a lot of these fascinating ideas.

Thanks very much, Ken for sharing your insights.

Ah.

Ken was my husband's Zach's physics teacher when he was at San Jose State.

Zach got like four ninths of a physics degree before his comic took off and he didn't have enough time to go to school anymore.

But Zach said Ken was a fantastic physics teacher.

Yeah, so we're found.

I had a great chat with him about retro causality, which inspired a lot of the ideas in this episode.

So thanks very much, Kenn.

Awesome side note, Hi, Ken, Zach really did say that you were a great physics teacher.

We talked about it over coffee a few days ago after you had the chat with Daniel over Blue Sky.

All right, so there's some issues here by thinking about the universe as a computer.

If it really is quantum mechanical number one, we don't really know like what was the initial state of the universe, Like, we don't know the current state of the universe.

We can't measure it precisely.

We can't tell the location and direction of every particle because of the uncertainty principle.

And no matter how far back in time that's true, and the initial state of the universe right can be undetermined in that same way.

So there's like a fundamental fuzziness which prevents the universe from operating as a computer in that way.

And the second is that its predictions are probabilistic.

Right, It does not tell you what's going to happen.

It tells you the probabilities of various things happening.

So in that sense, the past doesn't determine the future completely, and so in that way it gets kind of fuzzy if you want to hang onto Newton's idea that the past predicts the future, because we don't know the past perfectly and it doesn't actually predict the future in quantum mechanics.

Did I follow that the initial state's not really known?

Okay?

I did follow that.

Why should I care?

Yeah?

You might think, well, it's academic, doesn't really matter.

It predicts it in a different way.

Right, We're predicting probabilities instead of actual outcomes.

Isn't that fine?

Well, it matters in situations like the one we talked about a few episodes ago for Entangled part of and this is like what really troubled Einstein.

So the scenario there is you have like two particles that have been created by some process, and so they're entangled, like they can't just have any random state.

Maybe they have to conserve angularmentum, so one has to be spin up and one has to be spinned down.

Like if you have a spin zero state like a photon which turns into an electron and a positron, and those are spin one half, then one of them has to be like spin minus one half.

One of them has to be spin plus one half.

And so that means that either your electron is spin up and your positron to spin down, or you're electronic spin down and your positron to spin up.

They can't be both spin up, and it can't be both spin down.

So in that sense they're entangled.

The conditions you've used to create these two particles means that there's a connection between the states of the particles with me.

So far, yep, okay, And that doesn't seem too weird.

It's like if I have a bag and I know there's a red ball and a blue ball, and Kelly and I each draw a ball out of the bag, Then by looking at my ball, I can tell oh, I have a blue ball, Kelly must have a red one, or if I have a red ball, Kelly must have a blue one.

That's not so weird.

The weird thing is quantum mechanics says that it's not just that I don't know which spin my particle is, that it's not determined, right, that my particle is in this uncollapsed state of a mixture of spin up and spin down, and so is Kelly's.

But that weirdly, when I measure my electron and I say, oh, mine spin up, that that changes Kelly's electron to now be spin down because mine is spin up.

So when I make my measurement, it goes from undetermined to spin up.

Kelly's goes from undetermined to spin down.

And the weird thing is this is true even if our electrons are really far apart, like we can move them light years apart.

And the rule here tells us that this happens instantaneously across space and time.

Right as soon as I make my measurement, Kelly's changes from undetermined to spin down.

I remember this from our Entanglements episode.

Exactly, And you might ask, well, how do you know, how can you prove it?

How do you know that it wasn't really just spin up and spin down the whole time?

And then the Tanglement episode goes through Bell's experiment which proved this.

It proves that there is no local hidden information.

That there's not like some feature of mind which meant it had to be spin up the whole time, and some feature of Kelly's which meant it had to be spinned down the whole time.

That it really is undetermined that the universe chooses at the moment one of us makes a measurement.

So this is Bell's experiment, and go back and dig into that if you want more details about it.

And a couple crucial things to understand about Bell's experiment is number one, it only proves that there's no local hidden information.

So like you can have some global pattern.

There are theories like Boemian mechanics or whatever.

Let's say that it's controlled over space and time, there's a global information.

Those are more fringe theories.

People believe the universe is local, So there's that as a caveat to Bell's experiment.

Another important caveat to Bell's experiment, and this is what ken Wharton's paper is all about, is that Bell also assumes that there's no dependence on the future, right, that the universe operates like a quantum computer, that nothing that happens in the future can determine anything that's happening in the present or in the past.

And you might think, yeah, I'm cool with that, no big deal.

Of course, things can't go into the past because that would create paradoxes and we don't want that.

Mm hm.

So you know, we put the a ball goes in each of our bags, that's the past.

At some point one of us opens the bag that's the present, and we would be talking about at some point in the future, something happens that impacts both of those moments that I just talked about.

That's what we're talking about in the future, right.

Yeah, Or to spoil it a little bit, Belle is saying, it's not that when I make my measurement of my ball and I juster, mind is spin up that it goes back and changes the past.

To mine was always spin.

Up, goes back and changes the past, so yours was always spin up.

Okay, yeah, this is what physics does to me.

I'm like, let's go ahead and make sure I understand the definitions of past, present, and future.

Yeah.

And so you might be like, all right, that's cool.

Of course the future doesn't affect the past, and you want to hang on to that.

But even if you do.

There's already a problem with this experiment, which is this instantaneous component, Right like in the version of the story I just told you that seems weird, right I have instantaneous collapse over time.

I make my measurement and Kelly's changes instantaneously.

And that's a problem because in relativity, this concept of simultaneity, like things happening at the same time, is not universal.

It's frame dependent, right Like, if you have two events A and then B, I might think that happened at the same time.

But Kelly, moving at a train at half the speed of light going past me can see A happening before B, and Zach moving in a train the other direction can see B happening before A, and so relativity can scramble the orders of things.

And so already, if you're going to insist on instantaneous collapse across time, then you're already allowing for a form of retrocl causality.

Because if I make my measurement and then Kelly's is instantly collapsed, somebody moving a train past us could see Kelly's being collapsed before I make my measurement.

No more trains, everyone has to stand still.

This is too confusing.

Yeah, exactly.

And so the point here is that even if you don't want to allow retrocausality, this concept of instant collapse, when combined with special relativity, says that you're already in dangerous territory.

You're already potentially having information moving backwards in time.

Now we're saved here because all this information is hidden.

Like remember in the case of the classical example of the minimal principles, we're not saying that it's changing the past.

By solving this over time, knowing that the initial state and the final state and solving for the intermediate.

We're not saying that changes the past because you can't control the final states.

So there's no way to use that to change what was happening here.

There is no signaling back in time because the thing that might be retrocausal, the thing where the future might affect the past, is hidden.

It's the same reason why we can't have instantaneous communication.

Like when I make my measurement of my electron, it goes from undetermined to determined, and yours, far away in space light years away maybe goes from undetermined to determined.

But we can't use that to send information faster than light, even if that collapse is instantaneous.

And the reason for that is that you can't tell that I've made my measurement.

Right, I make my measurement, it goes from undetermined to determined.

You're looking at your ball.

You don't know if I've made my measurement, and there's no way for you to tell.

All you can do is measure your ball and get like, oh, I got spinned down.

I wonder if that's because Daniel collapsed it before I measured it, or if my measurement collapsed it.

You cannot tell.

So the fact that something has happened instantaneously across space and time is hidden from you.

That's the reason why we can't use it for communicating instantaneously across space and time.

That's the thing science fiction always gets wrong.

That that's crucial, right, That's what prevents the paradoxes and the signaling backwards in time.

That's why this isn't a fatal flaw.

Why special relativity saying from some perspectives, what's happening here is retrocausal, is things in the future affecting things in the past.

From some observer's point of view, Kelly's status collapsed before Daniel makes his measurement.

That's not a problem because no information is going backwards in time.

It can't be used to create any paradoxes because that information is hidden from Kelly.

Even if Kelly's present depends on Daniel's future, it's not in a way that she can tell, and so no information can go backwards in time.

So the punchline is you can actually have retro causality in quantum mechanics without ruining the universe.

Okay, all right, okay, so I think I get this, So okay, So does that mean that the only way you could get retro causality would be like, you know, for example, we're opening our bags and we're passing each other really quickly on like a train, and they're like momentary, this retrocausality thing happens, but we don't realize it, but it happened.

Yeah, and that's.

It exactly, Okay, And so you could take a whole different view of this if you allow retrocausality, because remember Bell's experiment, which in principle proves that the collapse is instantaneous and goes from undetermined to determined relies on an assumption.

The assumption is no dependence on the future.

So Ken Morton's amazing and hilarious and beautiful paper says, you know, he hasn't actually ruled out retrocausal theories.

Let's tell the story another way, because he just assumed no retrocausality, right, So what happens if you allow retrocausality.

Well, you know what I am dying to know, but I'm going to make everybody wait.

So let's find out what happens if you add retrocausality.

After the break, All right, we're back.

We're gonna talk about what happens when you add retro causality to Bell's experiment.

But first, let's remind ourselves real quick what did Bell do in his experiment?

So Bell has this experiment where we have these two entangled particles and they're separated across space, and you measure one of them and collapse it, and you ask whether the other one is collapsing right then, or whether it was already collapsed and you just didn't know it.

It's trying to distinguish between the cases of I don't know, but it's already determined, and I don't know because it's not yet determined.

And he did this amazing series of experiments where you have these particles and you make different measurements on them, and there's different random elements of those experiments to show the quantum correlations prove that the things are undetermined.

It's a really subtle experiment, and there's no like individual experiment that's like this.

One result is a smoking gun of quantum mechanics.

It's more like a pad learn over many iterations of the experiment proves about how the information is stored in the system.

And the takeaway from Bell's experiment is that there are no local hidden variables.

But again, Bell assumes no dependence on the future.

Right.

So the typical conclusion we draw from Bell's experiment is what we often say about quantum mechanics, that things have two possibilities and they can maintain those things.

And when I measure my particle, it's gone from having those two possibilities to deciding whether it's up or down, and that yours collapses instantaneously across space and time.

Right, that's a typical conclusion.

But there's another way to view these by allowing a little bit of retrocausality, because Bell didn't rule that out right.

Bell assumed no future dependence.

But what if we tell the different version of the story and it goes like this, I make my measurement, and my particle goes from undetermined too determined, and my particle goes from undetermined to up, and that propagates backwards in time, so that my particle was always spin up and article was always spin down.

Okay, And so that part is going backwards in time, right, and it yields the same results minus spin up, and yours is spin down.

You don't have this moment where like your particle collapses instantaneously across space and time.

Instead, we allow the information to propagate backwards.

And Bell's experiment again does not rule that out because he has just assumed that this can't happen, And so when you assume it, you close yourself off to that conclusion.

While it's understandably weird to think about particle states depending on the future, it actually solves some of the conceptual problems that we laid out before.

In the instantaneous collapse model, we had this bizarre frame dependence when the order of events depends on the observer and they're like velocity relative to the experiment.

Plus we had this instantaneous collapse across space time without any sort of physical mediator capable of doing that.

So the retrocosm model is frame invariant because it has this local, hidden invariable that is future dependent.

So my particle was always up and Kelly's particle was always down because they both depend on my future measurement.

So no matter what your velocity is relative the experiment, you see the same thing.

That's pretty cool, and there's no need for unexplained transit of information instantaneously, okay, And it also does not create paradoxes.

It's in principle information flowing backwards in time, but it's hidden information.

For the same reason that you can't use entangled particles at a distance to send information back and forth.

In the other view of this experiment, you can't use it here to send information backwards in time because the information that's going backwards is hidden.

Like you still can't tell whether I've made my measurement, whether it's collapsed now, whether it's collapsed backwards in time.

You just have a particle, you make a measurement, you get spin up, or you get spinned down, and I should say that propagated here is really just a figuous speech.

It's not physical.

There's no sense in which my particle was undetermined earlier and then information propagated back and later my particle was spin up.

My particle was always spin up.

It's just that the hidden information there depends on a future event.

You can't tell that that information propagated backwards in time, so there's no way to use it to create a paradox.

This might be a silly question, but so the particles that they were looking at, they knew they weren't I don't know, for example, traveling quickly on trains past each other.

Right, So were they able to control for the possibility of relativistic effects because of how they were measuring the particles and thus control for the possibility of retrocausality or am I missing something?

You can't control for that because you can always have an observer moving past the experiment, Like even if they keep the particles at rest relative to the Earth, and all their experiments are making these measurements on the Earth, so there's no relative velocity.

You could always put zac in a rocket ship flying past the experiment with no change to the experiment, and he would say, see it operating in a different order because of his velocity relative to the experiment.

But can I be like, during this experiment, sit your butt down and do not move because we're doing science.

Like, yeah, but it doesn't matter if nobody does it.

It just matters of whether it's possible.

And it's also always some particle out there moving past to whatever speed.

But I like your sort of school teacher approach, like, hey, everybody, don't break the universe now, please, I'm trusting you.

You can't trust you can't trust Zach to just sit still, you know.

Yeah, So let's again delineate the two scenarios here.

The canonical version without retrocausality says that I measure my particle and then instantaneously across space in time, yours goes from undetermined to determined.

And we already pointed out that sort of implies retrocausality because of the frame dependence of special relativity and the dependence on the order of events.

The retrocausal version says, when I make my measurement of my particle, that information propagates backwards in time and makes it so that our particles were always the way we ended up measuring them.

So it determines that hidden information.

So it is local hidden variables, right, And you might think, hold on a second, Bell ruled out local hidden variables.

Those are not allowed.

But Bell assumed no dependence on the future.

And because this is hidden information, it does not create paradoxes.

So I think it's super fascinating because it opens your mind up to like a really weird way for the universe to potentially operate that's actually still consistent with experiments, and it's just like a different interpretation of the same data that is wild.

Yeah.

So now I want to take this idea and pick apart a famous experiment that's often used to claim that quantum mechanics does send information into the past in a way they could be used to signal, which is very very wrong.

And it's a fascinating experiment.

It teaches us a lot about quantum mechanics, but it's often mis explained in this same context.

So I want to take it apart.

All right.

What's it called, because every great experiment has a name.

It is very cool.

It's called the quantum eraser, and it's one of these great experiments that tries to understand how information collapses and what an observer is.

And the idea is to instead of making your measurement directly, like Daniel is a big classical object heuse, You're going to use some detector which collapses the wave function.

What if we could try to extract this information without collapsing these uncertainties.

What if we could try to get the information and maintain the quantum uncertainties Because in quantum mechanics you have this distinction between classical objects which collapse things from superpositions into a single choice, and quantum interactions which don't collapse.

Things like if I have my electron and it's spin up, I can send another electron to interact with it, and that whole system can remain in a superposition of possible states.

It doesn't have to collapse.

Not every interaction leads to a lapse of superposition into a single choice.

Only when you interact with the classical object things which cannot be in a superposition like me or a detector or my eyeball big classical things.

So the idea is like, let's do the same experiment where you have two particles and they're entangled, so you know one of them went left and one of them went right, or one of them is to spin up, one of them is to spin down, And instead of interacting them with like some big detector which forces them to choose, instead entangle them with another particle, another quantum object, so you have no collapse, right, And so this is typically done in like the double slit experiment.

In the double slit experiment, you have like electron that's shot at two slits.

You don't know if it goes through the left slit or the right slit, and so that creates an interference pattern because the probability for it to go through the left interferes with the probability for it to go to the right, and you get this interference pattern.

But if you put a detector on it to measure which way went through, the interference pattern disappears.

That's the classic double slit experiment.

So in this case we replace the detector with a quantum detector, some electron or something which interacts with a particle in the experiment, but not in a way that collapses its wave function.

Right.

It stores the information about whether the particle went through the left slit or the right slit, but in a quantum object, not in a classical detector.

Is this a thought experiment or an actual thing?

This is an actual experiment you can do.

Oh okay, so now you in principle the information is encoded in this other electron right, and it hasn't collapsed yet, so you've preserved the uncertainty while encoding the information so you can find it later.

So the way this story is typically mistold is the following, and again this is incorrect.

It says that the particle continues through the double slits and you get the interference, and that you can go later to measure the information, the quantum information you've stored in this electron.

You can then go make a measurement and to figure out which slit it went through, after the particle has already hit the screen, right, so you're like, oh, it's too late for it to change.

Now I'm going to go figure out what which you want it went through.

And the incorrect version, which you see all over popular science, is that this goes back and changes what happens on the screen.

That like, you have an interference pattern on the screen, and then you go and you measure it, and the interference pattern disappears to be clear.

This would be retro signaling if it actually happened, but it doesn't.

So the incorrect version of the story is that you take an action in the future, well after the particles have already hit the screen and made an interference pattern, maybe a year later, you decide you want to know whether they went left or right, so then you access the stored information in that quantum detector.

And the incorrect story is that the collapse then happens backwards in time, going back to change the pattern on the screen from a year ago from interference to no interference, which is like, what, that's crazy and spooky.

That's because that's not what happens.

That's literally just not the outcome of that experiment.

It would be really fascinating if it were, but it's not.

What happens when you do this experiment is when you use the electron to figure out which slid it went through, the interference pattern disappears already, to be very clear.

Once you add the quantum detector, the interference pattern disappears, not when you open that detector a year later, but as soon as you add it to the experiment.

Whether or not you look at the information.

If you have the quantum detector, you never see the interference pattern, whether or not you access the stored information in the quantum detector, because you've measured that information, you've encoded it in an electron instead of some big classical detector.

But the interference pattern does disappear on the screen.

You do not see an interference pattern because that information has already been extracted from the experiment.

Which there was an interference pattern, and then you look at the electrons and it goes away.

There is an interference pattern on the screen.

Then you add the electron detector and the interference pattern goes away.

You don't have to look at the electron for the interference pattern to go away.

Oh, just having the electron detector makes it go away, even if you don't look at the response in the electron detector exactly.

So having a classical object add an electron detector is all it takes.

Even if the electron detector is a quantum detector.

That's right.

Even if it's a quantum detector, it still collapses the interference pattern.

What's cool is that you can then erase that information right by, like you know, scrambling it somehow, or you can go back and you can sort of recover the interference pattern after the fact.

You can use the information in those electrons to pull out an interference pattern after the fact, but it's not like an interference pattern appears on the screen.

You can use the information in your quantum detector to separate the particles on the screen into two groups, and if you pull those apart, each of those will have an interference pattern in them.

So there's no like retrocausality here.

It's a fascinating experiment because it shows like how quantum information propagates through the experiment, but in the way it's typically told that, like, by using a quantum detector instead of a classical detector, you are sending information back in time because if you look at the outcome of your quantum detector later, the interference pattern disappears.

That's not true.

The interference pattern disappears as soon as you inject the detector into the system.

Quantum or classical.

Come on, man, this stuff is crazy.

Yeah, yeah, exactly, And so this stuff is weird.

But there's a retrocausal interpretation of this.

The idea is that you make a choice whether to look at this quantum detector or not whether to collapse the quantum detector's information, and that sends information back in time to determine what happened in this experiment.

But again it's hidden information.

It's not information you can use to send messages or to do anything else.

And you know, there's another famous exam we don't have time to go through, which is called the delayed choice version of the double slit experiment, which is when you move the detector really close to the screen, and so basically you're deciding whether to detect the particle just before it hit the screen, well after it had to decide which slit to go through, or whether it're making an interference pattern or not.

It's a really cool experiment.

But in both cases you can interpret these experiments in a retrocausal way by saying that information propagates backwards in time, but again only to control a local hidden variable.

So basically, retro causality allows you to reintroduce local hidden variables in our experiments by allowing information to propagate backwards in time, which is kind of uncomfortable and kind of weird because it violates our sense that like the universe should flow from the past into the future, you know, and that we like to think about the universe as a computer, that it's like calculating things on the fly.

But we don't know how the universe works, and it's certainly not restricted to doing something that makes no.

It's not.

No, it's not.

And you know, another question you might ask is like, well, could we tell the difference between the one scenario where like things really are undetermined until you measure them and then it collapse instantaneously across space and the retrocausal version, where like things propagate back in time.

And I asked this question to Ken Wharton, the Zach's physics professor who's a philosopher of quantum foundations, and he said, maybe there will never be an experiment.

And you know, one issue is that we don't have like a full theory of retro causality.

Ken's paper, for example, just points out that this is allowed in theories.

It doesn't have a full theory that incorporates all of this and predicts everything.

And the problem is that all the experiments that we could set up to test this idea would also allow for signaling backwards in time, which would create paradoxes.

So those things definitely don't.

Work, but I thought they wouldn't allow for signaling back in time.

Yeah, exactly.

So the experiments we set up trying to test this would also test or retro signaling, which we already know can't work.

So we don't know how to test for this.

Just retrocausality without retro signaling, got it.

Okay, So yeah, we don't know how to test for retrocausality.

But the good news is that if we take a retrocausal formulation of quantum mechanics, it might make it easier to solve the big open problem of quantum gravity.

One of the big sticking points there is that general relativity demands to know where things are at all times, where are the masses in space so we can decide how much it's curving, And traditional quant mechanics says you can't know that some things are undetermined, But retrocausal quantum mechanics says that there are local, hidden variables that do determine where the particles are.

So if those can be made like covariant, then you can marry that with ordinary general relativity space time and maybe make some progress on quantum gravity.

And so you know.

The headline is that a lot of the popular science quantum mechanical articles about how quantum mechanics changes the past are wrong because they imply that you could send information back in the past, which could create paradoxes.

The more interesting but nuanced bit is that some interpretations of quantum mechanics are consistent with a form of retrocausality which changes local hidden variables in the past in a way that does not allow for information to be propagated into the past to create paradoxes, but is a fascinating insight into how the universe works, whether it really is a computer, whether it follows these minimal principles, you know, whether you should think of the universe as like a block.

Maybe the universe is not figuring it out as it goes.

It's just like one big physics problem, and some physics major at the end of the universe is like going to solve for the whole thing given the initial and the final state.

Oh god, I hope it doesn't mix up the pluses and the minuses.

Physics major is not famously good.

Yeah exactly.

Yeah, all right, So thank you for taking this journey with us.

Forwards and backwards in understanding.

It turns out the universe is far weirder than we imagine, and maybe far weirder and than we could ever understand.

Well, that was trippy and fun.

Thanks Daniel.

Until next time, extraordinaries, have a good one.

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