I want to talk today about an issue that bothers  me a lot, it’s that most physicists believe the  

speed of light is an ultimate, absolute,  and impossible to overcome limit. Indeed,  

I think it is THE biggest mistake that  physicists are making, that our entire  

species is making. Yes, trust me to reveal  this scandal exclusively on YouTube today.

More seriously. That physicists believe the  speed of light is a hard limit is the reason  

we invest so little in space travel: It means  that it doesn’t make any sense to travel to  

even the next closest solar system, because  as long as you’re bound by the speed of light  

that’d optimistically take several generations  no matter how far we push the current technology. 

It’s also why we are unable to make contact  to extraterrestrial civilizations, which I am  

convinced are out there. Because if there is any  way to message faster than the speed of light,  

obviously, that’s what everyone else is  using. So as long as you think they’ll  

certainly send messages with electromagnetic  signals, bound by the speed of light,  

you’ll never hear anything of them. That  is what I think explains the Fermi Paradox.  

Everyone else is messaging faster than light.  And we haven’t yet figured out how to do it.

And yes, I think it is possible to send signals  faster than light. In some sense it’s why I went  

into physics. You see, I thought the best way to  make rapid progress on this planet was to make  

contact with extraterrestrials who have already  solved the problems that we’re still working on.  

So the first thing we need to do is to develop a  way to receive those faster than light signals! 

I have not been successful in reaching my goal.  But 30 years later, at least I can tell you why  

I think physicists are wrong in thinking  that the speed of light is a limit and how  

one could beat it. It’s ultimately because  they’re misunderstanding quantum physics.

In Einstein’s theories the speed  of light is a sort of barrier.  

This theory says that overcoming it would  take an infinite amount of energy. Then again,  

I don’t know any physicist who thinks that  Einstein’s theories are ultimately correct  

because they don’t include quantum effects, we are  missing a theory of quantum gravity. So why should  

the limitation of the non-quantum theory continue  to hold when we know it ultimately isn’t correct?

Physicists have two reasons for believing  that it does hold. One is that they think  

faster than light signalling or travel  would lead to causality problems,  

those are inconsistencies where you can go back in  time and change something that already happened,  

so that you couldn’t have gone back in time to  begin with. The other reason is that we also have  

the speed of light barrier in quantum physics. And  then, claiming that quantum gravity might remove  

it makes no sense. I’ll start with the first, the  causality problem, then we get to quantum physics.

The argument that faster than light communication  or travel leads to causality problems is just  

wrong. Causality problems are only possible  if you disregard that time has a direction.  

We strictly speaking don’t know why time  has a direction, alright. But we know that  

it does. The time you’re spending on watching  this video will never come back. That’s what  

it means for time to have a direction. And  this makes causality violations impossible.

It’s quite easy to see this with one of  those spacetime diagrams that I keep drawing,  

where space is on the horizontal axis and  time is on the vertical. By convention,  

the speed of light is on the  diagonal, at a 45 degree angle. 

This region up here, contains all events  that can be reached by a signal from here.  

It’s called the forward lightcone. And  this region down here, that contains all  

events from which you can send signals to  here. It’s called the backward lightcone.

In this diagram, the horizontal lines  mark what happens simultaneously. up  

is forward in time and down is backward  in time. But forward in time for whom?

If an observer comes by moving  at some relative velocity,  

that they have a different notion  of simultaneity. Their surfaces of  

simultaneity are tilted to that of the other  observer. It’s one of the central features of  

Einstein’s theory. So it seems like for this  observer forward in time goes somewhere else. 

And that is true… but it’s not a problem  for causality so long as signals travel at  

most at the speed of light. Because then  really you only have to distinguish the  

forward lightcone from the backward lightcone.  Because there are no signals outside of those.

And those lightcones, here is the important  bit, are the same for all observers. Because  

the speed of light is always the same. The forward  lightcone always stays the forward lightcone. And  

the backward lightcone always stays the backward  lightcone. This is why, if signals can’t travel  

faster than light, causal order remains the same  for all observers, no matter how fast they move. 

The issue that physicists have is then that  whenever something can go faster than light,  

it will move outside the lightcone. And that  will be forward in time for SOME observers.  

But there will always be some for which it’s  backward in time. You can then imagine a way  

to close this loop with multiple signals,  which would make it possible that you,  

you know, tell yourself to sell Nvidia before  the AI bubble bursts or something like this.

But it’s really a non-issue. Because  once you acknowledge that we do have an  

arrow of time in the universe, for better or  worse, whether we know where it comes from or not,  

that introduces a time order which tells you  what forward in time means. In physics terms  

it’s what’s called a preferred slicing. Because  that’s what it does in the spacetime diagram. It  

slices it up, and it designates the direction  as THE forward. And with that, the entire  

causality problem disappears. Because even  if you could send signals faster than light,  

you could only send those forward in time,  according to that one preferred slicing,  

and never backward. So you cannot create loops  in time, and there are no causality problems.

But doesn’t such a preferred slicing  conflict with Einstein’s theory? No,  

it does not. We do as a matter of fact have  preferred slicings in all our models for the  

universe. They are usually related  to the cosmic microwave background.

Ok, so we see that really there is  no problem with faster than light  

signalling in Einstein’s theories,  now what about quantum physics.

Quantum physics is often said to be non-local, and  yet that non-locality somehow respects the speed  

of light limit. If that doesn’t make sense to  you, it’s because it actually doesn’t make sense.

I think the reason people get confused about  what nonlocality means in quantum physics is  

that the popular science media often say  something like entangled particles can  

share a non-local “link”. This suggests that  if you entangle, say, a pair of particles,  

like these rice crackers. And you move  them apart. And then you change, say,  

the spin of one. Then the other one will also  change, instantaneously, no matter how far away.

That is just wrong though. If the particles are  entangled and you do something to this particle,  

what happens to the other one is exactly nothing.

I have to take a break here because of a super  frustrating experience I had last year. I wrote  

a short contribution for an article about 100  years of quantum physics which appeared in  

nature news or something that was supposed  to clear up common misunderstandings about  

quantum physics. Naturally I tried to explain  that entanglement is NOT a spooky action at a  

distance. When you do something to  one particle of an entangled pair,  

what happens to the other is NOTHING. But every  time I got back an edited version of the text,  

the editors had put the same mistakes into  the text that I was trying to sort out.

Ok, this might give you the impression that  Sabine is promoting some fringe ideas about  

quantum physics. I promise I will get to  the fringe ideas, but so far, I think most  

physicists would agree with what I’ve said. Though I once talked to a guy who worked for  

a quantum computing startup who literally  seemed to believe that entanglement is a  

nonlocal interaction. Yeah, that’s part of the  reason why I’m skeptical this quantum computing  

business will go anywhere because it seems that  a lot of people working on this don’t understand  

quantum physics in the first place. But I digress,  let’s come back to the faster than light question.

The reason they say that quantum physics is  “non-local” has nothing to do with an interaction,  

it’s merely about knowledge retrieval.  Because suppose you selected a pair of  

entangled particles, so that they’re both the  same, but you don’t know which side is up. Then  

you move them apart. Now the moment you measure  whether this particle has chocolate up or not,  

you know instantaneously that the other has the  same. This is the supposed non-locality. You make  

a measurement here and you infer information  about the properties of something elsewhere.

This is nonlocal in some sense, but there is  no information actually traveling anywhere,  

it’s just that you learned something  about what is going on elsewhere. And  

this is the only way in which  quantum physics is non-local.

This also explains why you cannot send information  faster than light with these entangled particles.  

Because the measurement you make just reveals  some property of the particle on your side.  

Quantum mechanics tells you what the probability  is to get this or that measurement result,  

chocolate up or down. But your measurement  doesn’t affect the probability either on  

your side or on the other side. It’s just  what it is. So there is no way that your  

measurement on the one side can send a  message to the other side. Because the  

measurement doesn’t affect the probability  of either of the measurement outcomes.

The rest of this non-locality talk comes from  physicists’ conviction that the properties of  

these particles were not determined before  you measured them. Before you measured them,  

they were in a “superposition”, both chocolate up  and not. And only when you measure ONE particle  

will they BOTH take on a definite state. This  is the infamous “collapse” of the wave-function. 

And at least mathematically that happens faster  than the speed of light. This is why Einstein  

objected to quantum physics. But does this  collapse actually happen? There is not a shred  

of evidence that it does. So why do physicists  believe it’s a physical process? Because. And here  

is the punchline of the video, because they think  that faster than light signalling is not possible.

At this point I am afraid I have lost even  those with a PhD in physics, but you see I  

am making this video to save mankind from its  biggest mistake, so I think it’s worth a try.

It’s like this. Suppose that you think the  collapse of the wavefunction is NOT a physical  

process. That actually the particles did have some  specific state before you measured them, you just  

don’t know which. This is what’s called a “hidden  variables theory”. In the simplest case the hidden  

variable just directly encodes which side is  up. And the only way to make a hidden variables  

theory compatible with Einstein’s theory is by  using what has been called “superdeterminism”.

Superdeterminism is the only local explanation  for all the observations of quantum physics.  

This is why I think it’s trivially correct,  and I am waiting for the rest of physicists  

to wake up to that. Which means that  now we are properly crossing over into  

the range where most physicists would try to  disagree with me, meaning they would be wrong. 

Physicists have mostly dismissed  superdeterminism because of social  

reinforcement. The majority doesn’t even know  how it works, they never thought about it,  

they just heard someone else say it’s  wrong and they decided to believe this.  

This sort of thing really shouldn’t happen  among intelligent people. But it does.

In any case, superdeterminism can explain  quantum physics and then it does of course  

respect the speed of light limit. But that it  can reproduce standard quantum physics is not  

the relevant point of superdeterminism. The  relevant point is that if quantum physics can  

be explained by a hidden variables theory,  then that strongly suggests that quantum  

physics is not fundamental. It suggests that  there is an underlying, deeper layer that we  

have not yet discovered. This is why Einstein,  Podolsky and Rosen said quantum mechanics is  

“incomplete”. The collapse must be replaced by  an actual physical process. They were right.

The thing is now, if you believe this, then  quantum physics is really just an average  

description of some underlying deterministic  theory. It’s a statistical theory. This also  

explains why the Dirac Neumann equation, which  is equivalent to the Schrödinger equation,  

looks so very similar to the Liouville equation,  that is an equation in statistical mechanics.  

Why is that? It’s because quantum  mechanics IS a statistical theory.

This is important because whenever  you have some average theory,  

you can get deviations from that average. And  we know that deviations from that average,  

which is standard quantum mechanics, would  allow us to send signals faster than light. 

The reason is that with these deviations from  quantum mechanics, in a superdeterministic theory,  

the outcome of the measurement of the chocolate  cracker, depends on what you measure. And then,  

the measurement on one side can influence  the measurement outcome on the other side.  

It does so faster than light. That is still local  because both of these properties were determined  

already when the particles became entangled,  this is why it’s called superdeterminism.

So this is the connection. If quantum mechanics  is not actually the fundamental theory of nature,  

if hidden variables are real, then we can  almost certainly signal faster than light.

You’d think that this should be a very strong  motivation for physicists to find that deeper  

layer of reality underlying quantum physics.  But no, what has happened instead is the exact  

opposite. They take the speed of light limit  to be sacred. They assume that it must be a  

limit. And therefore they conclude there can  be no deeper layer underlying quantum physics.  

And because they take quantum physics to  be fundamental, they conclude the speed  

of light limit must persist even in quantum  gravity, which is why it’s a sacred principle.

So you see it’s not wrong, but it’s a circular  argument. The speed of light limit must hold  

therefore quantum physics is fundamental,  therefore the speed of light limit must hold.

You might find it hard to believe that a large  group of people would make such an obvious  

mistake. And at some point, I too would have found  that hard to believe. But having reached a certain  

age I have seen so many instances of group think  in academia that this no longer surprises me.

But the good news is that in the end it won’t  matter. Because what I think will happen in  

the foundations of physics is this. Physicists  will continue building quantum computers and  

push forward with other quantum technologies.  They will run into difficulties explaining  

some of their observations. The way things  are going they’ll throw the data at an AI.  

The AI will conclude that it’s actually quite  easy to explain the curious new observations  

with a hidden variables theory. The hidden  variables theory will tell them how to get  

out-of-distribution results for quantum particles,  and with those you can signal faster than light.

How long will that take? It seems likely  to me that we will see evidence for the  

first weird observations in the coming decade. Speaking of weird observations, have you noticed  

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Thanks for watching, see you around.