Proof of impossibility of superluminal signals?

[Mark_Laverty]

I really do believe we have the whole concept of superluminal anything and time travel completely wrong.

E.g a person hops in a wormhole and instantly travels across the galaxy nearly 100,000 light years away. The clocks at the place they left don't stop ticking, they carry on moving forwards in time. The person hops in a wormhole and travels instantly back to where they started a few seconds later by their clock.

They won't arrive back before the left, they arrive back a few seconds after they left measured by clocks at the place of departure/return.


And yes I understand concepts like light cones. The light from an event is not the event itself. Just because we perceive an event happening after it has happened does not mean it has not happened until we perceive it by observing its light.

Relativity predicts that superluminal travel is impossible and that it would lead to paradox's/cause before effect.

As such if superluminal travel is possible relativity has to be wrong. As such we can't use relativity to make reliable predictions about what would happen if superluminal travel were possible. We'd need a new theory...

That's what I think anyway...

 

[PeterDonis]

They won't arrive back before the left, they arrive back a few seconds after they left measured by clocks at the place of departure/return.

Here's the problem: you can't just *say* this. You have to *prove* it; that is, you have to show that it must be true using the assumptions and equations of relativity. Try doing that; you will see that it's not as simple as you think.

And yes I understand concepts like light cones. The light from an event is not the event itself. Just because we perceive an event happening after it has happened does not mean it has not happened until we perceive it by observing its light.

Nobody is claiming this, so you are attacking a straw man.

Relativity predicts that superluminal travel is impossible and that it would lead to paradox's/cause before effect.

That's not quite true. It's quite possible to construct scenarios in which objects travel at superluminal speeds, travel through wormholes, etc., that do not violate any requirements of relativity, do not violate causality, etc. You just have to be very careful to make the scenarios self-consistent, which does rule out a lot of the staple science fiction scenarios.

In fact, mathematically, general relativity even contains solutions, such as the Godel universe, which have closed timelike curves, i.e., curves along which objects traveling slower than light all the time end up in their own past. AFAIK nobody thinks these solutions are actually realized physically, but the fact that they exist means you have to be very careful saying what "relativity predicts".

if superluminal travel is possible relativity has to be wrong.

More precisely, if any of the staple science fiction scenarios are possible, the ones that violate requirements of relativity, then relativity has to be wrong.

As such we can't use relativity to make reliable predictions about what would happen if superluminal travel were possible. We'd need a new theory...

More precisely, if we ever got any evidence that any of the staple science fiction scenarios are possible, then we'd need a new theory. Feel free to wake me up when that happens...

 

[Fredrik]

Here's the problem: you can't just *say* this. You have to *prove* it; that is, you have to show that it must be true using the assumptions and equations of relativity. Try doing that; you will see that it's not as simple as you think.

I think he got that specific thing right. He's just talking about going through a wormhole to a distant location, and then immediately going back through the same wormhole.

 

[rjbeery]

E.g a person hops in a wormhole and instantly travels across the galaxy nearly 100,000 light years away. The clocks at the place they left don't stop ticking, they carry on moving forwards in time. The person hops in a wormhole and travels instantly back to where they started a few seconds later by their clock.

Define instantly. That's where you will find the problem.

 

[Mark_Laverty]

Hi,

The reference to the light cone thing was because last time I voiced these opinions they were put forward as an explanation of why they were wrong.

I wasn't aware of the rest of your points, I'll do some digging. Thank you :-)

As for the proof bit... Relativity is a theory, and theories themselves are not proof. You can not prove something with a theory, rather a theory requires proof.

What's more no theory can ever be 100% proven. Just tested to the point where you can be really really sure its right, but never 100% certain.

I'd even go so far as to say that the burdon of proof is the other way round. If relativity predicts cause before effect then that is an extrodinary claim that requires extrodinary evidence to support. Until I see proof of cause before effect due to superliminal travel personally I find it very hard to accept.

That last paragraph is probably a sign of my ignorance, but no matter how hard I try it j st doesn't seem right.

I'm not arguing that superluminal travel is possible, just that predictions of cause before effect rulling it out can't, I feel, be relied on.

 

[Mark_Laverty]

Define instantly. That's where you will find the problem.

Within a second according to their watch and the clocks of the place they left behind.

Just because the place they left behind doesn't see them appear 100,000 light years away until 100,000 years later (assuming they had a telescope to see the other side of the galaxy) doest mean they weren't there one second after they jumped into the wormhole.

 

[stevendaryl]

As for the proof bit... Relativity is a theory, and theories themselves are not proof. You can not prove something with a theory, rather a theory requires proof.

I think you misunderstood what people are saying. Special Relativity could certainly be wrong. The arguments about FTL signals are all conditional:

If SR is true, and FTL signals are possible, then ...

 

[PeterDonis]

I think he got that specific thing right. He's just talking about going through a wormhole to a distant location, and then immediately going back through the same wormhole.

If both ends of the wormhole are at rest relative to each other, yes, I think it would (or at least could) be that simple. But what if the ends are in relative motion?

 

[PeterDonis]

Relativity is a theory, and theories themselves are not proof. You can not prove something with a theory, rather a theory requires proof.

And relativity has the "proof" (you recognize in your next sentence that you can never really "prove" a theory, which is why I put the word in quotes), in the form of extensive experimental evidence confirming its predictions.

If relativity predicts cause before effect then that is an extrodinary claim that requires extrodinary evidence to support. Until I see proof of cause before effect due to superliminal travel personally I find it very hard to accept.

I'm confused about the position you are defending. Relativity predicts that *if* FTL travel were possible, *and* some other assumptions were true (the ones that ground the sci-fi scenarios I referred to), then causality would be violated. Since relativity says causality can't be violated, therefore, relativity predicts that, either FTL travel is impossible period, or, if it is possible, it is only possible *if* the other assumptions (the ones that ground the sci-fi scenarios) do not hold. In other words, if relativity is correct, than the kinds of scenarios that are possible that include FTL travel are very limited, because they have to be self-consistent: that is, there has to be one single spacetime model that includes any "causal loops" without breaking consistency, i.e., without assigning multiple physical outcomes to the same event. (Google "Novikov self-consistency principle" if you want to see more detail along these lines.)

Do you agree with the above paragraph, or are you arguing against it?

I'm not arguing that superluminal travel is possible, just that predictions of cause before effect rulling it out can't, I feel, be relied on.

Why not? Again, relativity has a huge amount of experimental support, so we are justified in having a high degree of confidence in its predictions. Also, once again, relativity does not "predict cause before effect"; it only says that *if* FTL travel were possible, *and* some other assumptions held, *then* you would have cause before effect. Relativity does *not* claim that that combination of conditions actually holds; so I don't see why you seem to be saying that relativity "predicts" that we should see evidence of cause before effect. It doesn't; that's the point.

 

[Chronos]

A wormhole that permits instantaneous travel to a distant point must be bounded by causality. The 'exit' obviously cannot be located wrt the past, which suggests a wormhole exit can only move relative to the present location of the 'entrance' - which would be very weird. On causal grounds I would argue this suggests wormholes are phantoms.

 

[rjbeery]

Within a second according to their watch and the clocks of the place they left behind.

Just because the place they left behind doesn't see them appear 100,000 light years away until 100,000 years later (assuming they had a telescope to see the other side of the galaxy) doest mean they weren't there one second after they jumped into the wormhole.

"Within a second" is still presuming that there is an actual moment 100,000 LY away which represents "instantly", when the reality is this moment changes based on the movement of the observers at each point.

That's the crux of the entire argument! If we had a way to transport "instantly" across the galaxy then I could travel 100,000 LY away and you, waiting for me there and under relative motion, could travel "instantly" back to my departure point before I left.

You are having a problem because you're thinking of time marching forward uniformly across the entire Universe as Newton did.

 

[phyti]

phyti, it's impossible to tell what your point is. I don't even understand what you're talking about. Increasing radar speed? We're talking about special relativity, not fantasy.

Currently, SR deals with events that communicate at light speed c.
If a method of faster than light communication was discovered that propagated in space at c', coordinate measurements would have to be done at c' to maintain compatibility.
Speed c' would assume the role of c as the limiting speed.
Pics 3 & 4 show the ftl signal becoming more horizontal in the B frame as the measurement signals increase to c'.
Using space-time pics shows the backward in time interpretation results from the measurement signal speed not equal to the info/message speed!

 

[Fredrik]

Currently, SR deals with events that communicate at light speed c.
If a method of faster than light communication was discovered that propagated in space at c', coordinate measurements would have to be done at c' to maintain compatibility.
Speed c' would assume the role of c as the limiting speed.

We're talking about the possibility that something that carries information can move faster than the invariant speed. If such a thing moves at c'>c, it wouldn't change the role of c, the only invariant speed in the theory.

 

[stevendaryl]

Currently, SR deals with events that communicate at light speed c.
If a method of faster than light communication was discovered that propagated in space at c', coordinate measurements would have to be done at c' to maintain compatibility.

Look, we've already verified that a clock moving at constant speed v relative to an inertial reference frame experiences time dilation according to

\dfrac{d\tau}{dt} = \sqrt{1-\frac{v^2}{c^2}}

If that formula is correct, then it can't also be correct if you substitute c' for c.

So no, you can't just replace c by some other speed c'. That would be nonsensical.

 

[Mark_Laverty]

@peterdonnis

I just wrote out a nice reply in which I agreed with you and said I think we were saying similar things but you explained it better than me!

E.g. I missed your emphasis on the word 'proof'.

I hit post and it didn't post and I'm struglinng for time to rewrite it (pun not intended!).

You are having a problem because you're thinking of time marching forward uniformly across the entire Universe as Newton did.

OK, this may be the problem, I'll think about this.