FTL paradox and time travel paradox

[RX-79G]

One of the biggest problems with faster than light travel is that is creates situations where information could travel essentially backwards in time. If you aren't familiar with this problem, there is a lot of information out there that demonstrates the problem using reference frames and light cones. For many, this is an absolute hurdle that demonstrates that FTL is not possible. Fair enough.

For time travel stories, there are a variety of ways of dealing with paradox, such as multiverse divisions or the inability to cause paradoxical situations. It seems like FTL and time travel are really the same problem, so could the remedies authors apply to time travel be smoothly applied to FTL stories, and if so, what does that look like?

 

[Mirannan]

They probably could, but there are also possibilities that apply to FTL in addition - particularly to wormholes.

Two solutions applied in fiction have been that if one tries to traverse a wormhole in such a way as to muck up the timeline then, in some way, you get destroyed; blown out of spacetime altogether, reduced to a quark/lepton soup, whatever. The other is that an attempt to move a wormhole to a place that allows such violations destabilises the wormhole itself - with extremely energetic and violent results (such as the entire mass of the wormhole at both ends being converted into high-energy gamma rays - not trivial, when talking about a Jupiter mass or more.)

 

[RX-79G]

Are those ways of preventing paradox that the traveler can avoid by adhering to certain rules? Or are those reasons that it can't happen at all?

 

[Venusian Broon]

Well there are solutions of Einstein's general relativity that exhibit 'proper' time travel i.e. not just going forward at 1 second per second! But I believe most of them use vast amounts of energy/mass and therefore may just be forever impracticable. Interesting but just 'angels dancing on the heads of pins' sort of discussion.

Much like 'wormholes' I believe they work by topographically stretching Spacetime into the required shape to achieve time travelling.

If one could feasibly put together an experiment that can time travel/FTL is there a more basic underlying logic that will prevent it anyway? Hence all of those above solutions would be imaginary anyway. For example, I think there may be serious problems with the conservation of energy in these cases, hence if time travel or FTL did actually occur, it might show up as a breakdown in that - and as far as we know that observation has been pretty solid and crucial. Perhaps this is truly fundamental and is the brake on these schemes?

On the other hand this raises the question, I suppose, that if it was viable how would the likely paradoxes that must appear and be actually viewed by us. You'd probably need to be looking for 'areas of weirdness' - abrupt changes in properties/matter/spacetime say? So far I don't think we have discovered anything that weird that needed the explanation that it was the result of time travel or FTL.

Of course General Relativity can not be a complete description of the universe, as we know that QM also is (more - QM fanboy) successful in describing what is happening, so if a new theory of everything can be put together perhaps that could have 'loop holes' for all this stuff...or it may have the logic/reasons in it for why they will never happen.

 

[goldhawk]

One of the biggest problems with faster than light travel is that is creates situations where information could travel essentially backwards in time. If you aren't familiar with this problem, there is a lot of information out there that demonstrates the problem using reference frames and light cones. For many, this is an absolute hurdle that demonstrates that FTL is not possible. Fair enough.

No that's not the reason. The same reasoning can be used to state that a cannonball cannot travel faster than sound because you could hear the ball hitting the target before you heard the gun fired: effect before cause.

The real reason is that the speed of light is constant. No matter how fast you go, you never get closer to the speed of light. You cannot break the light barrier (if there is one) because you're always the same distance (so to speak) away from it.

All this stuff with reference frames and light cones is to describe the consequences of the speed of light being constant. They are the effect, not the cause.

 

[Brian G Turner]

One of the biggest problems with faster than light travel is that is creates situations where information could travel essentially backwards in time. If you aren't familiar with this problem, there is a lot of information out there that demonstrates the problem using reference frames and light cones. For many, this is an absolute hurdle that demonstrates that FTL is not possible. Fair enough.

For time travel stories, there are a variety of ways of dealing with paradox, such as multiverse divisions or the inability to cause paradoxical situations. It seems like FTL and time travel are really the same problem, so could the remedies authors apply to time travel be smoothly applied to FTL stories, and if so, what does that look like?

So far as I understand it, if the speed of light isn't a constant, then barriers to FTL disappear.

Which is why recent news items in the science press about the speed of light being variable, rather than a constant, are especially interesting:
Non-constant Speed of Light

 

[Venusian Broon]

No that's not the reason. The same reasoning can be used to state that a cannonball cannot travel faster than sound because you could hear the ball hitting the target before you heard the gun fired: effect before cause.

Which would be true if 'cause then effect' was a priori.

One could however take a strict empirical view of the universe, a' la David Hume. Then the fact that effect follows cause becomes a posteriori. Then one need not always expect this to happen.

 

[Serendipity]

I keep saying to myself I must stay quiet about this subject... but the self-hypnosis is not working... I keep on coming back to quantum entanglement... my personal opinion as it currently stands is that we need a new type of mathematics to describe what is going on here... um... I'm going to leave it that... back to the self-hypnosis....

 

[goldhawk]

So far as I understand it, if the speed of light isn't a constant, then barriers to FTL disappear.

Which is why recent news items in the science press about the speed of light being variable, rather than a constant, are especially interesting:
Non-constant Speed of Light

But they still have its speed constant throughout the universe, just not throughout time. Which means nothing about FTL changes.

Which would be true if 'cause then effect' was a priori.

One could however take a strict empirical view of the universe, a' la David Hume. Then the fact that effect follows cause becomes a posteriori. Then one need not always expect this to happen.

Playing with words does not change the mathematics.

I keep saying to myself I must stay quiet about this subject... but the self-hypnosis is not working... I keep on coming back to quantum entanglement... my personal opinion as it currently stands is that we need a new type of mathematics to describe what is going on here... um... I'm going to leave it that... back to the self-hypnosis....

No, an old model explains entanglement and superposition (and maintains everything in the Standard Model): quanta as wave functions.

 

[Venusian Broon]

Playing with words does not change the mathematics.

If empirically the assumption of causality was seen to be broken, then the mathematics will have to be modified/changed.

In fact, as I am sure you know, such a case lies at the heart Einstein's disagreement with Quantum Mechanics, he arguing that it violated the local realist view of causality - hence the EPR entanglement paradox. Unfortunately for Einstein, it seems this violation does indeed hold (in some manner) and is the reason why we now have two competing, but clearly incomplete, systems - both with very different philosophies - that are required to describe the fundamental universe.

Perhaps this is what @Serendipity was meaning - that a new maths/philosophy is required to bridge between the QM in the small scale and GR on the large scale?

Anyway, having played a bit of devils advocate, it's one thing to suggest weirdness in the microscopic scale, but in the macro scale where GR seems to be doing an excellent job at prediction, I struggle to imagine how in the large scale, if possible, such a breakdown in causality would manifest.

 

[Venusian Broon]

Getting back to the OP - I came across quite a nice read:

Novikov self-consistency principle - Wikipedia

This explanation does have quite far-reaching implications on free will. So if closed time curves existed then free will would necessarily be constrained. And this would be easiest to explain if there was no free will at all.

 

[RX-79G]

No that's not the reason. The same reasoning can be used to state that a cannonball cannot travel faster than sound because you could hear the ball hitting the target before you heard the gun fired: effect before cause.

The real reason is that the speed of light is constant. No matter how fast you go, you never get closer to the speed of light. You cannot break the light barrier (if there is one) because you're always the same distance (so to speak) away from it.

All this stuff with reference frames and light cones is to describe the consequences of the speed of light being constant. They are the effect, not the cause.

That completely misses the causality problem. You really can't end up with a paradox unless you have travelers in multiple reference frames. A single ship or cannonball isn't going to be an issue. It is nothing like your cannonball example.

 

[RX-79G]

Getting back to the OP - I came across quite a nice read:

Novikov self-consistency principle - Wikipedia

This explanation does have quite far-reaching implications on free will. So if closed time curves existed then free will would necessarily be constrained. And this would be easiest to explain if there was no free will at all.

This is one of the versions of the "inability to cause a paradoxical situations" that I mentioned in the OP. It is largely a thought experiment, but certainly one a writer might employ. I think some writers already have, depicting space battles where missiles sometimes cannot strike to maintain causality.

Another avenue may be contained in this fellow's theories:
http://phys.org/news/2016-11-theory-gravity-dark.html
When I read more of his theory elsewhere, I recall the physicist claiming that our 2-way view of time was to blame for the dark matter and other problems with our understanding of the universe. I got the impression that with a 1-way view of time it wouldn't be an issue, but I might have misread.

 

[goldhawk]

In fact, as I am sure you know, such a case lies at the heart Einstein's disagreement with Quantum Mechanics, he arguing that it violated the local realist view of causality - hence the EPR entanglement paradox. Unfortunately for Einstein, it seems this violation does indeed hold (in some manner) and is the reason why we now have two competing, but clearly incomplete, systems - both with very different philosophies - that are required to describe the fundamental universe.

Perhaps this is what @Serendipity was meaning - that a new maths/philosophy is required to bridge between the QM in the small scale and GR on the large scale?

Anyway, having played a bit of devils advocate, it's one thing to suggest weirdness in the microscopic scale, but in the macro scale where GR seems to be doing an excellent job at prediction, I struggle to imagine how in the large scale, if possible, such a breakdown in causality would manifest.

Quanta as particles creates the EPR entanglement paradox. Quanta as wave functions solve it. Entanglement is a single wave function that can be measured in two different places.

The thing about entanglement that everyone seems to overlook is that entanglement does not have just two events, the measurements of the quanta but three: the measurements and the entangling event. All measurements of the entanglement must be in the light cone of the entangling event. Changing the property of one quantum causes a simultaneous change in the other but it is only simultaneous in the inertial frame of the entangling event.

Once you realize that the entangling event places restrictions on the entanglement, the spooky action is not so spooky.

 

[goldhawk]

That completely misses the causality problem. You really can't end up with a paradox unless you have travelers in multiple reference frames. A single ship or cannonball isn't going to be an issue. It is nothing like your cannonball example.

There is no causality problem. Cause and effect is an emergent feature of our universe, not a fundamental one. For that matter, time and space are also emergent features of our universe.

 

[RX-79G]

There is no causality problem. Cause and effect is an emergent feature of our universe, not a fundamental one. For that matter, time and space are also emergent features of our universe.

You seem to be misunderstanding the purpose of this thread.

We are not attempting to explain away the impossible - accelerating through the speed of light. We are supposing that there may be some way of stepping over it, and how a writer might deal with that problem given the causality issue of information moving through reference frames. This can happen even in sub-light objects travelling very fast, IF they have some type of FTL communicator.

So the question is - given one violation of known physical laws, how would a writer explain not violating the causality problem. If you don't believe in time and wouldn't write such a story, then this might not be a topic of interest to you.

 

[goldhawk]

So the question is - given one violation of known physical laws, how would a writer explain not violating the causality problem. If you don't believe in time and wouldn't write such a story, then this might not be a topic of interest to you.

You can't. If you allow FTL, then all of physics falls apart. You are no longer writing sci fi; you're writing space opera. The only restriction is your imagination.

 

[RX-79G]

You can't. If you allow FTL, then all of physics falls apart. You are no longer writing sci fi; you're writing space opera. The only restriction is your imagination.

I simply don't agree that every story ever written that involves FTL is no longer "sci fi". I don't think many would agree with that.


Additionally, we don't know that you can't have objects or information that travel faster than light, we just know that it is not possible via acceleration, and that there are some paradoxical problems if you could. Beyond that FTL is no more speculative than nanotech. Neither currently exist, and either might be impossible.

The purpose of this thread was to suggest what paradox solutions might be useful should the writer decide to imagine some sort of FTL communication device, wormhole, Abecurie drive, etc. It is not to re-categorize the sci fi genre to your specifications, or prove definitively something the world's leading physicists aren't 100% sure about. Either one is a ridiculous presumption on your part.

 

[goldhawk]

Additionally, we don't know that you can't have objects or information that travel faster than light,...

Yes, we do know. If you break the speed of light, you break the universe. The math is clear on this.

The purpose of this thread was to suggest what paradox solutions might be useful should the writer decide to imagine some sort of FTL communication device, wormhole, Abecurie drive, etc. It is not to re-categorize the sci fi genre to your specifications, or prove definitively something the world's leading physicists aren't 100% sure about. Either one is a ridiculous presumption on your part.

Whatever solution you want there to be. As I said before, it's only your imagination that restricts things. If you want a different set of physical laws in each scene, go ahead. If fact, that might be a more interesting read than a conventional novel. But a story without science cannot be called science fiction.

 

[RX-79G]

Yes, we do know. If you break the speed of light, you break the universe. The math is clear on this.

The "math" is clear that you can't accelerate through the speed of light. Special relativity does not prohibit things that happen faster than the speed of light, such as quantum entanglement.