Comment by lutusp | Hacker News Reader

Comment by lutusp | Hacker News Reader

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> If we ever invented FTL travel, even for just probes, this could be done too.

If we had FTL travel, we could outrace light waves, look back, and see events in the past. The we could race back and report what we saw. That's why it's not possible -- it violates some basic physical ideas about causality.

Remember that the speed of light isn't just a speed limit for light waves -- it's a barrier that sorts out temporal causes and effects.

Imagine a flat plane in the time dimension called the present -- on each side of the plane is a cone. The cone in the past encloses possible causes for present events. The cone in the future encloses possible effects from events in the present. Outside the past cone are events that may not produce results in the present. Outside the future cone are effects that cannot have been caused by events in the present. The slope of the cone surfaces is the speed of light.

Light cone picture:

http://4.bp.blogspot.com/--VBwEngltGc/Tzniy1imbII/AAAAAAAABL...

>>If we had FTL travel, we could outrace light waves, look back, and see events in the past. The we could race back and report what we saw. That's why it's not possible -- it violates some basic physical ideas about causality.

You are confused. You are not actually traveling to the past therefore causality is not a problem here.

> You are confused.

I'm confused? Which of us is discussing FTL without understanding its physical meaning?

> You are not actually traveling to the past ...

Locate where I made this claim.

> ... therefore causality is not a problem here.

Causality is certainly a problem. FTL is impossible in the same way that dividing a number by zero is impossible or taking the square root of a negative number is impossible.

Here's the simplest form of the relativistic equation that tells us the amount of time dilation we can expect for a given velocity v:

t' = t * sqrt(1-v^2/c^2)

t = time rate at v = 0

t' = time rate at v

c = speed of light

v = frame velocity

Compute the time dilation for v > c, then get back to me.

FTL means leaving the causality light cone, conventional notions of causality, and therefore all of physics:

http://en.wikipedia.org/wiki/Light_cone

Quote: "Because signals and other causal influences cannot travel faster than light (see special relativity and spooky action at a distance), the light cone plays an essential role in defining the concept of causality - for a given event E, the set of events that lie on or inside the past light cone of E would also be the set of all events that could send a signal that would have time to reach E and influence it in some way." [emphasis added]

"FTL means leaving the causality light cone, conventional notions of causality, and therefore all of physics:"

While I agree with your conclusion that ftl is equivalent of time travel and breaks causality I think its incorrect to say it breaks all of physics. There are several solutions to general relativity that are valid. http://en.wikipedia.org/wiki/Chronology_protection_conjectur...

> There are several solutions to general relativity that are valid.

They haven't been shown to exist in reality. Remember that science is -- must be -- empirical.

> Causality is certainly a problem. FTL is impossible in the same way that dividing a number by zero is impossible or taking the square root of a negative number is impossible.

This assertion is false. GR, even without FTL, allows for closed time-like loops, which violate causality. Violations of causality are not logical contradictions like dividing by zero. (Though, of course, there are alternative mathematical systems that allow for dividing by zero. And taking the square root of a negative number is surely not impossible. You just get an Imaginary result.) It's just that a universe in which causality is violated all the time would be a very strange one, and consequently, it is reasonable to assume that if there are, for instance, closed time-like loops in the universe, they are not traversed very often.

> GR, even without FTL, allows for closed time-like loops, which violate causality.

The theory may "allow" them, but this is not to say they actually exist in reality.

http://en.wikipedia.org/wiki/Causality_(physics)

Quote: "In the theory of general relativity, the concept of causality is generalized in the most straightforward way: the effect must belong to the future light cone of its cause, even if the spacetime is curved."

http://hipercom.inria.fr/~jacquet/retro1/crazy-science-corne...

Quote: "Weird space-time geometry distortion within general relativity can result in causality violation, but there is no evidence that they are physically possible."

> Quote: "Weird space-time geometry distortion within general relativity can result in causality violation, but there is no evidence that they are physically possible."

And there's no evidence that they are not possible. With no evidence either way, one must remain agnostic.

On the other hand, we do have evidence that closed time-like loops are possible: GR has never been observed to be wrong, and GR predicts their existence under the right circumstances.

It may certainly be the case, that even though closed time-like loops are possible, they do not actually exist in our universe and never will. Paradoxical universes are ruled out by logic alone, and so the space of possible universes without closed time-like loops (or least without humans traversing them) may outnumber the space of possible universes with time-like loops in them, allowing us to make a probabilistic argument against their occurrence in our universe. My transfinite number theory is rusty, however, so YMMV.

> And there's no evidence that they are not possible.

In science, the default precept toward ideas without evidence is the null hypothesis -- they are presumed to be false. This is why Bigfoot is not accepted as a scientific theory -- until it is observed, it doesn't exist.

http://www.null-hypothesis.co.uk/science/item/what_is_a_null...

Quote: "In statistics, the only way of supporting your hypothesis is to refute the null hypothesis. Rather than trying to prove your idea (the alternate hypothesis) right you must show that the null hypothesis is likely to be wrong – you have to ‘refute’ or ‘nullify’ the null hypothesis."

Saying "there's no counterevidence, therefore ..." is not a scientific statement, because it can be said about anything not conclusively falsified, including meadowland fairies and honest politicians.

> With no evidence either way, one must remain agnostic.

Not in science. It's nice to have an open mind, but not to the degree that one's brains spill onto the ground.

This is just untrue. When presented with a hugely precise and successful theory that has never been demonstrated wrong in even the slightest degree, that is certainly evidence (not proof, but evidence) towards whatever claims that theory makes. If you don't understand this, then you don't know what the word "evidence" means.

> This is just untrue. When presented with a hugely precise and successful theory that has never been demonstrated wrong in even the slightest degree, that is certainly evidence (not proof, but evidence) towards whatever claims that theory makes.

Nonsense -- perfect nonsense that the history of science repeatedly falsifies.

1. Science is empirical -- it must be. If it's not empirical, it's not science.

2. GR succeeded with every challenge that was put before it until it was compared to quantum theory, at which point it failed. Your position is that, because it had passed any number of difficult tests until then, it should have been assumed to reflect reality in all respects.

But this is not how a scientist approaches theories and experiments. Any aspect of a theory that has not been tested, is assumed to be false until there is evidence to support it.

The ether theory of the 19th century was successful until it failed one crucial test. The Phlogiston theory was successful until it failed one crucial test. The Ptolemaic theory of orbital mechanics seemed correct until Galileo looked through his telescope.

All scientific theories are assumed to be false until there is evidence to support them, then, after evidence supports them, they are assumed to be perpetually falsifiable by new evidence. An unfalsifiable theory is not a scientific theory.

> If you don't understand this, then you don't know what the word "evidence" means.

Science is not law. Learn about science.

> When presented with a hugely precise and successful theory that has never been demonstrated wrong in even the slightest degree ...

You need to learn about quantum theory, which falsifies GR. Start here:

http://library.thinkquest.org/04apr/01330/currentphysics/con...

The present enthusiasm for superstring theory is that it might -- might! -- replace both GR and quantum theory, with a more comprehensive theory that would resolve the conflict between the two earlier theories.

> Nonsense -- perfect nonsense that the history of science repeatedly falsifies.

You are completely incorrect in this and everything else you said above. Well, it is true that science is empirical, but it seem that you have profound misconceptions about what "empirical" means. One of the things that it means is that every additional confirming piece of observed evidence for a theory gives extra weight to the probability that the theory is correct, especially when the theory makes correct predictions against previously unobserved phenomena, which GR has done many times.

You also apparently don't understand that theories are not typically purported to be facts per se, but rather they are models. Models that have greater or lesser accuracy. Models are not expected to be perfectly accurate, which is why they are models and not facts. Though in fundamental physics, we do expect a level of accuracy far above what we would expect in almost any other science.

The ether model was proven to be so inaccurate as to no longer be useful. The limitations of Newtonian mechanics were shown by Relativity, but nonetheless, the model is still a hugely important and useful model, and using it correctly is the epitome of good science, despite the fact that we know that in some sense it is "false".

Re your examples, all the examples you give of theories that were eventually proven incorrect are arguments against a strawman, because no one here or anywhere has claimed that any amount of empirical evidence will grant a theory a 100% probability of correctness.

No one has asserted that GR is unfalsifiable, or that it has a 100% probability of being correct. None of this means that we can't have an extremely high confidence level in GR, considering all the empirical gauntlets that it has successfully passed. And when we finally have a successful TOE, GR is almost certainly not going to be scrapped. It will still be the extremely useful and productive model that it has always been. We will just have to be aware of the model's caveats, just as we must with Newtonian Mechanics, which is also one of the most successful scientific models ever constructed.

Additionally, all this is neither here nor there for the original point, which was about your claim that FTL is logically impossible. That is just not the case. FTL is completely within the realm of possibility. It just has serious and problematic consequences that should make us look askance when people claim to have figured out how to make FTL drives or communication channels. They are almost certainly mistaken, but not for the reasons that you claim.

You are forgetting that we could "travel faster than light" using an Alcubierre Drive [1][2], or something similar. i.e. bending space so that our path is now shorter even though in our local space we are not traveling faster than light.

[1] http://en.wikipedia.org/wiki/Alcubierre_drive

[2] http://techland.time.com/2012/09/19/nasa-actually-working-on...

Granted, it is all theory right now but that is how relativity started. Who knows, maybe in 100 years we'll be traveling to the stars.

Nowhere does causality violation come into the picture. There is more than one way to skin a cat.

So in normal space-time FTL is not possible.

It might be possible to transmit data through shortcuts, ala wormholes even if physical FTL is not possible.

The light speed limit comes up elsewhere. As best we know, you cannot transmit information faster than c. Not even with "spooky quantum stuff". There are no shortcuts. Information is just as real as matter or energy--if not more so, see John Wheeler's "it from bit"--and isn't tied to meaning, see Shannon.

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