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Is Faster-Than-Light Travel or Communication Possible? (1997)

math.ucr.edu

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Re: Is Faster-Than-Light Travel or Communication Possible? (1997)

#81

Post by snowwrestler »

ikeboy wrote: post #80
gizmo686 wrote: post #62

Earlier quoted context omitted.

Your equation is correct in a Newtonian universe, but does not consider relativistic effects. Generally when we talk about an object's acceleration, we are talking about its acceleration in its own reference frame. By definition, an object's velocity in its own reference frame is always 0, so acceleration does not become more difficult (ignoring the fact that the surrounding matter will be impacting you at higher spe…

So why is it difficult to maintain a constant acceleration? Is 1g really that impractical right now?

I'm being accelerated at 1g right now and I'm about 40...so in another decade or so I should be able to get to anywhere in the universe!

Jokes aside, there's no fuel source we know of that would allow a ship to accelerate continuously for 50 years at 1g.

Re: Is Faster-Than-Light Travel or Communication Possible? (1997)

#82

Post by gizmo686 »

ikeboy wrote: post #80
gizmo686 wrote: post #62

Earlier quoted context omitted.

Your equation is correct in a Newtonian universe, but does not consider relativistic effects. Generally when we talk about an object's acceleration, we are talking about its acceleration in its own reference frame. By definition, an object's velocity in its own reference frame is always 0, so acceleration does not become more difficult (ignoring the fact that the surrounding matter will be impacting you at higher spe…

So why is it difficult to maintain a constant acceleration? Is 1g really that impractical right now?

1) Maintaining an acceleration requires fuel.

2) Even though you don't observe yourself moving at increasing speeds, you do observe the rest of the universe moving at increasing speeds. This means that small particles impacting your ship will hit it with increasing energy.

Re: Is Faster-Than-Light Travel or Communication Possible? (1997)

#83

Post by undersuit »

EGreg wrote: post #79
joshuahutt wrote: post #73

Earlier quoted context omitted.

Because it's 4.367 light years away [1]. [1] https://www.google.com/search?q=distance+to+alpha+centauri

But from the point of view of the traveler, it an take far less time.

But from the point of view of both the Origin and the Destination(assuming they maintain similar frames of reference) it takes... 4 years.

Re: Is Faster-Than-Light Travel or Communication Possible? (1997)

#84

Post by pdonis »

Ono-Sendai wrote: post #78
pdonis wrote: post #77

Earlier quoted context omitted.

> special relativity doesn't say there is no absolute time, but rather that if there is, it is undetectable in any inertial reference frames. No, SR says there is no absolute time, because such a thing is undetectable in any inertial reference frame. The theory that makes all the same predictions as SR, but also says that there is such a thing as "absolute time", but it's undetectable in any inertial reference frame…

My understanding is that SR doesn't go quite as far as saying 'there is no absolute time', rather it dismisses the concept as undetectable and unnecessary (and useless). In other words, it doesn't disprove absolute time, it just dispenses with it.

"Absolute time" is supposed to be a physical concept, right? If it is, then showing that it is undetectable and unnecessary is "disproving" it. And the post of yours that I originally responded to certainly seemed to indicate that it was a physical concept--you were saying that if there is absolute time, it can restrict what kinds of FTL travel are possible, which is a physical restriction.

If "absolute time" is just a philosophical concept with no physical consequences, then no physical theory, SR or otherwise, is going to have anything to say about it anyway. But then, it can't be used to resolve the "paradox" you were claiming it could resolve, because it can't restrict what kinds of FTL travel are possible.

Re: Is Faster-Than-Light Travel or Communication Possible? (1997)

#85

Post by Ono-Sendai »

pdonis wrote: post #84
Ono-Sendai wrote: post #78

Earlier quoted context omitted.

My understanding is that SR doesn't go quite as far as saying 'there is no absolute time', rather it dismisses the concept as undetectable and unnecessary (and useless). In other words, it doesn't disprove absolute time, it just dispenses with it.

"Absolute time" is supposed to be a physical concept, right? If it is, then showing that it is undetectable and unnecessary is "disproving" it. And the post of yours that I originally responded to certainly seemed to indicate that it was a physical concept--you were saying that if there is absolute time, it can restrict what kinds of FTL travel are possible, which is a physical restriction. If "absolute time" is just…

Well, the overall point is: If FTL communication is possible, then you get grandfather type paradoxes, unless you have an absolute time (or it is resolved some other way). And SR does not rule out absolute time, it just discards it.

Re: Is Faster-Than-Light Travel or Communication Possible? (1997)

#86

Post by pdonis »

Ono-Sendai wrote: post #85
pdonis wrote: post #84

Earlier quoted context omitted.

"Absolute time" is supposed to be a physical concept, right? If it is, then showing that it is undetectable and unnecessary is "disproving" it. And the post of yours that I originally responded to certainly seemed to indicate that it was a physical concept--you were saying that if there is absolute time, it can restrict what kinds of FTL travel are possible, which is a physical restriction. If "absolute time" is just…

Well, the overall point is: If FTL communication is possible, then you get grandfather type paradoxes, unless you have an absolute time (or it is resolved some other way). And SR does not rule out absolute time, it just discards it.

> SR does not rule out absolute time

If absolute time has physical effects, which it must if it can prevent FTL grandfather paradoxes, then yes, SR does rule it out. SR is Lorentz invariant, and absolute time preventing FTL grandfather paradoxes would break Lorentz invariance.

That doesn't mean you couldn't construct a theory that included absolute time and in which FTL travel was allowed but grandfather paradoxes were prevented. But such a theory would not be consistent with SR. To be viable, such a theory would have to match the predictions of SR in the domain in which those predictions have been verified, but it would necessarily make different predictions from SR about FTL travel. (No such theory exists that I'm aware of; I've seen hand-waving about it in discussions of LET, but never any concrete theory.)

My personal prediction would be that such a theory would end up being falsified (either that or rendered meaningless by experiments showing that no type of FTL travel is possible at all). But that's a separate question.

Re: Is Faster-Than-Light Travel or Communication Possible? (1997)

#87

Post by Ono-Sendai »

pdonis wrote: post #86
Ono-Sendai wrote: post #85

Earlier quoted context omitted.

Well, the overall point is: If FTL communication is possible, then you get grandfather type paradoxes, unless you have an absolute time (or it is resolved some other way). And SR does not rule out absolute time, it just discards it.

> SR does not rule out absolute time If absolute time has physical effects, which it must if it can prevent FTL grandfather paradoxes, then yes, SR does rule it out. SR is Lorentz invariant, and absolute time preventing FTL grandfather paradoxes would break Lorentz invariance. That doesn't mean you couldn't construct a theory that included absolute time and in which FTL travel was allowed but grandfather paradoxes we…

You're right, it would violate Lorentz invariance, but only for FTL travel. In that sense it is consistent with SR for speeds I think it's quite easy to make such a theory, I'll get around to writing a blog post on it some day :)

The main result is that for communication (or travel) faster than c, invariance is broken, and it becomes possible to distinguish the absolute reference frame.

Re: Is Faster-Than-Light Travel or Communication Possible? (1997)

#88

Post by EGreg »

undersuit wrote: post #83
EGreg wrote: post #79

Earlier quoted context omitted.

But from the point of view of the traveler, it an take far less time.

But from the point of view of both the Origin and the Destination(assuming they maintain similar frames of reference) it takes... 4 years.

That may be true but I was questioning the parent comment which said ANY reference frame
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