Distorting spacetime: Links/references about fundamental limits

[Bill_K]

Thinker007, In all of this discussion about gravity stretching or compressing space, I think you need to answer the question "compared to what?" The distances in a curved spacetime are simply what they are, there is nothing else real to compare them to. In the example of a gravitational wave, you seem to be comparing them to distances in a flat background, but such a background is only a mathematical construct and has no physical existence. If a light ray in the curved spacetime should appear timelike or spacelike in that background, well fine, but it has no significance.

The effect of a passing gravitational wave means that the distance from here to Alpha centauri varies slightly as the wave goes by. A wise astronaut times his trip to coincide with the moment that the distance is reduced, but that doesn't mean he traveled faster than light.

Similarly, there's an effect on light travel near a massive object called the Shapiro delay. A light ray passing near a mass will take a slightly longer time to pass. But that's not to say that it traveled slower than c.

 

[Thinker007]

Thinker007, In all of this discussion about gravity stretching or compressing space, I think you need to answer the question "compared to what?"

See below.

A wise astronaut times his trip to coincide with the moment that the distance is reduced, but that doesn't mean he traveled faster than light.

Of course he's not traveling faster than light. He'd be traveling less far however, and from a practical perspective, traveling less far to reach the goal is just as good as traveling faster. It seems reasonable to me to compare the two trips, one undertaken before the wave has arrived and one undertaken while the wave is compressing the spacetime to reduce the distance that must be traveled.

However, even if the distance is reduced by spacetime compression, is it necessarily true that the travel time is reduced? Is there some effect on the traveler's time at the boundary as he crosses into the wave and the region of spacetime compression? Analogies with compressed rubber sheets can only take us so far - we can't (or at least *I* can't) just jump to the conclusion that spacetime compression automatically implies that the astronaut gains anything by departing at an optimum time. That's why I wondered if there were any links to papers discussing this general subject.

Similarly, there's an effect on light travel near a massive object called the Shapiro delay. A light ray passing near a mass will take a slightly longer time to pass. But that's not to say that it traveled slower than c.

I'm not sure why you are emphasizing this. Perhaps I'm missing something, but I thought all my posts had been clear - I understand that light travels at c in spacetime and there's no known way to exceed that speed. However, both the expansion and compression of spacetime have real effects, and it's those effects I was wondering about.

I'd often read of the effects of mass causing the type of delay you refer to (delay relative to flat spacetime), but I'd never read of light travel time decrease due to spacetime compression. Moreover, until I asked here, I wasn't even sure that spacetime was ever compressed in the absence of exotic mass/energy.

And I'll say it again. I'm not trying to build an FTL drive. :-) I am wondering, however, if our current understanding of physics allows for ordinary matter to compress spacetime - does that compression have any interesting implications? Are there any papers on the subject that I might read?

 

[Bill_K]

However, even if the distance is reduced by spacetime compression, is it necessarily true that the travel time is reduced?

Yes.

Is there some effect on the traveler's time at the boundary as he crosses into the wave and the region of spacetime compression?

No. This is really grasping at straws.

Analogies with compressed rubber sheets can only take us so far

Who mentioned rubber sheets? I never mentioned rubber sheets.

we can't (or at least *I* can't) just jump to the conclusion that spacetime compression automatically implies that the astronaut gains anything by departing at an optimum time. That's why I wondered if there were any links to papers discussing this general subject... both the expansion and compression of spacetime have real effects, and it's those effects I was wondering about.

I've tried my best to explain it, but it sounds like you'd better work it out for yourself at this point.

 

[Thinker007]

No. This is really grasping at straws.

Hmmm. I was trying to make it clear that I'm open to any comments on how the universe works. I wasn't trying to take one position or another.

I'm not aware of any fundamental limit on the compression that can result from a GW. Nor am I aware of any restrictions on creating a "beamed" GW. So does known physics permit a series of beamed parallel GWs, each timed and phased so that a particle traveling transverse to the beams can pass from the Milky Way to Andromeda in 2 weeks? Yes, it's extreme, and yes, it's sort of a warp drive, but the question is what fundamental limits apply. If physics allows high amplitude GWs and allows them to be shaped such that a sublight traveling particle (or a light beam) can pass from one compressed transverse region of spacetime to another adjacent compressed region, just as the second GW beam causes compression of that second region, what are the implications for causality, if any? Presumably, they would be the same as the implications for a wormhole, which as far as I know, is a permitted spacetime configuration, even if one that is not known to exist.

It appears in my simple mental model of GWs that one could step from one compressed region to another adjacent compressed region and repeat the process to achieve a slightly FTL travel time as compared to travel without passing transversely through the perfectly timed/phased adjacent GWs It appears one could slowly outrace a nearby light beam that was outside the GW beams and moving in flatter spacetime.

I don't expect anyone here to work this out for me, and I'm not here to try to convince anyone I've found some wonderful FTL hole in GR theory. I'm just wondering about how it works. I imagine a series of such beams being implemented in advance and running constantly, setting up a path that allows what has the practical effect of FTL communication when messages are sent transversely through the adjacent parallel beams.

I find it very difficult to believe that such a thing is possible, but I want to know. Whenever such a "workaround" for light speed limits is proposed, there is usually a "gotcha". Several are known for the Alcubierre drive and they mostly related to boundary conditions and requirements for exotic materials. I didn't think I was "grasping at straws" when I mentioned that there might be similar boundary condition "gotchas" limiting what appeared on its face to be a way to communicate between two points at speeds slightly faster than light could travel between those points in flat spacetime.

If the spacetime between Andromeda and the Milky Way can be compressed, then presumably a light beam from here to there can travel in less time than the 2 million years that light takes to make the trip in the absence of such compression. Does our current knowledge of physics rule this out? It seems not, but I find that hard to accept, but not impossible. Certainly no one discusses it (that I've heard) outside of discussions of the Alcubierre drive. But if physics rules it out, then how/why?

It seems that the light travel time in one leg of the LIGO experiment is indeed reduced as compared to the travel time in flat spacetime, so perhaps I should just accept that it's possible, but hard to do in practice? But that seems to open the possibility that physics let's us set up communication channels between distant points that operate faster than light channels between those points would operate in flat ST. Is that a problem? I don't know, but it makes me uneasy.