5 ms·

Detecting FTL with LIGO

scoot 3y ago
ELI5: If faster than light travel may theoretically be possible because the universe is expanding, isn’t that relative to your origin, rather than your destination, which is also moving? Ergo, you aren’t getting there any more quickly… And wouldn’t light then also be travelling “faster than light”?
- ggeorgovassilis 3y ago
  > isn’t that relative to your origin It applies to the universal scale, I'm not sure how more absolute it gets. Every observer regardless of frame observes that the expansion is faster than c. > And wouldn’t light then also be travelling “faster than light”? Space on light's path also inflates, so in terms of units of space travelled per unit of time nothing changes.
betaporter 3y ago
[flagged]
pavel_lishin 3y ago
> FTL should be possible because the universe expands [EXP] at speeds greater than that of light. I wish the article justified this statement, without just stating it and moving on. Expansion means that two points in space can be moving apart at greater than the speed of light - but that's not the same thing as traveling faster than the speed of light. > What we do know, however, is that a warp trip between A and B starts with a mass instantly disappearing from point A and appearing sometimes later at point B. This is also stated boldly, without any explanation of why this must be the truth. It sounds like a description of teleportation, which doesn't have to be an FTL phenomenon; and it doesn't explain why FTL would look like teleportation to an observer watching gravitational waves.
- ggeorgovassilis 3y ago
  Some of your observations are good, for those I thank you, and revised the post accordingly. > I wish the article justified this statement Your wish has been granted. Also, it's in the referenced Wikipedia article: "galaxies that are farther than the Hubble radius, approximately 4.5 gigaparsecs or 14.7 billion light-years, away from us have a recession speed that is faster than the speed of light". > that's not the same thing as traveling faster than the speed of light. The mechanism seems different, so indeed not the same thing, but the result seems to be the same. > it doesn't explain why FTL would look like teleportation The proposed detection method is agnostic towards what happens during the trip, it focusses on detecting the initial and terminal event - a mass disappearing (rapidly) from one point and re-appearing (rapidly) at another. That probably wouldn't work if FTL travel produces significant gravitational distortions, in which case we're lucky because that would stick out as a signal. It also wouldn't work if the FTL drive involves long phases of acceleration and deceleration.
  - pavel_lishin 3y ago
    > the result seems to be the same. I don't ... think that's true, but I'd have to think about this some more. The distance between two objects increasing at a speed greater than light is not the same thing as an object traveling - that is, traversing space - faster than the speed of light. To put it more specifically, the expansion of space doesn't help whatsoever get you from Point of Interest A to Point of Interest B faster than light can get there. (If you can compress space - the Alcubierre drive has been mentioned - then that may not be true, but that's not the same thing as the expansion of space, I don't think.) > it focusses on detecting the initial and terminal event - a mass disappearing (rapidly) from one point and re-appearing (rapidly) at another. I'm still not convinced that this is what FTL travel would look like to an external observer. I don't think you've done enough to justify this assertion.
    - ggeorgovassilis 3y ago
      > I'm still not convinced that this is what FTL travel would look like to an external observer. I'm not either, I'm saying that if that's the case, there is an easy way to find out. > I don't think you've done enough to justify this assertion. Again, speculation, not assertion. The proposed detection mechanism works only in the described case.
cmsj 3y ago
> FTL should be possible because the universe expands [EXP] at speeds greater than that of light. The universe isn't expanding faster than the speed of light, it seems to be expanding everywhere at the same rate. Galaxies beyond a particular distance from us are receding at/above the speed of light only because all of the expansion between us and them stacks up and is pushing them away from us at faster than the speed of light. They are not being pushed through space though, space itself is expanding. Furthermore, since the speed of light is really more accurately the speed of causality, once these distant objects are receding faster than the speed of light, no further information (which in practice means EM radiation, but in theory means any/all causality) from them can ever reach us. So, I'm not sure how it follows that because universal expansion exists and is being driven by something, it should therefore be possible for us to harness it. The universe seems to be extremely clear that causality is not allowed to happen faster than c.
- fnordpiglet 3y ago
  More specifically expansion isn’t movement per se, it just appears to be movement. It’s actually the expansion of distance between points without energy applied. There may be a time when expansion makes things appear to move faster than the speed of light away from each other, but that time will mean we can never observe anything and the universe has effectively ripped itself apart. As has been noted this can already be observed at large distances as the expansion accumulating between us and those distant points creates the appearance of faster than light travel. [0] That happens only in certain situations for certain constant assumptions and certain mathematics and physics assumptions, which can be totally overturned and we are in no way sure expansion is accelerating, slowing, going at the same rate, or even our empirical observations that support expansion aren’t caused by some as of now unknown physical process. [0] https://en.m.wikipedia.org/wiki/Hubble_volume https://en.m.wikipedia.org/wiki/Hubble_volume
  - ithkuil 3y ago
    > without energy being applied I'm confused. Why is the cosmological constant then also called dark energy if it's not equivalent to there being some energy in empty space?
anthk 3y ago
Not FTL. If an opposite lane sided car passes you at 180 km/h and you are running at 120 km/h, it looks like the gap "widens" at 300 km/h speed.
poulpy123 3y ago
LIRGO and VIRGO are barely able to detect black holes of neutron star merging, it is not going to detect the disapearance or appearance of a small spaceship
mentalpiracy 3y ago
This is not FTL in any real sense. The apparent motion of distant galaxies is just that - an appearance only. In our frame of reference, they appear to be receding faster than light, but in their own frames of reference, these galaxies themselves are not actually moving any faster than our own galaxy does.[0] Imagine you shine a laser pointer at the moon and are still able to see the dot. With a flick of your wrist, you move the dot from one side of the moon to the opposite side. If you did this quickly enough, the dot could appear to have moved ftl - but in reality, there was no superluminal acceleration. [0] galaxies are obviously still interacting and moving relative to local neighbors
6R1M0R4CL3 3y ago
[dead]

ELI5: If faster than light travel may theoretically be possible because the universe is expanding, isn’t that relative to your origin, rather than your destination, which is also moving? Ergo, you aren’t getting there any more quickly… And wouldn’t light then also be travelling “faster than light”?

ggeorgovassilis 3y ago
> isn’t that relative to your origin It applies to the universal scale, I'm not sure how more absolute it gets. Every observer regardless of frame observes that the expansion is faster than c. > And wouldn’t light then also be travelling “faster than light”? Space on light's path also inflates, so in terms of units of space travelled per unit of time nothing changes.

betaporter 3y ago

[flagged]

pavel_lishin 3y ago

> FTL should be possible because the universe expands [EXP] at speeds greater than that of light. I wish the article justified this statement, without just stating it and moving on. Expansion means that two points in space can be moving apart at greater than the speed of light - but that's not the same thing as traveling faster than the speed of light. > What we do know, however, is that a warp trip between A and B starts with a mass instantly disappearing from point A and appearing sometimes later at point B. This is also stated boldly, without any explanation of why this must be the truth. It sounds like a description of teleportation, which doesn't have to be an FTL phenomenon; and it doesn't explain why FTL would look like teleportation to an observer watching gravitational waves.

ggeorgovassilis 3y ago
Some of your observations are good, for those I thank you, and revised the post accordingly. > I wish the article justified this statement Your wish has been granted. Also, it's in the referenced Wikipedia article: "galaxies that are farther than the Hubble radius, approximately 4.5 gigaparsecs or 14.7 billion light-years, away from us have a recession speed that is faster than the speed of light". > that's not the same thing as traveling faster than the speed of light. The mechanism seems different, so indeed not the same thing, but the result seems to be the same. > it doesn't explain why FTL would look like teleportation The proposed detection method is agnostic towards what happens during the trip, it focusses on detecting the initial and terminal event - a mass disappearing (rapidly) from one point and re-appearing (rapidly) at another. That probably wouldn't work if FTL travel produces significant gravitational distortions, in which case we're lucky because that would stick out as a signal. It also wouldn't work if the FTL drive involves long phases of acceleration and deceleration.
- pavel_lishin 3y ago
  > the result seems to be the same. I don't ... think that's true, but I'd have to think about this some more. The distance between two objects increasing at a speed greater than light is not the same thing as an object traveling - that is, traversing space - faster than the speed of light. To put it more specifically, the expansion of space doesn't help whatsoever get you from Point of Interest A to Point of Interest B faster than light can get there. (If you can compress space - the Alcubierre drive has been mentioned - then that may not be true, but that's not the same thing as the expansion of space, I don't think.) > it focusses on detecting the initial and terminal event - a mass disappearing (rapidly) from one point and re-appearing (rapidly) at another. I'm still not convinced that this is what FTL travel would look like to an external observer. I don't think you've done enough to justify this assertion.
  - ggeorgovassilis 3y ago
    > I'm still not convinced that this is what FTL travel would look like to an external observer. I'm not either, I'm saying that if that's the case, there is an easy way to find out. > I don't think you've done enough to justify this assertion. Again, speculation, not assertion. The proposed detection mechanism works only in the described case.

cmsj 3y ago

> FTL should be possible because the universe expands [EXP] at speeds greater than that of light. The universe isn't expanding faster than the speed of light, it seems to be expanding everywhere at the same rate. Galaxies beyond a particular distance from us are receding at/above the speed of light only because all of the expansion between us and them stacks up and is pushing them away from us at faster than the speed of light. They are not being pushed through space though, space itself is expanding. Furthermore, since the speed of light is really more accurately the speed of causality, once these distant objects are receding faster than the speed of light, no further information (which in practice means EM radiation, but in theory means any/all causality) from them can ever reach us. So, I'm not sure how it follows that because universal expansion exists and is being driven by something, it should therefore be possible for us to harness it. The universe seems to be extremely clear that causality is not allowed to happen faster than c.

fnordpiglet 3y ago
More specifically expansion isn’t movement per se, it just appears to be movement. It’s actually the expansion of distance between points without energy applied. There may be a time when expansion makes things appear to move faster than the speed of light away from each other, but that time will mean we can never observe anything and the universe has effectively ripped itself apart. As has been noted this can already be observed at large distances as the expansion accumulating between us and those distant points creates the appearance of faster than light travel. [0] That happens only in certain situations for certain constant assumptions and certain mathematics and physics assumptions, which can be totally overturned and we are in no way sure expansion is accelerating, slowing, going at the same rate, or even our empirical observations that support expansion aren’t caused by some as of now unknown physical process. [0] https://en.m.wikipedia.org/wiki/Hubble_volume https://en.m.wikipedia.org/wiki/Hubble_volume
- ithkuil 3y ago
  > without energy being applied I'm confused. Why is the cosmological constant then also called dark energy if it's not equivalent to there being some energy in empty space?

anthk 3y ago

Not FTL. If an opposite lane sided car passes you at 180 km/h and you are running at 120 km/h, it looks like the gap "widens" at 300 km/h speed.

poulpy123 3y ago

LIRGO and VIRGO are barely able to detect black holes of neutron star merging, it is not going to detect the disapearance or appearance of a small spaceship

mentalpiracy 3y ago

This is not FTL in any real sense. The apparent motion of distant galaxies is just that - an appearance only. In our frame of reference, they appear to be receding faster than light, but in their own frames of reference, these galaxies themselves are not actually moving any faster than our own galaxy does.[0] Imagine you shine a laser pointer at the moon and are still able to see the dot. With a flick of your wrist, you move the dot from one side of the moon to the opposite side. If you did this quickly enough, the dot could appear to have moved ftl - but in reality, there was no superluminal acceleration. [0] galaxies are obviously still interacting and moving relative to local neighbors

6R1M0R4CL3 3y ago

[dead]