[HN Gopher] Detecting FTL with LIGO
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Detecting FTL with LIGO
Author : ggeorgovassilis
Score : 24 points
Date : 2023-12-23 07:21 UTC (15 hours ago)
(HTM) web link (blog.georgovassilis.com)
(TXT) w3m dump (blog.georgovassilis.com)
| scoot wrote:
| 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 wrote:
| > 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.
| pavel_lishin wrote:
| > _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 wrote:
| 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 wrote:
| > _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 wrote:
| > 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.
| ithkuil wrote:
| Yeah. Expanding space behind you may effectively allow you
| to recede from the origin point at arbitrary speeds but
| that doesn't imply you'll get any closer to your
| destination.
|
| Unless you're also able to compress the space ahead of you
| (which is what an alcubierre drive would do).
|
| You wouldn't be moving through space at all. Your
| destination would be closer. Arbitrarily closer. Eventually
| you'll then just reach the destination as if you moved.
|
| Now, I'm quite skeptical about the whole idea but I have to
| admit I didn't do the math.
|
| Wm
| sigmoid10 wrote:
| >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.
|
| It actually is and that is exactly the mechanism behind the
| Alcubierre drive mentioned in the post. But that doesn't change
| the fact that compressing and expanding space on practical
| scales is an astronomically difficult problem and we don't even
| have the slightest clue what the gravitational wave signal of
| the technology would look like. It's also unlikely that we
| could even measure it for the scenario outlined where mass just
| disappears and appears, unless these ships have masses of
| several suns.
| ggeorgovassilis wrote:
| > It's also unlikely that we could even measure it for the
| scenario outlined
|
| I understand the LIGO data is available. A skilled data
| analyst could look at it and tell if it's possible or not. If
| aliens FTL around our neighbourhood, maybe it does show up?
| sigmoid10 wrote:
| A skilled data analyst would never even attempt that before
| having a very strong theoretical model for the kind of
| signal they are looking for. Otherwise you could end up
| with countless "discoveries" that in reality are just
| statistical noise. LIGO's discoveries were only possible
| because of decades of theoretical research in General
| Relativity and simulations of black hole dynamics. They
| already knew precisely what the signal would look like
| before they ever saw it.
| ggeorgovassilis wrote:
| > A skilled data analyst would never even attempt that
| before having a very strong theoretical model
|
| I know for a fact this generalisation to be false
| sigmoid10 wrote:
| Then I guess we disagree on the definition of "skilled."
| What I'm trying to say is you definitely won't find
| something like that published by the LIGO collaboration
| or other serious astronomers.
| fnordpiglet wrote:
| Space seems to expand in the absence of anything, and mass
| and its agent gravity appear to counteract that natural
| expansion of distance. If this is in fact true, I don't think
| it's astronomically hard but impossible to directly
| manipulate space. Space isn't actually a thing to be
| manipulated, it's a description of the apparent distance
| between things of reference. The discussion of space warping
| around mass etc is a convenient metaphor for what we observe,
| the bowling ball on a blanket doesn't mean there's literally
| a blanket to be manipulated. It's an effect we observe about
| relative distances being distorted in the metrics of distance
| and rate of information propagation. What medium these exist
| in, if any, is entirely unknown and is most likely non
| existent but is a description of observed effects, while
| space and time are simply descriptions of those effects.
| cmsj wrote:
| > 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 wrote:
| 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
| ithkuil wrote:
| > 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 wrote:
| 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 wrote:
| 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
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