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”?
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.
> 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.
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.
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.
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.
This just made me think of something - would it be accurate to say that the speed of light is getting slower over time, when measured over insterstellar distances?
Speed is distance over time, and distance is measured by a landmark in this case.
In a hypothetical world where an observer has only a distant pointlike object as reference, and he measures that the time it takes for light to arrive from the object, would it not be equally valid for them to assume that the distance is increasing, or the speed of light is decreasing?
Would it be possible to say that light suffers from 'drag' over large distances thereby further objects appearing to take even longer to get to than it would be proportional?
>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.
> 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?
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.
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.
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.
This is wrong. General Relativity tells us exactly how mass (or rather energy) affects spacetime. This has nothing to do with relative distances. The issue with wormholes and warp drives in general relativity is that they require weird forms of energy that we know exist in quantum mechanics, but may not work on macroscopic scales.
> 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.
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.
Dark energy isn’t energy in a conventional sense (hence its disclaimer). In fact it isn’t known to actually exist and is more a placeholder explanation of the apparent expansion effects and ascribing some universal energy that can account for the expansion and the resistance to expansion gravity seems to exert. No energy is expended or moved to cause expansion as dark energy is constant and universal.
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
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
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[ 2.8 ms ] story [ 62.5 ms ] threadAnd wouldn’t light then also be travelling “faster than light”?
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.
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.
> 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.
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.
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.
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
Speed is distance over time, and distance is measured by a landmark in this case. In a hypothetical world where an observer has only a distant pointlike object as reference, and he measures that the time it takes for light to arrive from the object, would it not be equally valid for them to assume that the distance is increasing, or the speed of light is decreasing?
Would it be possible to say that light suffers from 'drag' over large distances thereby further objects appearing to take even longer to get to than it would be proportional?
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.
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?
I know for a fact this generalisation to be false
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.
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
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?
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