7 ms·

If what we’re seeing was 13.8B light years away 13.8B years ago and are now 46B light years away, wouldn’t that mean that those things traveled ~32.2B light yea

If what we’re seeing was 13.8B light years away 13.8B years ago and are now 46B light years away, wouldn’t that mean that those things traveled ~32.2B light years in 13.8B years. Wouldn’t that thus mean they travelled faster than the speed of light? Maybe I’m misunderstanding though. EDIT: I guess you also have to take into account the expansion of the fabric of space as well and not just the movement of the galaxies. Still a bit difficult to grasp because I still intuitively want to say that those galaxies are traveling faster than the speed of light with the assistance of space expansion like the old Hollywood trick of actors running on a moving rug to fake superhuman speed. They’re still moving faster, just not on their own. I guess the key difference is that an actor on a sliding rug is moving through space faster even if their legs are moving at the same speed, but in the case of the galaxies, space itself is moving thus they’re not technically FTL because the speed of light is relative to its movement through space.

amelius 7y ago
I didn't read the article but I suspect the reason is that space is expanding. If space wouldn't have expanded sufficiently fast in the beginning of time, all matter would be still clumped together as in a giant black hole. The fact that space expanded "faster" than the speed of light allowed this black hole to become free matter.
recursivecaveat 7y ago
In essence, it's 'okay' for things to be FTL due to expansion of the medium. Imagine an ant crawling across a balloon as we stretch it. The ant could get some pretty fantastic apparent speeds relative to another ant because the medium is expanding, but step by step its moving at regular ant speed.
- cellular 7y ago
  It is interesting to think of the opposite: space fabric is shrinking, so light needs to travel "slower" so as not to break causality. I know I know, it's not really slower because the fabric is shrinking.
  - badrabbit 7y ago
    But there is non space "fabric". There are interactions between qanta/matter but space is still space right?
  - lonelappde 7y ago
    It doesn't "need to", it just does. Causality is defined by the speed of light
Zenst 7y ago
Interesting ratio of 1:3.3141210374639769452449567723343 not exactly Pi. But then that expansion ratio may of been different over time, but it is a bit like sampling the second hand on a clock in a photo and working out the entire motion and workings of the clock - at least on the scale of the data/measurements we have and the age of the universe. Currently we are still not 100% sure when midnight was, but as always moving in the right direction.
- Retric 7y ago
  We have more recent data on closer objects. It’s a sequence of thin shells that expands as you go backward in time, or a 3D light cone aimed backward in 4D space time. https://en.m.wikipedia.org/wiki/Light_cone https://en.m.wikipedia.org/wiki/Light_cone
rhinokungfu 7y ago
Some of the confusion is, I think from referring to speed of light as "speed of light". When it really is speed limit of causality between 2 points in space. And light (em waves) just happen to be able to hit that limit. Now since expansion of space is itself not going to allow one point in space to cause any effect on another, it (the expansion) can be faster than speed of causality. PBS space time has a good video on this - https://www.youtube.com/watch?v=msVuCEs8Ydo https://www.youtube.com/watch?v=msVuCEs8Ydo
- nuccy 7y ago
  It is also peculiar that the energy conservation law is not applicable on cosmological scales, the light is redshifted due to expansion of the Universe and the energy difference between emitted and received light is "wasted", i.e. it doesn't feed the expansion in any way.
  - cygx 7y ago
    I don't know if 'feeding expansion' is necessarily the right way to think about this, but by the first Friedmann equation, loss of photon energy will in fact be balanced by the increased expansion rate...
    - nuccy 7y ago
      Sure, though the first Friedmann equation, or any other equation, where terms are balanced, e.g. a + b = 0, doesn't tell you anything about causality, though redshifting of light is exactly caused by expansion, so it is not strange that both are "balanced".
cortesoft 7y ago
Yeah, the intuitive view of universe expansion is that it expands from the edges out.... but in reality, it expands everywhere at once, so really the space between us is expanding as well.
harshreality 7y ago
FTL travel of anything (matter, photons, EM fields, gravitational fields) would result in potential causality issues. I don't know if there are models of universes where GR doesn't apply; that would be interesting, but the causality problems might make such a universe uninteresting even if it were possible. I suppose that's using the anthropic principle as justification for GR? Metric expansion does not result in causality issues because it's an isolating phenomenon (one that slows down interactions between things) rather than an interconnecting phenomenon (one that would speed up interactions between things). Therefore it's at least possible, and observations (which you can read a brief summary of in TFA) back it up.
- raattgift 7y ago
  > Metric expansion does not result in causality issues because it's an isolating phenomenon No, in the specific case of the concordance cosmology (and more general expanding cases such as an expanding Robertson-Walker spacetime, or a 4-dimensional de Sitter spacetime), the manifold is still Lorentzian [1] and globally hyperbolic [2]. These features let one sort trajectories into spacelike, null, and timelike. If spacelike trajectories are forbidden to interacting matter and gravitational waves, then we get our usual idea of causality [3]. Collapsing Robertson-Walker spacetimes and anti-de Sitter spacetimes are defined on globally hyperbolic Lorentzian manifolds. They are defined as free of matter and gravitational waves. However we can perturb them slighly by adding a bit of matter, even matter that generates small amounts of gravitational radiation, and as long as that matter and radiation moves only on timelike trajectories or lightlike trajectories, we have the normal causality even though the interactions among separated bits of normal matter perturbing these collapsing spacetimes will generally become more frequent and stronger over time. Indeed, this remains the case even for very highly accelerated compactions, where at late times distances between objects contract faster than light between the same objects can move. Other commonly encountered spacetimes -- the flat spacetime of Minkowski approximately describes laboratories on Earth and in the International Space Station, and the (exterior) Schwarzschild spacetime which approximately describes the spacetime in which Earth-orbiting satellites move -- are also globally hyperbolic and Lorentzian, and thus we only need to keep superluminality at bay to guarantee normal causality. In short: our usual idea of causality comes from being able to forbid a certain class of trajectories, and in physically realistic spacetimes (i.e., ones which approximately match parts of our observed universe), those are all as far as we can tell free from things moving spacelike (i.e., nothing's faster than a massless particle, and light is made up of massless particles, so nothing is superluminal). Taken the other way around: our local observations of causal interactions (both locally, like in laboratories, and astronomically) are strong evidence in favour of us inhabiting a manifold equipped with the requisite features [1][2]. Alternative explanations have generally either failed in some way (self-inconsistency, obvious incompleteness, or inconsistency with repeatable observations) or been more complicated than, but reduces to, General Relativity in some limit. None of the second variety of alternatives has yet to be demanded by actual observations that cannot be explained using General Relativity as the underlying spacetime theory. - -- [1] https://en.wikipedia.org/wiki/Pseudo-Riemannian_manifold#Lorentzian_manifold https://en.wikipedia.org/wiki/Pseudo-Riemannian_manifold#Lor... [2] https://en.wikipedia.org/wiki/Globally_hyperbolic_manifold https://en.wikipedia.org/wiki/Globally_hyperbolic_manifold [3] https://en.wikipedia.org/wiki/Causal_structure https://en.wikipedia.org/wiki/Causal_structure
lisper 7y ago
[EDIT] Everyone please ignore this comment. It is totally wrong. > If what we’re seeing was 13.8B light years away 13.8B years ago and are now 46B light years away, wouldn’t that mean that those things traveled ~32.2B light years in 13.8B years. No. What we're seeing was 13.8BLY away (in our reference frame) when the light was emitted. Those things are 46BLY away now (again, in our reference frame) but we can't see them now, we can only see them as they were 138B years ago.
- saagarjha 7y ago
  Just because we can’t see them now doesn’t mean they didn’t move…
- cygx 7y ago
  Still not quite right: 13.8 Gly is the light travel distance, whereas proper distance at time of emission was less than that. For example, for redshift z=9, light travel time is 13.17 Gy, proper distance at time of absorption (aka comoving distance) is 30.72 Gly and proper distance at time of emission (which, in a spatially flat universe, corresponds to angular diameter distance) is 3.07 Gly. The factor of 10 between these two distances is no coincidence as the ratio of the scale factor is given by 1+z. ( The values have been calculated via http://www.astro.ucla.edu/~wright/CosmoCalc.html http://www.astro.ucla.edu/~wright/CosmoCalc.html )
- nvrspyx 7y ago
  I'm sorry, but that's exactly what I said unless I'm misunderstanding your point. I said what we're seeing was 13.8B light years away 13.8B years ago, since it took 13.8B years for the light to reach us from the point it was emitted. I never said we could see now where they currently are. What I failed to account for was the space expansion during the time that the light was traveling meaning that the light had to travel further to reach us than the point of emission actually was when it emitted. So really, it wasn't 13.8B years ago; it was emitted later. I don't know by how much, but it seems like maybe the other commenter does.
  - lisper 7y ago
csomar 7y ago
My understanding is that space is expanding faster than the speed of light. I don't recall where I read it, though.
- saagarjha 7y ago
  https://en.wikipedia.org/wiki/Hubble_volume https://en.wikipedia.org/wiki/Hubble_volume
manigandham 7y ago
Things moving in space/time are limited to the speed of light. Space itself isn't. It's like a blowing up a balloon. There's no limit to how big it gets or how fast it grows, so objects on opposing sides can move apart at greater than the speed of light even if they're not moving themselves. This rate of expansion is accelerating too, driven by something called "dark" (as in unknown) energy which is overpowering all the gravity that should be pulling things together. It's fascinating to think that in 100s of billions of years, our local galaxy will be the only thing we can see in space. Any civilizations then would believe that there's nothing else out there.

amelius 7y ago

I didn't read the article but I suspect the reason is that space is expanding. If space wouldn't have expanded sufficiently fast in the beginning of time, all matter would be still clumped together as in a giant black hole. The fact that space expanded "faster" than the speed of light allowed this black hole to become free matter.

recursivecaveat 7y ago

In essence, it's 'okay' for things to be FTL due to expansion of the medium. Imagine an ant crawling across a balloon as we stretch it. The ant could get some pretty fantastic apparent speeds relative to another ant because the medium is expanding, but step by step its moving at regular ant speed.

cellular 7y ago
It is interesting to think of the opposite: space fabric is shrinking, so light needs to travel "slower" so as not to break causality. I know I know, it's not really slower because the fabric is shrinking.
- badrabbit 7y ago
  But there is non space "fabric". There are interactions between qanta/matter but space is still space right?
- lonelappde 7y ago
  It doesn't "need to", it just does. Causality is defined by the speed of light

Zenst 7y ago

Interesting ratio of 1:3.3141210374639769452449567723343 not exactly Pi. But then that expansion ratio may of been different over time, but it is a bit like sampling the second hand on a clock in a photo and working out the entire motion and workings of the clock - at least on the scale of the data/measurements we have and the age of the universe. Currently we are still not 100% sure when midnight was, but as always moving in the right direction.

Retric 7y ago
We have more recent data on closer objects. It’s a sequence of thin shells that expands as you go backward in time, or a 3D light cone aimed backward in 4D space time. https://en.m.wikipedia.org/wiki/Light_cone https://en.m.wikipedia.org/wiki/Light_cone

rhinokungfu 7y ago

Some of the confusion is, I think from referring to speed of light as "speed of light". When it really is speed limit of causality between 2 points in space. And light (em waves) just happen to be able to hit that limit. Now since expansion of space is itself not going to allow one point in space to cause any effect on another, it (the expansion) can be faster than speed of causality. PBS space time has a good video on this - https://www.youtube.com/watch?v=msVuCEs8Ydo https://www.youtube.com/watch?v=msVuCEs8Ydo

nuccy 7y ago
It is also peculiar that the energy conservation law is not applicable on cosmological scales, the light is redshifted due to expansion of the Universe and the energy difference between emitted and received light is "wasted", i.e. it doesn't feed the expansion in any way.
- cygx 7y ago
  I don't know if 'feeding expansion' is necessarily the right way to think about this, but by the first Friedmann equation, loss of photon energy will in fact be balanced by the increased expansion rate...
  - nuccy 7y ago
    Sure, though the first Friedmann equation, or any other equation, where terms are balanced, e.g. a + b = 0, doesn't tell you anything about causality, though redshifting of light is exactly caused by expansion, so it is not strange that both are "balanced".

cortesoft 7y ago

Yeah, the intuitive view of universe expansion is that it expands from the edges out.... but in reality, it expands everywhere at once, so really the space between us is expanding as well.

harshreality 7y ago

FTL travel of anything (matter, photons, EM fields, gravitational fields) would result in potential causality issues. I don't know if there are models of universes where GR doesn't apply; that would be interesting, but the causality problems might make such a universe uninteresting even if it were possible. I suppose that's using the anthropic principle as justification for GR? Metric expansion does not result in causality issues because it's an isolating phenomenon (one that slows down interactions between things) rather than an interconnecting phenomenon (one that would speed up interactions between things). Therefore it's at least possible, and observations (which you can read a brief summary of in TFA) back it up.

raattgift 7y ago
> Metric expansion does not result in causality issues because it's an isolating phenomenon No, in the specific case of the concordance cosmology (and more general expanding cases such as an expanding Robertson-Walker spacetime, or a 4-dimensional de Sitter spacetime), the manifold is still Lorentzian [1] and globally hyperbolic [2]. These features let one sort trajectories into spacelike, null, and timelike. If spacelike trajectories are forbidden to interacting matter and gravitational waves, then we get our usual idea of causality [3]. Collapsing Robertson-Walker spacetimes and anti-de Sitter spacetimes are defined on globally hyperbolic Lorentzian manifolds. They are defined as free of matter and gravitational waves. However we can perturb them slighly by adding a bit of matter, even matter that generates small amounts of gravitational radiation, and as long as that matter and radiation moves only on timelike trajectories or lightlike trajectories, we have the normal causality even though the interactions among separated bits of normal matter perturbing these collapsing spacetimes will generally become more frequent and stronger over time. Indeed, this remains the case even for very highly accelerated compactions, where at late times distances between objects contract faster than light between the same objects can move. Other commonly encountered spacetimes -- the flat spacetime of Minkowski approximately describes laboratories on Earth and in the International Space Station, and the (exterior) Schwarzschild spacetime which approximately describes the spacetime in which Earth-orbiting satellites move -- are also globally hyperbolic and Lorentzian, and thus we only need to keep superluminality at bay to guarantee normal causality. In short: our usual idea of causality comes from being able to forbid a certain class of trajectories, and in physically realistic spacetimes (i.e., ones which approximately match parts of our observed universe), those are all as far as we can tell free from things moving spacelike (i.e., nothing's faster than a massless particle, and light is made up of massless particles, so nothing is superluminal). Taken the other way around: our local observations of causal interactions (both locally, like in laboratories, and astronomically) are strong evidence in favour of us inhabiting a manifold equipped with the requisite features [1][2]. Alternative explanations have generally either failed in some way (self-inconsistency, obvious incompleteness, or inconsistency with repeatable observations) or been more complicated than, but reduces to, General Relativity in some limit. None of the second variety of alternatives has yet to be demanded by actual observations that cannot be explained using General Relativity as the underlying spacetime theory. - -- [1] https://en.wikipedia.org/wiki/Pseudo-Riemannian_manifold#Lorentzian_manifold https://en.wikipedia.org/wiki/Pseudo-Riemannian_manifold#Lor... [2] https://en.wikipedia.org/wiki/Globally_hyperbolic_manifold https://en.wikipedia.org/wiki/Globally_hyperbolic_manifold [3] https://en.wikipedia.org/wiki/Causal_structure https://en.wikipedia.org/wiki/Causal_structure

lisper 7y ago

[EDIT] Everyone please ignore this comment. It is totally wrong. > If what we’re seeing was 13.8B light years away 13.8B years ago and are now 46B light years away, wouldn’t that mean that those things traveled ~32.2B light years in 13.8B years. No. What we're seeing was 13.8BLY away (in our reference frame) when the light was emitted. Those things are 46BLY away now (again, in our reference frame) but we can't see them now, we can only see them as they were 138B years ago.

saagarjha 7y ago
Just because we can’t see them now doesn’t mean they didn’t move…
cygx 7y ago
Still not quite right: 13.8 Gly is the light travel distance, whereas proper distance at time of emission was less than that. For example, for redshift z=9, light travel time is 13.17 Gy, proper distance at time of absorption (aka comoving distance) is 30.72 Gly and proper distance at time of emission (which, in a spatially flat universe, corresponds to angular diameter distance) is 3.07 Gly. The factor of 10 between these two distances is no coincidence as the ratio of the scale factor is given by 1+z. ( The values have been calculated via http://www.astro.ucla.edu/~wright/CosmoCalc.html http://www.astro.ucla.edu/~wright/CosmoCalc.html )
nvrspyx 7y ago
I'm sorry, but that's exactly what I said unless I'm misunderstanding your point. I said what we're seeing was 13.8B light years away 13.8B years ago, since it took 13.8B years for the light to reach us from the point it was emitted. I never said we could see now where they currently are. What I failed to account for was the space expansion during the time that the light was traveling meaning that the light had to travel further to reach us than the point of emission actually was when it emitted. So really, it wasn't 13.8B years ago; it was emitted later. I don't know by how much, but it seems like maybe the other commenter does.
- lisper 7y ago

csomar 7y ago

My understanding is that space is expanding faster than the speed of light. I don't recall where I read it, though.

saagarjha 7y ago
https://en.wikipedia.org/wiki/Hubble_volume https://en.wikipedia.org/wiki/Hubble_volume

manigandham 7y ago

Things moving in space/time are limited to the speed of light. Space itself isn't. It's like a blowing up a balloon. There's no limit to how big it gets or how fast it grows, so objects on opposing sides can move apart at greater than the speed of light even if they're not moving themselves. This rate of expansion is accelerating too, driven by something called "dark" (as in unknown) energy which is overpowering all the gravity that should be pulling things together. It's fascinating to think that in 100s of billions of years, our local galaxy will be the only thing we can see in space. Any civilizations then would believe that there's nothing else out there.