3 ms·

how can universe be expanding faster than speed of light? general relativity doesn't apply here?

goodluckchuck 7y ago
Idk, but if we travel 13B light years one way, it moves 13B the other then were at 26, and space itself has to have expanded 20 billion light years within the time it takes light to travel 13... So yeah, sounds like something is moving faster than light.
- tialaramex 7y ago
  Things can't move faster than light, that isn't possible, but we can think about abstract concepts that aren't things. Fairness for example. Infinity. These are not things. Expanding space isn't a thing, it's just a concept. So that can go however fast you want. Let's try something much smaller and easier to reason about. Take a small fairly directional lamp (in my country we'd say a "torch" but words vary) and hold up a coin in front of it so that it makes a big obvious shadow on a far away wall, you may need to turn down ambient lighting. Relatively small movements of the coin result in the shadow moving quickly. Huh. How fast can that shadow move? While the coin is a _thing_ the shadow is not, it's just your imagination processing the absence of light. If you have a big enough room and a strong enough light source, you can do this trick so that a flick of the wrist appears to move the shadow much faster than the speed of sound. If a _thing_ did that in a room filled with breathable air you'd hear a sonic boom. But the shadow isn't a thing, it exists only in your imagination and so you hear nothing. Large enough objects can occlude a star to have this effect on a cosmic scale. The shadow of a planet can pass over the face of another planet at faster than the speed of light. But no actual _thing_ moved, so no rules are broken by this happening, "faster than the speed of light" was only in your head, all the actual things (including the photons) obeyed the rules and stayed at or below the speed limit.
cygx 7y ago
It's precisely because general relativity applies that this is possible. First, note that relative velocity and growth of distance between objects are distinct concepts: Shoot two bullets at 0.9c each in opposite directions, and from your perspective, the distance between them will grow at a rate of 1.8c, whereas their relative velocity (as computed from the 'angle' between their 4-velocities) will only be 0.994c. In special relativity, that relative velocity will correspond to the growth of distance between the bullets as measured by an observer comoving with either bullet. In general relativity, things are more complicated: Curved spaces lack distance parallelism, so you can only compare the velocity vectors of the two bullets when they are located in the same place at the same time. Also, there's generally no longer a canonical way to decompose spacetime into spatial slices across which you can measure distances. Now, you can still define a relative velocity through parallel transport along a path connecting the two objects, and in particular the path of a photon emitted by a distant galaxy and observed by us. This will in fact recover cosmological redshift as a generalized Doppler shift. If defined this way, the relative velocity goes to c as you approach the cosmic event horizon, hence you'll never observe relative velocities larger than c. However, while there's no canonical spatial slicing in general relativity, in Friedmann cosmology specifically, a preferred spatial slicing does exist: The one where the universe looks homogeneous and isotropic and the same amount of time will have passed since the big bang (as measured by an observer following the Hubble flow). This allows us to define the proper distance (at constant cosmological time) between astronomical objects, and the growth of this distance is what's known as 'recession velocities'. These velocities go to c at the Hubble sphere, which doesn't really have any particular significance: We can see galaxies that were located outside the Hubble sphere just fine (though as time goes on, the Hubble sphere will approach the cosmic horizon asymptotically).
Freestyler_3 7y ago
Space is expanding everywhere and not just around the edges. Space inside the edges has photons flying in it, while this photon travels, space is expanding in all of the photons directions. This would mean that a photon from 5 billion years ago would take longer to get here, and its source is even further away than the speed between us and the source would make it be. What I think about during all this: If space is expanding everywhere, then it is sort of pushing things away from each other, meaning that objects are moved, like domino's everything is moved a bit so that everything has more space in between. Do I have to separate expansion of the universe vs expansion of space?

Idk, but if we travel 13B light years one way, it moves 13B the other then were at 26, and space itself has to have expanded 20 billion light years within the time it takes light to travel 13... So yeah, sounds like something is moving faster than light.

tialaramex 7y ago
Things can't move faster than light, that isn't possible, but we can think about abstract concepts that aren't things. Fairness for example. Infinity. These are not things. Expanding space isn't a thing, it's just a concept. So that can go however fast you want. Let's try something much smaller and easier to reason about. Take a small fairly directional lamp (in my country we'd say a "torch" but words vary) and hold up a coin in front of it so that it makes a big obvious shadow on a far away wall, you may need to turn down ambient lighting. Relatively small movements of the coin result in the shadow moving quickly. Huh. How fast can that shadow move? While the coin is a _thing_ the shadow is not, it's just your imagination processing the absence of light. If you have a big enough room and a strong enough light source, you can do this trick so that a flick of the wrist appears to move the shadow much faster than the speed of sound. If a _thing_ did that in a room filled with breathable air you'd hear a sonic boom. But the shadow isn't a thing, it exists only in your imagination and so you hear nothing. Large enough objects can occlude a star to have this effect on a cosmic scale. The shadow of a planet can pass over the face of another planet at faster than the speed of light. But no actual _thing_ moved, so no rules are broken by this happening, "faster than the speed of light" was only in your head, all the actual things (including the photons) obeyed the rules and stayed at or below the speed limit.

cygx 7y ago

It's precisely because general relativity applies that this is possible. First, note that relative velocity and growth of distance between objects are distinct concepts: Shoot two bullets at 0.9c each in opposite directions, and from your perspective, the distance between them will grow at a rate of 1.8c, whereas their relative velocity (as computed from the 'angle' between their 4-velocities) will only be 0.994c. In special relativity, that relative velocity will correspond to the growth of distance between the bullets as measured by an observer comoving with either bullet. In general relativity, things are more complicated: Curved spaces lack distance parallelism, so you can only compare the velocity vectors of the two bullets when they are located in the same place at the same time. Also, there's generally no longer a canonical way to decompose spacetime into spatial slices across which you can measure distances. Now, you can still define a relative velocity through parallel transport along a path connecting the two objects, and in particular the path of a photon emitted by a distant galaxy and observed by us. This will in fact recover cosmological redshift as a generalized Doppler shift. If defined this way, the relative velocity goes to c as you approach the cosmic event horizon, hence you'll never observe relative velocities larger than c. However, while there's no canonical spatial slicing in general relativity, in Friedmann cosmology specifically, a preferred spatial slicing does exist: The one where the universe looks homogeneous and isotropic and the same amount of time will have passed since the big bang (as measured by an observer following the Hubble flow). This allows us to define the proper distance (at constant cosmological time) between astronomical objects, and the growth of this distance is what's known as 'recession velocities'. These velocities go to c at the Hubble sphere, which doesn't really have any particular significance: We can see galaxies that were located outside the Hubble sphere just fine (though as time goes on, the Hubble sphere will approach the cosmic horizon asymptotically).

Freestyler_3 7y ago

Space is expanding everywhere and not just around the edges. Space inside the edges has photons flying in it, while this photon travels, space is expanding in all of the photons directions. This would mean that a photon from 5 billion years ago would take longer to get here, and its source is even further away than the speed between us and the source would make it be. What I think about during all this: If space is expanding everywhere, then it is sort of pushing things away from each other, meaning that objects are moved, like domino's everything is moved a bit so that everything has more space in between. Do I have to separate expansion of the universe vs expansion of space?