Timeline for Can space expand with unlimited speed?
Current License: CC BY-SA 3.0
10 events
| when toggle format | what | by | license | comment | |
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| Feb 8, 2021 at 12:55 | comment | added | Edouard | Because, in his 1916 pop.-sci. book about GR, Einstein explicitly mentions the possibility of velocities faster than that speed-of-light-in-vacuum that we observe, I'd for some time realized that the velocity of light can be greater in causally-separated regions, but, partly in view of Lubos Motl's answer, I've since realized that it can differ between any pair of inertial frames of reference, regardless of whether there's a causal separation totally preventing their influence upon each other: Consequently, I've had to upvote both his answer and Pulsar's, for entirely different reasons. | |
| May 21, 2019 at 22:50 | comment | added | Henry | Try en.wikipedia.org/wiki/Ant_on_a_rubber_rope for a simple non-relativistic example where fast expansion of space does not prevent slow travel between two points | |
| Dec 13, 2013 at 22:15 | comment | added | Thriveth | Still, photons can travel between galaxies that are receding from each other faster than light already at the time of emission; see the Davis & Lineweaver paper linked by @Pulsar below. By the way, impressingly, Davis wrote this paper as part of her Ph.D. | |
| Jul 7, 2013 at 5:57 | comment | added | Luboš Motl | Dear Ben, maybe I am not using the "most common definition of the speed" in a curved spacetime, but the statement of mine is surely correct for the "most natural definition of the speed" in a curved spacetime for any comparison with special relativity. | |
| May 8, 2013 at 12:22 | comment | added | user4552 | "the fact that the mutual speed exceeds the speed of light - although it's not really well-defined - means that they can't see each other or otherwise communicate" Yes, it's not uniquely defined, and that's the fundamental answer to the question. However, for the most common cosmological definition of the speed, the remainder of this sentence is false. See arxiv.org/abs/astro-ph/0310808 . | |
| Apr 14, 2013 at 18:41 | vote | accept | cziko | ||
| Apr 14, 2013 at 18:41 | vote | accept | cziko | ||
| Apr 14, 2013 at 18:41 | |||||
| Apr 9, 2013 at 19:05 | comment | added | Luboš Motl | Dear Jim, yes, as I said, the fact that the mutual speed exceeds the speed of light - although it's not really well-defined - means that they can't see each other or otherwise communicate. Otherwise, they also can't send signals to each other gravitationally - gravitational signals propagate by the speed of light, too. However, both faraway galaxies still feel the gravitational field - but in general relativity, gravity is given by the local curvature of the spacetime which is there regardless of what the other distant galaxy is doing right now. | |
| Apr 9, 2013 at 13:46 | comment | added | Force | @Motl Then one can assume that sending messages between such galaxies is impossible, since they are receding from each other faster than the speed of light? How then they still affect each other by gravity, which has the speed of light? | |
| Apr 9, 2013 at 11:07 | history | answered | Luboš Motl | CC BY-SA 3.0 |