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> As a result, there could be empty regions of space in which billions of years more have elapsed If they are empty, those billion years didn't happen. But not

> As a result, there could be empty regions of space in which billions of years more have elapsed If they are empty, those billion years didn't happen. But nothing is really empty, right?

ben_w 2y ago
Assuming I correctly understood the argument in the link: Even if the space was truly empty, the expansion of that space would have gone on for longer, and thus things on opposite sides would eventually notice they were more distant. But also yes the space isn't really totally empty.
nine_k 2y ago
The higher the density, the more curved is the spacetime at that area, and the slower is the passage of time. You don't have to go to extremes like black holes vs absolute vacuum. A sufficient difference should be visible between regions closer to centers of galaxies, or just clusters of nearby galaxies, and really large "voids" between them, which contain some matter, and even some stars, but are vastly more empty. This is what the article explores. (This connects in a funny way to Vernor Vinge's SF idea of slower and faster areas of space. The "high" / "fast" space is mostly empty, so the time passes there faster than in the "unthinking depths" around galactic cores, and hugely more progress is done by civilizations in the "fast" space, as observed from the "slow" space.)
- skirmish 2y ago
  > Vernor Vinge's SF idea of slower and faster areas of space His ideas were more ambitious: there are advanced technologies (e.g. FTL travel) that work in "fast" space but completely stop working in the "slow zone" (where the Solar system is located). On the other hand, even human-level intelligence would stop functioning close to the galactic center, the crew would not be able to operate the ship and would be stranded.
  - lazide 2y ago
    Also, if I remember correctly wasn’t it implied that this was constructed/controllable a bit, and was due to some kind of ‘nature preserve’ from a super alien race? I might be misremembering.
pdonis 2y ago
> If they are empty, those billion years didn't happen. No, that's not correct. Here's a better way to look at it: In our cosmological models, we "slice up" the spacetime of the universe into slices of "space at a constant time"--each slice is like a "snapshot" of the space of the entire universe at a single instant of "cosmological time". The models, which assume homogeneity and isotropy, assume that the actual elapsed proper time at every point in space in each "snapshot" is the same--in other words, that "cosmologcal time" is also proper time for comoving observers everywhere in space at that instant of cosmological time--the time actually elapsed since the Big Bang on a clock moving with each observer. What these supernova papers are examining is the possibility that "cosmological time" and proper time (clock time) for comoving observers do not always match: roughly speaking, in areas with higher mass concentration (galaxy clusters), proper time lags behind cosmological time (the time we use in the math to label each "snapshot" slice of the space of the universe), and in areas with lower mass concentration (voids), proper time runs ahead of cosmological time. The idea is that this mismatch between proper time and cosmological time can be significant enough to affect the inferences we should be drawing from the supernova observations about the expansion history of the universe. As far as I know the jury is still out on all this; claims by proponents that what is presented in these papers is already sufficient to require "foundational change" are, I think, premature. But it is certainly a line of research that is worth pursuing.
- le-mark 2y ago
  As a layman, what I don’t get is; the speed of light is constant, so wouldn’t that nullify any time/space fluctuations due to lack of mass/gravity?
  - deleted 2y ago
    [deleted]
  - pdonis 2y ago
    > the speed of light is constant That's not a good way of describing the actual law in a curved spacetime, i.e., a spacetime that contains gravitating masses. In such a spacetime there is no single global definition of "speed"; you can't compare speeds at spatially separated points. A better way to state the law is that the light cone structure of the spacetime constrains the motion of all bodies: timelike bodies move within the light cones, lightlike bodies (like light itself) move exactly on the light cones. But once you state it that way, it becomes obvious that this law does not impose any constraints on "time/space fluctuations".

> As a result, there could be empty regions of space in which billions of years more have elapsed If they are empty, those billion years didn't happen. But nothing is really empty, right?

ben_w 2y ago

Assuming I correctly understood the argument in the link: Even if the space was truly empty, the expansion of that space would have gone on for longer, and thus things on opposite sides would eventually notice they were more distant. But also yes the space isn't really totally empty.

nine_k 2y ago

The higher the density, the more curved is the spacetime at that area, and the slower is the passage of time. You don't have to go to extremes like black holes vs absolute vacuum. A sufficient difference should be visible between regions closer to centers of galaxies, or just clusters of nearby galaxies, and really large "voids" between them, which contain some matter, and even some stars, but are vastly more empty. This is what the article explores. (This connects in a funny way to Vernor Vinge's SF idea of slower and faster areas of space. The "high" / "fast" space is mostly empty, so the time passes there faster than in the "unthinking depths" around galactic cores, and hugely more progress is done by civilizations in the "fast" space, as observed from the "slow" space.)

skirmish 2y ago
> Vernor Vinge's SF idea of slower and faster areas of space His ideas were more ambitious: there are advanced technologies (e.g. FTL travel) that work in "fast" space but completely stop working in the "slow zone" (where the Solar system is located). On the other hand, even human-level intelligence would stop functioning close to the galactic center, the crew would not be able to operate the ship and would be stranded.
- lazide 2y ago
  Also, if I remember correctly wasn’t it implied that this was constructed/controllable a bit, and was due to some kind of ‘nature preserve’ from a super alien race? I might be misremembering.

pdonis 2y ago

> If they are empty, those billion years didn't happen. No, that's not correct. Here's a better way to look at it: In our cosmological models, we "slice up" the spacetime of the universe into slices of "space at a constant time"--each slice is like a "snapshot" of the space of the entire universe at a single instant of "cosmological time". The models, which assume homogeneity and isotropy, assume that the actual elapsed proper time at every point in space in each "snapshot" is the same--in other words, that "cosmologcal time" is also proper time for comoving observers everywhere in space at that instant of cosmological time--the time actually elapsed since the Big Bang on a clock moving with each observer. What these supernova papers are examining is the possibility that "cosmological time" and proper time (clock time) for comoving observers do not always match: roughly speaking, in areas with higher mass concentration (galaxy clusters), proper time lags behind cosmological time (the time we use in the math to label each "snapshot" slice of the space of the universe), and in areas with lower mass concentration (voids), proper time runs ahead of cosmological time. The idea is that this mismatch between proper time and cosmological time can be significant enough to affect the inferences we should be drawing from the supernova observations about the expansion history of the universe. As far as I know the jury is still out on all this; claims by proponents that what is presented in these papers is already sufficient to require "foundational change" are, I think, premature. But it is certainly a line of research that is worth pursuing.

le-mark 2y ago
As a layman, what I don’t get is; the speed of light is constant, so wouldn’t that nullify any time/space fluctuations due to lack of mass/gravity?
- deleted 2y ago
  [deleted]
- pdonis 2y ago
  > the speed of light is constant That's not a good way of describing the actual law in a curved spacetime, i.e., a spacetime that contains gravitating masses. In such a spacetime there is no single global definition of "speed"; you can't compare speeds at spatially separated points. A better way to state the law is that the light cone structure of the spacetime constrains the motion of all bodies: timelike bodies move within the light cones, lightlike bodies (like light itself) move exactly on the light cones. But once you state it that way, it becomes obvious that this law does not impose any constraints on "time/space fluctuations".