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I'm not an expert, but I don't think that quote is quite right. There is still a notion of time, there is just no notion of a direction of time. For example, if

I'm not an expert, but I don't think that quote is quite right. There is still a notion of time, there is just no notion of a direction of time. For example, if you go deep into empty space, then one patch of space looks just like the next one. That doesn't mean there is no longer a physical concept of space. Similarly, just because one point in time doesn't look very different from the next, doesn't mean that there isn't a notion of time. The main point of thermodynamic time is to describe the idea that time appears to be moving forward. According to fundamental physics, as far as we can tell, time is symmetric (with a few caveats). So going forward in time should look similar to going backwards in time, just like going to the left in deep space is the same as going to the right. But the fact that we are in a pocket of high entropy means that time has an apparent direction.

supergarfield 7y ago
Speaking of this, does anyone know of an explanation for why the direction in spacetime in which entropy decreases is parallel to the time dimension (the one with a different sign in the relativistic metric) of spacetime? Maybe I'm missing something, but I've never seen a link made between the two.
- jbay808 7y ago
  As opposed to entropy decreasing as you travel, say, toward the galactic north? It's a good question. One answer could be the mostly spatially uniform initial conditions of the big bang. Also, a spatial entropy gradient would gradually diffuse over time to become uniform again. But in general, the arrow of time is a consequence of systems that store a memory of their pasts, which is possible because the past is lower entropy than the future. (The past can be more accurately predicted by querying the state of the data in memory than the future can). You could probably set up a carefully designed system with a spatial entropy gradient, and somehow get a memory where the arrangement of atoms on the left always "remembers" the arrangement of atoms on the right, but not the other way around. Maybe there are even systems like this found in nature? Interesting to think about.
  - empath75 7y ago
    Isn’t a black hole a spatial entropy gradient?
    - jbay808 7y ago
      That's a pretty difficult case to reason about. But setting aside the time warping effects, I think a black hole's entropy would be too high to support any kind of memory-bearing structure in its vicinity.
  - supergarfield 7y ago
    Thanks for the answer. I mean exactly that! Thoughts on your thoughts: > One answer could be the mostly spatially uniform initial conditions of the big bang. That's fair, but in that case, to continue the galactic north analogy, is there a reason why the Big Bang couldn't have happened uniformly in the dimension-3 hyperplane of (time, plane of the milky way) instead of the space hyperplane? > a spatial entropy gradient would gradually diffuse over time to become uniform again. But wouldn't that be "diffuse over direction-of-decreasing entropy time", not "diffuse over relativistic-time-dimension time"? In that case, I don't think it would help. Apologies if I'm missing your points or not making sense, I can't claim I understand any of this well. I just feel like I see a lot of circular arguments around this :).
    - jbay808 7y ago
      > is there a reason why the Big Bang couldn't have happened uniformly in the dimension-3 hyperplane It's a really good question. I think it has to do with how space expanded after the big bang (which is to me still a mysterious subject). To avoid confusion, let me call "time" the arrow of time as perceived from within a system, and "TIME" the coordinate axis of spacetime that has the opposite sign from the other three. The early days of the universe were full of heat and radiation everywhere throughout space, which seems like a high entropy scenario for sure. But entropy was able to increase as the universe cooled because space itself expanded adiabatically (and so the configuration space became larger). In other words, we end up with an entropy gradient that points along the TIME axis because the volume of space expands along that axis. Like space-time is shaped like a pyramid rather than a cube, with the pointy axis in the TIME direction. I don't know enough general relativity to say for sure, but I expect that that pyramid shape (expanding towards later times) is probably a consequence of TIME behaving differently than other spatial dimensions. If spacetime shrank volumetrically toward the galactic north, in a fairly uniform way throughout all times and throughout the universe, then we'd have a different conclusion. (This might seem like the case near a black hole). I guess another thing to remember is that TIME and space really are different things. For example, conservation of angular momentum means the universe has constant angular momentum at all times, but not necessarily in all directions of space.
- Retra 7y ago
  Low entropy systems are likely to transition into high entropy systems, and the reverse is unlikely to occur. It is largely a statistical argument; low entropy states are by definition less likely to occur by chance than high entropy states. (You might then ask why we care what happens 'by chance'; the answer is either that physics is fundamentally unpredictable, or that the scale of the universe so large that the influence of the whole on any particular subsystem must necessarily be modeled with noise.)
- obastani 7y ago
  I'm not sure, and this is a lot of speculation, but I feel like it has to do with the fact that mass/energy is conserved over time (but not in space) [1]. The mass now corresponds to the mass in the past/future, which, for example, allows memories to form. In any case, time is still a qualitatively different dimension than space. [1] https://en.m.wikipedia.org/wiki/Conservation_law#Exact_laws https://en.m.wikipedia.org/wiki/Conservation_law#Exact_laws
aassddffasdf 7y ago
We are in a pocket of high order/ low entropy (not high entropy).
- obastani 7y ago
  That's what I meant -- thanks for the catch!

supergarfield 7y ago

Speaking of this, does anyone know of an explanation for why the direction in spacetime in which entropy decreases is parallel to the time dimension (the one with a different sign in the relativistic metric) of spacetime? Maybe I'm missing something, but I've never seen a link made between the two.

jbay808 7y ago
As opposed to entropy decreasing as you travel, say, toward the galactic north? It's a good question. One answer could be the mostly spatially uniform initial conditions of the big bang. Also, a spatial entropy gradient would gradually diffuse over time to become uniform again. But in general, the arrow of time is a consequence of systems that store a memory of their pasts, which is possible because the past is lower entropy than the future. (The past can be more accurately predicted by querying the state of the data in memory than the future can). You could probably set up a carefully designed system with a spatial entropy gradient, and somehow get a memory where the arrangement of atoms on the left always "remembers" the arrangement of atoms on the right, but not the other way around. Maybe there are even systems like this found in nature? Interesting to think about.
- empath75 7y ago
  Isn’t a black hole a spatial entropy gradient?
  - jbay808 7y ago
    That's a pretty difficult case to reason about. But setting aside the time warping effects, I think a black hole's entropy would be too high to support any kind of memory-bearing structure in its vicinity.
- supergarfield 7y ago
  Thanks for the answer. I mean exactly that! Thoughts on your thoughts: > One answer could be the mostly spatially uniform initial conditions of the big bang. That's fair, but in that case, to continue the galactic north analogy, is there a reason why the Big Bang couldn't have happened uniformly in the dimension-3 hyperplane of (time, plane of the milky way) instead of the space hyperplane? > a spatial entropy gradient would gradually diffuse over time to become uniform again. But wouldn't that be "diffuse over direction-of-decreasing entropy time", not "diffuse over relativistic-time-dimension time"? In that case, I don't think it would help. Apologies if I'm missing your points or not making sense, I can't claim I understand any of this well. I just feel like I see a lot of circular arguments around this :).
  - jbay808 7y ago
    > is there a reason why the Big Bang couldn't have happened uniformly in the dimension-3 hyperplane It's a really good question. I think it has to do with how space expanded after the big bang (which is to me still a mysterious subject). To avoid confusion, let me call "time" the arrow of time as perceived from within a system, and "TIME" the coordinate axis of spacetime that has the opposite sign from the other three. The early days of the universe were full of heat and radiation everywhere throughout space, which seems like a high entropy scenario for sure. But entropy was able to increase as the universe cooled because space itself expanded adiabatically (and so the configuration space became larger). In other words, we end up with an entropy gradient that points along the TIME axis because the volume of space expands along that axis. Like space-time is shaped like a pyramid rather than a cube, with the pointy axis in the TIME direction. I don't know enough general relativity to say for sure, but I expect that that pyramid shape (expanding towards later times) is probably a consequence of TIME behaving differently than other spatial dimensions. If spacetime shrank volumetrically toward the galactic north, in a fairly uniform way throughout all times and throughout the universe, then we'd have a different conclusion. (This might seem like the case near a black hole). I guess another thing to remember is that TIME and space really are different things. For example, conservation of angular momentum means the universe has constant angular momentum at all times, but not necessarily in all directions of space.
Retra 7y ago
Low entropy systems are likely to transition into high entropy systems, and the reverse is unlikely to occur. It is largely a statistical argument; low entropy states are by definition less likely to occur by chance than high entropy states. (You might then ask why we care what happens 'by chance'; the answer is either that physics is fundamentally unpredictable, or that the scale of the universe so large that the influence of the whole on any particular subsystem must necessarily be modeled with noise.)
obastani 7y ago
I'm not sure, and this is a lot of speculation, but I feel like it has to do with the fact that mass/energy is conserved over time (but not in space) [1]. The mass now corresponds to the mass in the past/future, which, for example, allows memories to form. In any case, time is still a qualitatively different dimension than space. [1] https://en.m.wikipedia.org/wiki/Conservation_law#Exact_laws https://en.m.wikipedia.org/wiki/Conservation_law#Exact_laws

aassddffasdf 7y ago

We are in a pocket of high order/ low entropy (not high entropy).

obastani 7y ago
That's what I meant -- thanks for the catch!