4 ms·

I don't see why you need QM for that. In the classical GR picture, at a BH (black hole) event horizon there are no timelike worldlines that leave the BH; insid

I don't see why you need QM for that. In the classical GR picture, at a BH (black hole) event horizon there are no timelike worldlines that leave the BH; inside the horizon there are no lightlike ones that leave the BH either. All worldlines are thus directionally constrained and any object can only march further inwards towards the gravitational singularity. This is a feature of a BH metric. It isn't that much of a stretch to have a classical BB (big bang) metric constrain all worldlines to march outwards from the gravitational singularity. Shazam: the arrow of time is then set by global geometry. QM isn't even needed for half of the BH information loss problem to manifest itself: if you drop a spherical shell of mass M into a Schwarzschild BH or two spherical shells each of mass M/2 into a Schwarzschild BH, at late times who can know this? In the classical case, no BH evaporation means no information lost (it's just locked up somewhere else); but if we allow complete evaporation then we have to have stored that information somewhere. Non-evaporating growing black hole has a clear arrow-of-time due to global asymmetry; evaporating black hole formed by collapse should in principle be fully time-reversible. QM just sharpens the picture by providing a mechanism for evaporation, bringing in unitarity and other features, and letting us drop in collections of particles into arbitrary black holes such that a no-hair black hole can hide an awful lot of quantum numbers inside. It also gives us hope for a structure that replaces the gravitational singularity. Likewise in the BB case, we'd like to avoid marching back to a gravitational singularity so we can bring in a similar QM structure to replace it (although you can do that with "throat" models fully classically, see slide 17 at https://www.slideshare.net/seanmcarroll/what-we-dont-know-about-the-beginning-of-the-universe https://www.slideshare.net/seanmcarroll/what-we-dont-know-ab... and refs therein). Similarly, we expect there to be an enormous number of quantum numbers in the hot dense phase of the universe, and unitarity, and so forth. In these cases the thermodynamic arrow of time is the set of transformations that leave the equilibrium state unchanged. We can do that in the classical regime as well as the quantum one, given some additional symmetries (including but likely not limited to translation invariance and Gallilean boost invariance) and keeping well away from the ultrarelativistic limit. With a small positive cosmological constant, much of the early universe gas can be constrained to remain at essentially the same comoving coordinates as its daughter products that collapse into structures like galaxies, although things get ugly in the infinite temperature limit. Statements arguing that the arrow of time is driven by increasing quantum correlations are great slogans, certainly enough to help focus theorists' attention, but I think they do not capture a required feature of an expanding FLRW universe's early epochs.

pdonis 9y ago
> It isn't that much of a stretch to have a classical BB (big bang) metric constrain all worldlines to march outwards from the gravitational singularity. Not only isn't it "much of a stretch", this is exactly how the idealized Friedmann-Robertson-Walker spacetimes in GR work. They have an initial singularity which is to the past of all events in the spacetime, which is the time reverse of the way in which the black hole singularity is to the future of all events inside the horizon. However, there are also time reversed versions of the FRW spacetimes, which have a final singularity instead of an initial one--just as there are time reversed versions of black holes (white holes), where the singularity inside the horizon is initial, not final (and where nothing can travel from outside the horizon to inside). So you still need to explain why we observe the "forward" versions of these solutions, but not the "reversed" ones.
- raattgift 9y ago
  > Not only isn't it "much of a stretch", this is exactly how the idealized Friedmann-Robertson-Walker spacetimes in GR work. Right. You could have at least read all the way down my comment. :-) > So you still need to explain why we observe the "forward" versions of these solutions, but not the "reversed" ones. I don't think so; that's just the cosmic variance problem: we're apparently in the DE-dominated era of an expanding universe rather than in some other era in the same universe or in a non-expanding universe. If you start travelling an arbitrary great circle on Earth you'll experience significant temperature changes on most GCs; if you happen to pass through one of the polar regions and can't remember anything before that then until you are near the equator you'll be very tempted to conclude that the past is colder than the present. Conversely if you happen to cross the equator on a GC that takes you into a polar region, and you forget everything before the equator crossing, you'll be very tempted to say that everything freezes in the future. There's no time reversal at play here, just a memory horizon that is tempting to extrapolate through. What's this mean? I think it means that RW spacetimes have an implicit choice of gauge in them that we hate to really think about because it is so convenient to foliate them.
  - pdonis 9y ago
    > we're apparently in the DE-dominated era of an expanding universe rather than in some other era in the same universe or in a non-expanding universe Isn't this just a restatement of the problem? Or are you saying that those other solutions also exist, they're just in different universes from the one we live in? > I think it means that RW spacetimes have an implicit choice of gauge in them that we hate to really think about because it is so convenient to foliate them. Hm. Since the standard foliation is picked out by a symmetry of the solution (the standard foliation is the one in which each spacelike surface is homegeneous and isotropic), I suppose it could be termed an "implicit choice of gauge". But the same gauge choice would be implied by a contracting FRW spacetime, so I don't see how it picks out one direction of time vs. the other.

pdonis 9y ago

> It isn't that much of a stretch to have a classical BB (big bang) metric constrain all worldlines to march outwards from the gravitational singularity. Not only isn't it "much of a stretch", this is exactly how the idealized Friedmann-Robertson-Walker spacetimes in GR work. They have an initial singularity which is to the past of all events in the spacetime, which is the time reverse of the way in which the black hole singularity is to the future of all events inside the horizon. However, there are also time reversed versions of the FRW spacetimes, which have a final singularity instead of an initial one--just as there are time reversed versions of black holes (white holes), where the singularity inside the horizon is initial, not final (and where nothing can travel from outside the horizon to inside). So you still need to explain why we observe the "forward" versions of these solutions, but not the "reversed" ones.

raattgift 9y ago
> Not only isn't it "much of a stretch", this is exactly how the idealized Friedmann-Robertson-Walker spacetimes in GR work. Right. You could have at least read all the way down my comment. :-) > So you still need to explain why we observe the "forward" versions of these solutions, but not the "reversed" ones. I don't think so; that's just the cosmic variance problem: we're apparently in the DE-dominated era of an expanding universe rather than in some other era in the same universe or in a non-expanding universe. If you start travelling an arbitrary great circle on Earth you'll experience significant temperature changes on most GCs; if you happen to pass through one of the polar regions and can't remember anything before that then until you are near the equator you'll be very tempted to conclude that the past is colder than the present. Conversely if you happen to cross the equator on a GC that takes you into a polar region, and you forget everything before the equator crossing, you'll be very tempted to say that everything freezes in the future. There's no time reversal at play here, just a memory horizon that is tempting to extrapolate through. What's this mean? I think it means that RW spacetimes have an implicit choice of gauge in them that we hate to really think about because it is so convenient to foliate them.
- pdonis 9y ago
  > we're apparently in the DE-dominated era of an expanding universe rather than in some other era in the same universe or in a non-expanding universe Isn't this just a restatement of the problem? Or are you saying that those other solutions also exist, they're just in different universes from the one we live in? > I think it means that RW spacetimes have an implicit choice of gauge in them that we hate to really think about because it is so convenient to foliate them. Hm. Since the standard foliation is picked out by a symmetry of the solution (the standard foliation is the one in which each spacelike surface is homegeneous and isotropic), I suppose it could be termed an "implicit choice of gauge". But the same gauge choice would be implied by a contracting FRW spacetime, so I don't see how it picks out one direction of time vs. the other.