Paging Apokrisis

Heh. Well as PF is buggered...

Applying entropy concepts to the whole universe is tricky. But two simple points we can make are:

1) The Heat Death state is as cold as it gets and so that limits rather severely the possibility of hot thermal fluctuations that might spawn something new.

2) Having said that, it is only the Planck scale that puts a floor under the ultimate temperature drop and so if the Planck scale is not a true physical limit for some reason (that is, our vacuum state is only a false vacuum) then theoretically some further symmetry breaking could arise and the universe would plunge on through, getting larger and colder than 0 degrees K.

Then starting to get into the complications, the old cosmological problem was that the universe really ought to gravitationally roll itself back up into an a negentropic super hot ball given enough time - setting up a simple eternal recycling.

The new dark energy universe tells us there will indeed be eternal expansion and so an actual heat death. But the new get out clause could be that dark energy is a residual inflation (a scalar field that expands exponentially without "cooling" - believe that if you like) and so at some point in an eternal Heat Death there could be a fluctuation that again breaks the symmetry and inflation takes off all over again in its "cost less" fashion - the so-called Big Rip.

The point is that any scenario can be concocted. But the observable state of the universe should suggest some scenarios are highly unlikely (because the Heat Death just now has the look of a one way trip).

However in popular science, no one likes a dull answer. So eternal recycling and other wild rebirth scenarios get plenty of airtime.

Who said there is no magic??

Hey Apo/NL

Does entropy explain the arrow of time...its direction.

I read an interesting paper recently, which reduced the 2nd Law of Thermodynamics by information theory to a quantum description, where time is treated as a loss of information. I would cite it but can't seem to find it.

I liked the idea, but a lot of it was over my head.

Entropy is the only thing that could explain time (as global change) having a direction.

So the problem for all of physics is that it is "mechanical" - an assumption of local time reversibility is built into the very form of its equations. Any equation is a statement of a symmetry which is then broken by you the observer plugging in some actual physical number of a variable.

To then talk about the whole of the Universe, including the emergence of time, you have to have equations that already encode some sort of temporal direction - a symmetry breaking.

Thermodynamics or statistical mechanics give you such a framework. It says if you let a system freely randomise, it will freely keep randomising until that randomness reaches its equilibrium maximum.

But a theory of the universe has to include not only the story of its initial conditions (the local material contents that get shifted around randomly) but also the reason for their being "the system", quantum or otherwise, that is the boundary conditions.

So ordinary thermodynamics - which likes to take a bounded spacetime for granted - is only half an answer, which is why ordinary entropy accounting doesn't get you too far here.

On the specific thing of quantum information loss (or decoherence, as I believe you likely mean) then yes, that is a useful model.

The general idea is that as things tend towards a cold residual fizzle of radiation, all particular local information has been erased and you only need two global measurements - the general temperature (or energy density) of the universe and its scale factor (how big it is in terms of the visible event horizon).

At the Heat Death, with both these at their maximum, time has pretty much halted in terms of meaningful change. Like gas particles in a flask, everything might still be in motion, but nothing new is happening in any physical sense.

Isn't Lee Smolin saying that the directional nature of time is real, and the fact that according to physics, it appears to be reversible, is one of the fundamental problems of modern physics?

Its only a problem if you believe scientific models have to be "the thing as it really is", rather than a pragmatic model.

So the source of the time symmetry in mechanical descriptions is hardly a metaphysical-level problem - science built its models that way. But the next step would have to be a quantum gravity model of existence. And at that stage, spacetime is going to have to be statistically emergent from a more fundamental symmetry (and symmetry breaking).

Sure, Smolin talks about the need for real time. But on closer inspection, does that mean more than thinking that flux or change is fundamental? It is the (quantum foam) ground out of which spatial coordinates and persistent lawful organisation develops.

Another entropy based way of visualising it is like gas condensing to water. Gas moves about in constant disorderly fashion. Then it cools and gets organised. It develops a dimensionality of flow. Talking about directions makes sense because there are now counterfactual directions not being taken.

So is the gas timeless or too busy flying in any old direction? What we know as a dimension of time is a global entropic flow, against which any negentropic counter-action becomes a difference that makes a difference. Something "really happens" only to the degree that it can't very easily unhappen anymore.

Boltzmann brains are analogous to the intelligible books in Borges finite library.

Why do we talk about brains when simpler objects would be just as numerous and far more probable, that which brains might require to come into being the way they have? A Boltzmann brain is a place holder for any an all possible objects and their corresponding probabilities in the timeless entropy soup, including the total serendipitous arrangement of the universe itself.

Given that the universe is always far weirder than one can possibly imagine, there is bound to be unforeseen or new interactions near the end state which will happen when nothing is there to see it.

I like the idea of spontaneous reset.