Out today, Sean Carroll’s new book, From Eternity to Here, is excellent. After reading a draft in March, I wrote: We are far from understanding thermodynamics. … The distributions we would usually use to successfully predict [physical system] futures are completely, totally, and almost maximally WRONG for predicting their pasts! … Worse, this “past hypothesis” is ambiguous in several ways … Only a tiny handful of physicists (and philosophers) are trying to explain this past hypothesis; … no one is even remotely close.
Sean on video: "The Origin of the Universe and the Arrow of Time"
Less crazy than I thought - but he doesn't really present or consider alternatives to the "baby universe" hypothesis - which seems to make little sense considering what wild speculation it is.
Possible super-humanly intelligent breakthrough on the problem of gravity and the arrow of time!
'The Entropy Force' (New Scientist)
The Entropy Force
Verlinde's work offers an alternative way of looking at the problem. "I am convinced now, gravity is a phenomenon emerging from the fundamental properties of space and time," he says.
To understand what Verlinde is proposing, consider the concept of fluidity in water. Individual molecules have no fluidity, but collectively they do. Similarly, the force of gravity is not something ingrained in matter itself. It is an extra physical effect, emerging from the interplay of mass, time and space, says Verlinde. His idea of gravity as an "entropic force" is based on these first principles of thermodynamics - but works within an exotic description of space-time called holography.
There is an arrow of time problem regarding why thermodynamics predicts future evolution but not past evolution, as Hanson and Tyrrell both correctly explained.
I'm sticking to my guns and saying its got something do with Occam's razor and complexity. Hanson and you both rebutted me in the earlier thread by stating that Occam can't be the solution because high entropy states can be very simply spcified, but I don't buy it. It all depends on exactly how your complexity meausre is defined.
It's Occam I'm telling you all. Universal priors are the answer. Of course, pinning down exactly why and how is where I'm running into a spot of bother ;)
For every trajectory through state-space moving from high entropy to low entropy, there’s a trajectory moving from low entropy to high entropy. Why is ours one of the former rather than the latter?Ignore this point; it is incoherent. We just define the direction of time on a trajectory to point in the direction of lower entropy. My second point above is the real one. By the 2nd Law, we would expect the present to be a local maximum with respect to both "temporal sides". That is, we would expect both temporal directions on our trajectory to be "thermodynamic futures".
That seems pretty trivial to me: you get analogs of the second law in practically any reversible cellular automaton that exhibits complex behaviour.
The issue isn't the 2nd Law. It's making the connection to an arrow of time. For every trajectory through state-space moving from high entropy to low entropy, there's a trajectory moving from low entropy to high entropy. Why is ours one of the former rather than the latter? Or, rather, what justifies our belief that it is?
Given a state on a trajectory, the 2nd law says that its future is very likely to be high entropy. But the 2nd law also says that its past is very likely to be high entropy. For some reason, we think that there is countervailing evidence showing that our past was actually low entropy. But we also think that this evidence doesn't imply that the future is low entropy.
What justifies this asymmetry? That's the problem of the thermodynamic arrow of time.
Re: "The hard problem is closer to what salacious described - it’s showing why there’s a thermodynamic arrow in the first place."
There is no “arrow of time problem”. Those who think there is are just in a muddle about the issue. The reason the psychological arrow of time and the thermodynamic arrow of time are linked is rather well understood.
But that's not the really hard part of the arrow-of-time problem. Given a thermodynamic arrow, you can probably derive a psychological arrow. (This 12.5 minute BloggingHeads segment makes the case that it's not as straightforward as many think, though.) The hard problem is closer to what salacious described---it's showing why there's a thermodynamic arrow in the first place.
Even if true, that wouldn't explain the difference between physics and economics.
To clarify, I read it while I was in junior high or high. Not a school book, but one I found and purchased at Wordsworth in Harvard Square, or possibly at the New England Mobile Book Fair. So: a popular book.
Thinking Physics, by Lewis Carroll Epstein, which I read either in high school or in junior high, does not seem to fit the profile that Robin described. It remains my favorite popular physics book, and I read it, well, a long time ago. It's still in print. My favorite popular economics books, on the other hand, do fit the profile.
Why do physics book get written as though from on high? Because the writers are trying to convince themselves, because all they have to rely on is belief generated out of incomplete facts and models that don't completely fit together or which, at minimum, rely on assumptions about evidence that hasn't actually been discovered. Because they're trying to marshall support for their perspective because prestige in the fields is rooted in creativity AND acceptance. And all this is because we have no understanding of some basic why's. It sometimes frightens me to realize how many fundamental absences we paper over.
Robin you are spot on as to why I read physics, but that is also why I read economics. I think one of the main reasons people get so worked up over economics isn't just that they care more about it but also that they are exposed to more controversial information when reading at a popular level. Disagreement in the field of physics is mostly outside popular understanding but disagreement in economics is over popular level stuff.
As far as I understand it, the reversal of entropy would eliminate any physical evidence that it existed, whilst simultaneously - via a reduction of information between the entropic system, and any correlated systems - wiping the memory of any observer. Therefore if the universe is a closed system, time may only be perceived as moving forwards.
That's an explanation for why we remember the past, and not the future:
"Our ability to remember past but not future also coincides with the arrow of entropy. The reason, Hawking says, is that whenever a memory is made, in either a brain or a computer, the smidgen of energy required to light up a neutron or move an electron is released as heat. Heat -roiling, chaotic heat - increases entropy. Memories, then, because they release heat, increases disorder, too. Entropy increases from yesterday to tomorrow. That`s why memories are made in the past."
My understanding is that this post is about another issue. We do, in point of fact, have a low entropy past (the big bang) - and the anthropic principle dictates that evolved creatures will look back on low-entropy beginnings.
There is no "arrow of time problem". Those who think there is are just in a muddle about the issue. The reason the psychological arrow of time and the thermodynamic arrow of time are linked is rather well understood. The explanation is even in A Brief History of Time, IIRC - though Hawking was pretty confused about the issue back then, and got other bits of it all wrong.
That's a huge "if" - one not supported by any evidence I have heard of.
IMO, you would need *good* evidence to hypothesise infinities in physics, not a load of ungrounded speculation.