Monday, December 29, 2008

Richard Feynman, Mr X, and the arrow of time

As we have mentioned, on Christmas Eve, Sean Carroll began to understand that the previous 40 years of his thinking about the arrow of time had been plagued by elementary, childish mistakes. In his very recent text
Have a Thermodynamically Consistent Christmas,
he announced that he finally understood that subsystems with opposite arrows of time cannot co-exist and interact in the same Universe, i.e. that the arrow of time is and must be universal. That was a pretty radical departure from his previous writings that included "pearls" such as
Incompatible Arrows I
Incompatible Arrows II
Incompatible Arrows III
Incompatible Arrows IV
The Arrow of Time in Scientific American
Latest Declamations about the Arrow of Time
and dozens of similarly dumb articles - including bizarre fairy-tales about the Boltzmann Brains - that he has written over the years. In all these texts, Carroll had promoted the absurd opinion that incompatible arrows of time are a priori allowed to co-exist and the origin of the second law of thermodynamics lies in cosmology (wrong), and not in statistical physics (correct).

In the newest article at Cosmic Variance,
Richard Feynman on Boltzmann Brains,
Carroll mentions Feynman's attitude to the problem. Feynman thought the same thing about these issues as every sufficiently intelligent and independently thinking person does after having looked into the problem at least for a few days. 

For example, the hypothesis of the Boltzmann Brains - namely that we are just a low-entropy fluctuation in a high-entropy world - can be easily ruled out because it predicts that virtually everything in the Universe (that is unnecessary for our brains to exist) should be found in a disordered state without a consistent history - but the prediction is falsified as soon as you see that there seem to be other objects that apparently follow from an organized, meaningful past - an argument we have mentioned repeatedly at TRF.

It's really not a big deal. The Boltzmann Brain hypothesis can be falsified by any tiny amount of empirical evidence. Also, no physical or cosmological theory - as opposed to a philosophical theory - can ever predict that we should be Boltzmann Brains. If it could make such a prediction, that would make it inconsistent with the very basic observations. 

Obviously, no kind of cosmology - neither inflationary nor cyclic nor other cosmology - is inconsistent with the existence of objects of an increasing entropy and with their basic qualitative "local" behavior. It follows that no such theory can be predicting that we are the Boltzmann Brains, and everyone who thinks that such a prediction is generated is making a trivial logical mistake. Sadly, there are hundreds if not thousands of wrong papers built on this meme.

But Feynman's opinions about this issue were not only much closer to mine than Carroll wants you to believe. They were also much more militant. ;-) You may read a text by Stephen Hawking,
The No Boundary Condition And The Arrow Of Time
in a book called "Physical Origins of Time Asymmetry".

In this article, Hawking talks about what he considers the greatest scientific blunder of his life (rightfully, in my opinion): Hawking used to think (because of some arguments based on the imaginary time) that the logical arrow of time is bound to be aligned with the cosmological arrow of time given by the expansion of the Universe.

But at the beginning, he talks about a conference on the direction of time that took place at Cornell in 1963. A lot of nonsense, pretty much identical to the nonsense printed by Sean Carroll during his previous life, has been said on the conference. Stephen Hawking explains that Richard Feynman considered the proceedings so worthless - he was so disgusted - that he didn't want his name to be associated with them. So he was referred to as Mr X even though everyone has always known that Mr X was no one else than Richard Feynman.

You know, when Feynman was determining the right QED propagators, a work that was initiated by his and Wheeler's research of advanced and retarded waves, he has made some important insights about the direction of time in the fundamental formulation of the physical laws. He realized that the propagators must be half-advanced, half-retarded, a rule that is equivalent to the "i epsilon" prescription in the momentum space. This rule reflects one of the aspects of the microscopic time-reversal symmetry.

However, when it came to the observed time asymmetry, Richard Feynman realized what your humble correspondent and every sensible person does: the arrow of time is always a consequence of ordinary statistical mechanics. Read the bottom of page 346 in Hawking's article

In fact, the arrow of time always follows from ordinary statistical mechanics. QED is just an example but all these arrows of time - all thermodynamic and logical arrows of time associated with friction, heat transfers, decoherence, or memory of humans or computers or anything else - are always aligned with each other. 

Using the words of Mr X, matter is thermodynamically one-sided. It has the disposition to damp when you shake it. The hypothetical anti-thermodynamic matter is just an image of the normal matter, and the two arrows can't ever co-exist. The time-asymmetric logic or thermodynamics does not contradict the T- (or CPT-) symmetric microsopic laws in any way. And whatever words you identify as the "ultimate reason for the thermodynamic asymmetry", it is a basic and true fact about the real world. A very important one.

It's of course fair to say that we don't understand the very origin of the Universe and the beginning of the Universe is connected with a very low-entropy state. But it is an entirely different thing to say that the very ordinary phenomena that we observe today are not properly understood by the physical laws as we know them. The latter proposition is absurd and the misunderstanding is only a testimony of the incompetence of many cosmologists as physicists rather than a testimony of an incompleteness of the laws of physics as we know them.

Of course, the 1963 conference was not the first cosmological conference that drove Feynman up the wall. :-) In 1962, he wrote a famous letter to his wife about another cosmological conference in Warsaw:
"I am not getting anything out of the meeting. I am learning nothing. Because there are no experiments, this field is not an active one, so few of the best men are doing work in it. The result is that there are hosts of dopes here (126) and it is not good for my blood pressure. Remind me not to come to any more gravity conferences!"
Well, not much has changed about similar conferences during the last 45+ years. There are still hosts of dopes over there who discuss nonsense. But maybe, once Sean Carroll realized that he's been saying nonsense about similar issues for years and as soon as he starts to pretend that he has been saying the very opposite thing than what he was ;-), rational reasoning could perhaps return to these debates.

No comments:

Post a Comment