Tuesday, April 07, 2009 ... Deutsch/Español/Related posts from blogosphere

Richard Feynman on the arrow of time

The distinction of past and future

Playlist (click, 5 times 10 minutes)

Update: The videos are available via Project Tuva
The lecture is pretty much isomorphic to Chapter 46, Volume I, of Feynman's lectures on physics.

The lecture took place a few days after the experiment finding the CP violation. Feynman was somewhat skeptical ;-) but he correctly said that the effect couldn't be responsible for the macroscopic irreversibility.

In the first part, Feynman talks about the reversibility of the microscopic laws, the irreversibility of all macroscopic phenomena, and the need "not to look too carefully" in order to understand the irreversibility.

In the second part, he gets to the low initial entropy. At the end of this part, he suggests the "Boltzmann brain" theory. At the beginning of the third part, he explains why this is a "ridiculous theory" because it incorrectly predicts that the rest of the world should be completely disordered.

He also explains that the low initial entropy is independent of the dynamical laws: no dynamical theory can ever "predict" that we are Boltzmann brains because the second law depends on an additional layer of knowledge, the initial conditions, that remain pretty much outside physics (even today). Feynman ends the third part by ruling out a particular Maxwell's demon. So in the fourth part, he is led to clarify the difference between energy and useful energy.

He generalizes the insights about the increasing entropy by noticing that the laws of physics often do not have a direct relevance for the experience - one needed to analyze statistics instead of the microscopic laws themselves. We would say that some phenomena are emergent and many microscopic details become irrelevant along the way.


By knowing the laws, we don't understand "much" immediately. It takes a while and the understanding remains partial, anyway. What we see in Nature is an accidental consequence of a mixture of many elementary laws. So because many complex effects are involved (8:30), it is not surprising that Y(4140) has some complicated properties and seemingly accidental numbers.

Well, except for one 7.82 MeV level of Carbon-12 (9:00) that makes all the difference in the world, as Hoyle figured out. :-) So in the fifth part, Feynman shows that it was possible to predict that there was a 7.82 MeV level of the carbon nucleus. This very level allowed all the heavier elements to exist. I will discuss this point later.

In the fifth part, Feynman explains why it is important to invent emergent, approximate concepts in physics, such as "heat", that are not directly linked to simple objects appearing in the fundamental laws. This structure of concepts needed in science has many layers: it is hierarchical in character.

Needless to say, I think that the lecture is funny, perfect, deep, true, and nothing essential has happened to the irreversibility since that time.

Bonus: the 7.82 MeV level of the carbon nucleus

This observation is a cute example to test the anthropic ideas. Heavier elements are needed for life as we know it. And to create them, you apparently need the 7.82 MeV level of Carbon-12 because such a carbon nucleus created out of three alpha particles can live a little bit longer, so that it has a lot of time to be rearranged into the ordinary Carbon-12 ground state before it decays, and the creation of other elements becomes possible, too.

Now, the existence of this level is a nontrivial condition for life to exist. The energy of the level depends on many parameters, including the bare quark masses. (Of course, if it doesn't, then it means that the level follows from QCD with no dimensionless parameters - which means that mathematics of QCD has built-in "miracles allowing life": that would be a big argument against the anthropic reasoning.) Is this condition enough to constrain the vacua of string theory? How much does this condition and similar conditions constrain them?

I am sometimes afraid that the vacuum selection problem could indeed depend on objects as complicated as the existence of a required level of the Carbon-12 nucleus, a bound state of 12 nucleons or 36 quarks (plus many gluons and quark-antiquark pairs, among other stuff). It seems likely (but not certain) that there is no simple calculation that would link the levels of the carbon nucleus with those of the alpha particles in a general QCD-like theory.

However, I still think that this doesn't necessarily make the vacuum selection problem unsolvable. Imagine that there actually exist e.g. 100 similar constraints on the existence of different levels of nuclei, atoms, and organic molecules that may be needed for intelligent life of some kind. And the probability that a compactification passes all the tests and allows one of a relatively small number of possible "formats" of life is 10^{-500} or so.

I find it perfectly plausible. There can be just a few vacua that admit life. Or many of them. But whatever is the number of compactifications with life, we can still use all the data we have and all the parameters we have measured and try to find the most likely vacua by logical inference.

What I find important is that even if the constraints requiring the life to exist are real and relatively stringent, it doesn't mean that they're the only constraints or that they're the only hints that can lead us to the right answer.

BTW, if you liked the lecture, you may also watch The law of gravitation.

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reader Rosy Mota said...

the arrow of time is truthly,as observe the violations of cp for meson,and quarks given by matrix kkc.the subsystem
that are subspacetime.where the these spacetime are invariant.follow the lorentz's transformations( orthochrous and antichrous )

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