Saturday, 11 August 2007

Entropic Principle

I'm back. Just in time to report on the curious TH seminar last Wednesday. Jim Cline was talking about The entropic approach to understanding the cosmological constant. I am lucky to live in a special moment of history and to observe fascinating sociological processes going on in particle physics. I'm dying to see where this will lead us...

Everyone knows about the anthropic principle. In short, certain seemingly fundamental parameters are assumed to be enviromental variables that take random values in different parts of the universe. We can live only in those corners where the laws of physics support more or less intelligent life. This could explain why the values of certain physical parameters appear fine-tuned.

One undeniable success of this approach is Weinberg's prediction concerning the cosmological constant. Weinberg assumed that intelligent life needs galaxies. Keeping all other cosmological parameters fixed, galaxies would not form if the energy density in the cosmological constant exceeded that in matter today by roughly a factor of 5000. If all values of the cosmological constant are equally probable we should observe a value close to the upper limit. Weinberg's explanation points to a value some 1oo0 times larger than the observed one. However it remains attractive,
lacking any fundamental understanding of the smallness of the cosmological constant.

Weinberg's paper appeared in 1987, well before the first indications for accelerated expansion
from supernovae started to show up. That was a prediction in the common-sense use of this word. Since then, the anthropic principle was mostly employed to predict things we already know. The successful applications include the electroweak breaking scale, the dark matter abundance and the CMB temperature. As for the things we don't know, predictions based on the anthropic principle turn out to be less sharp. For example, the supersymmetry breaking scale is predicted by anthropic reasoning to be at a TeV, or at the Planck scale, or somewhere in between.

The entropic principle, p
roposed earlier this year by Bousso et al, is a new twist in this merry game. The idea is to replace the selection criterion based on the existence of intelligent life with something more objective. Bousso et al argue that the right criterion is maximizing the entropy production in a causal diamond. The argument is that life of any sort cannot exist in a thermal equilibrium but requires free energy. They argue that free energy divided by the temperature at which it is radiated (that is the entropy increase) is a good measure of a complexity that may arise in a given spacetime region. They proceed to calculating the entropy production (dominated by infrared radiation of dust heated by starlight) for various values of the cosmological constant and find that it is peaked close to the observed value. This allows them to conclude that ...this result improves our confidence in the entropic principle and the underlying landscape.... More details on this computation can be found in the paper or in this post on Reference Frame.

Jim in his talk reviewed all that minus the sneer. He also played his own part in the game. He predicted
the primordial density contrast. His conclusion is that the observed value 10^(-5) is not unlikely, which further improves our confidence in the entropic principle and the underlying landscape. Pushing the idea to the edge of absurd, he also made an attempt to predict the dark matter abundance. He took an obscure model of gravitino dark matter with non-conserved R-parity. In this model, the dark matter particles decay, thus producing entropy. He argued that the entropy production is maximized close to the observed value of the dark matter abundance, which further improves our confidence in the entropic principle and the underlying landscape. I guess i missed something here. The observed universe is as we see it because the entropy production from the synchrotron radiation of the dark matter decay products may support some odd forms od life? At this point, pulling cards does not seem such a weird idea anymore...

9 comments:

mcc said...

One undeniable success of this approach is Weinberg's prediction concerning the cosmological constant. Weinberg assumed that intelligent life needs galaxies. Keeping all other cosmological parameters fixed, galaxies would not form if the energy density in the cosmological constant exceeded that in matter today by roughly a factor of 5000.

See, I've heard of this one, but I still don't get it. The only application of the anthropic principle here seems to be the first step in the argument, the "intelligent life needs galaxies, therefore there must be galaxies" bit. However it seems to me that this step is not only taken somewhat on faith (are we quite sure that intelligent life needs galaxies?) but also completely unnecessary to the argument.

Why couldn't we skip the part about intelligent life and instead have the first step be "we observe that galaxies exist", and proceed on from there to the "therefore galaxies must exist, therefore the cosmological constant must be such that allows for galaxy production..." etc?

island said...

Because this doesn't account for the rest of the multitude of commonly balanced coincidences, which is no different than what they did. In this case, you have to consider the entire spectrum of the goldilocks enigma; from the WMAP anomalies to the ecobalances that surround us, and this includes many other galaxies that exist in the fine layer or habitable zone of the observed universe.

The Goldilocks Enigma... again

The only problem with the AP is that both "sides" of this are predispositioned to what they will look at and what they will see, so all of the evidence doesn't get presented.

The bottom line when this has actully been done is that humans are far-from-equilibrium-dissipative-structures that must be energy-enabled by the environment in order to exist.

This is the real connection that both sides miss, since there is an asymmetrical thermodynamic energy conservation law and a true entropic anthropic COSMOLOGICAL principle that is being defined, that nobody who believes in a multiverse will ever see.

So the subject is doomed by fools on both sides who can call themselves scientists as long as self-honesty isn't a prerequisit.

Jester said...

mcc: Right, in practice there is not much difference whether you call it the anthropic principle or the galaxy principle. The point is that some physical parameters may be determined by enviromental arguments rather than by fundamental physics. I don't have problems with that and, in fact, Weinberg showed that this kind of argumentation may have some predicting power. What annoys me so much is all this circus that followed, i mean calculating probabilities of events that have happened. This turns science into its caricature.

M said...

Anthropic arguments smell right and it is important to try to see if they can lead to good physics.

For the moment this is not happening, but maybe somebody will do something better than speculating that the Purpose of the Universe might be heating dust...

island said...

m is right, but at least one cosmological model that I know of says that the creation of matter/antimatter pairs from vacuum energy *is* something that has a very significant impact on the symmetry of the observed universe when the purpose of the universe becomes *survival* as this process enables it to "evolve". This is why it is so important not to exclude the rest of the galaxies that fall into the habitable zone, because this makes the combined effect of many like us, cumulative, and it also removes much of the geocentric arrogance that is typically associated with strong interpretations.

The problem is that a whole nother form of anticentrist dogma and willful denial takes over when you start talking about strong interpretations that include higher "purpose" in nature.

Torbjörn Larsson said...

Interesting post; I have waited for someone to test this environmental principle further.


What annoys me so much is all this circus that followed, i mean calculating probabilities of events that have happened. This turns science into its caricature.


But they do this in cladistics all the time, don't they? And one of the most popular methods is AFAIK to use bayesian likelihoods to establish the tree diagram of phylogeny. (I'm not a biologist, but as I understand it some of these methods are fairly insensitive to an artifact called long-branch attraction, which frequentist methods may have more problem with.)

Here further genome sequencing or fossil discoveries will test the small subset of likely trees, which I think is important especially when using bayesian models. There is a unique common descent phenomena that we can access but it still permits test of the method, the evolutionary theory and the specific observed model.

Maybe this is the idea? We have a unique universe that we can access but using the same principle it seems from the posts account it still permits test of the method, the inflationary theory and the specific observed model. But what will they do if the principle fails to predict all parameters? (Say if some isn't decided by the environment but by unknown constraints.) Likelihood of likelihoods? :-P Or will they simply be confident in that it is 'currently the best model'?

Anonymous said...

"We can live only in those corners where the laws of physics support more or less intelligent life. This could explain why the values of certain physical parameters appear fine-tuned."

If you actually believe this, your education system failed you.

island said...

"We can live only in those corners where the laws of physics support more or less intelligent life. This could explain why the values of certain physical parameters appear fine-tuned."

If you actually believe this, your education system failed you.


If you reject the multiverse, then you have to explain fine-tuning from first principles, rather than selection effects.

This still includes an entropic anthropic principle, because you still have to explain fine tuning, so there is no problem with your statement as long as we keep the "pseudo-science" separate from the observered fact.

Unfortunately, this gets blurred by people like Peter Woit, who is also too vague about what he objects to and why.

aaacss said...

And you have the nerve to call ME a crackpot? This this argument is the same as saying the effects generate the causes. It's faulty logic. The fact is, we can measure the effects and we simply don't know the causes...yet. A multi-verse is one far-out but acceptable hypothesis for a cause, but just being here isn't.

It's the same as saying the sun is warm because if it wasn't we wouldn't be here to observe it. It explains nothing. Very faulty. I'm surprised anyone with even the most basic of science training would entertain such a backwards notion.