Aug

14

Here is a review of The Cosmic Landscape: String Theory and the Illusion of Intelligent Design, by Leonard Susskind. Susskind is the “Felix Bloch Professor in Theoretical Physics at Stanford University since 1978, and is a member of the National Academy of Sciences.” So this is a high-powered author.His area of physics is string theory, an intensely mathematical area that has tried to bridge the gap between quantum mechanics and general relativity. My impression is that in order to really understand what is going on in string theory one would need to undertake a multi-year apprenticeship, plowing through some very difficult math. I have not done that, and I doubt I would be able to, so my feel for the subject is limited. I can try to read books like this one for the layman, and try to understand as much as I can through the simplified analogies that the author presents.

The author proposes that string theory provides perhaps the only way to understand the apparently coincidental facts about our universe that make life possible. It has long been observed that the universe appears finely “tuned” to enable the possibility of life. One can make a long list of fundamental physical parameters (e.g. the ratio of the electric to the gravitational force, the charge to mass ratio of the electron, the energy of a certain excited energy state of the carbon nucleus, etc.) which, were they to take on slightly different values, would make life, or sometimes even stars, planets, and galaxies, impossible.

The most astonishing case of apparent “tuning” of the universe has been clarified over the past decade or two, the value of the “cosmological constant”. Einstein proposed the idea of a cosmological constant in his first papers on general relativity. It was an ad hoc device that he put in the theory so that the universe would be static, with unchanging distances between galaxies. Soon afterward, in 1929, Edwin Hubble demonstrated with his observations that the universe is expanding and not static, so Einstein retracted the idea of the cosmological constant, calling it his “biggest blunder”.

Though Einstein did not propose any mechanism for his cosmological constant, there is a clear candidate mechanism. In quantum mechanics, the “vacuum”, or the volume of space that has been evacuated of all matter, is a very lively place, with “virtual” particle-antiparticle pairs forming and annihilating. There is energy and even an effective mass associated with these quantum effects, and that energy can be a factor that can cause the universe to either contract or expand, depending on its sign. “Fermion” particles (such as the electron) would cause a contraction, but “boson” particles (such as the photon) would cause expansion.

The problem is that if one calculates the cosmological constant from, say, electrons, its magnitude is absurdly large. If the total cosmological constant were anywhere near this magnitude the universe would either collapse immediately or expand at such a rate that matter could not form. Other particles (e.g. photons) make contributions of opposite sign and similar but not the same magnitude, so there is a possibility that the contributions from all the particles could cancel each other exactly. They would, however, have to cancel to about 1 part in 10 to the 120’th power, in order to be consistent with current observations! Nobel Prizewinner Steve Weinberg has also shown that even with an only slightly less perfect cancellation, say a one part in 10 to the 119’th, the stars and galaxies would never have formed from the early universe.

This incredibly precise cancellation of the cosmological constant is the most outrageous example of what looks like a “tuning” of the universe to galaxies, stars, planets, and life to form.

String theorists, in their initial efforts, hoped that string theory would provide a unique explanation of the values of the several dozen particle masses and coupling constants of the “Standard Model” of elementary particle physics. Susskind describes, however, the process by which string theorist were forced to conclude that their general idea could be consistent with about 10 to the 500 different possibilities, each of which would be kind of like its own little Standard Model, with its own value of the cosmological constant and just about everything else. The 10 to the 500 possibilities are the “Landscape”.

Initially this finding was thought to be a disaster for string theory. Susskind proposes, however, that it is actually a great blessing. He proposes a “multiverse”, that there are a very large number (presumably much greater than 10 to the 500) universes out there. Within one universe, all the other universes are beyond the “event horizon”, and therefore can not be observed. A key point (which I haven’t fully grasped) is that random factors cause each of the universes to be different, and to “populate” a different part of the “Landscape”. Therefore each universe would have its own little Standard Model.

From this perspective, it is not surprising that our universe appears remarkably tuned to life. The explanation is that there are many, many, many other universes where no life exists. Since there are so many, each with very different properties, it is not so surprising that you could find one (or more) that is remarkably tuned for the possibility of life. And once you believe that, you realize that there is nowhere else that we could possibly be living.

Susskind presents his work as a refutation of the idea of Intelligent Design. What a newcomer might take away, though, is how respectable the idea of Intelligent Design really is, that the best physicists must postulate a near infinity of unobserved universes out there existing in parallel with ours, in order that ours can have its exceptional properties as the result of random chance rather than design. The concern about the “fine tuning” problem is widespread among the physics/cosmology elite (see for example Steven Weinberg’s discussion.)

I have not emphasized here the experimental observations that have been emerging over the past couple of decades that have had great impact. The most important are observations and mapping of the “cosmic microwave background”, the faint afterglow of the Big Bang, which is sort of like observing the red glow of burning coals, except that instead of red we are seeing microwaves, and the coals are the early universe, observed now at a temperature of just a few Kelvin above absolute zero. These experiments, the “COBE” and “ WMAP” map out fluctuations of the night sky of 1 part in 10 to the 5 to predict the temperature of the early universe. This experimental subject is probably a little more accessible to us mortals than is string theory. I think astronomy buffs and others will find it fascinating.


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