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Old 04-18-2017, 11:38 AM
Riemann Riemann is online now
Join Date: Nov 2015
Location: Santa Fe, NM, USA
Posts: 4,263
Originally Posted by SamuelA View Post
...the simplest model of universe formation...
I don't think I accept your invocation of Occam's Razor here.

First, here's Andre Linde with a good account of the fine-tuning problem, the anthropic principle and the multiverse:

From the early days of string theory, physicists knew that there are exponentially many different ways to compactly the extra 6 dimensions, but we did not know what can prevent the compactified dimensions from blowing up. This problem was solved about 10 years ago, and the solution validated the earlier expectations of the exponentially large number of possibilities. Some estimates of the number of different options are as large as 10^500. And each of these options describes a part of the universe with a different vacuum energy and different types of matter.

In the context of the inflationary theory, this means that our world may consist of incredibly large number of exponentially large "universes" with 10^500 different types of matter inside them.

A pessimist would argue that since we do not see other parts of the universe, we cannot prove that this picture is correct. An optimist, on the other hand, may counter that we can never disprove this picture either, because its main assumption is that other "universes" are far away from us. And since we know that the best of the theories developed so far allow about 10^500 different universes, anybody who argues that the universe must have same properties everywhere would have to prove that only one of these 10^500 universes is possible.

And then there is something else. There are many strange coincidences in our world. The mass of the electron is 2000 times smaller than the mass of the proton. Why? The only known reason is that if it would change few times, life as we know it would be impossible. The masses of the proton and neutron almost coincide. Why? If one of their masses would change just a little, life as we know it would be impossible. The energy of empty space in our part of the universe is not zero, but a tiny number, more than a hundred orders of magnitude below the naive theoretical expectations. Why? The only known explanation is that we would be unable to live in the world with a much larger energy of vacuum.

The relation between our properties and the properties of the world is called the anthropic principle. But if the universe were given to us in one copy, this relation would not help....

But it seems to me that there are three possible resolutions to the apparent fine-tuning problem, and (given the lack of evidence to support string theory) it would be extremely difficult to defend the position that any one of these is a more economical explanation than the others.

(1) Multiverse + anthropic principle. No constraint on parameter space, a vast variety of universes DO all exist, the vast majority inconsistent with life. The anthropic principle explains why we see ours.

(2) There is a better model for how universes are built, that does not have all the free parameters of current string theory. In other words, the parameter values found in our universe arise from the model somehow as the only valid solution. The only real basis to claim that this is unlikely is that we have been looking for such a model for decades and have failed. But that seems a weak argument to me.

(3) We are wrong about our interpretation of the parameter space and "life" is not as unlikely as it appears. This would probably need to be combined with a weaker form of (2) above, where a better model that current string theory might constrain parameter space allowing a much narrower variety of possible universes.

Last edited by Riemann; 04-18-2017 at 11:43 AM.