Christopher Lord

Read an interesting article on the Nature Website regarding a new cosmology based on string theory. Check out the <a href="http://www.nature.com/nsu/020422/020422-17.html" target="_blank">article</a>.

As string theory matures, will we be seeing more sweeping changes to our theories, much like this may be?

[ April 28, 2002: Message edited by: Christopher Lord ]</p>

Daydreamer

I really need to pick up a copy of The Elegent Universe. I had a friend in High School who wouldn't shut up about Super-String Theory, which turned me off to it for a while.

eh

Doesn't anyone worry about the problem of infinite regress we have here? Anyway, there is a thread in the EC forum about this ekpyrosis. I don't like it, but we will probably see more string theory related cosmology in the future. Perhaps there is some way to make string theory compatible with inflation?

I just finished reading the Elegant Universe, and can honestly say it's a worthwhile read. It was very easy to follow, which is a big surprise for a physics book.

Friar Bellows

"String theory" has been mentioned many times on this forum, so you might get the impression that it's the only viable path to properly uniting general relativity and quantum mechanics. But there's an alternative approach that has been at least as successful. It's called a number of names: "Quantum Geometry", "Canonical Quantum Gravity", or perhaps more specifically, "Loop Quantum Gravity". It's not as ambitious as string theory, but in some ways it's better. Look it up on Google or look for review papers on the subject at arxiv.org (e.g. gr-qc/0112038, gr-qc/0011089). Not that you'll understand much of it.

[ April 28, 2002: Message edited by: Friar Bellows ]</p>

Christopher Lord

I've read a few papers on loop theory, and it sounds good. But it cant be stretched to large scales yet.

A unified theory needs to be able to be stretched like mad. The cyclic hypothesis mentioned in the article is one such example of how string theory could be used on large scales.

Philosoft

I'll just continue the praise for The Elegant Universe. I'm reading it now and it's truly a pleasure. Not to be missed.

Bill

I've read <a href="http://www.secweb.org/bookstore/bookdetail.asp?BookID=186" target="_blank">The Elegant Universe</a> and I don't see it as being in any way specifically hostile to the idea of inflation. Nobody knows the real equations of string theory yet, so you can't really begin to exclude other theories from the potential embrace of string theory.

As for the issue of "infinite regress," I think it is way too soon to be calling on that card. At the beginning of the 20th century, we had two known "basic" forces (gravity and electromagnetism) and no way to unify them into a common field theory. During the 20th century, we discovered two additional "basic" forces (the strong and weak nuclear forces) and managed to unify all but gravity using some combination of relativity and quantum mechanics. What has been lacking is a good theory of quantum gravity so that gravity, too, can be unified. This has proved to be far more difficult than was first imagined due to the weird behavior of quantum forces at subatomic scales of distance.

The main promise of string theory is to place a true lower limit on the size of physical phenomena. Instead of being forced to deal with the infinities generated by a concept of an infinitely small "point particle," we have a "string" of some finite (but extremely small) size. What <a href="http://www.secweb.org/bookstore/bookdetail.asp?BookID=186" target="_blank">The Elegant Universe</a> points out is that this has the side effect of eliminating these troublsome infinities by refusing to concentrate all of the mass and density of a particle into a single point. With string theory, you can go so small, but no smaller.

Yes, I suppose you might argue "infinite regress" based upon first atomic theory, which was supplanted by a theory of quarks, which seems to be in turn about to be supplanted by a theory of strings (or perhaps even some intermediate forms of matter between quarks and strings). But again, one of the usefulnesses of string theory is that it prevents infinite regress by setting a limit to what is the smallest possible "thing." Prior to string theory, the "smallest possible thing" was a matter of imagination rather than a matter of mathmatical calculation. String theory at least holds out the hope that there will be no "input variables" in the equations of string theory, and that all of the known variables of relativity and quantum mechanics will naturally emerge from the calculations of actual string theory.

But of course, it is way too early to make any hard-and-fast claims. It is just that I also feel it is way too early to be raising the boogyman of "infinite regress."

== Bill

eh

I was talking about an infinite regress of time. Specifically, the problem that comes from a cyclic ekpyrotic universe.

String theory ought to be compatible with the BB, or at least something similar. I'm guessing that in 20 years, ekpyrosis won't be the only M theory related cosmology.

Answerer

Well eh, I don't think anyone here understand time vey well, so it might not even have a beginning at all. Next, time is a relative concept, even when time has ended in our universe, it is still in motion in other universes(if there are). Therefore, if there is a super entity-observer who exist outside the universes, he may actually see time as infinity, with no beginning and end.
Besides cyclic universe theory is not the final one and it is far too early to tell who's right or wrong.

eh

But if time is merely a measure of change, the problem remains. We would have an infinite number of big splats before our present universe in this brane scenario. This seems absurd.