WaveJumper said:
I have a question concerning the mathematical predictions for space-time under String Theory. Physicist David Gross of University of California said in an interview that String Theory will change the way think about space-time. Then another physicist, Sylvester Gates of University of Maryland, said that space is also made of strings:
"Strings create everything, including space and time and even us."
I can see how elementary particles could be represented as different patterns of oscillating strings, but how does this notion arise that space-time is also made out of 1-dimensional loops of vibrating strings?
Ok, I can give you the way that I understand this, in the hopes that someone smarter will come along and tell me I'm wrong, and then we all can learn something.
General relativity is an effective field theory. This means that it is only good in a limited range of energy. Just like, say, classical mechanics. You can use E = 1/2 mv^2, so long as v isn't too big. In this case, Newtonian mechanics is a pretty good approximation to nature. Now, when v gets big, or gravity becomes strong then you have to use SR or GR, respectively. Again, the approximations you made in writing down your equations of motion using Newton's laws fail (namely v isn't too big and g isn't too strong), and you need a better theory.
This is a very important point: one can derive Newton's equations from Einstein's Equations. by making a set of approximations. This is very important, and required for consistency of Einstein's equations: one MUST be able to understand the effective field theory as a simplification of the underlying "more fundamental" theory. In this case, one understands Newton's equations as arising from Einstein's equations when gravity is weak and velocities are small.
So we expect that somewhere general relativity will break down, and "space-time" will no longer be described as "geometry", like Einstein thought. We already have indications that this is the case, for example, because of the paradoxes associated with black holes and the big bang. The bottom line is that there's really nothing sacred about general relativity, just as there was really nothing sacred about Newton's Laws. This may sound shocking to you, but it's the way that physicists have been thinking since the early 1980's.
There's two things you can do: Naty suggested one approach, which doesn't seem particularly appealing (to me, at least). In particular, I haven't seen any REAL convincing ways to get matter and forces (other than gravity) out of the non-stringy quantum gravity program.
The other thing you can do is try to use strings. Then you build your quantum theory around little one dimensional strings. This next part is very important: one can derive Einstein's equations from string theory. This is a non-trivial check on the theory, and one that had to come out properly if we were to take it seriously.
So what's the point. The idea of "four dimensional curved space-time" is one that comes from Einstein's equations---indeed, "space-time" itself is an Einsteinian concept. As such, there's no reason to expect that that intuition is still valid when it comes to string theory. So the statement that "space-time is made out of strings" is tantamount to the statement that "GR can be derived from string theory". THIS is the important point.
