# Free Parameters In String Theory

## Main Question or Discussion Point

Hello to all:

I heard Michio Kaku talk on string theory recently and I thought I heard him say that the theory had no free parameters. But in thinking about the matter, I could not decide what that meant -- if it is true. Could someone please tell me what ingredients go into string theory? Are c, h and G taken to have the known experimental values, for instance?

RoKo

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In string theory there is at first one free parameter: the length of the fundamental string.

Once we start trying different ways of compactifying the extra dimensions, all these different compactifications are sort of like parameters.

In string theory there is at first one free parameter: the length of the fundamental string.

Once we start trying different ways of compactifying the extra dimensions, all these different compactifications are sort of like parameters.
Okay. Thanks for the information. Could I ask you: When you say compactification, are you speaking of ways of rolling up the various dimensions? I have a fair understanding of quantum physics, but that's about it. I earned my Ph.D. in physics in 1980 -- back in the Dark Ages.

Thanks again.

RoKo

Yes, compactification could loosely be thought of 'rolling up the extra dimensions.' In mathematical terms there is a compact (closed and bounded) manifold attached to each point in spacetime.

Yes, compactification could loosely be thought of 'rolling up the extra dimensions.' In mathematical terms there is a compact (closed and bounded) manifold attached to each point in spacetime.
Many thanks.

RoKo

malawi_glenn
Homework Helper
one should say I think "no dimensionless free parameters", c, G etc have dimension

one should say I think "no dimensionless free parameters", c, G etc have dimension
Thanks.

RoKo

malawi_glenn
Homework Helper
Thanks.

RoKo
so this is not the case in the standard model, where eg. mass ratios and coupling constants are fixed.

so this is not the case in the standard model, where eg. mass ratios and coupling constants are fixed.
You have been of much help. Thanks.

RoKo

It might be fun to know that the "freedom of string theory" is one of the largest problems in string theory at the moment. People are trying to come up with realistic configurations (i.e. compactifications etc) which would give a description of the Standard Model and a positive cosmological constant (General Relativity itself already comes along quite natural, I think). The point is that different configurations lead to different excitation spectra of the strings and thus different universes.

But apart from the fact that a 'realistic' configuration has not been found, there is not even a mechanism known which explains why it's this particular configuration that corresponds to our universe. In some sense, string theoy provides way too many different ways of coming up with toy universes.

This is known as the landscape problem.

It might be fun to know that the "freedom of string theory" is one of the largest problems in string theory at the moment. People are trying to come up with realistic configurations (i.e. compactifications etc) which would give a description of the Standard Model and a positive cosmological constant (General Relativity itself already comes along quite natural, I think). The point is that different configurations lead to different excitation spectra of the strings and thus different universes.

But apart from the fact that a 'realistic' configuration has not been found, there is not even a mechanism known which explains why it's this particular configuration that corresponds to our universe. In some sense, string theoy provides way too many different ways of coming up with toy universes.

This is known as the landscape problem.
Thanks very much.

Could I ask what kind of energies are involved to probe strings? Maybe your answer could be in Gev or Fermis -- as I am an experimentalist. And could I ask you to say a few words about what kind of particle states would be created by strings? And what do you think string theory might explain? Would all of the known particle masses come out of the theory? Would QED come out of the theory -- along with simple quantum mechanics? Would Newton's laws come out, along with special and general relativity?

What evidence now exists to support the existence of strings?

I am a former physics professor and am going to be on a national radio program this Friday. May use part of your answers on the air -- if I am asked any questions about string theory. My main focus on the program will relate to exotic physics phenomena to include scalar electromagnetic weapons.

Thanks again.

RoKo

malawi_glenn
Homework Helper
The energies are around the Planck Scale, like 10e19 MeV

However, String Theory 'predicts' extra dimensions to exists, and if they are "big enough" they might be discovered at LHC @ CERN.

String Theory requires special relativity, but as a result, general relativity will come out - as the weak limit of a higher order theory of relativistic gravity.

One hopes to solve/find QCD from string theory http://en.wikipedia.org/wiki/String_theory#Gauge-gravity_duality

No experimental observation supports strings so far.

The energies are around the Planck Scale, like 10e19 MeV

However, String Theory 'predicts' extra dimensions to exists, and if they are "big enough" they might be discovered at LHC @ CERN.

String Theory requires special relativity, but as a result, general relativity will come out - as the weak limit of a higher order theory of relativistic gravity.

One hopes to solve/find QCD from string theory http://en.wikipedia.org/wiki/String_theory#Gauge-gravity_duality

No experimental observation supports strings so far.
Thanks much.

Best wishes,

RoKo