Why strings and not zero dimensional fundumental particles?

[snackster17]

Im aware that there has been some difficulties in zero dimensional particle calculations, which I suppose gave way to string theory which explains gravity. But i would like to have some more input on the exact problems when dealing with zero dimensional particles and why strings were added. Are there not two types of strings?

 

[Kevin_Axion]

This is from Polchinski's book stating the attributes of having a Superstring Theory to having a point particle Quantum Field Theory - SU(3)xSU(2)xU(1) which doesn't include gravity:

"1. Gravity. Every consistent String Theory must contain a masless spin-2 [vibrational] state, whose interactions reduce at low energy to general relativity.

2. A consistent theory of quantum gravity, at least in perturbation theory. As we have noted, this is in contrast to all known quantum field theories of gravity.

3. Grand unification. String theories lead to gauge groups large enough to include the Standard Model. Some of the simplest string theories lead to the same gauge groups an fermion representations that arise in the unification of the Standard Model.

4. Extra dimensions. String theory requires a definite number of space-time dimensions, ten [or 11 in M-Theory]. The field equations have solutions with four large flat and six small curved dimensions, with four dimensional physics that resembles the Standard Model.

5. Supersymmetry. Consistent String Theories require space-time supersymmetry, as either a manifest or a spontaneously broken symmetry

6. Chiral gauge couplings. The gauge interactions in nature are parity asymmetric (chiral). This has been a stumbling block for a number of previous unifying ideas: they required parity symmetric gauge couplings. String theory allows chiral gauge couplings.

7. No free parameters. String theory has no adjustable constants.

8. Uniqueness. Not only are there no continuous parameters, but there is no discrete freedom analogous to the choice of gauge group and representations in field theory: there is a unique string theory." - (String Theory, Volume 1: An Introduction to Bosonic String, Polchinski)

Hope this helps, Kevin

 

[atyy]

The main difficulty with using point particles (ie. quantum field theory) to describe gravity is that it perturbatively nonrenormalizable.

There is some chance that a quantum field theory of gravity might be nonperturbatively renormalizable, in which case one could potentially use point particles to describe gravity. This line of research is called Asymptotic Safety.

Whether Asymptotic Safety can work is unknown, and the perturbative nonrenormalizability of gravity suggests that gravity is not a fundamental field, but is made up of more fundamental things. There is a theorem by Weinberg and Witten stating that if gravity is made up of more fundamental things, these things cannot be special relativistic quantum field theories in 4 dimensions.

The idea that gravity is made of strings evades the Weinberg-Witten theorem, as does the idea that gravity is made of a relativistic quantum field theory in 3D. Amazingly, there is much evidence to support the idea that some 10D string theories are relativistic quantum field theories (ie. point particles) in a lower dimension (4D)! This is called AdS/CFT or gauge/gravity duality.

 

[snackster17]

Thanks for the replies. String theory seems to work well but its not accepted because its impossible to test, correct?
Do strings and point particles have any attributes such as spin and charge such as other particles? I once read that point particles appear and disappear instantly which makes me think that there formulation was more of a mathematical concept than a actual physical process.

Thats pretty crazy 10 dimensional strings effecting gravity. Can i have a link if possible? Once again thanks for the replies

 

[Kevin_Axion]

It's not impossible to test, just the primary ideas in superstring theory are very difficult. For instance, supersymmetry is a direct prediction of superstring theory and is testable at the LHC. Also the permeation of a graviton into a higher dimension is possible but is extremely unlikely because gravitons don't interact strongly. Yes the vibrational states of fundamental strings are what give the particles their charge, mass, spin - all of their properties.

 

[bcrowell]

String theory seems to work well but its not accepted because its impossible to test, correct?

It's not just that string theory makes predictions, but those predictions can't be tested; the theory is not yet able to make predictions at all.

 

[atyy]

Thats pretty crazy 10 dimensional strings effecting gravity. Can i have a link if possible? Once again thanks for the replies

http://arxiv.org/abs/0908.0333
http://arxiv.org/abs/hep-th/0207142

http://online.kitp.ucsb.edu/online/spheno10/

 

[snackster17]

Despite all of this particles still do exist though in string theory correct? the strings are what gives them there properties though.

 

[Kevin_Axion]

The strings are the particles, for instance if you had a string that was one centimeter in circumference, up close it would appear to be topologically the same as a circle. Now if you move backward by 100 meters the string is topologically the same as the point. Essentially strings are so small that the current energies or distances we can see with the LHC isn't small enough to distinguish them from points.

 

[snackster17]

i think i see it now. all of the particles of the standard models are just part of the string, you can't see the rest.

 

[Kevin_Axion]

Every particle is a different harmonic of a fundamental string, there is a fundamental frequency and the more harmonic partials you have the more energy it has hence more mass, other properties are determined similarly. Look at this: http://www.pbs.org/wgbh/nova/elegant/resonance.html.