Why more than 4 spacetime dimensions

[scope]

hi,

I have a very simple question: why is there really a need for more than 4 spacetime dimensions?

are there any concepts for which 4 dimensions are not sufficient?

thank you!

 

[DaveC426913]

hi,

I have a very simple question: why is there really a need for more than 4 spacetime dimensions?

are there any concepts for which 4 dimensions are not sufficient?

thank you!

General Relativity and Quantum Mechanics are two of the most successful theories in a century. Each explains very well their areas of interest - GR explains the very large while QM explains the very small. They are triumphs of science and most of modern physics is built on one or the other.

Only one niggling detail: they are incompatible. When you try to reconcile the two, you get infinities. (In a nutshell: GR says that all particles are points with zero radius. QM's Uncertainty Principle says that, the smaller area of space or the smaller unit of time you examine, the larger uncertainty there is in the amount of energy. Reduce the space or time to zero and the energy is infinite. Put GR and QM together and you get infinities popping up all over the place.)

Something is wrong. Scientists have tried for a goodly part of the 20th century to figure out how they could be reconciled.

String theory does so. It explains virtually everything.It is as close to a Theory Of Everything as we've ever seen. The core of string theory is that all matter and energy is composed of unimaginably tiny, vibrating strings. (That's how it reconciles the "zeroness". Basically, zero is not zero, it's just very small.) ) These vibrations manifest as everything we see today, from photons to gravity to the mass of particles. Only problem is, to do so, they must vibrate in at least five dimensions.

 

[scope]

General Relativity and Quantum Mechanics are two of the most successful theories in a century. Each explains very well their areas of interest - GR explains the very large while QM explains the very small. They are triumphs of science and most of modern physics is built on one or the other.

Only one niggling detail: they are incompatible. When you try to reconcile the two, you get infinities. (In a nutshell: GR says that all particles are points with zero radius. QM's Uncertainty Principle says that, the smaller area of space or the smaller unit of time you examine, the larger uncertainty there is in the amount of energy. Reduce the space or time to zero and the energy is infinite. Put GR and QM together and you get infinities popping up all over the place.)

.

why does GR say that all particles are points with zero radius?

 

[Careful]

why does GR say that all particles are points with zero radius?

GR doesn't say such thing at all ! Actually, it is such an open question in GR as in QM what a particle is supposed to be. People have looked in several directions: black holes, (topological) geons, gravitational waves (of spin 1 and 2) are all classical candidates coming from within GR itself. Then of course, you may add external matter fields, also here it is far from clear how a particle should be defined, plenty of relativists have worked on that and no conclusive answer has been given. Of course, simple geometric objects like point particles and strings are just plain old fashioned ideas about what matter is supposed to be. In my opinion, the ontology of string theory is as old fashioned of the one of Bohm and de Broglie, not very revolutionary at all... The difference is that the former are honest about it, while the latter conveniently forget to mention that even people like Heisenberg had more revolutionary ideas in the 1940 ties.

Careful

 

[DaveC426913]

GR doesn't say such thing at all !

It is an oversimplification.

Can you succinctly address how GR and QM reconcile to produce infinities?