M Theory: Exploring Gravity's Necessity in the Standard Model

• orthovector
In summary, M theory forces gravity as a necessity when the standard model couldn't fit in gravity because it uses vibrational modes of strings to include gravity. String theory is not part of the standard model, but it has been proven that the Einstein-Cartan action is not renormalizable.
orthovector
How does M theory force gravity as a necessity when the standard model couldn't fit in gravity?

Why should the standard model "fit in gravity"?

M-theory should have some supergravity theory as low energy limit. Are you asking if the standard model fits into some supergravity theory?

String theory didn't "force" gravity; although never intended to represent gravity, when a spin 2 massless particle appeared it was later discovered: wow, a graviton! And string theory is not part of the standard model...

orthovector said:
How does M theory force gravity as a necessity when the standard model couldn't fit in gravity?

I think it works like this:

The standard model describes the forces using gauge groups. When you construct your forces this way, you cannot include gravity without the math breaking. This is because the gravitational force (the graviton, rather) is a "spin 2 boson", and a spin 2 particle cannot be "renormalized", which is a mathematical procedure you have to be able to do for the standard model to be usable.

M theory describes the forces using vibrational modes of strings. When you construct your forces this way, not only can you have a spin 2 particle, you always have a spin 2 particle. Every string theory you construct has a spin 2 boson as one of its vibrational modes, and this is the graviton.

Was it proven that "spin-2 interactions" can not be renormalized ? Was it proven that string theory is finite to all orders ?

It's not ironic, it's a serious question. As far as I know, none have been proven, but I only follow as an amateur. After periods of hopes, there has been for a few decades a general belief that no practical definition (allowing computations) of "spin-2 gravitation" would be perturbatively renormalizable, but even this belief has been challenged again lately.

It has been proven that the Einstein-Cartan action is not renormalizable in perturbarion theory. Each order in perturabtion theory produces new types in infinities and counter terms.

There are recent results that certain supergravity models might be finite to all orders w/o infinities (they cancel in each order due to the large symmetry).

It has not been proven that string theory is finite to all orders. In is known hat certain infinities are absent, but as far as I know it is not clear if no new infinitires could appear at higher genus.

It has not been proven that summing the prturbaruon series produces a finite answer. I guess that the seroies is (as usual) an asymprtotoc series which diverges eventually. One reason is that you simply neglect non-perturbatibe aspects. This is comparable to QCD: the series itself is divergent (as you can see from the scaling of running coupling constant);

humanino said:
Was it proven that string theory is finite to all orders ?

I will say, it has always seemed a bit weird to me that we match the problem "if we add a graviton to the standard model, no one anymore knows how to calculate anything" with the solution "we will switch to String Theory, where no one knows how to calculate anything". Did we solve the problem here or just hide it behind a bigger one...?

My feeling is that string theory is currebtly the most ambitious program to unify all interactions; nevertheless it has some serious problems:
- no central equation of principle for the whole program
- perturbative finiteness only wishful thinking
- unified theory (M-theory) not rigorously constructed
- no background independence (= requires flat space-time!)
- not falsifiable by experiment

But there is currently no alternative! Loop Quantum Gravity (which is in my opinion very successfull) addresses only gravity.

1. What is M Theory?

M Theory is a theoretical framework that attempts to reconcile the fundamental laws of physics, particularly gravity, with the Standard Model of particle physics.

2. Why is M Theory important?

M Theory is important because it provides a potential solution to several long-standing problems in physics, such as the unification of gravity with the other fundamental forces and the existence of multiple dimensions.

3. How does M Theory explain the necessity of gravity in the Standard Model?

M Theory proposes that gravity is an inherent property of 11-dimensional space-time, and that our perceived 4-dimensional universe is just a small part of this larger space. This allows for the unification of gravity with the other forces, as they all arise from the structure of space-time.

4. What evidence supports M Theory?

Currently, there is no direct evidence for M Theory, as it is still a theoretical framework. However, it has been used to make predictions about phenomena such as the behavior of black holes and the early universe, which can be tested through observations and experiments.

5. What are the potential implications of M Theory?

If M Theory is proven to be true, it would revolutionize our understanding of the universe and potentially lead to new technologies and advancements. It could also provide a deeper understanding of the fundamental laws of nature and the origin of the universe.

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