Grand unified theories and quantum mechanics

[Vals509]

Why is it that general relativity and quantum mechanics are so different and that physicists are having difficulty combining them.

Please give me a simple answer. I scoured the whole net and each website has its own opinion.

 

[malawi_glenn]

hehe and you do not think that people answering here will give his/her opinion? ;-)

To me, the most difficult thing is that the quanta of gravity must have spin 2 and be massless, and you can't construct a quantum field theory which is Lorentz-invariant and renormalizable. This is wienberg-witten theorem.

 

[confinement]

I agree with malawi_glenn, and another important reason is that classically the equations of General Relativity develop singularities (black holes) within a finite time. The classical analogues of our quantum fields for the color force, weak force, and e&m do not have this behavior.

 

[malawi_glenn]

Mass has not anti-mass in the same way as we have positive and negative charges etc.

 

[koolmodee]

To me, the most difficult thing is that the quanta of gravity must have spin 2 and be massless, and you can't construct a quantum field theory which is Lorentz-invariant and renormalizable. This is wienberg-witten theorem.

Could you put that clearer? Do you find it difficult that
- quanta of gravity must have spin 2 and be massless
and
- can't construct a quantum field theory which is Lorentz-invariant and renormalizable

If so, why do you find it difficult?

 

[malawi_glenn]

You can show that gravitons must have that property within the standard model and then the rest follows from Wienberg-Witten theorem.

The difficultly is that you can't construct a quantum field theory which is lorentz invariant and renormalizable with that property of the graviton.