Grand unified theories and quantum mechanics

In summary, physicists are having difficulty combining general relativity and quantum mechanics due to the fundamental differences between the two theories. One major challenge is the fact that the quanta of gravity must have spin 2 and be massless, and it is not possible to construct a quantum field theory that is both Lorentz-invariant and renormalizable with these properties. Additionally, classical equations of general relativity lead to singularities, unlike the classical analogues of other quantum fields. This is known as the Wienberg-Witten theorem.
  • #1
Vals509
52
1
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.
 
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  • #2
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.
 
  • #3
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.
 
  • #4
Mass has not anti-mass in the same way as we have positive and negative charges etc.
 
  • #5
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?
 
  • #6
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.
 

Related to Grand unified theories and quantum mechanics

1. What is a grand unified theory (GUT)?

A grand unified theory is a theoretical framework that attempts to unify all of the known fundamental forces in the universe, specifically the strong, weak, and electromagnetic forces, into a single mathematical model. It aims to explain the fundamental structure of the universe and the interactions between all particles.

2. How does quantum mechanics relate to grand unified theories?

Quantum mechanics is a fundamental theory of nature that describes the behavior of particles at the microscopic level. It is an essential component of grand unified theories as it provides the mathematical framework for understanding the behavior of particles and their interactions within the unified theory.

3. What are the main challenges in developing a grand unified theory?

One of the main challenges in developing a grand unified theory is the inability to experimentally test it at the energies required for unification. Additionally, there are still many unanswered questions in both quantum mechanics and particle physics that must be addressed in order to create a complete and accurate grand unified theory.

4. Are there any experimental evidence or observations that support grand unified theories?

While there is currently no direct experimental evidence for grand unified theories, there have been some successes in experiments that indirectly support the idea. For example, the existence of neutrino oscillations and the idea of baryogenesis can be explained by some GUT models.

5. Can grand unified theories be used to explain the origins of the universe?

Yes, grand unified theories are often linked to theories of cosmic inflation and the Big Bang. These theories propose that the universe underwent a rapid expansion in its early stages, and a grand unified theory can help explain the fundamental forces and particles that were present during this time.

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