Grand unified theories and quantum mechanics

Click For Summary

Discussion Overview

The discussion revolves around the challenges of reconciling general relativity with quantum mechanics, focusing on the fundamental differences between the two theories and the implications for grand unified theories. Participants explore various aspects of this problem, including theoretical constraints and the nature of gravity at the quantum level.

Discussion Character

  • Debate/contested
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant questions why general relativity and quantum mechanics are fundamentally different and highlights the difficulty in combining them.
  • Another participant points out that the quanta of gravity must have spin 2 and be massless, referencing the Weinberg-Witten theorem, which complicates the construction of a Lorentz-invariant and renormalizable quantum field theory.
  • A different viewpoint emphasizes that general relativity's equations can develop singularities, such as black holes, within a finite time, unlike the classical analogues of quantum fields for other forces.
  • One participant notes that mass does not have an anti-mass counterpart analogous to positive and negative charges, suggesting a fundamental difference in how gravity operates compared to other forces.
  • Another participant reiterates the challenge of constructing a quantum field theory that is both Lorentz-invariant and renormalizable given the properties required of gravitons.

Areas of Agreement / Disagreement

Participants express differing views on the specific challenges posed by the properties of gravity and the implications of the Weinberg-Witten theorem. There is no consensus on a singular explanation for the difficulties in unifying general relativity and quantum mechanics.

Contextual Notes

Participants reference specific theoretical constraints, such as the requirement for gravitons to have certain properties, and the implications of singularities in general relativity, but do not resolve these complexities or assumptions.

Vals509
Messages
52
Reaction score
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.
 
Physics news on Phys.org
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.
 
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.
 
Mass has not anti-mass in the same way as we have positive and negative charges etc.
 
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?
 
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.
 

Similar threads

Graduate Multiparticle Relativistic Quantum Mechanics in an external potential
  • · Replies 22 ·
Replies
22
Views
3K
Undergrad Why is m_j not a good quantum number in strong-field Zeeman effect?
  • · Replies 2 ·
Replies
2
Views
2K
Undergrad What Are the Key Questions and Challenges in Understanding Quantum Gravity?
  • · Replies 5 ·
Replies
5
Views
2K
Graduate Resolving the Paradox: Combining Quantum Mechanics and Special Relativity
  • · Replies 10 ·
Replies
10
Views
2K
Graduate Can Quantum Mechanics be postulated to exclude humans?
  • · Replies 44 ·
2
Replies
44
Views
6K
Graduate Quantum superposition: Is a problem of space-time worldview?
  • · Replies 1 ·
Replies
1
Views
2K
Graduate Quantum Field theory vs. many-body Quantum Mechanics
  • · Replies 36 ·
2
Replies
36
Views
7K
High School Quantum Mechanics is not a theory?
  • · Replies 19 ·
Replies
19
Views
2K
Undergrad QED/Quantum Mechanics: Probability or Spatial Function?
  • · Replies 8 ·
Replies
8
Views
3K
Undergrad Relativistic Quantum Mechanics vs Quantum Field Theory
  • · Replies 5 ·
Replies
5
Views
2K