A question about Hawking's path integral methods

Click For Summary

Discussion Overview

The discussion revolves around Hawking's path integral methods, particularly focusing on the implications of superpositions of different metrics in the context of quantum gravity and decoherence. Participants explore the validity of these superpositions and their stability, as well as the relationship between topology and coherence in metrics.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Technical explanation

Main Points Raised

  • One participant questions the meaningfulness of superpositions of different metrics, suggesting that decoherence may destroy them and referencing Penrose's claim about the instability of such superpositions.
  • Another participant proposes that coherent superpositions may only exist for topologically similar metrics, indicating a limitation in the applicability of Hawking's methods.
  • A participant clarifies that their interest lies in the method of path integrals rather than specific results related to black holes, emphasizing the challenge of superposing vastly different states, such as macroscopic objects.
  • Some participants suggest that adding degrees of freedom could resolve the issues raised regarding superpositions, though the rationale behind this suggestion is not fully articulated.
  • A later reply seeks clarification on what is meant by "adding degrees of freedom" and questions how it addresses the initial concerns about superpositions.

Areas of Agreement / Disagreement

Participants express differing views on the validity and implications of superpositions in Hawking's path integral methods, with no consensus reached on the stability or coherence of such superpositions.

Contextual Notes

Participants note potential limitations in understanding the relationship between decoherence, topology, and the construction of superpositions, but these remain unresolved within the discussion.

Fredrik
Staff Emeritus
Science Advisor
Homework Helper
Insights Author
Gold Member
Messages
10,876
Reaction score
423
Hawking's path integral methods seem to rely on the assumption that superpositions of different metrics are meaningful. (If I'm wrong about this, let me know). But are they? Aren't these superpositions destroyed by decoherence. And aren't they also in contradiction with Penrose's claim that there is no well-defined time evolution operator on a superposition of two spacetimes, and that such superpositions should therefore be highly unstable?
 
Physics news on Phys.org
Since nobody answered yet, I give my opinion, but be warned : I might be wrong too since I am not a specialist. I think there is no coherent superposition of topologically dinstinct metrics. If I understood correctly, there is coherent superposition only of topologically similar metrics. You should use one of the other threads where discussions already began on this problem.
 
The reason I started a new thread is that my question is not really about Hawking's recent result about black holes. It's about the method he used to obtain that result and many others. I hope this will be a discussion about superpositions, and not about spacetime geometry and topology.

Let me explain more clearly what I mean.

When we use path integral methods to compute the probability of an event, we don't include amplitudes for both "the cat's dead" and "the cat's alive".

The reason is that it's not possible to construct a superposition between two very different states of a macroscopic object. Some people would blame this on decoherence caused by interactions with the environment. Others (especially Roger Penrose) would blame it on a quantum gravity effect. Penrose claims that gravity makes macroscopic superpositions impossible.

If we don't add the amplitudes for "the cat's dead" and "the cat's alive" because a cat's big and heavy, why would it make sense to add amplitudes for two different distributions of matter throughout the entire universe over a period of 13.7 billion years?!
 
Add degrees of freedom and your problem is solved. Hawking is very smart. He is not infallible.
 
Chronos said:
Add degrees of freedom and your problem is solved.
I have no idea what you're trying to say here. What degrees of freedom? And why? How does it solve the problem?
 

Similar threads

  • · Replies 6 ·
Replies
6
Views
2K
  • · Replies 1 ·
Replies
1
Views
2K
  • · Replies 19 ·
Replies
19
Views
4K
  • · Replies 11 ·
Replies
11
Views
4K
  • · Replies 8 ·
Replies
8
Views
2K
  • · Replies 3 ·
Replies
3
Views
2K
  • · Replies 26 ·
Replies
26
Views
5K
Replies
1
Views
2K
  • · Replies 4 ·
Replies
4
Views
3K
  • · Replies 12 ·
Replies
12
Views
2K