Many worlds interpretation incompatible with quantum gravity

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Discussion Overview

The discussion centers around the compatibility of the Many Worlds Interpretation (MWI) of quantum mechanics with theories of quantum gravity, particularly in the context of linearity and non-linearity of wavefunctions. Participants explore theoretical implications, objections, and the speculative nature of string theory and quantum gravity.

Discussion Character

  • Debate/contested
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • Some participants note that if quantum gravity and string theory are nonlinear and confirmed as correct, then MWI could not be true, but emphasize that this is conditional on several "if" statements.
  • There is uncertainty regarding whether string theory is inherently nonlinear with respect to the wave function, with some suggesting it can be linear.
  • One participant expresses doubt about the ability to test string theory experimentally, labeling it as speculative.
  • Another participant mentions that the concept of wave function may not be meaningful in certain quantum gravity theories, such as loop quantum gravity.
  • Some assert that string theory is linear with respect to the wave function, suggesting compatibility with MWI, which assumes the wave function is physically real.
  • Participants discuss a paper that addresses linear relationships in quantum gravity theories, clarifying that it does not directly pertain to the linearity of the theories themselves.
  • There is a reference to the belief that modern theories of quantum gravity are essentially nonlinear, but this belief is distinguished from experimental validation.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the linearity of string theory or its implications for MWI. Multiple competing views and uncertainties remain regarding the nature of quantum gravity theories and their compatibility with MWI.

Contextual Notes

Limitations include the speculative status of string theory and quantum gravity, the lack of experimental evidence for certain claims, and the ambiguity surrounding the definition and relevance of wave functions in different theoretical frameworks.

Nickyv2423
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On wikipedia, I found one of the objections to MWI.
"We cannot be sure that the universe is a quantum multiverse until we have a theory of everything and, in particular, a successful theory of quantum gravity.[73] If the final theory of everything is non-linear with respect to wavefunctions then many-worlds would be invalid
MWI response: All accepted quantum theories of fundamental physics are linear with respect to the wavefunction. While quantum gravity or string theory may be non-linear in this respect there is no evidence to indicate this at the moment.[17][18]"
So is this saying since quantum gravity and string theory are non-linear that many worlds cannot be true?
 
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Nickyv2423 said:
is this saying since quantum gravity and string theory are non-linear that many worlds cannot be true?

No, it's saying that if quantum gravity and string theory turned out to be nonlinear, and if as nonlinear theories they were confirmed to be the correct fundamental theory, then MWI could not be true. But that's a big if.
 
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PeterDonis said:
No, it's saying that if quantum gravity and string theory turned out to be nonlinear, and if as nonlinear theories they were confirmed to be the correct fundamental theory, then MWI could not be true. But that's a big if.
But isn't string theory already non-linear
 
Nickyv2423 said:
isn't string theory already non-linear

I'm not sure it's "nonlinear with respect to the wave function"; or more precisely, I'm not sure that it has to be. And in any case, string theory is speculative at this point; there is no way to test it experimentally now or for the foreseeable future.
 
PeterDonis said:
I'm not sure it's "nonlinear with respect to the wave function"; or more precisely, I'm not sure that it has to be. And in any case, string theory is speculative at this point; there is no way to test it experimentally now or for the foreseeable future.
Are there quantum gravity theories that are linear with respect to the wave function?
 
Nickyv2423 said:
Are there quantum gravity theories that are linear with respect to the wave function?

AFAIK string theory can be; "string theory" doesn't really describe one theory, it describes a whole family of theories.

I don't know enough about other quantum gravity candidates to know where they stand on linearity with respect to the wave function, or if "wave function" is even a meaningful concept in them at the fundamental level (it doesn't seem to be in loop quantum gravity, for example). My general impression is that this is an open question at this point.
 
Nickyv2423 said:
But isn't string theory already non-linear
String theory is linear with respect to the wave function. I have never heard that someone even considers the possibility that it might be non-linear.
 
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Demystifier said:
String theory is linear with respect to the wave function. I have never heard that someone even considers the possibility that it might be non-linear.
So does that mean that string theory is compatible with the many worlds interpretation (which assumes wave function is physically real)?
 
Nickyv2423 said:
So does that mean that string theory is compatible with the many worlds interpretation (which assumes wave function is physically real)?
Yes.
 
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physika said:
it seems that it is not linear (Quantum Gravity)

That's not what the paper is saying. The term "linear" in the paper refers to a postulated linear relationship between photon energy and the speed of light in some quantum gravity theories. That has nothing to do with whether the theory itself is linear or nonlinear.
 
  • #12
PeterDonis said:
That's not what the paper is saying. The term "linear" in the paper refers to a postulated linear relationship between photon energy and the speed of light in some quantum gravity theories. That has nothing to do with whether the theory itself is linear or nonlinear.

from bibliograpy of
http://www.openu.ac.il/personal_sites/yoni-granot/papers/GRB090510_Nature.pdf

Zloshchastiev, K. G. Logarithmic nonlinearity in theories of quantum gravity: origin of time and observational consequences.

http://link.springer.com/article/10.1134/S0202289310040067
https://arxiv.org/pdf/0906.4282v5.pdf

"Starting from a generic generally covariant classical theory we introduce the logarithmic correction to the quantum wave equation. We demonstrate the emergence of the evolution time from the group of automorphisms of the von Neumann algebra governed by this non-linear correction. It turns out that such time parametrization is essentially energy-dependent and becomes global only asymptotically - when the energies get very small comparing to the effective quantum gravity scale. Similar thing happens to the Lorentz invariance - in the resulting theory it becomes an asymptotic low-energy phenomenon. We show how the logarithmic non-linearity deforms the vacuum wave dispersion relations and explains certain features of the astrophysical data coming from recent observations of high-energy cosmic rays. In general, the estimates imply that ceteris paribus the particles with higher energy propagate slower than those with lower one, therefore, for a high-energy particle the mean free path, lifetime in a high-energy state and, therefore, travel distance from the source can be significantly larger than one would expect from the conventional theory"

"In the conventional quantum mechanics the linearity of the wave equation is something which is implicitly presupposed, yet the possibility of the non-linear generalization has not been ruled out by experiment"

"the modern theory of quantum gravity is believed to be essentially non-linear - because the propagating particle will cause the quantum fluctuations in gravitational medium which will react back.".
 
  • #13
physika said:
the modern theory of quantum gravity is believed to be essentially non-linear

"Is believed to be" is not the same as "has been experimentally demonstrated to be".
 
  • #14
The OP's question has been answered. Thread closed.
 

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