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No Theory of Everything is going to make realistically testable predictions. That's a problem of the domain, not the theory. The unification energy between the

by jfengel 11mo ago

No Theory of Everything is going to make realistically testable predictions. That's a problem of the domain, not the theory. The unification energy between the graviton and quantum field theory is on the order of 10^19 GeV, over a dozen orders of magnitude beyond anything we can generate. We might get lucky that some ToE would generate low-energy predictions different from GR and QFT, but there's no reason to think that it must. It's not like there's some great low-energy predictions that we're just ignoring. The difficulty of a beyond-Standard-Model theory is inherent to the domain of the question, and that's going to plague any alternative to String Theory just as much.

munchler 11mo ago
I think that’s highly debatable. For example, dark matter particles with testable properties could be a prediction of a ToE. Or the ToE could resolve the quantum measurement problem (collapse of the wave function) in a testable way.
- yablak 11mo ago
  What's the "quantum measurement problem"? And why is it a problem? I get the wave function collapses when you measure bit. But which part of this do you want to resolve in a testable way?
  - munchler 11mo ago
    It’s the question of how the wave function collapses during a measurement. What exactly constitutes a “measurement”? Does the collapse happen instantaneously? Is it a real physical phenomenon or a mathematical trick?
    - yablak 11mo ago
      I thought that what constitutes a measurement is well understood; it's just the entanglement between the experiment and the observer, and the process is called decoherence - and the collapse itself is a probabilistic process as a result. AFAIK an EoT is not required to design experiments to determine if it's a real physical phenomenon vs. a mathematical trick; people are trying to think up those experiments now (at least for hidden variable models of QM).
      - munchler 11mo ago
        https://en.wikipedia.org/wiki/Measurement_problem https://en.wikipedia.org/wiki/Measurement_problem
        canjobear 11mo ago
        https://en.wikipedia.org/wiki/Quantum_decoherence https://en.wikipedia.org/wiki/Quantum_decoherence
rhdunn 11mo ago
The testable predictions would be at the places where QM and GR meet. Some examples: 1. interactions at the event horizon of a black hole -- could the theory describe Hawking radiation? 2. large elements -- these are where special relativity influences the electrons [1] It's also possible (and worth checking) that a unified theory would provide explanations for phenomena and observed data we are ascribing to Dark Matter and Dark Energy. I wonder if there are other phenomena such as effects on electronics (i.e. QM electrons) in GR environments (such as geostationary satellites). Or possibly things like testing the double slit experiment in those conditions. [1] https://physics.stackexchange.com/questions/646114/why-do-relativistic-effects-come-into-play-when-dealing-with-superheavy-atoms https://physics.stackexchange.com/questions/646114/why-do-re...
- Jabbles 11mo ago
  re 2: special relativity is not general relativity - large elements will not provide testable predictions for a theory of everything that combines general relativity and quantum mechanics. re: "GR environments (such as geostationary satellites)" - a geostationary orbit (or any orbit) is not an environment to test the interaction of GR and QM - it is a place to test GR on its own, as geostationary satellites have done. In order to test a theory of everything, the gravity needs to be strong enough to not be negligible in comparison to quantum effects, i.e. black holes, neutron stars etc. your example (1) is therefore a much better answer than (2)
  - rhdunn 11mo ago
    Re 2 I was wondering if there may be some GR effect as well, as the element's nucleus would have some effect on spacetime curvature and the electrons would be close to that mass and moving very fast. For geostationary orbits I was thinking of things like how you need to use both special and general relativity for GPS when accounting for the time dilation between the satellite and the Earth (ground). I was wondering if similar things would apply at a quantum level for something QM related so that you would have both QM and GR at play. So it may be better to have e.g. entangled particles with them placed/interacting in a way that GR effects come into play and measuring that effect. But yes, devising tests for this would be hard. However, Einstein thought that we wouldn't be able to detect gravitational waves, so who knows what would be possible.
deleted 11mo ago
[deleted]
jcranmer 11mo ago
I'm far from an expert in this field--indeed, I can but barely grasp the gentle introductions to these topics--but my understanding is that calling string theory a "theory of everything" really flatters it. String theory isn't a theory; it's a framework for building theories. And no one (to my understanding) has been able to put forward a theory using string theory that can actually incorporate the Standard Model and General Relativity running in our universe to make any prediction in the first place, much less one that is testable.
- colechristensen 11mo ago
  Getting into the weeds about what is and is not "A Theory" is an armchair scientist activity, it's not a useful exercise. Nobody in the business of doing physics cares or grants "theory status" to a set of models or ideas. Some physicists have been trying to build an updated model of the universe based on mathematical objects that can be described as little vibrating strings. They've not been successful in closing the loop and constructing a model that actually describes reality accurately, but they've done a lot of work that wasn't necessarily all to waste. It's probably either just the wrong abstraction or missing some fundamental changes that would make it accurate. It would also be tremendously helpful if we had some new physics where there was a significant difference between an experiment and either GR or the standard model. Unfortunately the standard model keeps being proven right.
griffzhowl 11mo ago
There's a more basic problem with string theory, which is that it's not a theory. It's a mathematical framework which is compatible with a very wide range of specific physical theories. About tests of quantum gravity, there have been proposals for feasible tests using gravitationally-induced entanglement protocols: https://arxiv.org/abs/1707.06036 https://arxiv.org/abs/1707.06036
- RossBencina 11mo ago
  I don't think that's quite the problem. In mathematics, the word "theory" is often used when referring to particular mathematical frameworks (e.g. Group Theory, Graph Theory, Morse Theory). In that sense I think String Theory is very much a theory. As you imply, in physics, the word "theory" is typically used in a different sense. I'm not a physicist but I presume a physical theory has to be verifiable, consistent with observations, able to predict the behavior of unexplained phenomena. If I understand correctly, the basic problem is that in some quarters string theory is being passed off as a physical theory. I know of pure mathematicians who are interested in string theory and who couldn't care less whether its a physical theory.
  - griffzhowl 11mo ago
    Yes, that's what I meant: it's not a physical theory in the sense of making a well-defined set of predictions about the actual world.
    - colechristensen 11mo ago
      The word "theory" doesn't matter in the way you are portraying it as. Like a book is a book because it's got pages with words on them glued to a spine with covers. It's not "not a book" because the plot makes no sense. Scientists don't care about what "a theory" is, it's not philosophically important to them. It's just a vague term for a collection of ideas or a model or whatever.
      - griffzhowl 11mo ago
QuadmasterXLII 11mo ago
i mean, a theory of everything should at least make retrodictions, which afaik string theory never got to. if someone wants to point me to where someone solved e.g. the hydrogen spectrum using a string theory, then I will be wrong but very happy
- throwaway81523 11mo ago
  I remember a video talk by Witten where he said string theory predicts the existence of gravity, and nothing else does.
fguerraz 11mo ago
> The unification energy between the graviton and quantum field theory is on the order of 10^19 GeV, over a dozen orders of magnitude beyond anything we can generate. lol the confidence.

by jfengel 11mo ago

munchler 11mo ago

I think that’s highly debatable. For example, dark matter particles with testable properties could be a prediction of a ToE. Or the ToE could resolve the quantum measurement problem (collapse of the wave function) in a testable way.

yablak 11mo ago
What's the "quantum measurement problem"? And why is it a problem? I get the wave function collapses when you measure bit. But which part of this do you want to resolve in a testable way?
- munchler 11mo ago
  It’s the question of how the wave function collapses during a measurement. What exactly constitutes a “measurement”? Does the collapse happen instantaneously? Is it a real physical phenomenon or a mathematical trick?
  - yablak 11mo ago
    I thought that what constitutes a measurement is well understood; it's just the entanglement between the experiment and the observer, and the process is called decoherence - and the collapse itself is a probabilistic process as a result. AFAIK an EoT is not required to design experiments to determine if it's a real physical phenomenon vs. a mathematical trick; people are trying to think up those experiments now (at least for hidden variable models of QM).
    - munchler 11mo ago
      https://en.wikipedia.org/wiki/Measurement_problem https://en.wikipedia.org/wiki/Measurement_problem
      - canjobear 11mo ago
        https://en.wikipedia.org/wiki/Quantum_decoherence https://en.wikipedia.org/wiki/Quantum_decoherence

rhdunn 11mo ago

The testable predictions would be at the places where QM and GR meet. Some examples: 1. interactions at the event horizon of a black hole -- could the theory describe Hawking radiation? 2. large elements -- these are where special relativity influences the electrons [1] It's also possible (and worth checking) that a unified theory would provide explanations for phenomena and observed data we are ascribing to Dark Matter and Dark Energy. I wonder if there are other phenomena such as effects on electronics (i.e. QM electrons) in GR environments (such as geostationary satellites). Or possibly things like testing the double slit experiment in those conditions. [1] https://physics.stackexchange.com/questions/646114/why-do-relativistic-effects-come-into-play-when-dealing-with-superheavy-atoms https://physics.stackexchange.com/questions/646114/why-do-re...

Jabbles 11mo ago
re 2: special relativity is not general relativity - large elements will not provide testable predictions for a theory of everything that combines general relativity and quantum mechanics. re: "GR environments (such as geostationary satellites)" - a geostationary orbit (or any orbit) is not an environment to test the interaction of GR and QM - it is a place to test GR on its own, as geostationary satellites have done. In order to test a theory of everything, the gravity needs to be strong enough to not be negligible in comparison to quantum effects, i.e. black holes, neutron stars etc. your example (1) is therefore a much better answer than (2)
- rhdunn 11mo ago
  Re 2 I was wondering if there may be some GR effect as well, as the element's nucleus would have some effect on spacetime curvature and the electrons would be close to that mass and moving very fast. For geostationary orbits I was thinking of things like how you need to use both special and general relativity for GPS when accounting for the time dilation between the satellite and the Earth (ground). I was wondering if similar things would apply at a quantum level for something QM related so that you would have both QM and GR at play. So it may be better to have e.g. entangled particles with them placed/interacting in a way that GR effects come into play and measuring that effect. But yes, devising tests for this would be hard. However, Einstein thought that we wouldn't be able to detect gravitational waves, so who knows what would be possible.

deleted 11mo ago

[deleted]

jcranmer 11mo ago

I'm far from an expert in this field--indeed, I can but barely grasp the gentle introductions to these topics--but my understanding is that calling string theory a "theory of everything" really flatters it. String theory isn't a theory; it's a framework for building theories. And no one (to my understanding) has been able to put forward a theory using string theory that can actually incorporate the Standard Model and General Relativity running in our universe to make any prediction in the first place, much less one that is testable.

colechristensen 11mo ago
Getting into the weeds about what is and is not "A Theory" is an armchair scientist activity, it's not a useful exercise. Nobody in the business of doing physics cares or grants "theory status" to a set of models or ideas. Some physicists have been trying to build an updated model of the universe based on mathematical objects that can be described as little vibrating strings. They've not been successful in closing the loop and constructing a model that actually describes reality accurately, but they've done a lot of work that wasn't necessarily all to waste. It's probably either just the wrong abstraction or missing some fundamental changes that would make it accurate. It would also be tremendously helpful if we had some new physics where there was a significant difference between an experiment and either GR or the standard model. Unfortunately the standard model keeps being proven right.

griffzhowl 11mo ago

There's a more basic problem with string theory, which is that it's not a theory. It's a mathematical framework which is compatible with a very wide range of specific physical theories. About tests of quantum gravity, there have been proposals for feasible tests using gravitationally-induced entanglement protocols: https://arxiv.org/abs/1707.06036 https://arxiv.org/abs/1707.06036

RossBencina 11mo ago
I don't think that's quite the problem. In mathematics, the word "theory" is often used when referring to particular mathematical frameworks (e.g. Group Theory, Graph Theory, Morse Theory). In that sense I think String Theory is very much a theory. As you imply, in physics, the word "theory" is typically used in a different sense. I'm not a physicist but I presume a physical theory has to be verifiable, consistent with observations, able to predict the behavior of unexplained phenomena. If I understand correctly, the basic problem is that in some quarters string theory is being passed off as a physical theory. I know of pure mathematicians who are interested in string theory and who couldn't care less whether its a physical theory.
- griffzhowl 11mo ago
  Yes, that's what I meant: it's not a physical theory in the sense of making a well-defined set of predictions about the actual world.
  - colechristensen 11mo ago
    The word "theory" doesn't matter in the way you are portraying it as. Like a book is a book because it's got pages with words on them glued to a spine with covers. It's not "not a book" because the plot makes no sense. Scientists don't care about what "a theory" is, it's not philosophically important to them. It's just a vague term for a collection of ideas or a model or whatever.
    - griffzhowl 11mo ago

QuadmasterXLII 11mo ago

i mean, a theory of everything should at least make retrodictions, which afaik string theory never got to. if someone wants to point me to where someone solved e.g. the hydrogen spectrum using a string theory, then I will be wrong but very happy

throwaway81523 11mo ago
I remember a video talk by Witten where he said string theory predicts the existence of gravity, and nothing else does.

fguerraz 11mo ago

> The unification energy between the graviton and quantum field theory is on the order of 10^19 GeV, over a dozen orders of magnitude beyond anything we can generate. lol the confidence.