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-re...

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)

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.

Can't black holes explain Dark Energy? Supposedly there was an experiment showing Black Holes are growing faster than expected. If this is because they are tied to the expansion of the universe (univ. expands -> mass grows), and that tie goes both ways (mass grows -> universe expands), boom, dark energy. I also think that inside the black holes they have their own universes which are expanding (and that we're probably inside one too). If this expansion exerts a pressure on the event horizon which transfers out, it still lines up.

No.

You don't need a full fledged theory of quantum gravity to describe Hawking radiation. Quantization of the gravitational field isn't relevant for that phenomenon. Similarly you don't need quantum gravity to describe large elements. Special relativity is already integrated into quantum field theory.

In some ways saying that we don't have a theory of quantum gravity is overblown. It is perfectly possible to quantize gravity in QFT the same way we quantize the electromagnetic field. This approach is applicable in almost all circumstances. But unlike in the case of QED, the equations blow up at high energies which implies that the theory breaks down in that regime. But the only places we know of where the energies are high enough that the quantization of the gravitational field would be relevant would be near the singularity of a black hole or right at the beginning of the Big Bang.

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.

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?

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?

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).

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.

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.

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

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.

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.

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.

I guess I'm not being clear. I don't care about the word "theory". The point is string theory makes no predictions. It's not just inaccessible energies which make it untestable, but the fact that, as a framework, string theory is compatible with a huge range of possible universes.

Eh, again you're just saying "it's not real because..." whether or not you attach the word theory. It is a space with lots of possible parameters being explored and is not one set of parameters with predictions because all of the sets that have been explored so far are either broken or don't represent reality. That in itself does not mean it doesn't have merit. It's simply incomplete, but given its history it's fair to doubt that it ever will close the loop and have a final form that models our reality.

I'm not sure what you're disagreeing with tbh. If you look up the thread, my point is that the reason string theory is untestable is not simply that high energies are experimentally inaccessible. Rather, string theory doesn't make any definite predictions for those high energies either. It seems you agree with that?

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

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

> 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.