A What is the biggest problem to be solved in Quantum Physics in 2023?

[ThiagoMNobrega]

TL;DR Summary

As the field progresses it seems to be an interesting time to hear from the different (or unified) opinions of the community on this question.

Hey everyone,

A quick question that I hope all can participate: In your educated opinion, what is the biggest question or the biggest problem to be solved in quantum mechanics for 2023?

Warm regards,
Thiago Munhoz da Nóbrega

 

[Demystifier]

I would reverse the question. What was the biggest quantum problem that was solved in 2022?

 

[PeroK]

TL;DR Summary: As the field progresses it seems to be an interesting time to hear from the different (or unified) opinions of the community on this question.

Hey everyone,

A quick question that I hope all can participate: In your educated opinion, what is the biggest question or the biggest problem to be solved in quantum mechanics for 2023?

Warm regards,
Thiago Munhoz da Nóbrega

The biggest outstanding question, IMO, is to develop a theory of quantum gravity.

 

[Demystifier]

The biggest outstanding question, IMO, is to develop a theory of quantum gravity.

It will not be solved in 2023. But I am finishing a paper in which I study quantization of toy models (*) with emergent diffeomorphism invariance, with a motivation to shed some light on conceptual problems with quantum gravity. In this paper I solve toy versions of the problem of time in quantum gravity, of the cosmological constant problem, and of the black hole firewall problem. Stay tuned!

(*) By toy models I mean something similar to the toy model for Casimir effect in https://arxiv.org/abs/1702.03291

 

[martinbn]

It will not be solved in 2023. But I am finishing a paper in which I study quantization of toy models (*) with emergent diffeomorphism invariance, with a motivation to shed some light on conceptual problems with quantum gravity. In this paper I solve toy versions of the problem of time in quantum gravity, of the cosmological constant problem, and of the black hole firewall problem. Stay tuned!

(*) By toy models I mean something similar to the toy model for Casimir effect in https://arxiv.org/abs/1702.03291

I disagree with you on what you did in the paper, but i will tell you more once you've written it.

 

[Demystifier]

I disagree with you on what you did in the paper, but i will tell you more once you've written it.

Happy New Year to you too! I think your argument misses the point, but I will ignore it once you've written it.

 

[mitchell porter]

If you want unsolved physics problems to work on, there's 42 of them, on page 49 here.

 

[berkeman]

If you want unsolved physics problems to work on, there's 42 of them,

Another Insights article written by @fresh_42 ?

 

[physika]

The biggest outstanding question, IMO, is to develop a theory of quantum gravity.

Not 2023 ->
Maybe in 2100

 

[Demystifier]

It will not be solved in 2023. But I am finishing a paper in which I study quantization of toy models (*) with emergent diffeomorphism invariance, with a motivation to shed some light on conceptual problems with quantum gravity. In this paper I solve toy versions of the problem of time in quantum gravity, of the cosmological constant problem, and of the black hole firewall problem. Stay tuned!

(*) By toy models I mean something similar to the toy model for Casimir effect in https://arxiv.org/abs/1702.03291

Finished!
https://arxiv.org/abs/2301.04448

 

[timmdeeg]

Are we sure that there exists a theory of quantum gravity?

Supposed someone in the far future claims to have a theory of quantum gravity. It looks good and it is self consistent mathematically. How would we prove its correctness?

 

[vanhees71]

It doesn't exist yet. If somebody claims to have found one, it must be possible to make testable predictions, i.e., one would do experiments checking whether the predictions are correct or not.

 

[timmdeeg]

But which testable predictions do we expect? Probably not predictions describing the state of matter in an astrophysical black hole. Quantized spacetime?

 

[vanhees71]

That's a very good question. I don't know the answer. I think it's the greatest obstacle in finding an idea for a viable quantum description of gravitation that there's no guidance from observation, i.e., the lack of observable quantum effects concerning quantization of gravity.

 

[PeterDonis]

But which testable predictions do we expect?

Predictions about probabilities for different spacetime geometries in suitable experimental configurations. Or predictions about quantum interference effects between different spacetime geometries. Or predictions about quantization of lengths and times on short enough scales (currently speculated to be the Planck scale). Or predictions about discrete spectra for something like the masses of small black holes.

Of course we currently don't observe anything like these things. But we also don't expect them to be observable on scales that we can test. We are many, many orders of magnitude away from being able to run experiments at or near scales like the Planck scale where we expect such effects to be observable.

 

[timmdeeg]

Do you say these predictions are testable in principle (perhaps with much advanced technology) or with today's technology? How could the Planck scale ever be accessible for experiments?

 

[PeterDonis]

Do you say these predictions are testable in principle (perhaps with much advanced technology)

Yes.

or with today's technology?

Obviously not.

How could the Planck scale ever be accessible for experiments?

With advanced enough technology, anything the laws of physics permit is possible.

 

[Demystifier]

But which testable predictions do we expect?

Perhaps predictions concerning large deviations from our classical understanding of macroscopic black hole horizons. Experimentally it could be observed in the form of black hole echoes, see e.g.

 

[timmdeeg]

With advanced enough technology, anything the laws of physics permit is possible.

That seems a bit difficult though regarding the size of a particle accelerator needed to come close to the Planck scale.

 

[timmdeeg]

Perhaps predictions concerning large deviations from our classical understanding of macroscopic black hole horizons. Experimentally it could be observed in the form of black hole echoes, see e.g.

Thanks, very interesting and possibly within reach of advanced gravitational waves detection technology.

 

[martinbn]

I disagree with you on what you did in the paper, but i will tell you more once you've written it.

Finished!
https://arxiv.org/abs/2301.04448

I read it, and I was right. I do disagree with you on pretty much everything in it.

 

[physika]

Are we sure that there exists a theory of quantum gravity?

Maybe not

How would we prove its correctness something ?

Ultra high energy cosmic rays.

 

[Frabjous]

Ultra high energy cosmic rays.

Would you expand on that?

 

[Demystifier]

Would you expand on that?

@physika is a master of haiku physics.

 

[Conn_coord]

I hope that in 2023 an error will be fixed, the essence of which is the fixation of the value of the Planck constant.
While waiting for the appearance of new values https://physics.nist.gov/cuu/Constants/

 

[physika]

@physika is a master of haiku physics.

A little time please, I am articulating in a few words (not more than 20?) the response

...apart, don't like too much write and work on smartphones.
Latter on a PC i wil write.

Excuse me, please.

 

[Conn_coord]

What’s the problem with Planck’s constant?

I hope that the experiments will show a proportional change in electrical and magnetic "constants".
After all, they are no longer fixed.
But their multiply remains a constant.
This would be a sentence for some "constants", including Planck's "constant".

 

[vanhees71]

I don't understand what you mean. Also after the redefinition of the SI the relation $c=1/\sqrt{\epsilon_0 \mu_0}$ still holds exactly, and $c$ is the value fixed already in 1983 within the SI. What's right is that now $\mu_0$ has to be measured. It's no longer $4 \pi 10^{-7} \text{N}/\text{A}^2$ as in the old SI but has to be measured since not the elementary charged is defined by a fixed value within the new SI.

 

[Conn_coord]

In 2018
vacuum electric permittivity
8.854 187 8128(13) x 10^-12
vacuum magnetic permeability
1.256 637 062 12(19) x 10^-6

Wiki:
Since the redefinition of SI units in 2019 (when the values of e and h were fixed as defined quantities), μ0 is an experimentally determined constant...

In 2022, I calculated
8.854 187 8103(13)x10^-12 (decrease)
1.256 637 062 47(19) x 10^-6 (increase)

 

[vanhees71]

Yes $\mu_0$ is experimentally determined, and then also $\epsilon_0=1/(\mu_0 c^2)$ is determined with the defined value of $c$ in the new SI.

 

[ohwilleke]

In terms of problems that could actually be solved in 2023, I think that the most plausible would be for there to be some sort of breakthrough in how to calculate the path integrals that are used to quantify the predictions of quantum mechanics, perhaps in some way to utilizes quantum computing.

In the area of quantum gravity, it would be a long shot, but it isn't inconceivable that experimental evidence of gravitationally induced decoherence could be discovered.

 

[vanhees71]

It looks as if the reactor-neutrino problem is (close to be) resolved:

https://physics.aps.org/articles/v16/s10

 

[Meir Achuz]

I don't understand what you mean. Also after the redefinition of the SI the relation $c=1/\sqrt{\epsilon_0 \mu_0}$ still holds exactly, and $c$ is the value fixed already in 1983 within the SI. What's right is that now $\mu_0$ has to be measured. It's no longer $4 \pi 10^{-7} \text{N}/\text{A}^2$ as in the old SI but has to be measured since not the elementary charged is defined by a fixed value within the new SI.

The best thing about SI units is that you don't have to use them unless you teach elementary physics or want to sell a book. Rather than try to remember muzero, try to remember $\mu_0/4\pi$, which is easier. You just have to know how many nines there. I think the reason they didn't use the parallel wire force to define the ampere, is that they still don't understand electromagnetism.

 

[PeterDonis]

I think the reason they didn't use the parallel wire force to define the ampere, is that they still don't understand electromagnetism.

What are you basing this on?

 

[Meir Achuz]

If something has been $10^{-7}$ for so many years, why change it?
SI shows 'they' don't understand electromagnetism.

 

[vanhees71]

The best thing about SI units is that you don't have to use them unless you teach elementary physics or want to sell a book. Rather than try to remember muzero, try to remember $\mu_0/4\pi$, which is easier. You just have to know how many nines there. I think the reason they didn't use the parallel wire force to define the ampere, is that they still don't understand electromagnetism.

When teaching elementary physics, of course I use Heaviside Lorentz or even natural units :-).

 

[PeterDonis]

If something has been $10^{-7}$ for so many years, why change it?

Because of improvements in measurement technologies, that make it more accurate to fix the values of other constants.

SI shows 'they' don't understand electromagnetism.

No, it shows that you don't understand the rationales underlying the SI system of units.

 

[Meir Achuz]

Because of improvements in measurement technologies, that make it more accurate to fix the values of other constants.No, it shows that you don't understand the rationales underlying the SI system of units.

You are right.

 

[Meir Achuz]

You are right.

The definition of a unit does not have to be the most accurate way to measure it.

 

[Frabjous]

The definition of a unit does not have to be the most accurate way to measure it.

Why wouldn’t you choose the most accurate way?

 

[Meir Achuz]

Then the definition would change as a more accurate way was determined.
The definition of the ampere instead of the coulomb would just change the number for the coulomb as more experimental accuracy were obtained instead of changing $$\mu_0/4\pi##.

 

[PeterDonis]

Then the definition would change as a more accurate way was determined.

Yes, which is what happens with most systems of units, including SI units.

You still have not stated what, exactly, the problem is with this. The fact that you don't appear to like it is not a problem for anyone but you.

 

[Conn_coord]

The SI uses trial and error.
And that's very true!
Fixation, light speeds, electric charge values are direct hits.
Boltzmann's constant tightly binds temperature to energy.

However, it is not correct to fix changing values - this is an error.
But the error is correctable.
Such "constants" early - electric and magnetic, now - Planck , Stefan-Boltzmann "constant".
The accuracy of measuring such "constants" has already reached the desired level.
It is well known that the system of natural units (c, h, e, ... ) is not closed. And it does not close with G, the gravitational constant, (although G is the true constant)
But there is an opportunity to close it.
This solves the basic problem of quantum metrology.

 

[vanhees71]

The definition of a unit does not have to be the most accurate way to measure it.

Why, do you think, did it take so long to redefine the SI? Of course, it's important that the redefinition doesn't change the units more than necessary, and that's why the now fixed values of the fundamental constants (as well as $\Delta \nu_{\text{Cs}}$) are chosen such as to make the new definition of the Ampere and the other basic units as accurately as possible the same as given by the older one.

 

[Demystifier]

How did the discussion of the biggest problems turned into a discussion of units?

 

[Vanadium 50]

How did the discussion of the biggest problems turned into a discussion of units?

This is PF, where we take any opportunity to detour into metrology.

 

[Demystifier]

Finished!
https://arxiv.org/abs/2301.04448

It's now accepted for publication.

 

[Fra]

It's now accepted for publication.

I know you don't have the full answer, but in section 6 you sketch and emergence for 4D diff but a perturbation around a flat metric, like in perturbative string theory? My question is - in your own thinking that is - is your "vision" that you also want to explain the emergence of spacetime dimenstionality (ie. why 4D?) or do you picture this 4D structure as given, and it's just the general metric that is emergence?

/Fredrik

 

[Demystifier]

I know you don't have the full answer, but in section 6 you sketch and emergence for 4D diff but a perturbation around a flat metric, like in perturbative string theory? My question is - in your own thinking that is - is your "vision" that you also want to explain the emergence of spacetime dimenstionality (ie. why 4D?) or do you picture this 4D structure as given, and it's just the general metric that is emergence?

/Fredrik

No, I have no idea how to explain the 4-dimensionality.