# A The Crisis in Theoretical Physics - Gian Francesco Giudice

1. Nov 2, 2017

### Greg Bernhardt

The Italian physicist Gian Francesco Giudice has come out with an article about the crisis in theoretical physics. Please read the paper before commenting. Thoughts?

https://arxiv.org/abs/1710.07663

Last edited: Nov 3, 2017
2. Nov 2, 2017

### Staff: Mentor

Around 1900 there was the idea around that all of fundamental physics might have been discovered (although it was not Lord Kelvin who said that) - shortly before relativity and quantum mechanics revolutionized physics.

Today we know we didn't discover everything, but we might be in a similar phase, as all particles found so far were predicted 50+ years ago already. We don't have experimental guidelines where new physics should be explored theoretically. As a symptom of this, many theorists jump on every result that is slightly away from expectations.
- OPERA doesn't understand the neutrino speed measurements? Let's invent models of tachyonic neutrinos! Many arXiv submissions, the average quality of these models was very questionable. The interest in all these things dropped to zero when OPERA found the issue with the time measurement.
- ATLAS and/or CMS see a 2 sigma fluctuation in some diboson spectrum? Let's tune the models to make a W' or Z' there! Again many models, and then the interest stopped once more data didn't have such a fluctuation.
- ATLAS and CMS see some excess in the diphoton spectrum? Let's invent all sorts of different models for it! Hundreds of arXiv submissions, all for nothing as the excess disappeared with more data.

There are many things in the Standard Model that need better understanding, and work there is ongoing. Outside of that: Yeah... a bit difficult at the moment.

There are still some weird measurements that might turn out to be actual deviations from the Standard Model, especially related to muons. We'll see.

3. Nov 3, 2017

### haushofer

I like his comparison between the lack of experimental results now and those of Michelson and Morley. Maybe we have to accept something which now is still hard to swallow.

I think the cosmological constant could already hint to that: a deeply IR-phenomenon which in effective field theories gets UV contributions. Maybe the separation of scales (locality?) idea is not as correct as we think it is.

4. Nov 3, 2017

### Demystifier

Maybe multiverse?

5. Nov 3, 2017

### atyy

Giudice's overall view is that the present state of affairs is exciting and healthy. It is very different from Hossenfelder's tone.

6. Nov 3, 2017

### Fra

Or the concept of evolution of laws? Which biologists know already, but physicists insist on trying to find timeless laws in a true reductionist spirit.

Thats related to the the "fallacy" i talked about in the other thread, that is also from smolins book. This is indeed hard to swallow, probably becuase it strips us of the tools that worked so far. And its hard to swallow because this "idea" is not yet materialised into concrete ideas.I happend to be motivated because i have a vision of this, but i fully understand that without a vision, the reasons for abandoning the old principles are insufficient.
This is related to same ideas that has been the subject of many recent threads on emergence of symmetry. Which makes it interesting. You can approach this issues from different angles.

I agree that there seems to be a growing awareness of the limitations of the guiding principles that has served us well. But if we consider the scientific process we can understand WHY these tools are powerful at times, but why they seem inappropriate for cosmological models or for big theories that attempt to connect very high and low energy domains.

/Fredrik

7. Nov 3, 2017

### Staff: Mentor

Everyone is waiting for that next great breakthrough. String theory is perhaps the gedanken way of thinking taken too far and has mesmerized folks into thinking it is the one true way.

I think physics is in its heyday of experimental physics right now collecting so much cool data about the largest, smallest, farthest and highest scales that a new TOE will eventually emerge. Its just that the suspense is killing us.

It's becoming a whodonit/howdoesitwork mystery.

Where are Newton, Einstein, Maxwell, Bohr, Heisenberg, Schrodinger... when we need them now?

8. Nov 3, 2017

### Staff: Mentor

That's what I thought, too. To me it always appears like: "QFT works fine with small Lie algebras. Maybe we should consider simply higher dimensions? I mean, they all can be embedded in one $\mathfrak{sl}(n)$ as long as only $n$ is large enough!" And as if this wasn't sufficiently general: "Let's grade them!" Personally I hope, that the truth is buried in their solvable subalgebras rather than the entire simple ones.

I also agree with you, that a real good idea is needed. That we failed to develop a TOE is in my opinion due to the fact, that one part is about coordinates, whereas the other part is about representations. Not easy to merge them.

9. Nov 3, 2017

### Greg Bernhardt

So we need more powerful computing and analysis methods/tools?

10. Nov 3, 2017

### Demystifier

With current publish-or-perish standards, some of them could not get a job today.

11. Nov 3, 2017

### star apple

Did you know Steven Weinberg has written an article in the Scientific American. I think he mentioned the TOE would arrive about 2050 (I'll search the article tomorrow). So it's only 33 more years to go... Remember we have waited for 50 years patiently (I got this from Bee " Without some new physics, they have nothing to work with that they haven’t already had for 50 years, no new input that can tell them in which direction to look for the ultimate goal of unification and/or quantum gravity.").

Here's the article by Steven Weinberg "A United Physics by 2050?" I'll read it again now.. http://www.drtulsian.com/interestingReading/physics/unified-physics-by-2050.pdf

Last edited by a moderator: Nov 3, 2017
12. Nov 3, 2017

### ZapperZ

Staff Emeritus
Y'know, I've seen many forms of this type of "Oh no, the sky is falling"-type articles on this-and-that crisis because certain things haven't been solved or certain things haven't been found..... yet! People today are either impatient, have short attention span, or simply forget that things have become more complicated and that more, paradigm-shifting discoveries are going to be less common and will take a longer time. A human lifetime is actually quite short, and to think that many of these things should occur in such a time scale is highly unreasonable.

Superconductivity was discovered in 1911. It took more than 40 years until a successful and satisfactory explanation for the physics of superconductivity to be published in 1957. Forty years, folks!

Now, you can argue that at that time, the "tools" of quantum mechanics were not yet established, so trying to solve the physics that require such tools were impossible till all of them were in place. Well, why can't we make similar arguments here? How are you to know that (i) all the tools that you need have already been found and (ii) all the possible experiments that you need to be able to formulate a clear set of theory have already been done? After all, between 1911 and 1957, more and more experiments and discoveries in superconductivity were being made to help in the development of the BCS theory. And let's not forget how long we've been searching for gravitational waves until their detection only recently.

In this fast-paced, almost-instantaneous news world, we seem to lose sight that the idea such as "dark matter", "dark energy", etc. are actually quite new! In fact, "dark energy" data are still in their infancy! It just feels as if it has been around for a "long" time, because of the fact that it is sexy news and seems to be in the media often. And just like gravitational waves, many of the experiments needed to detect them, supersymmetry particles, etc.. etc... are NOT easy experiments. They are also not cheap, and in a world where science budgets have either been slashed or remain stagnant, what are the odds that we can make a fast-paced discovery?

The things we are tackling now are getting to be more and more difficult, both theoretically and experimentally. We should not expect the pace of discovery and resolution to proceed at a "normal" pace. I do not see a "crisis" here.

Zz.

13. Nov 3, 2017

### ZapperZ

Staff Emeritus
Addendum to my previous post. I can't believe I forgot this, but high-Tc superconductivity has just passed 40 years since its Nobel-winning discovery (1986). Yet, we still do not have a satisfactory theory to explain this phenomenon. You don't see condensed matter theorists running around like chickens with their heads cut-off proclaiming a crisis, do you?

Zz.

14. Nov 3, 2017

Staff Emeritus
First, since not many people have read the link, they should. If not, they should at least read this much:

Second, in reference to Zz's history of superconductivity, it wasn't until 1950 until the critical experiment - showing the isotope effect - was even performed.

15. Nov 3, 2017

### Demystifier

Speaking of condensed matter, I believe that condensed-matter style of thinking could be the best approach towards a ToE, as suggested by
https://www.amazon.com/Universe-Droplet-International-Monographs-Physics/dp/0198507828
https://www.amazon.com/Quantum-Field-Theory-Many-body-Systems/dp/019922725X

16. Nov 3, 2017

### ZapperZ

Staff Emeritus
That would be ironic, since condensed matter theorists such as Phil Anderson and Bob Laughlin dismissed the concept of a "Theory of Everything". I believe it was Laughlin who said that it is "ToE" for reductionism, but it is not a ToE of physics.

Zz.

17. Nov 3, 2017

### haushofer

Maybe. On the other hand, it's a concept which is mainly motivated by inflation and string theory, two theories which are heavily criticized at the moment when it comes down to the question whether they describe reality.

I don't agree with the way ZapperZ is describing the attitude of the paper by Giudice. Giudice uses the word "crisis" in a very distinct and optimistic way. Also, there is probably a deep and profound message to be found in the lack of new particles at the LHC. Patience is good, but one also has to know when signs are coming which tell you to question well-established physics. That's where the paper imo is about.

18. Nov 3, 2017

### websterling

I think he's less optimistic these days...

BTW- The November, 2017, CERN Courier has 2 Weinberg articles. One on the 1967 Electroweak paper and the other an interview with him.

19. Nov 3, 2017

### Demystifier

I think that Anderson and Laughlin only dismiss reductionism in practice, not reductionism in principle. The behavior of high Tc superconductors can, in principle, be predicted by a computer simulation of a system with $10^{23}$ particles, but we don't have such a strong computer, and even if we had one, we still couldn't say that we understand high Tc superconductivity. For efficient understanding, we need an effective non-fundamental emergent theory which operates with only a few effective degrees of freedom.

And particle physicists who search for a ToE, mean it only in the sense of reductionism in principle. No string theorist attempts to apply string theory to explain the protein folding. So there is no true disagreement between condensed matter physicists and particle physicists.

Last edited: Nov 3, 2017
20. Nov 3, 2017

### Staff: Mentor

Of course, they're too frozen to reply.

21. Nov 3, 2017

### Crass_Oscillator

One issue is that people have an implicit set of assumptions about what fundamental physics is, and therefore, will not see the directions from which new, radical ideas about physics may arise. If you expect it to be reductionist particle physics when the answer (might!) come from elsewhere, you may very well think we are in a crisis. Here are several areas that may play a significant role in fundamental physics by my reckoning to which people do not see a connection due to various implicit assumptions:

1. Artificial intelligence. I may be bearish on its alleged near future applications, but the question of engineering an intelligent system may shed light on fundamental questions regarding modeling the observer/system interaction that underlies quantum mechanics.

2. Chemical and biological physics. For instance, a protein is the simplest object in the universe I can think of that does signal processing, and rather than being engineered, it evolved through natural physical processes. There are major theoretical similarities between the geometric structure of their phase spaces and the phase spaces of learning models such as neural networks. Again, this could ultimately stimulate developments in the synthesis of computer science, statistics, and physics.

3. Quantum information/quantum computing. More people suspect this may have major implications, but it surprises me how people sometimes dismiss this idea.

Honorable mentions:
The emergence of analogs to quantum field theory in condensed matter has been studied for decades, but it's still incomplete. Topological effects are the topic du jour and there's more to do than that.

22. Nov 3, 2017

### atyy

Higher dimensions, as well as lower dimensions. I consider https://arxiv.org/abs/1705.03026 to exemplify the current exciting and healthy state of affairs in theory. What do you think of such work?

23. Nov 3, 2017

### atyy

In condensed matter, the multiverse already exists.

24. Nov 3, 2017

### mitchell porter

First, here's a list of particle physics anomalies from a few years back. I'm not sure how many of these are still considered live issues.

Second, neutrino masses and the dark sector need to be accounted for, and baryogenesis in cosmology.

Third, one may hope to derive the parameters of the SM from a deeper theory, and then even test that theory by predicting the next decimal place. String theory still seems pretty unique in its capacity to make such predictions (e.g. of coupling constants and yukawas), if only string theorists can figure out how to calculate them to a precision beyond O(1) estimates. (Actually, it would be massive progress in string phenomenology, even to have a "standard model vacuum" which gave calculable predictions for all SM parameters that were of the correct order of magnitude.)

Above and beyond all that are questions like, why these gauge groups, why the quantum, why does anything at all exist.

I mention all this just to put in context my view on Giudice's headline topic of naturalness.

I think about it like this: A model, a theory, either contains finetuning or it doesn't. That it contains finetuning in no way implies that it is wrong. Our current empirically best models - the slight extensions of the standard model which propose definite mechanisms for neutrino mass, etc - contain finetuning. In such models, the Higgs boson mass, the cosmological constant, the QCD theta angle, all require finetuning.

However, that it requires finetuning does imply that it is incomplete. If your theory rests on an absolutely massive near-coincidence between a bare parameter and a mess of quantum corrections, there needs to be an explanation.

In a natural theory, that situation is replaced by one in which the quantum corrections are small for a reason like symmetry, so the bare parameter doesn't have to be large and finetuned to cancel against them.

In an anthropic solution to finetuning, the quantum corrections remain a mess but they (or the quantum corrections plus the bare parameter) add up to a small number anyway, so that life (or atoms or whatever) may exist.

In principle you could have a theory in which finetuning is absent because "bare parameter + quantum corrections" just isn't the fundamental framework. This would fall under the category of nonperturbative solutions to finetuning. Ideas exist, they all seem pretty hazy, but maybe I am forgetting something.

Anyway, all I want to say is that, yes, finetuning is a problem, it is one of the unsolved problems of physics, and various types of solution are available. But it is not the only problem, and it is not a problem that absolutely must be solved before anything else can be discovered.

25. Nov 3, 2017

### Staff: Mentor

The situation is completely different in condensed matter. There is an unexplained experimental phenomenon - there is something to work on and you know the direction new theories should have.
That is the point. The discussed crisis is not the lack of new models - we have tons of new models in particle physics. But we have no idea which direction is good.
Most of them should still be there. The cosmic ray e+ excess is highly significant now. The WIMP "signal" has been ruled out by other experiments, I'm highly confident it is a measurement error. Proton charge radius is still open, and B to K μ μ and related measurements continue to be puzzling, with more and more deviations accumulating over time.
The Tevatron measurements should have been superseded by LHC measurements now. Charm CP violation went away with more data from LHCb. Muon g-2 should get an update in the near future, the new experiment is running.