[zachf]

There are in fact exceptionally strong incentives to discover alternatives to quantum gravity which could be tested in experiments. These are the same incentives that always drive the scientific process, and new theories cost next to zero to produce. The reason string theory is popular is not because string theorists somehow prevent funding of other directions. It is because string theory has given us tools like AdS/CFT that are useful in other contexts to understand real physics—-and the alternatives have not (yet). There are many physicists who spend their lives studying alternatives to string theory with 100% of their time. I hope for their sake that there is a similar pot of gold at the end of their rainbows. It has not yet materialized.

[naasking]

Oh Ads, you mean that space that emphatically does NOT describe our universe? Ads/CFT is overblown. It's just an interesting mathematical result that hasn't borne much meaningful fruit for actual physics.

I'm sorry, but string theorists absolutely do prevent funding other research because funding is finite, grad students have to research something their advisors think is worthy, and their advisors have their heads full of "beautiful math" so that's what they tell their students to work on if they want their PhDs, and that's what they hire their post grads to work on if they want a job.

Only now as the strong theory haze has started dissipating are we starting to see novel approaches, like Oppenheim's post quantum gravity theory.

[zachf]

No, this is a shallow understanding of AdS/CFT. If you want to study quantum gravity when it is weakly coupled to matter, you can use AdS/CFT regardless of whether the background space is asymptotically AdS by embedding a brane near the boundary and working in a perturbative expansion. If you want to study the physics of quantum de Sitter space with a field theory dual, you can study any of the recent work on TTbar deformations. And anyway, surely you aren’t arguing that conformal field theories are irrelevant for physics? Because that would obviously be an untenable position, and the whole point is that quantum gravity AdS (basically) is CFT (it’s an equality! It goes both ways), just in different variables. You can actually study non-gravitational physics with it, using a gravitational language. That’s awesome stuff! Please don’t dismiss this fascinating field too quickly.

By the way, I know Oppenheim personally. He gets funding from string grants. Nobody is angry about that. Anybody can do this. I don’t think his theory is going to pass any experimental validation (it requires a really severe violation of a physical principle we have tested over and over) but the entire community has always supported and listened. He gives talks at major universities. He’s not an outcast or renegade or something.

[naasking]

> Because that would obviously be an untenable position, and the whole point is that quantum gravity AdS (basically) is CFT (it’s an equality! It goes both ways), just in different variables. You can actually study non-gravitational physics with it, using a gravitational language. That’s awesome stuff!

Which makes it an interesting mathematical construct, but in what way does that actually help physics? I included a link to one critique of Ads/CFT in another post, and others have critiqued its applications to QCD and other alleged "successes" because the important properties to do meaningful work in those domains just aren't there.

The versions of this correspondence that are easy to work with also depend on supersymmetry, for which every experiment has failed to find any evidence in the expected regimes. In the old days we'd call this "refuted", but these days it just means reworking it (adding a new epicycle?) to get "new bounds".

Ads/CFT is a mildly interesting mathematical derivation, but its actual utility for physics is questionable.

> He gets funding from string grants. Nobody is angry about that. Anybody can do this.

Maybe anybody can do this now, and I think that's because, as I said, string theory's stranglehold has weakened because of well-motivated criticisms over the past 15 years or so. The evidence of string theory's former dominance is right in what you said: string theory grants.

> but the entire community has always supported and listened.

I think some physicists are open minded, and some are not. You need only look at how physicists who work MOND are treated to see how not open minded some physicists are. MOND is not a final theory, but it and the people who work on it are scorned despite it's unreasonably good predictive success over the last 40 years.

[zachf]

Okay, I’ll tell you about my own research. From studying the way that geometric surfaces work in AdS, we conjectured a relationship between the stress tensor of QFT and entanglement entropy. This is because those quantities translate into geometrical analogs in the quantum gravity theory. We then proved this same relationship holds in some simple field theories and then other physicists proved it in the general case. So we learned something about non gravitational physics from gravitational physics. We study a specific, tractable case (AdS, mapping onto CFT) and then use it to learn about the general case (every QFT). That’s how physics works! You study the spherical cows. Eventually you learn something universal. All this is because I started with an open mind, and pursued the full consequences of AdS/CFT.

Your complaint about supersymmetry is like saying that Newtonian physics can’t work because objects are not rigid, continuous solid bodies. And yeah, that’s true, there are none of those in nature. Does that mean Newtonian physics is not useful? NO! It’s a model that’s useful. Is it wrong? Kinda. And the models that have unbroken SUSY are “wrong” too, in the same way. But the point is—-it’s obviously useful!

Please try to be open minded about string theory, especially if you wish to lecture about small-mindedness around MOND. Diminishing the real accomplishments of physicists doesn’t make other fields more likely to get funded—it makes it more likely that bureaucrats defund everyone. That’s the lesson of the SSC.

[glenstein]

>Ads/CFT is overblown

This is a preposterously uncharitable characterization of something that again, was I think a triumph of string theory, the likes of which cannot be claimed by any competing theory. It is a framework for understanding black hole information loss, and it even has specific applications in condensed matter physics for modeling high temperature superconductors.

[naasking]

Sorry, but I'll have to side with Nobel laureate Anderson who disagrees about the utility of Ads/CFT for condensed matter physics:

https://pubs.aip.org/physicstoday/article/66/4/9/414412/Stra...

Like I said, Ads/CFT's alleged "successes" are overblown.

As for it being a framework for understanding black hole information loss, it's merely one idea that has questionable application to our universe. We'll see if anything actually useful comes from it.