AdS/CFT correspondence is a pretty old idea, whether it will yield something useful is still to be seen. Our world is not an N=4 Yangs Mills CFT, and our space is not AdS, so both sides of the correspondence are basically toy models that have been known for a while and haven't produced verifiable predictions (other than supersymmetric partners we haven't seen).
Neither of these models predicts superpartners. The AdS theory doesn't include the standard model. The N=4 theory doesn't even have particles. They're toy models, albeit rather sophisticated ones. Their utility is that they can give us concrete demonstrations of various conjectures about gravity, holography, and gauge theories.
> The AdS theory doesn't include the standard model
I never said AdS includes standard model?
> The N=4 theory doesn't even have particles
Yang Mills is a super symmetric CFT used as a toy model for other super symmetric models (which do have super partners; Yang Mills informs us about our real world only in as much as it informs us about other supersymmetric CFTs).
> Their utility is that they can give us concrete demonstrations of various conjectures about gravity, holography, and gauge theories.
Sure, but there is significant concern about how meaningful those demonstrations are since the theories that are most similar to the toy examples are themselves probably dead ends.
I won't give away the spoiler but this article is written within a few months of the above video (oct vs dec).
The paper in reference is: "Limits on the number of spacetime dimensions from GW170817"
Pardo, Fishbach, Holz & Spergel 2018
https://arxiv.org/abs/1801.08160
I'm not sure if "agrees with" is the right term, I think with the current experimental evidence we have it's more more like it does not invalidates them.
It's difficult for a non-specialist to place this article in a broader historical context. I thought string theory was by now more or less considered a dead-end, so I wonder if this article is an attempt to revive it, or the first sign that it may actually go somewhere after all. Maybe someone knowledgeable can weigh in?
I've worked on string theory briefly (published one paper that got a respectable number of citations). It is not considered a dead end by people working on it obviously.
Many of the best people in theoretical physics with Edward Witten being the most prominent example have been working on it for ~40+ years now. None of the public detractors have nearly the same stature that Witten has. Of course this is basically an argument by authority and there is a chance that once titans like Witten leave the playing field the following generations will lose more and more influence.
I think there is a fair bit of criticism of String Theory among very good people that is not voiced publicly because there would be no point to it, they have nothing to gain. In many ways it is the "Only Game in Town" (literal quote from my supervisor), just doing high-energy particle phenomenology is not that much more exciting.
String theory has already led to a number of very high profile successes in mathematics (Perelman's Proof of the Poincare Conjecture, Monstrous Moonshine etc.), so it is not like the techniques that have been developed are totally useless.
I wonder if it’s not that string theory gets its more reasonable results simply by reducing the singularity of a particle in one dimension.
If we look at the idea of a ”point particle”, it’s clearly something that is never observed elsewhere.
This is what leads to the whole idea of renormalization: that any force that goes to infinity as r->0.
For string theory, adding a second dimension to the particle adds density, which avoids the same amount of infinites. Similarly, loop gravity’s ”chunky” timespace also prevents infinites by essentially adding a minimum distance.
I feel both approaches are wrong. The conceptual model of the particle as a dot still lives on in some regard.
With M-theory, string theory moved more to something like the concept of a particle with density in multiple dimensions.
However, the extreme complexity and the many assumptions needed in string theory made it very unappealing to me.
I would like to see some theory that used the wave nature of particles resolve the infinities - including quantum gravity, then describe some mechanism how these waves could gain dual particle-like behaviour when leaving the Planck length world.
The starting point of Perelman's proof is the observation that (a modified version of) the Ricci flow is a gradient flow. The connection to 2d non-linear sigma models is briefly discussed in section 2 of the introduction in https://arxiv.org/pdf/math/0211159.pdf and section 1.4*. The overall intuition is that string theory captures the geometry of the back ground Riemannian manifold.
Yeah, loop quantum gravity is pretty badly broken as a theory of gravity. It doesn't have the right classical limit (i.e., general relativity). Doesn't even have thermodynamics compatible with the right classical limit.
I'd be extremely hard pressed to name a significant result loop quantum gravity has exported to the rest of physics and mathematics.
There's also the asymptotic safety program, which isn't completely implausible, but is kind of stuck in the 'steal underpants' phase.
Quantum Loop Gravity is much smaller ~200 people most of them not very famous and it has not produced the same depth of research directions in particular not recently.
It’s interesting mathematically, but we don’t live in an AdS universe so there’s that. Recently there’s been some progress extending the notion fromthe AdS toy model to a dS toy model, but the degree to which any of that applies to anything real’s questionable. “Interesting, but with no indication of a valid physical interpretation in the universe” is probably the best way to sum up the world of string theory.
The other big problem is that a lot of what made it seem natural and elegant has been progressively ruled out, so you’re left with far less simple and elegant versions which could apply to the universe. At that point you have to ask what the appeal is, you know? It’s not unlike the problems with Inflationary cosmology. Originally it was a simple way to resolve outstanding issues of homogeneity and isotropy of the observable universe. That was based on a model with a very natural PE curve, which has been ruled out. Now we’re left with a “curve” every bit as contrived as simply saying “because” in reference to the underlying questions.
In both cases, M-Theiry and Inflation, they persist because of huge investments of time and energy into them, and because as yet no strong competitors have emerged.
Who is saying string theory is dead? Is it the theorists themselves or are they discounted due to conflict of interest so we only get to hear pop sci articles from people who can't do integrals let alone understand SS beyond shit analogies.
A lot of high energy theorists themselves are discouraged. Basically almost all theorists outside of high energy/cosmology are VERY discouraged about the prospect of superstrings.
I'm not an expert, but normally particles don't have an entangled counterpart. Only virtual ones do, and those which fall in can have a counterpart which ends up being "radiated".
Perhaps the confusion is caused by the current understanding that any information (and mass?) that comes in will eventually get emitted back away.
Part of the problem however is that science communication has become its own “field” typically staffed by miserably unqualified people on a shoestring budget. There are also few laurels and less money to be had in trying to give the public at large a faithful representation of the real science. By contrast rehashing decades old basics about black holes with lots of hand-waving sells papers, clicks, magazines, etc.
As long as fiendishly complicated subjects are relentless dumbed down in the name of “inspiring enthusiasm” rather than instilling facts, we’re stuck with this.
To be fair, it is the usual way HR is described in pop science articles. The reality of pair production outside of the event horizon, and one particle falling in while the other “steals” a bit of energy from the hole’s gravitational field is not described as often. It’s also equivalent to the same pair being produced with the inflating partner having negative energy.
It's a form of redshift effect in curved spacetime. Any massive object stretches the spacetime in a way that the spectrum of the light emitted by that object shifts down from the point of view of a remote observer (remote - this is important). The larger the mass, the stronger the effect.
A black hole would create an infinite redshift (due to singularity inside of it) -- but only if infinitesimal EM frequencies were possible, which in turn would imply continuous spectrum. The energy of photons is quantized, therefore no truly infinite redshift is possible. The same applies to other particles.
Basically it's a raster effect that becomes apparent when you zoom up on a magazine photograph, or equivalently -- when you stretch a photograph printed on a rubber sheet, the only difference is that you stretch spacetime, not just space.
That’s part of it, but then you have to account for the role the event horizon plays, and inherently complicated things like wave packets scattering off of it. I can’t pretend to understand the process well enough to talk about it without resorting to heuristics.
Does assuming some ultimate law ruling the universe what mainly drives research?
Or, what if our so-called laws of nature, just like life, only emerged from the initial big bang? Was there a natural selection of the fittest particles after the big bang?
M-Theory posits that yes, the laws of nature and physics have evolved as the miltudimensional space evolved, they may not always have been the same as now.
Thanks, didn't knew about M-theory, I'll look it up!
Science taught me about how everything is made of particles while their interactions follow the laws of nature. Much later, reading Wittgenstein, I understood that the world is not so much about it's particles but more about how things interact together. Because if something never interact at all with anything, it may as well not exist at all. So the very existence of something always comes with its interactions to the world and whatever rules they happen to follow...
