Saturday, January 23, 2016

Sachdev on string/graphene duality

My ex-colleague Subir Sachdev is one of the main world's condensed matter physicists, if not the main one, who leads the application of the stringy holographic methods to condensed matter physics. And the word "Sachdev" appears in 9 TRF blog posts. Subir would surely deserve more. ;-)

Kevin Hartnett has interviewed him for the Quanta Magazine:
Taming Superconductors With String Theory
Before he was charmed by these stringy methods sometime in 2007, Sachdev has done lots of important work about phase transitions in various condensed-matter systems. I actually remember some parties we attended in 2007 where he couldn't hide his enthusiasm about the new stringy tricks he has introduced.

In the interview, it's being stated that condensed matter physicists like to describe lots of phase transitions. The concept of "quasiparticles" is often useful. It hasn't been the case of the high-temperature superconductors – materials that allow you a strictly zero resistance even at temperatures as "mild" as –70 °C, the current record.

Sachdev and collaborators are completing a paper in Science that employs some holographic methods to correctly calculate the heat and electric conductivity of a recent "darling material" of many condensed matter and similar physicists, graphene, the two-dimensional i.e. single-layer honeycomb web of carbon atoms pictured at the top. Graphene is meant to be a toy model for other materials.

He describes his interesting "new materials" as those in which the interactions and entanglement between the conductance electrons can't be neglected. Sachdev sketches his big-picture approach to problems which is obviously close to my values: he wants to understand things like high-temperature superconductors by understanding and classifying a whole landscape of phase transitions. You can say that you really understand a given system if you also understand a couple of (or all) analogous but inequivalent ones.

It's hard to access the interacting-electrons systems numerically because you need to remember states for many electrons rather than one, and those have exponentially larger Hilbert spaces.

A 2014 70-minute Perimeter introduction talk by Subir.

Many of the mysteries are probably linked to the phase that these superconductors turn into above the phase transition – namely strange metals. Those are really strange because it seems that no quasiparticles may describe their behavior. Sachdev's ideas was to basically "visualize" the entanglement between the particles as open strings attached to D-branes (the entanglement is "located" in extra dimensions outside the D-branes) - while the end points are the particles that are entangled. Many of the phenomena may be stringy-mapped on questions about charges added to charged black holes.

Now, the interviewer asks about rumors that a prominent condensed matter physicist Philip Anderson doesn't believe a word about AdS/CMT and related ideas. Is that remarkable gossip true, Sachdev is asked? Andersson has been a string hater in all respects for decades so he may be just keeping the image. The interviewer is patient and asks two almost identical questions about Andersson's skepticism.

Subir didn't really address it much – except that he made sure that he's had discussions with Andersson but they have apparently produced nothing interesting to talk about. Perhaps, Andersson is skeptical because this stuff is conceptually too new. But new evidence has emerged since 2007, Subir says. OK, let me say what Subir should have answered to the question why Andersson hasn't embraced this stuff yet:
Perhaps, it's because he's not as smart as I am, after all, is he?
Such comments, especially from the people who have some reasons to voice them, have become unpopular but they're so badly needed. You know, the very idea that we should try to be "mining" for the reasons why Andersson is skeptical is a bizarre, unscientific attitude to knowledge. If someone hasn't presented any clear enough picture of his skepticism, the most likely explanation is that he has no interesting evidence or insights to offer, right? Why should one assume that Philip Andersson has to have something crucial or interesting to say about this new direction of research that has clearly nothing to do with him? And even if he had something to say, wouldn't it be more natural to ask Andersson rather than Sachdev?

In other words, my point is that many of these interviewers keep on imagining science as a collection of cults of authorities. They think that in science, you "weight" the authorities saying this or that.

But the scientific opinions aren't evolving according to authorities. They are evolving according to the evidence and evidence (both experimental and theoretical – and mixed) emerges wherever it does. The "weight" that a physicist has isn't given by some universal "authority index" he possesses but by the evidence he has concerning a particular proposition or theory. You should not try to "order" the evidence to emerge at a pre-determined place, in a pre-determined skull, or even suggest some pre-determined conclusions. Even if one decides that D-brane/graphene or AdS/graphene and similar descriptions remain inconclusive, it's still right that Sachdev has presented evidence that may shape physicists' opinions about this duality while Andersson hasn't done such a thing yet.

At the end of the interview, Sachdev describes his research as string-inspired, not full-fledged string theory – which is obviously right – and he says he hopes that the understanding of the phase transition and the "optimal density" associated with it will lead to new insights.

Horgan and recycling

Some of Sachdev et al. work may be said to be "recycling some ideas" from elsewhere. This brings me to an insightfully stupid blog post by John Horgan, a hater of physics. He says that "physics has lost its fizz" and these days, "it just recycles ideas". It's funny for him to say that he just recently noticed something disappointing about physics. In reality, it's already exactly 20 years ago when he wrote his End of Science. So what kind of bullšit about his recent realizations is he reporting? He is just recycling his many decades old hateful anti-science talking points and Scientific American isn't willing or capable of throwing this pile of stale greasy Šmoitian feces into a lavatory.

Strangely enough, science has postponed its "end" and new advances kept on coming – and they still continue, a fact that must drive this idiot insane.

In that essay, we also learn that while he majored in literature, he liked physics because of the idea that it could replace his Catholic religion. Sorry but physics isn't meant to be "the equivalent" of the Catholic belief. If you become a Catholic renegade, it can in no way be guaranteed that you will have a close relationship to physics which is absolutely independent of any Catholic beliefs. Sometimes it may be positively correlated, very often, it is negatively correlated.

Horgan enumerates some recent advances and frames them as repetitions of some older insights in physics. But you could have always described any advance in physics in this way. Either these advances were building on some other recent advances, or they resuscitated and elaborated upon some kind of reasoning whose glimpses could have seen in the distant past before they were temporarily suppressed.

And the very fact that physics - and Nature – are capable of recyling similar ideas many times and at different places is absolutely exciting. It's really one of the reasons why certain top minds and many other folks love physics. The recycling character of physics as a science is another way to see its universality and unification power. It's obvious that a lapsed Catholic who hates this feature must hate physics because the explanation of everything in terms of "properly recycled ideas" is really what the ultimate goal of physics is!

So return to your church, Horgan the aßhole, and don't oxidize in science that you have nothing to do with.


The most closely related, previous authoritative TRF post: Andreas Karch's review of applications of holography. See also Subir's 2011 review of AdS/CMT and his recent papers about high-\(T\) superconductors, emergent gauge fields, graphene, Kondo insulators, and other things.

No comments:

Post a Comment