## Friday, January 23, 2009 ... /////

### Steven Weinberg on condensed matter physics culture

We have written a lot stuff on the emergent phenomena and various cultures in physics, including the difference between the relativistic and particle-physics cultures.

But when I looked at Clifford Johnson's blog an hour ago or so, I saw his comments about an interesting article by Steven Weinberg in the CERN Courier:

From BCS to the LHC
Clifford Johnson seems to disagree with Weinberg but I think that his reasons to disagree are based on a misunderstanding. Well, it shouldn't be shocking that my general philosophical opinions about physics are probably indistinguishable from Weinberg's opinions. Let me try to defend his viewpoint.

There are many ways in which high-energy and condensed-matter theorists use similar methods and tools that are often helpful in the other discipline. The two cultures overlap and flows of ideas are sometimes helpful. But as Weinberg correctly says, there is a huge gap between the goals, aims, values, motivation, and sources of satisfaction in between the two cultures.

I always knew this to be the case but a few years ago, many friendly discussions with those roughly five condensed-matter theorists in the Society of Fellows - like Yaroslav Tserkovnyak (now UCLA: Privet!) - have convinced me that the differences are much deeper than I had previously thought.

As Weinberg correctly says, condensed matter theorists are driven by the weird phenomena themselves; by their applications in the everyday life; and by the emergence of new phenomena that can be created by old players. On the other hand, high-energy theorists are motivated by the desire to explain new phenomena away; by purely theoretical desires to simplify the world and understand it accurately with a few assumptions; and by reductionism in general.

From some perspectives, these intellectual values couldn't be more different. Weinberg wants to be nice to the condensed matter physicists who invited him to speak, so he criticizes Gell-Mann for his term "squalid state physics" but be sure that in the privacy of his home and his mind, he finds Gell-Mann's term damn accurate just like I do! :-)

Clifford Johnson apparently misinterprets what Weinberg is saying. And when he does so, he makes it completely clear that his (Clifford's) thinking is one of a high-energy theorist (who probably hasn't interacted with a sufficient ensemble of condensed matter theorists to understand that not everyone would share his/our attitudes).

Clifford thinks that Weinberg is claiming that the statements of high-energy theoretical physics are uninteresting for us. Well, Weinberg is saying something else. He's saying that the things that excite us are actually not individual, isolated, observable phenomena!

And Clifford's example is not a "phenomenon", either. Phenomena are things like superconductivity, superfluidity, or the Hall effect. You can actually see them and entertain others by showing them. Something is going on. On the other hand, Clifford's example, namely the identical character of the electrons in the Universe, is not a "phenomenon" in this sense. Nothing is moving there. Children won't scream Wow.

It is an insight that follows from our theories. And experimentally speaking, it is an abstract conclusion that you can obtain by a careful comparison and generalization of many phenomena - i.e. many measurements of various electrons. Again, the identical character of particles is not a phenomenon per se but a logical conclusion of a theoretical analysis of many phenomena that don't produce the Wow effect in isolation. It is a pattern, a principle, a sophisticated insight, not a phenomenon.

This is exactly the kind of stuff that Weinberg correctly claims to be interesting for high-energy theorists: universal laws and patterns that one doesn't realize just by looking around. It is necessary to use the brain for a little while to see these things. And you bet that condensed-matter physicists are thrilled by very different things, namely things that must be more obviously thrilling, even for the laymen.

Another example of the differences is the following.

We once chatted about the ability of contemporary physics to derive the accurate material constants for various materials - such as the heat capacity of ice - from the first principles. Now, water is not the most mysterious thing under the Sun. On the other hand, this kind of a calculation - if it could be done - would be an interesting piece of low-energy theoretical physics for a high-energy theoretical mind like mine. It would be a genuine test whether real-world objects such as a glacier are actually composed out of the electrons and baryons that we believe to describe all regular matter.

Yaroslav Tserkovnyak couldn't disagree more. He wouldn't care about any calculation of any material constants because it's not "real physics" for him. There are no new phenomena. He thinks that he's learning nothing here. Other condensed matter physicists wouldn't give a damn, either. They know that ice has some heat capacity and they can measure it. They know some phenomena where the heat capacity may be useful. So that's everything they need. Who could possibly care about a theoretical calculation of numbers that can be measured, he asked?

And I was like beep beep beep beep. Well, almost. :-)

On the other hand, when you talk about some real observed phenomena, like the Hall effect, the situation gets reversed. The condensed-matter physicists are thrilled and dream about discovering another phenomenon of a comparable importance. A high-energy theorist asks: isn't it just some other messy, low-energy, low-brow configuration of the same ordinary electrons controlled by electromagnetism - the kind of physics from the late 1920s?

Of course that both cultures agree that the elementary building blocks can behave in many ways and new emergent phenomena with new effective descriptions can appear and become useful at many places. And these new setups are important for our understanding of the landscape of ideas - especially because similar ideas can be recycled and are recycled at many contexts in physics. But they have dramatically different belief systems about what matters in physics. High-energy theorists want to reduce insights: they view new phenomena as challenges or hints to find a better, more natural unifying theory.

Challenges that should be explained and "undone" by explaining them with some more fundamental physics. On the other hand, the very ramification of the tree of known phenomena is the real driving force for the condensed matter physicist. They see both new phenomena as well as new theoretical ideas to be mere tools to discover (or invent?) even newer phenomena in the future.

So even if Clifford Johnson finds the boundary between "fundamental" and "emergent" phenomena "blurred" (the boundary looks mostly razor-sharp to me! A quantity that distinguishes them is the maximal accuracy that the description can in principle achieve), the condensed matter physicists and high-energy theorists have a very different idea about the purpose of the derived phenomena. For CMT physicists, they're the very goal of their research while for HEP theorists, they're consistency checks and tools to refine their principles.

Of course, this difference is related to the applied science vs basic science debates.

High-energy theorists are driven by pure knowledge. And a unifying theory of many phenomena is just extremely important from a philosophical viewpoint. On the other hand, it usually doesn't help us in our everyday lives. You can't usually build new and better hard disks with these insights which is why the condensed matter physicists don't care. They care about new phenomena because they can be useful in new applications and sometimes even as new sources of profit. But any usable description, arbitrarily phenomenological in character, even if it is designed for engineers or their assistants, is just good enough for them.

So I think I know very well what Weinberg was talking about.

Clifford mentions the gauge-gravity duality, including the AdS/CMT correspondence. Well, it's clear that some high-energy people who are deeply studying this field began to think like condensed matter physicists and maybe they always have. But I think that even if they're intensely collaborating with real condensed matter physicists, there's a big gap here.

The condensed matter physicists view the holography as a new method to look for new phenomena and to derive qualitative phenomenological laws - e.g. the very character of the useful degrees of freedom - that govern them. On the other hand, high-energy physicists view the AdS/CMT description of various systems as a new test of a powerful principle that could perhaps help us to generalize or refine the principle a little bit.

Fine, they may be happy if condensed matter physicists find it useful. But if they only look at their own intellectual satisfaction, genuine high-energy theorists are simply not excited by the phenomena per se. They care what the phenomena tell us about the deep architecture of the Universe.

And that's the memo.