A while ago I got an email out of the blue from an enthusiastic young fellow who wanted to do an interview with me on camera for his YouTube channel. After we bounced emails back and forth a bit and I got a sense that this was both legitimate, worth supporting, and that I had time to do it, we agreed that we’d meet to do it. So we met at the excellent Mystery and Imagination bookshop in Glendale, and he set the camera running and threw a bunch of questions at me. We talked about all sorts of things from dark matter, the LHC, supersymmetry and string theory to trumpets, jazz clubs, and noir films.

It was fun, and you can find the results on his YouTube channel (here), that he hopes to populate with more interviews with people working in science and other topics. He’s got an interview with mathematical physicist John Baez up there already, so go and look.

Below I’ve embedded the interview with me, for your convenience.

Enjoy!

-cvj

Dr. Johnson,

Great interview! I took a brief look at your recent papers and came across the term ‘entanglement entropy’. I’ve never understood what that meant and I can’t find any good explanations of it. Would you mind giving me a brief explanation of what that means? I have a physics background, but I’m not an expert in string theory, AdS/CFT, etc.

Hi Phil!

Thanks.

Entanglement entropy has nothing to do with string theory or ads/CFT per se. It is used in those contexts, sure, but it is a concept from quantum mechanics. Since you have a physics background, you are probably familiar with the idea of quantum systems being entangled…? The basic EPR discussion might spring to mind….. Where two spins having interacted (but no measurement yet made) and allowed to fly apart as far as they might are still connected by virtue of being entangled… They are still in a quantum superposition – they’re part of one quantum state. I’m just saying some phrases to jog your memory here. Well, assuming you remember that (or look it up), then you’ve more or less got all you need to understand the purpose of entanglement entropy. It is simply a measure of how much two systems (typically their union makes one total ssystem you are interested in) are entangled. So, not just two spins now, but larger systems…. Like huge numbers of spins interacting (atoms, electrons, quarks, etc) There are other measures, and also several definitions of the entanglement entropy, but the basic idea is the same…. You have a system and you are interested in seeing how a subsystem of it is entangled with the rest of the system quantum mechanically. The entanglement entropy is a number that quantifies that. As the system evolves, changes in that number are important to keep track of if you are interested in how the system may be reorganizing itself as it evolves.

There are several good sources for more details now you have the basic motivation. Try googling a bit and see what you find, but a good text on quantum information will give you the best grounding if you want to take it further.

Best,

-cvj

Thanks! I do have a few quantum information books lying around (though I’m not in that field), so I’ll take a look.

Your description jogged my mind about something I read recently about a recent poll of physicists for their “favorite” interpretation of QM. Do you have one? For me, personally, it doesn’t really matter. We have a bunch of mathematical rules that work well, rules that allow us to successfully explain current observations and predict the outcomes of new experiments, and that’s all we can really ask for. But I do find EPR really interesting. Really, for me, the only explanation is that the world is made out of “mathematical objects”, “states”, or “wavefunctions”, whatever you want to call it. The world, fundamentally, is mathematical. You can’t think of the two spins in an EPR setup as two particles. Really, it’s a state, a mathematical object obeying mathematical rules (quantum mechanics). That’s the only interpretation I need or makes sense to me.

Hi,

I’m a pragmatist (on most days) and so interpretations don’t interest me so much if they do not add anything to the physics. Interpretations that try to place the classical world we see everyday as central in some way may not be the best start, since (arguably) it is only accidental that we discovered classical rules of physics first. Interpretations that might help us better understand how to understand pieces of physics that we do not yet understand (like the full story of quantum gravity) are ones that might be the most fruitful.

Best,

-cvj

Hi, nice interview about the physics and the music even though I came probably closest to the “Jazz Thing” with something like this ;D http://www.youtube.com/watch?v=DYeAzLMq1r8 . Btw. I have send you an email cca (Sun, Jan 27, 2013 at 12:13 PM) at that one presented on http://physics.usc.edu page. Have you recieved it? If not, I can send it to you again. If you have received it and you don’t want to talk about that subject on your blog, just tell me and I will accept that. Anyway, thanks for your articles and I am looking forward for more of them and also your next appearance in some new episode of The Universe (btw. are they on lack-of-money delay right now or are they shooting some episodes?) or somewhere else.

And one more question. What do you think about that? Is WIMP era finally here? http://phys.org/news/2013-02-scientists-breakthroughs-dark-matter-mystery.html

Thank for the great interview