I discovered blogs back in summer of 2004, when someone at the annual string conference, taking place in Paris that year, told me that Jacques has an interesting “webpage“. Jacques confirmed with a pat on my shoulders and a look of pity, I guess his blog was well-known to just about everyone by then. Shortly afterward more familiar faces kept popping up on the internet, most notably the group blog Cosmic Variance, which included as writers two people I know fairly well (Mark and Clifford) and another one I had met a few times previously (Sean). Like many other people I started reading these two blogs (and maybe a couple more) on a semi-regular basis. I still do, along with just a few more, all of which are featured on our blogroll.

Since I knew some of the writers fairly well, many times chatting with them about their posts in person (especially with Mark, whom I was visiting fairly regularly at the time), it was very natural to start commenting, provided I had something to say (and sometimes even when I didn’t). On many occasions the blog conversations simply merged into personal conversations.

Perhaps that is one reason I see very little point in “lurking” in the background. If you are going to spend any time reading, chances are you’d find something useful to say, some point that needs clarifying, or just a clever comment that crossed your mind. If you find that urge to comment, by all means do, that is the point of the endeavor. This is about the only medium where you are able to direct the conversation to places you find interesting (with the risk of going there alone…).

So, please consider adding this to your new year’s resolutions: participating and contributing to this blog, making your presence felt in the best possible way. For the majority of the readers, I’m pretty confident we are all going to be richer for that effort (for the rest of the comments, let us remember how easy it is to scroll down on your browsers. It’s even easier deleting the occasional offensive comment).

As a first occasion to practice this new year’s resolution, here is an audience participation question: what should I write about?

Every problem needs boundary conditions, so let me add that I find most interesting to write posts about physics, at a level aimed to advanced undergraduate or beginning graduate student, or an interested layperson with sufficient background on the topic. This helps me clarify my thoughts, and typically results in an interesting discussion. Ideally I’d also like to write some things to more advanced, and necessarily more restricted audience, but I’d need some evidence that the relevant people are reading and are willing to participate. In any event, here is a chance to shape this corner of your virtual environment, fire away…

Oh yeah, while I am at it, happy new year to everyone. May 2009 be an exceptionally happy and productive year to you and your loved ones.

on January 1, 2009 at 12:09 pmmeichenlWell, I am a final-year undergraduate. “Advanced” is debatable. But here are a few things I would love to have a professional theorist explain, that aren’t necessarily textbook topics. I hope I understand them enough that the questions at least make some sense.

1) What is decoherence, and what are your personal opinions on it? I’ve mostly seen pop-level explanations, and would like a more technical treatment.

2) I have seen Noether’s theorem in the context of analytic Newtonian mechanics, as well as the discussion of generating functions and symmetries in the quantum book by Shankar. How easily does Noether’s theorem generalize to GR and to quantum field theory? Don’t conservation laws hold in GR only locally? (I’m just beginning my way through a GR textbook now.) Also, can you “invert” Noether’s theorem? That is, instead of showing that least action + symmetry -> conservation law, can you take the hypothesis that each symmetry of the Lagrangian has an associated conservation law, and use that to prove that nature follows a principle of stationary action? If you can, how can you further constrain the form of the Lagrangian by demanding general covariance? If not, I have always been a bit confused about the relationship between mechanics and conservation laws. How much of mechanics can you derive simply by assuming certain conservation laws hold? Also, do discrete symmetries like parity lead to conservation laws, and do conservation laws like charge conservation and some of that particle physics stuff I’ve heard of here and there (I’ve never taken a particle physics course) also come from symmetries?

3) How is it that the spin of a particle determines whether it obeys Fermi-Dirac or Bose-Einstein statistics? Do I need to study field theory in depth to understand that?

4) Why are snowflakes symmetric? Once they start forming, how does one side know what the other side is going to do? Why do they have 6-fold symmetry and not 8-fold? I have heard this has to do with the geometry of a water molecule, but that’s about it.

5) How does the accelerometer work in my friend’s iPhone? It seems uncanny that you can get angular position simply by integrating acceleration over time – eventually you would need to recalibrate. Surely it’s reading the direction of gravity somehow.

6) How does an abacus do math? Could I learn to do arithmetic in my head by imagining a bunch of little beads and mentally moving them around, then reading off the result? (I suppose I could look this one up pretty easily, but you asked for things I was wondering about.)

7) I never quite understood the relationship between EM fields and momentum. How do you find the momentum of an EM field from Maxwell’s equations, and beyond p = E/c, how do photons deal with momentum?

8) Suppose you are looking for periodic signatures in a sample of data, but the intrinsic period of the phenomenon drifts longer and shorter about some mean over a time scale longer than a period. If you try to take a fourier analysis of the time series over many many periods, the data will drift in and out of phase with the sine wave, even if that sine wave is right at the mean period. How then can we extract the periodic behavior of the phenomenon? For example, could we look at something like the average correlation between the value, first derivative, and second derivative of a function at f(t) and f(t+P) over the course of the entire time series to see how well this data fits with the period P (or P/n)?

9) Euclidean geometry is usually presented in terms of axioms, and then we derive theorems. But Minkowski geometry isn’t treated the same way in most textbooks. How can we replace the parallel postulate to yield Minkowski geometry? Is this even possible, given that it is not a metric space? If I want to understand the mathematical foundations firmly, do I need to learn differential geometry and treat Minkowski geometry as a special case, or is a more basic, non-calculus approach possible in this flat spacetime?

10) What is this “symplectic geometry” thing I’ve heard mentioned only in hushed tones? For that matter, what is “Clifford Algebra”? Or more generally, what do physicists mean when they talk about “an algebra” in some particular theory?

11) I’ve seen explanations of the freedom to choose a gauge in standard undergrad EM, but is this really the same thing as “gauge theories” in theoretical physics? Are you dealing with scalar and vector potentials rather than directly with wave functions? That seems odd given my current knowledge of quantum mechanics. Can you give an example of a gauge outside EM?

12) How does a plant seed underneath several inches of soil know which way is up, so it can decide which way to grow?

13) Why does the flame appear above the candle? Why does it leap around?

14) Can you determine the size and shape of water droplets in the air from the type of rainbow they form? What sort of rainbow is formed by spherical droplets of a given radius considering only geometrical optics? How does this change accounting for interference effects? How does it further change if I allow the drops to become spheroids, or to be deformed by some strain tensor?

I guess that should be enough for now.

on January 1, 2009 at 1:55 pmGiotisHi Moshe

Just a few ideas.

Generally i like posts about the nature of space, time and gravity. You could present with a series of posts

the different views/theories that are out there and how they are trying to deal with this problem.

You could have a series of post about outstanding, unsolved problems of physics and the related progress

so far. Moreover you could even have a column for each one of them and post every time something new appears.

In addition to that, what are the current problems that each theory is facing? What is their status? Which are the areas under intensive research right now? For example the current challenges of ADS/CFT.

I like to see posts about the conceptual foundations of the different theories and how they alter our fundamental notions.

Origin of the Universe and Cosmology. What are the current views? What are cosmological models of the current theories?

I would love to see a presentation via series of posts of the various “no go theorems”.

Also a presentation via series of posts of the important experiments/effects that have confirmed the various theories so far. What are the experimental challenges we are facing now? What are the chances that the various theories have to be experimentally confirmed?

Finally you could maintain a column which will review important papers that have appeared in the arxiv.

This column could be updated every month or two and will present shortly papers that made the difference.

BR

on January 1, 2009 at 2:09 pmGiotisComplementary: Not just “no go theorems”. It would be nice if you could present other important “milestone” theorems that shaped contemporary physics.

on January 1, 2009 at 4:41 pmJust LearningI think invariably you’ll have to write about the things that interest you the most. The important thing is to keep a pace your comfortable with.

I agree that “no-go theorems” would be an interesting topic. Particularly an expose on Coleman-Mandula, and how super-symmetry side steps it.

For better or for worse, what drives some of the best conversation on blogs is when there is a strong disagreement between parties about the interpretation of something fundamental in physics.

I also think that you could spend a great deal of time on an expose about philosophy and science. Why should Popperism have such a strong hold on contemporary physics? (or does it?)

Does our “macroscopic” view of “truth” change at the microscopic level? Do we have to abandon some of our cherished notions about “observation = empirical evidence = truth” at the microscopic level in order to really push into “new physics”? What replaces “classical” notions of science at that transition scale? Is mathematics and proofs enough?

on January 1, 2009 at 5:18 pmJSDI would not want to say what you should write about; nonetheless, I would really like to read a clear explanation of ‘Asymptotic Freedom’ and its consequences. I have tried to understand it, but all what I have found is extremely basic or very tough.

on January 1, 2009 at 5:43 pmLionelmeichenl: Heady questions for a final year undergrad. I’ll attempt to answer your coredump with a few one liners which will presumably raise more questions than answers.

1) Environmentally induced superselection is one mechanism for decoherence. But in order to understand decoherence, one must first understand coherence.

2) Diffeomorphism invariance is just another symmetry, like the U(1) symmetry that gives rise to E&M and conservation of charge.

4) Group theory.

7) Noether’s theorem, again. Derive the charge corresponding to translation symmetry, promote to an operator, and act the operator on single-photon states.

10) Usually refers to the Lie algebra corresponding to the symmetry group of the theory.

11) Yes. The vector potential is a connection (like Christoffel symbols) on a gauge bundle, and “wave functions” are components of the the curvature of this connection. Ryder’s QFT book has a nice explanation. QCD is almost exactly the same thing, just a tad harder.

12) My wife says it’s the temperature gradient across the plant. My wife is always right.

13) It doesn’t, unless you are in a microgravity environment.

14) This goes under something called Mie theory.

on January 1, 2009 at 5:48 pmLionelI’ve always wondered about the relationship between scale invariance and conformal invariance (that one implies the other). It is said that a free scalar field on flat space is conformally invariant but this is only obvious in d=2. Otherwise the theory has a diffeomorphism-invariant generalization that is not Weyl invariant unless d=2. Maybe it’s a quantum result that I’m not getting.

on January 2, 2009 at 3:09 amMichael SchmittHi Moshe, I won’t suggest topics for you to write about, but I will take up your suggestion for a New Year’s resolution. Your blog is a good one and I’ll keep reading it and try to provide comments to help generate discourse.

Wait – let me pose two questions:

1) Why do we believe that gluons are massless (aside from the fact that the QCD assumes they are)?

2) Does the lifetime of a black hole as a function of its radius depend on the number of spatial dimensions?

regards,

Michael

on January 2, 2009 at 3:23 pmPlatoI am “an interested layperson with sufficient background on the topic? ” I’d like to think so, but it’s not always the case,so I will lurk to try and catch the jest of what is going on, too add to my further comprehension. I’ll try and not be a nuisance. 🙂

Gendanken experiments are always interesting to me because they introduce problems to date and some of the ways and approach it will be handled by it’s contributors. D’s current article,”

Probability of seeing a stage in a concert” is a point of expression in such a case.Gedanken Experiments involving Black Holes by L. Susskind, L. Thorlacius, (Submitted on 20 Aug 1993)

Best,

on January 4, 2009 at 2:43 amAnswer to #12Usually the discussions on this blog go way beyond my understanding, but I can answer one of those questions!

Plants do not “know” which way is up due to temperature graidents. In fact, they are able to correctly orient their growth patterns while kept in total darkness and at a constant temperature. They manage this through a process called gravitropism.

The most popular explanation for this involves little masses which are free to roll around inside the cells located in the tips of the stem and roots. These masses settle to the bottom of the cell by the influence of gravity and trigger the chemical processes which direct the growth of the plant.

on January 13, 2009 at 9:12 amamusedInflammatory posts on the status of string theory are always entertaining.

Besides that, I’d be glad to have an expository post (or maybe series of posts) on conformal field theory. Apparently conformal symmetry in 2 dimensions imposes such strong constraints at the quantum level that the theory is completely determined? I’d like to understand that.

(I made a few half-hearted attempts to learn this previously, but could never find a good text. The ones I looked at were horribly “physical” and did their best to obscure the underlying mathematical structure)

on January 13, 2009 at 5:06 pmMosheHi amused, welcome!

I can write something about CFT, let me just write one paragraph here, I think you probably have the background to decipher it. In two Euclidean dimensions the conformal group is simply the set of purely holomorphic (and purely anti-holomorphic) reparametrizations of the complex plane, which you probably remember from your complex analysis days. This is infinite dimensional, so there are infinitely many conserved charges, and you may then contemplate that something as complicated as a field theory may be exactly integrable. That is the case for simple enough CFTs (the so-called minimal models). For more complicated CFTs conformal invariance gives you partial knowledge, organizes the spectrum and correlation functions, but is not sufficient to solve the theory completely.

On another topic, I’d be glad to display a guest post on the topic of your choice in lattice field theory, whatever you think people with interest in mathematical physics and high energy physics may find useful. Just email me if you are interested. This can be done (both the email and the post) using your pseudoname.

on January 13, 2009 at 5:06 pmMosheThanks everyone for the suggestions, I’d better get on with it!

on January 14, 2009 at 7:37 amamusedThanks Moshe!

It would be great if you could write about CFT, elaborating on what you wrote in the paragraph. I would be very glad to finally learn what these things called “conformal blocks” and “fusion rules” really are.

I’ll be happy to contribute a post on lattice field theory at some point — thanks for the invitation. I’ll get in touch when I have the chance to start on this, hopefully in the not-too-distant future.