What Physics Topics Would You Like to Read About?

Posted by Chad Orzel on July 29, 2010
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I had intended to write up a recent paper for ResearchBlogging today, but I cleverly forgot to bring either the hard copy of the PDF home last night, which wrecked that plan. And I’ve got real lab work to do today, so it’s not happening at work.

This seems like a good opportunity, though, to ask if there are things I ought to be explaining here that haven’t occurred to me for one reason or another. So, as the post title says:

What topics in physics or related areas would you like me to write about here? This could be a recent paper, something from a recent news story (“I heard these guys in India invented a room-temperature superconductor…”), or some background idea that you’ve always wondered about (“What’s angular momentum, anyway?”).

Leave your questions and suggestions in the comments. I won’t promise to answer everything (there are a lot of topics even in physics that I’m not really qualified to comment on), but I’ll make an effort to write up answers to any questions I have good answers for.

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  1. #1 James D. Miller
    July 29, 2010

    1) Is it true that our understanding of quantum physics comes from studying systems with only a small number of particles and there is a good chance our theories won’t hold in more complex systems.

    2) Is immortality consistent with our current understanding of physics?

    3) Is it true that physicists often run experiments and get results inconsistent with current well established theory but don’t follow up on the experiments because the experiment might have been flawed and there isn’t time to look into everything?

  2. #2 chezjake
    July 29, 2010

    What are your thoughts on whether or not the new experimental fusion reactor proposed for France will actually come up with a practical reactor?

    http://www.bbc.co.uk/news/science-environment-10793883

  3. #3 Brad
    July 29, 2010

    Part of the reason I read your blog (and Doug Natelson’s) is to hear about three guys in India making a room temperature superconductor. That kind of thing does not come up over dinner.

    * Why is a room temperature superconductor so hard? Why do things have to be cold for there to be no resistance (I can guess, but my knowledge of super conductors consists of the words “Cooper pairs” which does not get me very far.)

    * A lot of AMO is testing QM at a really basic level. Are there any future exciting experiments?

    * Laser cooling and magnetic cooling are really fascinating. Are there any other tricks along those lines?

    * Finally, why did all of my stat mech courses suck?

  4. #4 anon234
    July 29, 2010

    Why isn’t the “detection loophole” considered a solid flaw in Bell test experiments? Wouldn’t it be more reasonable to believe in local hidden variables that control both detection and output, rather than believe in entanglement and “spooky action at a distance”?

  5. #5 miller
    July 29, 2010

    It’s often said that virtual particles can “borrow” energy, as long as it’s for a short enough time to be compatible with the uncertainty principle. This never made sense to me, because the uncertainty principle says that product of uncertainty in energy and uncertainty in time is greater than h-bar over 2, not less than. Please explain.

    What are anyons, and why should we care?

  6. #6 rpenner
    July 29, 2010

    What’s the connection between action, symplectic and Lagrangian’s?

  7. #7 Steven Colyer
    July 29, 2010

    ONLY One:

    1) What is the BEST way to teach Introduction to Quantum Mechanics, other than your book? And Penrose’s and Hey and Walters’ books?

    Case in point: The Double-Slit Experiment. Do NOT start there! That was Feynman’s thing. WE can do better. We have Feynman to build on, after all.

    Do YOU Chad, when you teach that all-important first-day lecture intended we hope, to NOT discourage the Einstein-wannabes, NOT mention the following people (in anticipation of Indeterminacy, which is where science loses what would be its greatest advocates), because if you don’t, I question how much you know about the subject in the first place? Which I’m sure you do, but I mention this for other professors, who may not, or did once but have since forgotten:

    - Augustin-Louis Cauchy
    - Viktor Bunyakovsky
    - Hermann Amandus Schwarz

    And mention those and explore what they were about, before the name “Werner Heisenberg” et. al. even leaves one’s lips.

    “The general formulation of the Heisenberg uncertainty principle (Indeterminacy) is derived using the Cauchy–Schwarz inequality in the Hilbert space of quantum observables.”

    … Wikipedia, entry: Cauchy–Schwarz inequality.

  8. #8 Matt Leifer
    July 29, 2010

    anon234,

    I am not Chad, but here is my take on why the detection loophole is not taken seriously. First of all, it is taken seriously enough that people regularly post papers on the arXiv proposing loophole free Bell experiments, so I am fairly certain that the tests will be done as soon as it becomes technologically feasible. Secondly, there are two main loopholes in current state of the art Bell inequality tests: the detection loophole and the communication loophole. The latter has to do with not being able to make changes to the measurement setting fast enough in one arm of the experiment to rule out the possibility of signaling to the other arm.

    Now, the communication loophole has been closed in optical experiments and the detection loophole has been closed in experiments on ions. Although there is no current experiment that closes both loopholes, one would have to believe that nature exploits the detection loophole for optical systems, whilst exploiting the communication loophole for ions. The types of models that exploit the two different loopholes are very different, so this seems fairly implausible to me. Whilst it is true that photons and ions are physically very different, the detection methods used in the experiments are very similar — the main difference is just that we get a large number of photons to detect when we measure an ion, so the detection efficiency doesn’t have to be so high to close the loophole.

    The other reason for not taking the detection loophole seriously is that we lack an alternative model general enough to explain all the successes of quantum physics that exploits it. Sure, one can cook up models to explain particular Bell experiments, but they do not go beyond that and indeed they are usually specialized to one type of Bell test, e.g. testing the CHSH inequalities, and do not even generalize to a model for all possible quantum correlation experiments. If someone wrote down a plausible theory that did this then perhaps it would be taken more seriously, but then again I really think it is a lost cause due to the previous reason.

  9. #9 Eric Lund
    July 29, 2010

    It’s often said that virtual particles can “borrow” energy, as long as it’s for a short enough time to be compatible with the uncertainty principle. This never made sense to me, because the uncertainty principle says that product of uncertainty in energy and uncertainty in time is greater than h-bar over 2, not less than. Please explain.

    I am not Chad, but I can try to answer this one.

    The uncertainty principle, expressed thus, is a bound on the precision with which we can simultaneously measure both energy and time. The physical reason for this limitation is the ability of the virtual particles to temporarily “borrow” energy in just this fashion.

  10. #10 theresnoonehere
    July 29, 2010

    One of the most dissatisfying things I found when first learning QM in my undergrad was that it was based on a continuous space, and all solutions seemed to be an attempt to linearize the underlying pde’s. My QM stopped in grad school (applied math) and in the intervening years, I never really got back into it.

    However, I am curious about the current state of the art in this area. It seems as though true nonlinear and discontinuous phenomena (I know they are two separate problems) are being ignored by current approaches in the same way that the Euler equations don’t display the same phenomena as the Navier Stokes equations, which miss out on the added complications of the Boltzman equation.

    String theory (I think – but don’t really know) doesn’t seem to do the trick. It’s still a continuous underlying space and “only” linearized, despite being extraordinarily complicated. Are there *any* attempts at this problem that try and tackle the space and/or linearity problems?

    If so, could you give a summary of these approaches?

    Thanks.

  11. #11 Richard
    July 29, 2010

    I’d be interested in (probably a series) of posts on how people practically actually do cold atoms experiments because I don’t really know. Perhaps you’ve discussed this before..

  12. #12 Jeff
    July 29, 2010

    I’d like to learn more about mechanics after Newton: the development of Lagrangian and Hamiltonian mechanics and related concepts, ideally with enough of a historical slant on it to see the progression of ideas.

  13. #13 Tim
    July 29, 2010

    Turbulence

  14. #14 E4
    July 29, 2010

    @theresnoonehere

    Which PDE’s did you linearize in your quantum mechanics course?

  15. #15 ObsessiveMathsFreak
    July 29, 2010

    Do you (or indeed most physicists) use dimensional analysis of equations a lot, and is there any formalised system for applying it?

  16. #16 E4
    July 30, 2010

    +1 for Richards idea on “how people practically actually do cold atoms experiments”.

    I am also interested in your perspective, as an experimental physicist specialized on cold atoms, on papers like these http://arxiv.org/abs/cond-mat/0505055 and http://arxiv.org/abs/cond-mat/0604671 . Do you think it is possible to do these experiments, or is this a just a matter theoreticians fantasies? Emergence is an amazingly powerful thing.

  17. #17 Johan C
    July 30, 2010

    My personal favourites are the way you explain “basic concepts” (spin, polarization, energy, fields, etc). Perhaps one day you could bundle those into a new book “How to teach basic concepts in physics to Blog readers” :-) (I’m also saying this, because at Amazon I noticed some disgruntled readers who found your title “How to teach physics to your dog” quite misleading, since (to them) quantum physics is not (all of) physics.

  18. #18 Bee
    July 30, 2010

    quantum dots

  19. #19 Andrew Perrin
    July 30, 2010

    I’d love to hear you talk about the “pseudomagnetic fields” found in these graphene bubbles:
    http://www.berkeley.edu/news/media/releases/2010/07/29_graphene.shtml

    For one thing, I don’t understand why electrons flying in circles in the bubbles aren’t inducing a *real* magnetic field?

  20. #20 Paul
    July 30, 2010

    miller: “It’s often said that virtual particles can “borrow” energy…”

    I would also like to hear more about the mainstream view on this subject.

    Here is my take on what likely actually happens which may or may not be in line with standard reasoning.

    I see the effect as inherently due to wavelike nature of the background matter and radiation fields. For example we are now awash with electromagnetic radiation from cell phones. The effect of this radiation cancels out on timescales larger then the period of the waves T which is given by T=h/E (based on photon energy relation E=hf), however for times t smaller then half the period or t

    This energy can be “borrowed” in the sense that it can accelerate the charged particle for half a period but it has to be “given back” in that in the next half the particle will be accelerated in the opposite direction canceling the effect.

    All in all it can be seen that the energy available times the time for which it can be borrowed roughly satisfy the E*t

    Now for EM this seems pretty simple and uncontroversial (as far as I can tell). But I suspect that the same holds for all matter fields and that vacuum is actually full of background matter and radiation fields and that it’s temporary non-cancellation of such fields that leads to virtual particles popping in and out of existence. This is probably not a mainstream view (stochastic electrodynamics seems to deal with similar ideas but I read much on the subject and it seems somewhat controversial).

  21. #21 Steven Colyer
    July 30, 2010

    Other ideas to answer your excellent question:

    -1) Interpretations of Quantum Mechanics. Stay away from this one, even though you’re forced to cover it in your classes, which further proves that every job has its drawbacks. QMI is Physics meets Philosophy which quickly turns into a bottomless pit. Physics meets Mathematics is 10,000 times more interesting.

    0) Hiroshima and Nagasaki. This is the general public’s biggest knowledge of “Physics” and what it is capable of. Stay away from this one too. What’s done was done and can’t be undone. Time to move on.

    1) Physics and Ethics. I can’t think of a single field of study that wouldn’t benefit from a real cool and required class to discuss Ethics within that field. In Economics, doubly so. All fields are interdisciplinary, Physics most of all.

    2) How to use Physics to become a Bazillionaire like Gordon Moore at Intel. This subject will appeal to your greedier readers. I’ll play, since I have an MBA in addition to a Mechanical Engineering degree, so I will actually know what I’m talking about, as opposed to say: Clifford Algebras, which I’ve never formally studied.

  22. #22 Sili
    July 30, 2010

    Do you know anything about breeder reactors and whether nuclear is a viable stopgab measure until renewables can take over, or is the waste issue still too expensive?

  23. #23 KTNAJR
    July 31, 2010

    How is it that light sets an upper limit for speed?
    I read somewhere that special relativity only gives that there is a limit, so, how do we know the limit is the speed of light?

  24. #24 Steven Colyer
    August 1, 2010

    The speed of light is a physical constant of the Universe, a “given” that we have to deal with, like it or not. I don’t like it personally, but shrug, what can you do?

    From Wikipedia’s entry on “The speed of light”:

    Ole Rømer first demonstrated in 1676 that light traveled at a finite speed (as opposed to instantaneously) by studying the apparent motion of Jupiter’s moon Io. After centuries of increasingly precise measurements, in 1975 the speed of light was known to be 299,792,458 m/s with a relative measurement uncertainty of 4 parts per billion.

    Ain’t the Internet grand?