Built on Facts

Photons, Universes, Etc.

I thought about linking this Forbes article on the economic situation simply because it’s interesting. What actually made me link it was the sentence at the end:

And reality tells us that we barely avoided, only a week ago, a total systemic financial meltdown; that the policy actions are now finally more aggressive and systematic, and more appropriate; that it will take a long while for interbank and credit markets to mend; that further important policy actions are needed to avoid the meltdown and an even more severe recession; that central banks, instead of being the lenders of last resort, will be, for now, the lenders of first and only resort; that even if we avoid a meltdown, we will experience a severe U.S., advanced economy and, most likely, global recession, the worst in decades; that we are in the middle of a severe global financial and banking crisis, the worst since the Great Depression; and that the flow of macro, earnings and financial news will significantly surprise (as during the last few weeks) on the downside with significant further risks to financial markets.

One sentence! Beginning with a conjunction! I guess the editors allowed it for the dramatic effect, because you could replace all the semicolons and probably three or more of the commas with periods without generating a sentence fragment.

While we’re linking, I’d like to make sure everyone has a chance to read this great post by Dr. Orzel of our own Uncertain Principles. It’s about teaching the photon concept in modern physics education. Turns out that in fact many of the “classic” demonstrations of the photon’s existence in fact don’t require the photon as an explanation at all. With enough subtlety the semi-classical picture works just fine. So why use photons at all, especially since they’re not “particles” in the usual sense and don’t really even have wavefunctions (though that too is a very subtle point fraught with complications)? Well, we use the concept because it’s right. There are in fact photons and there are experiments which unambiguously demonstrate their existence.

Which is not to say that the photon is something to casually throw around without taking a look and understanding all those issues. Every decent quantum optics text I’ve read spends considerable effort hashing out all those considerations in detail. (This one, for instance, does a particularly good job)

But those considerations aren’t too important at the introductory level, and it’s possible – and a good idea – to use the photon at a very early level. It simplifies a lot of problems, makes intuitive sense, and is correct. What’s not to like?

By the way, I notice in yesterday’s post that many readers were not satisfied with giving a “yes/no” answer to the multiple universes question. Instead many of you objected to the whole idea in the first place. If it’s not observable, what difference can it possibly make? This attitude warms my heart. I am strongly a subscriber to the “shut up and calculate” interpretation of quantum mechanics and I find the conclusion of the physicist quoted by EvolutionBlog to be downright nuts:

When I ask Linde whether physicists will ever be able to prove that the multiverse is real, he has a simple answer. “Nothing else fits the data,” he tells me. “We don’t have any alternative explanation for the dark energy; we don’t have any alternative explanation for the smallness of the mass of the electron; we don’t have any alternative explanation for many properties of particles.

Really? Aside from the extreme sketchiness of the claim that in fact a multiverse requires any of those things, how about this for an explanation: those quantities take those values because the underlying final theory happens to contain those values. Is that actually the case? I have no idea, but if we find a final theory and then have to chose between “It is what it is” and “There’s an infinite number of other universes where other theories hold sway and oh by the way there’s no possibility even in theory of ever checking this idea”, well, I can guess which one Occam is going to pull a Sweeney Todd on.

Well, I’m off to take the classical mechanics exam. Actually at the time this goes online I should be pretty close to done. I hope it goes well!


  1. #1 Zifnab
    November 14, 2008

    From a purely philosophical perspective, the multiverse question feels a lot like the extraterrestrial life question.

    We don’t seem to have a great deal of evidence for or against them and most of the claims we do have are sketchy or quickly discredited. But when you already have one instance, statistically speaking, it begs the question of what makes what we do know so special.

    Why would the universe be such a unique occurrence? Space is vast and almost completely unexplored. We sit on one tiny little satellite at the ass-end of our own Galaxy peering off into space collecting information that is billions of years old by the time it gets to us. What the hell do we know that precludes the existence of multiple universes? Or life forms, for that matter. And what about our own realm makes us so special? What about the Big Bang (or whatever theory you hold most accurate) makes it so special that it can only happen once?

    I have a hard time NOT believing in a multiverse.

  2. #2 Tercel
    November 14, 2008

    Why would the universe be such a unique occurrence? … What about the Big Bang (or whatever theory you hold most accurate) makes it so special that it can only happen once?

    Nothing. But if a universe external to our own does exist, and is fundamentally unobservable, that essentially relegates it to fantasy, metaphysics, or philosophy. Just because something is allowed for in theory doesn’t mean it is “real” in any practical or meaningful way.

    I have a hard time NOT believing in a multiverse.

    I have a hard time believing in anything I don’t have evidence for. Even if it does seem nice in theory. Although I do accept the existence of things which I assign a limited definition of “real.” Things like photons and electrons, for example, are real in that they are part of theories that explain our observations incredibly well. But are they real like a brick wall, or a hamburger? You can never feel or touch or see or taste an individual photon or electron, so in some way, no.

    Classical mechanics, yikes. That class was damn hard. Very cool though. Did you ever have the question about dropping a puck onto a rotating platform with given initial velocity in the stationary frame, and a given mass, coeff. of friction, etc., and you had to solve for the path of the puck in the rotating frame of the platform?

    Very cool class.

  3. #3 Jorg
    November 14, 2008

    Tercel: “But if a universe external to our own does exist, and is fundamentally unobservable, that essentially relegates it to fantasy, metaphysics, or philosophy. Just because something is allowed for in theory doesn’t mean it is “real” in any practical or meaningful way.”

    Indeed. But direct observation of a phenomenon is unnecessary for us to accept its reality. Just observing its effects will do, together with lacking in alternatives. Multiverse seems to fall into that, admittedly less than comfortable, position more and more. What is more, it may actually be required, in some versions of QG, at least.

  4. #4 Carl Brannen
    November 14, 2008

    Good hunting on the exam!

    Regarding photons, a great deal of physics is taught as if there is only one way of interpreting the mathematics. But there are always multiple interpretations that lead to the same equations. And it’s the equations only that are verified by experiment, not the interpretations.

    Despite this, physics is usually taught according to a set of interpretations that are the most widely accepted. If you think it’s bad now, you should have seen it back in the day when I was taught. At that time we did not discuss any alternative interpretations. Now, people talk all the time about stuff like multiple universes.

    Regarding the existence of the photon, I’m of a rather crankish bent on this and feel that all the gauge bosons are unnecessary, though in replacing them with fermions one sometimes would have to postulate that the bosons are composite.

    This is a sort of sub rosa theme in the paper I just released to Foundations of Physics. The paper is written from a density matrix point of view rather than a quantum field theory view. But translating into QFT, it is analyzing Feynman diagrams with the boson leads clipped off. The justification is that everything is happening inside of a point particle so there is no reason to go to the trouble of propagating the bosons; instead of having to integrate them out you can just ignore them and watch what they do to the fermions. The idea is “what collection of Feynman diagrams is consistent with a bound state” for the problem described. I guess I should link it.

  5. #5 rhett
    November 14, 2008

    I posted this in the comments on my blog, but here is a great paper about photons. I think it gives a good perspective on the historical development.

    W.E. Lamb, Jr. “Anti-Photon” Applied Physics B Lasers and Optics (1995)

    I didn’t mean to get everyone so riled up.

  6. #6 Peter
    November 14, 2008

    The problem I have with comment #2 is simply this: absence of evidence is not evidence of absence. A scientist may answer Yes to the existence of a entity with data but they can not answer No based on no data… The best that can be said is Don’t Know, or No Data!

    I for one would not be at all ready to declare this universe to be the only one based on a lack of observable data on any other.

    Further given the progression our sun is a star and there are many stars, our sun is in a galaxy and there are many galaxies, and our galaxy is in a universe, I might even suspect the existence of other universes. As scientist such a pattern would be enough to warrant an experiment if one could devise such… Scientists do NOT simply accept only what is supported by data but question and test any pattern they see. Science does not advance by limiting imagination to only what is known with current data.

    At this point we seem to have a theories of the origin of the universe which make little demand on the starting conditions, arising out of “nothing” rather spontaneously. It would seem to me there is the potential for a lot of “nothing” since it seems to be unlimited in concept, and no known unique apriori conditions for the spontaneous event, so why “believe” there are no other universes?

    I have a hard time believing anything we don’t have evidence for by now will NOT turn out to have new and exciting science in the first data we find.

  7. #7 Matthias
    November 15, 2008

    This is a rather easy sentence if German is your mother tongue… just enumerations.

    We tend to dig deep into subsubsubordinate clauses, sometimes such that you need to break it into a tree of meaning, often many levels deep, when trying to get it as a whole – which is complicated by tha fact that in some temporal cases verbs may get spread in front of and after subordinate clauses.

    I had a hard time learning to keep english sentences short.

  8. #8 Gus
    November 15, 2008


    Based on laboratory simulations, I believe I have identified dark energy as the energy contained in traveling gas shock waves.
    This is because a low pressure wake is established on the downstream side of the star, after the shock waves passes. For details see: ISBN 978-1-4343-0661-6(sc)

  9. #9 Zifnab
    November 17, 2008

    Nothing. But if a universe external to our own does exist, and is fundamentally unobservable, that essentially relegates it to fantasy, metaphysics, or philosophy. Just because something is allowed for in theory doesn’t mean it is “real” in any practical or meaningful way.

    I completely agree. If multiverses exist, but have absolutely no impact on our own, then for all extensive purposes they don’t exist. But we have a number of unexplained astronomical observations that a multiverse might help explain.

    There are certain questions in physics that basically ask “What do we look for?” We are spending millions of dollars looking for the Higgs Bozon, for instance. We’ve got millions more invested in SETI. Not so much money invested in searching for unicorns or Big Foot.

    So the real question is whether a multiverse is worth looking for or trying to account for. And I would say that it is.

  10. #10 Brandon
    November 24, 2008


    I was just tuned into your blog by, none other than, your dad. I work with him out here and he has nothing but the fondest and warmest comments about you. Thanks for the wit you deliver in your science editorials; all others either speak above their audience or condemn those fail to follow their logic – your blog is truly refreshing.

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