What It Takes

In the ongoing string theory comment thread (which, by the way, I’m really happy to see), “Who” steps off first to ask an interesting question:


One way to give operational meaning to a theory being predictive in the sense of being empirically testable is to ask

What future experimental result would cause you to reject the theory?

I think what worries a lot of people about string thinking is that it seems so amorphous that it might be able to accomodate any future experimental measurement. In fact I am not aware of any string theorist’s answer to this basic question.

It’s an interesting question, and not just for string theory.

The book I’ll be using for my “Quantum Optics” class in the spring term spends a good deal of time on the history of experimental attempts to prove the existence of photons. It turns out to be a really difficult thing to do, because people can keep finding theoretical loopholes. When you start looking itno the history, the Qwiki page for “Photon” has the definition about right:


A little word that can cause a lot of problems…

The simplest experiment you can do– setting up a detector or a camera that records individual photons– is a complete non-starter. You see discrete spots, or hear discrete “clicks” on your detector, but there’s nothing about that that proves you’ve got photons.

Historically, a lot of textbooks point to Einstein’s theory of the photoelectric effect (which is officially what got him the Nobel) as the demonstration that photons exist. The photon theory works very nicely in this case, but even after Einstein’s model was experimentally confirmed, there were people who really didn’t buy it. In fact, you can develop a semi-classical model (treating the atoms as quantum objects with discrete states, but the light as a continuous classical wave) that reproduces all the features of the photoelectric effect. The Compton effect was actually more convincing, historically, but there’s a dodge around that one, too (I don’t recall what it is, though, and the book is at work).

There are a number of other experiments that have been claimed as proof of the photon nature of light (Hanbury-Brown and Twiss, for example), but all of them turn out to have semi-classical explanations. The experiment that finally settled the question for just about everybody was the observation of photon anti-bunching. In 1977, a full 72 years after Einstein’s paper on the photoelectric effect.

(More after the cut.)

(The anti-bunching experiment is a pretty cool one. What you do is set up a photon detector looking at a light source, and measure the probability of detecting a second photon some interval dt after detecting the first photon. It turns out that classical sources of light either give you a flat distribution as a function of dt (a completely random distribution), or a peak at dt=0 (photon bunching).

(If you use the right kind of light source, though, you can find that the probability goes to zero as dt goes to zero. That is, you never detect a second photon immediately after the first one. This is impossible to reproduce with a classical model of light– you need photons to do it.

(The simplest example of such a light source is the light scattered from a single atom. The atom emits light only when it drops down from an excited state to the ground state, so after you detect the first photon, you have to wait for the atom to be excited again, and drop back down, and that takes some time. There’s no way for the atom to emit two photons right on top of one another (for most atomic transitions, anyway), so you get anti-bunching, which is a purely quantum effect.)

Now, of course, most people in physics had accepted the existence of photons long before 1977. The anti-bunching experiment was just the last nail in the coffin– a piece of experimental evidence that there was absolutely no way to dance around. It’s not like it took seventy years to get anybody to accept photons– just the die-hards.

But if you’re going to say “It’s been twenty years, when are you going to give up string theory?” remember that it took seventy years to get people to give up classical light…

Comments

  1. #1 Dave S.
    January 19, 2006

    Not only this.

    Dalton proposed his atomic theory of matter in 1805, but it wasn’t conclusively put to rest until Jean Baptiste Perrin’s experiments in Brownian motion 100 years later.

  2. #2 Peter Woit
    January 19, 2006

    As you note though, the problem with string theory is that there is no conceivable experimental result that would cause people to give up on it.

    The other main difference here is that twenty-some years out after the photon hypothesis, most prominent theorists had given up on the classical theory of light. This case is quite different, with pretty much the entire particle theory faculty at places like Harvard and Princeton showing no signs yet of giving up.

  3. #3 Chad Orzel
    January 19, 2006

    I don’t want to claim that there’s a perfect correspondence between string theory and the classical theory of light. The photon theory very quickly suggested and passed a large number of experimental tests, which string theory has not done. At least some of the people producing semi-classical models to explain experiments up to 1977 were playing devil’s advocate as much as anything else.

    Still, I think it’s an interesting example of the resiliance of old theories. And you can see it lots of other places, as well– the atomic thoery Dave mentions is another good example, and the various loopholes people have found in Bell’s inequality experiments are a good current example.

    It’s not surprising to me that people who have spent a good deal of time working on string theory would be very resistant to letting go. And they do claim to have produced interesting and useful mathematical developments over the past few decades– I wouldn’t consider that as significant as an experimental prediction, but it’s not nothing, either.

  4. #4 Dave S.
    January 19, 2006

    The photon theory very quickly suggested and passed a large number of experimental tests, which string theory has not done.

    I think this is a very important point that deserves repeating. Throughout the time from when these theories were first proposed to when they were universally (or nearly so) accepted, experimenal work continued and predictions which flowed from the theory were constantly tested against the data and contrasted to the older theories.

    As beautiful and fruitful as string theory may be mathematically, and I am in no position to judge but will cheerfully accept that as a fact, unless and until some empirically testable predictions can be elucidated, it remains more firmly in the realm of philosophy than science. My opinion.

  5. #5 NL
    January 19, 2006

    Making things even more confusing for students, I believe there is no hypen in Hanbury Brown…

    (of course, I thought Cohen and Tannoudji were buddies for at least a year…)

  6. #6 JK
    January 19, 2006

    Are there semi-classical models which explain high precision QED predictions and corresponding experimental measurements such as the Lamb shift and electron magnetic moment?

    These aren’t direct observations of photons but they seem like pretty good evidence that the electromagnetic field is quantized (obtained well before 1977).

  7. #7 Chad Orzel
    January 20, 2006

    JK: Are there semi-classical models which explain high precision QED predictions and corresponding experimental measurements such as the Lamb shift and electron magnetic moment?

    I don’t believe so, but then I’m not really that familiar with the history of attempts to avoid photons.

    These aren’t direct observations of photons but they seem like pretty good evidence that the electromagnetic field is quantized (obtained well before 1977).

    Absolutely.
    But here, we’re sort of back to the philosophical question at the heart of the recent argument about the E=mc2 test. Indirect proofs are great to have, but a direct detection is more satisfying.

    Which of course, brings us to Dave S.: As beautiful and fruitful as string theory may be mathematically, and I am in no position to judge but will cheerfully accept that as a fact, unless and until some empirically testable predictions can be elucidated, it remains more firmly in the realm of philosophy than science. My opinion.

    I definitely agree that experimental tests carry a great deal more weight than discoveries in mathematical formalism. I’ve come around a bit as to the precise status of string theory, and I’m a little more willing to give it provisional “Science” status than I used to be. In spite of what sometimes appears to be a concerted effort by prominent people identified with string theory to make the whole business look ridiculous.

    NL: Making things even more confusing for students, I believe there is no hypen in Hanbury Brown…

    I’ve seen it both ways. The sources I was looking at yesterday hyphenated it, so I just copied them.

  8. #8 Simon
    January 20, 2006

    Even the Lamb shift can be explained without having to have photons: Barut & Huele PRA 32, 3187 (1985). I can’t say, having skimmed the opening paragraphs, that I really understand what exactly they’re doing, but hey, I’m only an experimentalist.

  9. #9 Nathan Myers
    January 21, 2006

    I wonder if astronomers would allow similar questions to be entertained about Big Bang. I wonder whether it is possible, even in principle, for an observation to lead them to accept that an alternative to Big Bang must be considered as competition for it. Thus far, it seems, each falsification has led simply to more fantastical invention.

    The resistance, seen in only certain branches of science, to maintaining more than one plausible hypothesis seems more psychological than practical.

  10. #10 Chad Orzel
    January 21, 2006

    I wonder if astronomers would allow similar questions to be entertained about Big Bang. I wonder whether it is possible, even in principle, for an observation to lead them to accept that an alternative to Big Bang must be considered as competition for it. Thus far, it seems, each falsification has led simply to more fantastical invention.

    It’s not my field, so I couldn’t really say.

    The resistance, seen in only certain branches of science, to maintaining more than one plausible hypothesis seems more psychological than practical.

    I don’t think it’s confined to any particular branch of science. It’s just that the questions tend to linger for a longer time in certain branches, before they get settled by experiment.

    But in pretty much any area you’d care to name, I don’t think you’ll find a large number of people who really, truly, maintain that two competing hypotheses are equally plausible. Most scientists have a definite preference for one hypothesis or another, and will tend to discount the others.

  11. #11 Nathan Myers
    January 22, 2006

    Of course everybody tends to favor one hypothesis over the others, even where the evidence doesn’t. The difference is whether, as a journal referee, they permit papers that mention other hypotheses.

    A good example to the contrary is in the study of the rise of H. Sap. The field has been wide open forever, there are several competing models, and everybody agrees the data are way too sparse to definitively favor one over the others.

    When the COBE microwave-background results were first presented, Georg Smoot was quoted (as I recall) saying “This proves the Big Bang once and for all.” Of course it did nothing of the sort, and no true scientist would say any such thing.

    Astronomy isn’t my field, either. My impression, though, is that one source of insecurity among astronomers and cosmologists is that vanishingly few of them has studied plasma fluid dynamics in any detail. (Most are said to have taken a semester of “MHD”, a mathematically-elegant model entirely inapplicable in most astronomical scenarios. The mathematics of actual plasma fluid dynamics are anything but elegant.) Any hypothesis that depends on understanding real plasma fluid dynamics leaves almost all working astronomers ill-equipped to contribute, so it’s not so surprising that they are reluctant to follow data that lead in that direction.

    Work with plasma involves scaling arguments from fiendishly difficult vacuum-lab work, and enormous finite-element computer simulations. It’s an easy guess that one attraction for young physicists who enter astrophysics is precisely the lack of lab work. A development that limits the value of purely mathematical constructs will naturally be seen as “somebody else’s problem”.

    So, now we have 96+% of the universe composed of unobservable matter with increasingly tortured properties. Meanwhile, there is no explanation of how the enormous EM fields engendered by light-years-long “jets” of charged particles manage to have no effect whatsoever on any astronomical event.

    It would be most unfortunate for generations of astronomers to find that their careers turned out to be equivalent to those of the phrenologists. It’s fortunate, though, that the space probes and telescopes are built and operated by engineers, and the data will remain available and (mostly) useful when it happens. Regardless, the pictures are pretty (and astounding) enough to keep us paying to collect them, without worrying too much about what they really mean.

  12. #12 CS
    January 24, 2006

    As a cosmologist, I can address a few of the issues in the thread here.

    NM: I wonder whether it is possible, even in principle, for an observation to lead them to accept that an alternative to Big Bang must be considered as competition for it.

    The notion of a Big Bang was objectionable to most of the scientific community (including many astronomers) until the CMB was accidentally observed by Penzias and Wilson in the 60′s. But the CMB had several unique properties predicted by the Big Bang Model, such as having a black body spectrum and a uniform (to one part in 100000 or so) temperature of a few Kelvin. Without a universe expanding from a period of extremely high density and temperature, there is absolutely no reason to predict the presence of such a microwave background. Don’t forget about Hubble’s observations showing all galaxies moving away from each other, which is also consistent with the Big Bang.

    But the key point here is that astronomers did not just say “oh, that model sounds cool, I think I’ll take it.” Most of them were convinced only after observational evidence became available that was simply unexplainable by other ideas at the time (and basically all models of the universe conceived through today).

    Astronomers did not stop there. The small variations in the CMB temp (to one part in 100000), the ones you see with COBE and WMAP, are not completely random. The variations from the average temp are characterized by their relative size. The Big Bang model predicted a specific statistical distribution of these variation sizes that can be characterized by only seven parameters; the CMB observations (WMAP with more than a million data points) were well described by the seven parameter prediction. Those observations are absolutely incompatible with simply random variations.

    That is like asking someone to draw any kind of line on a piece of paper for you, but before doing so, you supply an equation that can be used to describe that line. It is pretty difficult to come up with an equation that can account for all the possible loops and squiggles such a person may draw. Suppose, however, I assume the person will use a ruler (or some other straight edge) to draw the line. Then I predict the line can be described by y=mx+b, with m & b the two parameters. If the assumption is wrong (the person does not use a ruler), that equation is very likely to very poorly describe the drawn line. If your equation does well describe the line, you can (with a high degree of certainty) conclude the person used a ruler.

    That is essentially what occurred with the Big Bang. It was not generally accepted until very specific predictions appeared in observations. The number and specificity of the predictions that have been been supported by data have led the Big Bang theory to be as well accepted in the astronomy community as gravity.

    I am not going to say the Big Bang theory is “absolutely positively 100% the exact correct theory” as no respectable scientist should, but astronomers expect the likelihood of the Big Bang to be incorrect to be about the same as gravity being incorrect. A new theory is likely to be simply a modification of the current Big Bang theory rather a completely new one. That is the same as we can say for gravity: we know there are problems with gravity at the quantum levels, but the “correct” theory of gravity will almost certainly encompass the current theory (Einstein’s General Relativity) in the same way that Newton’s theory is encompassed by Einstein’s.

    NM: My impression, though, is that one source of insecurity among astronomers and cosmologists is that vanishingly few of them has studied plasma fluid dynamics in any detail.

    Surprisingly, there is quite a bit in cosmology that one can do without having detailed knowledge of plasma fluid dymanics. When you are talking about the early universe (dealing with CMB, nucleosynthesis, etc.), the fluctuations are very small and everything can be treated linearly. After that, everything on large scales is primarily influenced by gravity, which is very simple to model.

    Small scales are where lack of expertise in fluid dynamics can hurt, but there are a subset of cosmologists/astronomers who specialize in this area and do very sophisticated modeling (see e.g. the Virgo Consortium). I myself know little on the subject, but I have no qualms about collaborating with those who do. I do not feel “insecure” in my lack of knowledge because there are too many subjects in astronomy & cosmology for anyone to be an expert in all.

    NM: It’s an easy guess that one attraction for young physicists who enter astrophysics is precisely the lack of lab work.

    Those large telescopes and satellites don’t just magically appear. It takes the same laboratory design and testing to develop astronomy equipment as it does for, say, high-energy physics (there are actually more labs at my institution for the former than the latter).

    While the public only sees a few pretty pictures, this represents only a very small fraction of the data being collected. CMB, radio, gravitational lensing, etc. data take a lot of analysis. Unlike a simple photo, it takes a LOT of work to draw results out from the data. And oftentimes simulations are necessary.

    NM: So, now we have 96+% of the universe composed of unobservable matter with increasingly tortured properties.

    I do not quite understand why you believe dark matter and dark energy have “increasingly tortured properties”. In fact, their properties are actually quite simple compared to ordinary matter. The only property of dark energy, in fact, is that it has a negative pressure in a cosmological sense. Its presence has observational consequences (the expansion of the universe is accelerating rather than decelerating) that essentially no one was expecting to see. To people familiar with cosmology, though, it is now difficult to refute its presence. What exactly dark energy is is where all the speculation lies. But please don’t believe that lack of a consensus on what dark energy may be has any bearing on whether it actually exists.

    The same goes for dark matter, although its presence is even more irrefutable. Again, we do not know exactly what it is, but – make no mistake – we know there is SOMETHING there. And it is something that does not interact much with light (which excludes any ordinary matter, composed of protons and neutrons).

    NM: It would be most unfortunate for generations of astronomers to find that their careers turned out to be equivalent to those of the phrenologists.

    It’s not going to happen. Astronomy and cosmology are not to be confused with astrology and cosmetology: they are grounded in predictions and observations, the same as any other field in science.

    As you mentioned, astronomy is not your field. Please do not assume then that it is a field of bumbling, ignorant fools. Dark energy, for example, may sound fanciful and extravagant, but unless you take the time to learn about the field, you should not base the validity of such ideas on your “impressions”. That is a decidedly unscientific way of going about things.

  13. #13 Roman Werpachowski
    January 25, 2006

    After that, everything on large scales is primarily influenced by gravity, which is very simple to model.

    Now that is one hell of a bold statement.

  14. #14 Roman Werpachowski
    January 25, 2006

    The only property of dark energy, in fact, is that it has a negative pressure in a cosmological sense.

    And this is most irritating in it ;-) The less properties something has, the more difficult it is to disprove its existence.

    Maybe we should not think yet of the dark matter as some particular type of matter, but only as some general concept? A template?

  15. #15 penny
    January 30, 2006

    In re: Dark Matter
    Why does it have to be a form of matter? Why not just believe in the Einstein Field equations with a non-zero cosmological constant term?
    Penny