What do you do for a living, anyway ?

What is the job of a theoretical physicist ? Isn’t physics an experimental science ? Should discoveries not always occur as a result of reproducible, controlled laboratory observations ? Perhaps in no other scientific discipline is the division between “theorists” and “experimentalists” so well-defined and rigid as in physics.

It did not use to be that way. In the days of Isaac Newton, a physicist could (and was expected to) do an experiment and come up with an explanation for its outcome, preferably one (a theory) accounting not just for one particular observation, but for as wide as possible a class thereof. These days, however, as the boundaries of the discipline are constantly being pushed (both toward the very small or to the very big), the sheer complexity of both experiments and theoretical calculations requires a high degree of specialization, which makes it essentially impossible for most mortals to excel at both [0].

In practice, despite their being called both “physicists”, in principle possessing for the most part the same, broad knowledge of the laws of nature, experimentalists and theorists are typically very different scientists. Physics graduate students who are trained as experimentalists generally take different courses (and even emphasis placed on formal course work is not the same as for their theory colleagues), develop vastly different interests and skills, and I think it is fair to say a rather distinct way of thinking and investigative approach with respect to their theory counterpart.
Not surprisingly, such a pronounced difference in mentality can and does lead to awkward interactions between theorists and experimentalists collaborating on the same problem, as well as, in my opinion, fundamentally different, often conflicting views of the discipline as a whole — but that is for another post, maybe after I retire.

OK, so, what experimentalists do is clear, right ? They set up a laboratory and conduct research by performing measurements of cogent physical quantities on systems of interest — samples of condensed matter, molecular, atomic or subatomic particles, biological membranes, you name it. In some (rare) instances, the experiment can be carried out by a few individuals, perhaps as few as two (e.g., a graduate student and his/her supervising faculty); normally, however, a collaboration among several scientists, often based at different institutions, each bringing his/her own unique expertise will be required.

But what about theorists ? What is their contribution to the whole enterprise (now, now, no foul language, my experimentalist friends…) ? Well, based on my experience I tend to think that we all fall somewhere in an interval where, at one end, you have those who work in close contact with experimentalists. These theoretical physicists (I often refer to these as “Experimentalists who cannot use a screwdriver”) are familiar with experimental methods and measurement techniques, and can make sense of and carry out analysis of raw data coming out of an instrument. More generally, their main contribution in a laboratory setting consists of the identification of physical mechanisms underlying the observed behaviour [1], consequently providing an interpretation of the data by means of simple (usually phenomenological, i.e., not justified based on first principles, microscopic pictures) models. The task of these scientists is to attempt to explain the outcome of a measurement.

At the opposite end, there are those theoretical physicists (also known as “Failed mathematicians”) who are rather removed from the “trenches”, namely the laboratory. They read articles describing experimental work but do not talk to their experimental colleagues on a daily basis. They are not so interested in interpreting one specific experiment, or making sense of one set of data, as much as in achieving a unifying description, capable of accounting for all observations made on a physical system of interest (or, a class thereof). Their aim is that of extracting, from a relatively large set of observations, the one or few common traits that underlie the same, interesting common behaviour of systems whose detailed compositions may be quite different. They shy away from mathematical descriptions of reality too rich in what they regard as superfluous details, preferring instead to work with simple, toy models — something that often frustrates their experimental colleagues, who do not see the point of trying to understand something that may have little or nothing to do with what they perform their measurements on.
Ultimately, the ambition of these theorists lies not so much in explaining what has been observed, but rather in predicting what has not yet been observed.

Most of us fall somewhere in-between the above two “extremes”, and I can see the point of being a bit of both. On the one hand, falling too close to being a Failed mathematician does entail the risk of losing touch with reality, of becoming enamoured with modelling for modelling’s sake, of drifting into irrelevance (I was going to write “string theory”, not sure why). Conversely, the other extreme, being just the “resident theorist” at the service of an experimental effort, or making one’s main goal that of incorporating into a phenomenological model every bit of new experimental evidence, seems itself limiting (and not likely to originate much insight, in my opinion).

I have to confess, however, that if I were to pick a camp, I would join the rank of the failed mathematicians. Even though physics is an experimental science, which means that connection with an experiment (at least possible in principle) should always be made, theoretical physics should not always be at the service of experiment. I believe that an important part of the job of a theoretical physicist consists of imagining things that a bit off the track, thinking of new phenomena to discover, suggesting new experiments, not just interpreting the results of those already carried out.
But in order to conceive such novel phenomena, sometimes it is necessary to forget about the known ones. There is something to be said for the freedom of thinking about a physical system in abstract, without any connection to previous or ongoing experiments, exploring scenarios that may be unphysical (at the present time, or that we know of, anyway). So, yes, even string theory has a place.

Notes

[0] The only name that comes to mind, when thinking of a prominent physicist who lived in modern times and displayed uncanny ability both as a theorist and as an experimentalist, is that of Enrico Fermi.

[1] Hey, wait a minute, isn’t that what experimentalists themselves should be able to do ? Just kidding…

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12 Responses to “What do you do for a living, anyway ?”

  1. Calvin Says:

    Well, responding to your footnote [1], in fact in large particle physics collaborations, many “experimentalists” never touch the actual experimental equipment or even gather the data, but just analyze it. For a couple of decades now this has often formed the backbone of the PhD for many an “experimental” student in particle and even nuclear physics. Of course it’s much less so in condensed matter.

    In my experience mathematical physicists are even more abstract than you describe. Many of them, including string theorists, are hardly interested in making predictions; in fact with the rise of anthropic arguments and the string “landscapes” they seem to be abandoning actual empirical evidence altogether. See Peter Woit’s blog “Not even wrong” for amusing diatribes on the topic.

    More generally, mathematical physicists at the extreme end are more interested in theorems and proving properties of a system (saying proving properties of a toy model), then in comparing to experiment.

    Like you, I’m in the middle. I got an undergraduate degree in maths (double in physics and maths, actually), and was pretty good at it, but the the extreme attention to rigor was not that appealing to me, so I headed towards physics. I deliberate sought out lab experience and had happen exactly what I hoped: the experimentalists told me, Don’t touch our equipment, why don’t you go play on the computer instead? Which I did so happily.

    Overall I agree with your picture of theorists: we like to try to strip down the picture to its simplest elements, and work a lot with toy models. I certainly do that.

    • Massimo Says:

      Hi Calvin,

      many “experimentalists” never touch the actual experimental equipment or even gather the data, but just analyze it.

      True. For some reason I regard HEP as almost a world of its own, when the typical size of a collaboration is of the order of several hundred, the degree of specialization is inevitably accentuated, even by accepted standards.

      See Peter Woit’s blog “Not even wrong” for amusing diatribes on the topic.

      I used to read it as religiously as possible for an atheist — but the amount of trolling (mostly by one individual) was unbearable.

      the experimentalists told me, Don’t touch our equipment, why don’t you go play on the computer instead?

      Ditto, especially after the first time I tried to transfer liquid helium from the tank to the cryostat — scary, scary stuff…

      • Sophia Says:

        In defense of the string theorists see this:

        http://ptonline.aip.org/journals/doc/PHTOAD-ft/vol_63/iss_5/29_1.shtml?bypassSSO=1

      • Calvin Says:

        Sophia, I’m well aware of, and constantly find hilarious, your example. The string theorists are nothing if not completely contemptuous of nuclear physics–my field– and think it a dead subject, as Leonard Susskind openly admits in his book “The Black Hole Wars”…except when they are desperate to trot out an example of the “success” of string theory _techniques_. And even more recent papers make it clear that holographic techniques actually have nothing to do with string theory per se–Verlinde and others are applying holographic methods to gravity without any strings whatsoever. (And while I find Verlinde’s suggestion, which is really just a rehash of Ted Jacobson’s idea from 15 years ago, very intriguing, it too lacks any substantive, testable prediction that I am aware of. Although it would be really cool if it did.)

        Look, string theory is an interesting idea and there were some good reasons to consider it. And who knows, maybe some SUSY particles will be discovered at LHC–that’d be really exciting. But still, I think a lot of Woit’s dissection of string theory has merit.

      • Massimo Says:

        Calvin beat me to the punch. I remember finding that article bizarre the first time I read it. Aside from what Calvin has already eloquently said, I find that perhaps the tone is what disturbs me the most. It’s statements like this one

        Even the famous helium-3, which can flow out of a container via capillary forces, does not count as a perfect fluid.

        that make you wonder how much they really know or even care about the rest of physics.
        Why would anyone familiar with superfluidity cite helium-3 as a paradigm, and not the most abundant isotope helium-4, far more easily accessible, a clean bosonic system with a measurable condensate fraction, with a transition temperature a few hundred times higher than helium-3, for which no atom pairing is required in order to establish superflow ?
        And, why is it that liquid helium, 3, 4 or whatever, “does not count” as a perfect fluid ? Weird…

      • Sophia Says:

        To clarify my comment: here are the things I agree on
        1. String theory is probably over hyped, especially to the public (probably a combination of them doing a better job in promoting their field than other fields do and the inherent intellectual appeal of understanding nature at the fundamental level)
        2. String theorists can be obnoxious.
        3. They don’t get as much funding or positions as other fields, but they do get some of the best students. This is obviously not a sustaining model of business and as a result there are too many good physicists trained in string theory trying like mad to be absorbed into other fields.
        4. My (limited) understanding of string theory is that it has many built-in problems as famously Woit, Smolin and others point out. It is also disconnected from experiment, which is in itself problematic with any subfield of physics.
        My attempt to defend it was based on
        1. Obviously the string theory community is interested in making connections with reality. That was the point of linking to the Phys. Today article. I was trying to say that they do not snob experiments and/or other subfields as it is widely believed, probably because of some vocal famous people in string theory. Of course you can argue that anybody in trouble would cling on whatever they found that could save them.
        2. There have been some successes of applications of AdS/CFT to correlated systems recently, even if only at the qualitative level.

        Finally, I don’t understand the claim of He being imperfect fluid in the article. As I said I was linking to that article as a counter example to the statement “Many of them, including string theorists, are hardly interested in making predictions; in fact with the rise of anthropic arguments and the string “landscapes” they seem to be abandoning actual empirical evidence altogether.”.

      • Calvin Says:

        Sophia, I am sure there do exist string theorists who value experiment and who strive to make testable predictions. But overall the field has done poorly on this front and with the rise of anthropic arguments and the landscape they seem, in general, to be moving away from falsifiable theory.

        The AdS/CFT example remains a nonissue to me. Let me give an example. Suppose Isaac Newton had discovered Gauss’s law in his investigation of gravity. (He sort of did, but not phrased in that way.) Now suppose he or someone else showed that Gauss’s law applied to electrostatics, which it does. That successful application to electrostatics does not in any way validate Newton’s theory of gravity, only that the mathematical theorem of Gauss’s law is useful. The AdS/CFT correspondance is exactly the same thing, a useful mathematical tool that was devised by a string theorist–but its successful application has absolutely NOTHING to do with the validity of string theory itself.

        If you want my opinion, the biggest potential evidence for string theory could be dark matter. Superpartners make viable candidates for nonbaryonic dark matter, and if and when we directly detect dark matter, it may provide more substantive evidence for superstring theory. Frankly, that still-unfinished possibility is relatively much more persuasive than the red herring of AdS/CFT, at least to me.

      • Sophia Says:

        Calvin, I agree completely with what you say about using a technique from string theory to solve different problems. As I said in my first comment it was a defense of (some) string theorists and not string theory itself.

  2. ScientistMother Says:

    I think I now know where the idea of the crazy mad scientist came from! :)

  3. PhD student Says:

    Talking about the difference between experimentalist and theoretician, I have always been puzzled by how they sort authorships.

    In experimental science First Auth. did a significant part of the experiments and Last Auth found a significant part of the budget.

    In theory, authors are sorted by alphabetical order. So i was wondering if it is a tradition being perpetuated or there really is something different about the way theoretician and experimentalist collaborate for papers ?

  4. Massimo Says:

    In experimental science First Auth. did a significant part of the experiments and Last Auth found a significant part of the budget. In theory, authors are sorted by alphabetical order.

    That is news to me…

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