What’s The Difference?

Computer Reconstruction of the aftermath of a particle collision inthe CDF experiment at the Tevatron, FermilabA commenter, Edward Hessler, asked for a bit of explanation about what the difference is between a phenomenologist and a more “formal” theorist, since I recently mentioned some of us overlapping during workshops at the Aspen Center for Physics.

I offered this (see below) as a starter explanation. (Feel free to refine, disagree, agree, etc, in the comments. It is always interesting to hear different people’s takes on this.)

So there is really no sharp distinction – it is a complete continuum, really. We’re all theorists. We are just tackling different aspects of the puzzle of what Nature is up to.

There’s the matter of developing theories right from the drawing board. This often means looking for new principles, symmetries, physical mechanisms, mathematical techniques, etc, that you then build into a working framework. This does mean being informed by Nature, and having an eye on what has gone before, just as in any other area of the science. Much of what I said are the primary concerns -but not exclusively so- of your more formal modern theorist.

Your typical modern phenomenologist is working more directly on the issue of how a given theoretical model or framework interfaces with what we know about (adn would like to know about) Nature in a precise way. In other words, you have thought processes like “if this symmetry principle is true, then it implies the following term in my equations, which in turn imply that -putting in the observed data from experiement X- I should see the following phenomenon (decay rate, new particle, pattern of fringes, feature of my graph, etc) in experiment Y.”

If experiment Y has already been done, and those things are not seen, our phenomenologist concludes that the symmetry principle is not true (or perhaps that some assumption along the way needs to be adjusted – it can be a messy business), or that something more complicated is going on that can contribute to the physics and modify the expected signal (such as other mechanisms, etc, at work, which then need to be identified.). If experiment Y has not been done, this chain of thought might be the beginning of drive to get the experiment built, and test the hypothesis. This brings in the other crucial thinker and doer in the game: the Experimentalist or Experimenter. (Have a look -use the search engine- on Cosmic Variance for discussions of the LHC and the ILC for several examples of this process in the context of particle physics. But you can go back to any early era and any field of science to see it.) The Expermenters come in a wide range of sorts as well. Topic for a different day.

Anyway, I hope the distinction is clearer, although when it comes down to it, there’s no sharp distinction. In the more specialised world of particle physics, the distinctions can often be clearer than in other fields, but even then not so. There are some things the formal theorist can immediately rule out as most likely being incompatible with observation -more than three large spatial dimensions all on an equal footing, for example- and others do need the help of physicists with a more delicate knowledge of the resulting signals in the lab – large extra spatial dimensions where only some of the forces we know can act, for example. Similarly, there are important principles that phenomenologists can and do use in their guiding thoughts and part of their tools (and which directly impinge upon the lab results) are not just the province of the formal theorist -rotational invariance, Lorentz invariance, locality, charge conservation, momentum and energy conservation, gauge invariance, etc.

So we can all be formal, and we can all be phenomenological, in varying degrees. As theories and experiments have become more and more complex, there has been an increasing need to specialise (given that we only have finite lives and careers) in one area or another, leading to, for example, basketball throwdowns. (Or is it throwsdown?)


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5 Responses to What’s The Difference?

  1. edward hessler says:


    How very good of you to respond to my question, especially when I thought/think it is probably a simple one and not worth a lot of time (although for me it informs my sense of the nature of science–“no sharp distinction–it is a complete continuum.”). I very much appreciate and value your comments. I’m a biologist/ecologist by training and have spent most of my life working as a K-12 science and environmental educator. The descriptions you provided of the discussions at Aspen were so yeasty (I suppose on this blog that a physical analogy would be more appropriate.), making the continuum feel and mind like one, smushing ideas together…juxtaposing them, providing a new lens through which to think about one’s specialty.

    Thanks again.

  2. Clifford says:

    Thanks Edward,

    Thanks for the question. I’m pretty sure that you were not the only one wondering that, and so it was worth spending the time, since you clearly wanted to know.



  3. Say Lee says:

    It reminds me of the dichotomy apparent in the fluid mechanics community as well: hydrodynamists and hydraulicians, with phenomenologists tending to be lumped with the latter. However, the distinction is becoming moot these days as both converge to become hybrids of sort.

  4. Clifford says:

    Say Lee:- Sorry about the delay to your comment. Young, enthusiastic spam blocker was confused. Still teaching it.



  5. Say Lee says:

    Don’t worry. One of those teething problems.