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I would like to know what is the difference between research in theoretical physics and pure mathematics. In particular, what does a theoretical physicist actually do all day long for his research? In other words, what does research in theoretical physics involve? Do theoretical physicists work only on big problems like string theory or any other major topics or there are other topics which are more fit for, say, PhD dissertations?

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If there were no problems that could be solved in the span of a PhD, how would theoretical physicists get their degrees? Also note that the vast majority of physicists work on nuanced particulars in fields that are not as sexy or publicly advertised as e.g. string theory. –  Chris White Jul 3 at 22:46
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I'm arguably a theoretical physicist myself and I pretty much sit around at my computer punching in code & facepalming when things go wrong (which is often). –  Kyle Kanos Jul 3 at 23:01
    
In Cambridge (and possibly other universities) the is a Deparment of Applied Mathematics and Theoretical Physics (DAMPT - leads to jokes like do you like experimental physics? Damned if you do and DAMPT if you don't...). I believe their existence gives credibility to your question - I suggest you google them... –  Floris Jul 4 at 0:11
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Related: physics.stackexchange.com/q/56293 –  Chris White Jul 4 at 2:17

4 Answers 4

First, you can be sure that a PhD student will almost always work on a very particular and nuanced topic in either of the fields. Even if you do a PhD in string theory, you might just ask about some very specific class of solution or about a prediction of observation of a very special measurement. It is hard to find "big" questions which haven't been asked yet and attempted to answer in the exact same way as you would attempt to.

Science is just not "waiting for the next Einstein", it is team work.

The line between mathematics and theoretical physics gets blurred especially in the nowadays "beyond standard" theories. Generally, we can say the theoretical physicist wants to predict a particular result connected with observation based on theoretical constructs and the mathematician wants to find general truths about theoretical constructs (which may have nothing to do with reality). The part where these two meet is often called "mathematical physics" but it should be clear that the conventional line might often be unclear.

As to what does a theoretical physicist do all day, I actually don't know from personal experience as I do research only part time so far but my research includes mainly formulating problems, trying to tackle them analytically, running numerical simulations and drawing physical (observation-like) conclusions. However, there will for sure be theoretical physicists whose work tends to get more analytical or numerical. For more information, it would perhaps be convenient to ask a more experienced scholar in either of the fields.

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+1 Science is just not "waiting for the next Einstein", it is team work. Although I disagree. There are many people out there who are the next Einstein: Einstein would be working in a team were he alive now. The days of the polymath, sadly, but excitingly, are over: compare the mathematics and background you have to learn now to get to the edge of anything with the pithy, conceptual jewels of a hundred years ago. I think it was Lee Smolin who observed you could summrise the beautiful concepts and mathematics of GR so that any motivated individual could thoroughly understand the physical .... –  WetSavannaAnimal aka Rod Vance Jul 4 at 3:23
    
... and mathematical principles in a few tens of pages. You need tens of pages of background for even the most trivial of problems now and I find reading papers in topics which ultimately are much less meaty than GR much harder than reading Einstein's original papers. It would be true to say that I was a bit spoilt reading them at age 23 and thinking that all science was this easy to explain: there aren't that many people with the gift for technical writing that Einstein had. –  WetSavannaAnimal aka Rod Vance Jul 4 at 3:26

I'll take a stab at each of your questions one by one:

How does the research in theoretical physics differ from mathematics?

Research in physics and research in mathematics are very different activities. While they both use mathematics as a tool to communicate ideas, they are doing so to accomplish very different goals. Remember that conclusions in physics are expected to eventually boil down to something that can be confirmed or refuted by experiment while mathematics has no such requirement. We say that physics is an experimental science and so the problems worked on by a theoretical physicist are rooted in a physical problem.

That said, if the phenomenon being studied by a physicist is easily described using heavy duty mathematics then the theoretical physicist will do so if he's got the math chops for it (and they usually do). Often it can seem like there is no real difference at all with what a mathematician is doing and many physicists will even go so far as to create the mathematical tools they need to properly frame their question. At that point they're arguably doing mathematics -- and that's ok. I've got guys like Witten or Feynman in mind here.. The physicist is still just using the tools that are appropriate. Nevertheless the research the physicist is pursuing has a completely different motivation than the research a mathematician chases down.

What does a theoretical physicist actually do all day long for his research?

That totally depends on the subject the guy specializes in.

I knew a guy in grad school who spent the better part of two years coding up stuff in Mathmatica to calculate higher-order loop corrections. He definitely thought of himself as a theoretical physicist.

Another guy I knew worked in the nuke theory group and studied relativistic heavy ion collisions. He spent his time reading papers from the experiments (PHENIX, STAR, etc.) and examining published data to support his own position on the existence of quark-gluon plasmas and explanations for the observed transverse momentum distributions. He also thought of himself as a theoretical physicist and eventually finished up his degree to take on a post-doc in what most people think of as theory.

Yet another person I knew was very much into relativity and spent all his time working on curvature calculations. This dude was like a human calculator... Not much in the way of experimental conclusions I'm afraid but lots of keeping up with Phys. Rev D papers, studying differential geometry, and so on. Another theoretical physicist in the making...

The point is that what a theoretical physicist does all day long is totally dependent on their area of physics. It's hard to make general statements that can up an answer to your question.

Do theoretical physicists work only on big problems like string theory or any other major topics or there are other topics which are more fit for, say, PhD dissertations?

Take the three guys I mentioned above:

  • The mathematica work (If I remember correctly) rolled into other work his group used to attack questions on renormalization.
  • The nuke theory guy did important work summarizing results that cut across a bunch of experiments that helped others get a global view on the existence of the QGP.
  • The human calculator came up with a few tricks that helped himself and others in their calculations and work with a class of solutions to the Einstein field equations.

I'd say even though these appear to be bite sized chunks of work in their respective fields they were really all tackling the big picture stuff: renormalization, quark-gluon plasma, and the field equations. So perhaps it's just a matter of perspective.

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Really good answer. Thank you very much. Still, I would like to understand better what physicists do with mathematics: do they prove theorems (and hope that such theorems will have experimental relevance)? do they execute mere calculations? –  user10024 Jul 4 at 10:21

The fundamental difference is, that the physicist can develop stuff based on heuristics and physical intuition, while the mathematician has to prove every single step he does. The work of justifying the stuff the theorist does, is again the mathematician's part, the theorist is happy as long as his theory works. For a nice example for the difference, I suggest you study the history of the Dirac $\delta$-"Function". Thing is basically, that Dirac introduced it because it seemed physically intuitive and right for him, but it took the mathematicians more than 10 years to justify it.

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Also see my answer here which summarises the end point of the work on the Dirac delta by Schwarz, Grothendieck, and Gel'Fand. There is a quote at the beginning of Lighthill, "Introduction to Fourier Analysis Generalised Functions" roughly (I don't have it before me): To Paul Dirac, who saw that it must be true, to Laurent Schwartz who proved it and to George Temple, who showed how easy it could be. –  WetSavannaAnimal aka Rod Vance Jul 4 at 3:19

Just an extra thing to add: take a look at Feynman's wonderful point of view on this subject. It's very similar to what Daniel and Void answered, in my opinion- just more interesting (no offense to Daniel and Void- I think Feynman is more interesting than everyone!).

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