Whenever I meet someone new and tell them I'm a physicist, there's a decent chance that they'll ask me to explain string theory. I can't manage much more than "It's one possible way to combine General Relativity with the Standard Model" (which we know must be wrong). Those two theories are both brilliantly successful as long as they're working separately, but they're not mathematically consistent in situations where both would be significant, and that's a problem.
One follow-on question I sometimes get is "So, if string theorists succeed in combining all of physics into a single theory of everything, what happens to physics? Do you all pack it in and go home?" It makes a certain kind of sense, because lots of pop-physics books present it as an Ultimate Theory, but in another sense, it's way off the mark.
After all, we're not done with physics from three hundred years ago.I've spent the week at the APS March Meeting, which is dominated by fields other than particle physics, but my first three days at the meeting were still dominated by quantum physics. Most of the talks I went to concerned understanding the behavior of materials (largely exotic but simple ones), and that revolves around the quantum behavior of electrons as they move around inside matter.
The talks I went to on Thursday, though, were all about classical physics. The core of classical physics was first codified in the late 1600's in Isaac Newton's Philosophiae Natrualis Principia Mathematica, where he set out his famous Laws of Motion. In the last three-and-a-bit centuries, physicists have added a lot of mathematical apparatus beyond the calculus that Newton invented for his work (which is now routinely taught to high school students), but the basic principles remain unchanged.
And while we now know that they're merely an approximation to a deeper theory of reality, Newton's Laws work brilliantly for physics involving objects over a range of sizes from viruses to large planets.While the basic principles haven't changed since Newton's day, though, classical physics still supports an astonishingly rich set of phenomena. It continues to yield surprises even when applied to really simple things.
So, for example, the session on Geometry and Mechanics of Folded Filaments, Writhing Ribbons and Braided Bundles opened with a talk by Robin Ball on the physics of ponytails. (A topic of some interest around my house, as our seven-year-old daughter has repeatedly said that she plans to grow her hair out until it reaches the ground.) Predicting the shape of something as commonplace as a bundle of hair turns out to be surprisingly complicated mathematically.
Ball and his colleagues have had to do a bunch of heavy mathematical lifting to find a method that matches the shape of hair bundles used by Unilever's cosmetics researchers. Most of the time-- there are still a few details they can't quite match.Later in the day, I checked out some talks in a session with the kind of uninspiring title Rheology of Dense Particulate Media.
These were concerned with the motion of large numbers of tiny particles in a fluid, and how they flow in response to an applied force. Or don't flow-- this is the scenario that leads to the famous cornstarch-in-water effects, making fluid that you can walk on, provided you do it quickly:On a microscopic level what happens here is a "jamming" transition where particles that flow easily under small forces basically lock up when you try to make them go too fast. Figuring out what's going on with these systems at a microscopic level is a very active area of research, and Dan Blair from Georgetown showed results from experiments where they use a microscope to track the motion of particles in these sorts of systems in response to different sorts of forces.
And, of course, things get even more complicated when the particles you're looking at don't merely respond passively to external forces, but can propel themselves-- flocks of birds, swimming bacteria, etc. There were at least ten sessions worth of talks on such "Active Matter" in the program, exploring a huge range of phenomena and giving journalists the once-in-a-career opportunity to write headlines like Sperm swim in teams when fluid is gloopy.In all of these cases, the key factor is the number of interacting objects.
The particles in all of these talks behave in a perfectly classical manner-- there's no quantum weirdness going on-- but there are so many of them that their collective response can defy intuition. This is a feature of classical physics that's been known since the mid-1800's, when Henri Poincar showed mathematically that it's impossible to find a formula to describe the motion of even three particles interacting via Newton's law of gravity, which led to the modern study of chaotic dynamics.The systems I saw talked about on Thursday are both better and worse than Poincar's three-body problem.
The motion they're dealing with isn't strictly chaotic-- the aggregate behavior of ponytails and non-Newtonian fluids and the rest is very predictable and reproducible. But the sheer number of particles involved makes the detailed behavior ferociously complicated to model, which is why you still have physicists in 2016 working to deepen their understanding of bundled hair.And that's why I find it kind of amusing when I get asked what comes after the "theory of everything.
" When string theory or some other technique finally succeeds in merging General Relativity with the Standard Model, it's not like the job of physics will be over. We've gotten more than three hundred years worth of physics out of Newton's Laws, and they're still going strong. Any future theory, when applied to the complexity of the real world, will yield an effectively infinite supply of new surprises, keeping physics and physicists around for a long time to come.
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