Tuesday, September 8, 2009

The future of physics

My grandfather sometimes tells me that in my lifetime, I will be shocked by the many changes in the world and changes in science. To some extent I believe this. Cultural, political, and technological changes are quite swift, with a characteristic time scale on the order of a decade. I will probably live through several of these decades.

But I'm not much of a futurist. For instance, I don't really hold with the idea of a technological singularity. The faster things change, the more likely it is that things will change into something that doesn't change quite so quickly.

And in my specific field, physics, I don't think we're going to have a major revolution after revolution, indefinitely.

That's a rather vague and unfalsifiable statement. No matter how many revolutions I witness, I would never be shown to be wrong. And as all you skeptics know, unfalsifiable statements are on the useless end of things. So allow me to put forth a much stronger statement. We will never again, not in a million years, have another physics revolution which quite rivals the one we had at the beginning of the 20th century. That was when the foundations were laid for Relativity theory and Quantum theory.

It's not because I think we already figured out all the fundamental laws of physics. I don't even think we've figured out most of them. But it's a question of applicability.

Applicability is another important concept that every physicist should understand. When we consider some physical problem, we don't just blindly apply all the known laws of physics and see what rolls out. That would be far too difficult. Instead, we selectively apply laws which are appropriate to the realm under consideration. If we're talking roller coasters, we use classical mechanics. If we're talking two orbiting black holes, we use General Relativity. If we're talking atoms and molecules, we use quantum mechanics and hell, even chemistry.

And within each of these theories, we can divide it up further. For example, when the two orbiting black holes are moving slowly enough, we can use the Post-Newtonian Approximation of General Relativity. If the roller coaster is much smaller than the earth, we can assume that gravity is constant, even though it's not. If you want to know an electron's ground state to only a few digits of accuracy, you can ignore relativistic corrections. Every good physical theory and approximation predicts its own applicability conditions. Given any desired level of accuracy, we can predict when it is acceptable to use an approximation, and when it is necessary to consider another theory.

So what happens if we slowly gather evidence and confirm, say, String Theory? What are its applicability conditions? As the joke goes, the reason we've spent billions of dollars building huge particle accelerators is so that physicists can determine what happens when a multi-billion-dollar device accelerates and collides particles. Of course, the joke isn't strictly fair, because the theory of everything will also be important to cosmology, astronomy, and who knows what else. But I don't think it would be quite as revolutionary as Relativity or Quantum Mechanics, simply because the applicability conditions will not be nearly as far-reaching. See, if the applicability conditions were so great, we would have already tested it with cheaper experiments.

My basic point was echoed by Feynman (lecture 7, 52:10 mark). Feynman didn't make as strong a statement as I did, but he says that he doesn't think that theoretical physics can just go on and on indefinitely. Either it will reach some end point where we know everything, or it will become progressively harder and less interesting. That is to say, the applicability conditions will cover a smaller and smaller range of reality. The limited applicability implies that there is a very limited set of experiments (read: very expensive) which can discover and test the theory. As they get more expensive, progress will slow down or halt, not accelerate. This may very well be the endgame of physics.

Happily, the applicability condition of my argument is only in fundamental theoretical physics. There's a lot more to physics than that. If you thought first-year physics is hard, just remember that they usually only give the physics problems which are easily solvable. If you add only a few more complications to the system, it quickly becomes very difficult, as in, impossible to solve exactly. Gosh, even something as simple and clockwork as our planetary system requires some serious study to understand fully. Physicists aren't going to be out of research material for a long time.

1 comment:

DeralterChemiker said...


And, surely you're joking, Mr. Feynman.

For starters, tell me why light behaves as both a particle and a wave. In a way that even I can understand it.