21st century redux
The crucial thing about physics in the middle of the last century was not the intellectual revolution that went on in the field, with Relativity and Quantum Mechanics supplanting the classical theories, but rather the material consequences of that revolution. Quantum Mechanics is important not because it forced scientists to re-think our relation to the universe, but because an understanding of quantum theory makes it possible to build devices like transistors and lasers. Relativity is important not because it transformed our understanding of space and time, but because understanding the theory makes it possible to build atom bombs and nuclear power plants. Everything that happened in the latter half of the twentieth century, from the Cold War to the Internet, is in some sense a result of the revolution in physics that took place in the first few decades of that century.
Seen in that way, none of the problems Michael mentions look like they stack up. Yes, a working theory of quantum gravity would be a major revolution in physics. But it doesn’t seem likely to have material consequences for the average person, unless some quirk of the theory makes either free energy or levitation possible. Quantum measurement and cosmology are fascinating topics, and the people who nail them down will richly deserve their Nobel Prizes, but I don’t think either is likely to have results that will re-make the world in the way that the transistor and the atomic bomb did.
The rub is in that phrase: “it doesn’t seem likely to have material consequences”. Neither did problems like resolving the ultraviolet catastrophe (which set in train the events leading to quantum mechanics), or the fact that the symmetry properties of Maxwell’s equations are different from those of ordinary Newtonian mechanics (which helped lead to relativity). However, resolving these problems caused the fundamental changes to our view of the world that enabled all the new technologies that Chad mentions.
It is, of course, an article of faith on my part that understanding quantum gravity, say, or quantum measurement, would have similar unexpected consequences. At present, I can no more predict those consequences than I could have predicted the laser before Planck discovered the first hints of quantum mechanics.