Books for the interested layperson on the theory of relativity are not in short supply. The local library has a shelf full of them, and I don’t think that’s entirely because I live in Livermore, with an important national laboratory just a couple of miles down the street.
A physics book with equations in it would be completely over my head. But I’d sure appreciate a book that didn’t cut corners. I’m not looking for Relativity for Dummies.
Case in point: The classic thought experiment in which I fly past you in a spaceship at a significant fraction of the speed of light. From your point of view, a clock on my spaceship is running slower than your clock. However, from my point of view your clock is running slower than mine.
I get that. And every book on the subject will tell me that that’s what will happen. (I won’t say they explain it, because there isn’t really an explanation, is there? That’s just the way the universe operates.) What none of the books I’ve read bother to explain is what happens when I turn my spaceship around, come back to where you’re standing, stop the ship, and hop out carrying my clock. We then compare the two clocks.
Which clock will be faster or slower? Neither answer makes sense. The only answer that makes sense is, the two clocks will show exactly the same time. But that doesn’t make sense either. At what point did each clock speed up relative to the other clock? I’d sure like to know.
Here’s another slippery bit. Any number of books will tell you that the speed of light is an absolute. No matter how fast you’re going relative to anybody else, you’ll always measure the speed of light to be the same. If the book is trying to be meticulous, it will say, “the speed of light in a vacuum.” It’s known that light slows down when traveling through a medium such as air, water, or glass; if it didn’t, your glasses wouldn’t work, and telescopes wouldn’t work either.
But wait: There’s no such thing as a vacuum! Our current understanding of quantum mechanics suggests that even a “vacuum” is full of virtual particles, which pop into existence and vanish too quickly for us sluggardly humans to observe them. But a photon is pretty fast, isn’t it? What effect might those virtual particles have on a photon? Even if the answer is, “no effect,” a supposed vacuum is also full of other photons (to say nothing of gravitational waves and neutrinos) zipping around and through one another. Even in a black box from which all the air has been pumped out, the walls of the box will be shedding infrared photons to beat the band, and billions of neutrinos will be zipping through the box as if the walls weren’t even there.. Is that a vacuum?
Here’s the kicker, though. Einstein arrived at the theory of relativity (according to the book I’m reading) with the aid of his intuition, which assured him that the laws of the physical universe must be simple and beautiful. Unfortunately, one of the things we definitely know about physics is that our intuitions are very seldom of any value when it comes to understanding the universe. Light is a wave, and also a particle. Intuition will tell you that’s flatly impossible, that it has to be one thing or the other, and indeed physicists rejected the notion for a couple of centuries — but your intuition is wrong. Your intuition will tell you that a particle such as an electron must be at some specific location in space at all times, but that’s wrong too. An electron is sort of smeared or spread out, except when it isn’t.
Why should the laws that govern the physical universe be either simple or beautiful in the way that “simple” and “beautiful” are understood by the bags of protoplasm squidging around on this particular unimportant little planet? In all likelihood, they aren’t. Physicists are now aware of about 30 mathematical constants — pure numbers — that govern the way the universe works, and there appears to be no sensible reason why any of those numbers has the value that it does. Physicists have no theory that would even remotely begin to explain those numbers.
There’s also the fact that while both general relativity and quantum mechanics make excellent predictions, predictions that can be confirmed experimentally, those two theories are not compatible with one another. But why should we expect that they would be compatible? Just because we humans want them to be compatible?
Maybe the universe is just plain messy. But Relativity for Dummies is not going to tell us that.