Getting Real

Guest Post: Is It Solipsistic in Here, or Is It Just Me?

Our first major breakthrough came when we realized that physics can pin down what’s real and what isn’t. It’s one of those things that’s somehow stupidly obvious and yet deeply profound: something is real if it’s invariant. That is, something is real if it remains unchanged from one reference frame to the next. Just look at a rainbow. You’ll see one in the sky if you’re in just the right reference frame with the Sun shining in from behind you, and droplets of water in the atmosphere refracting the light. It’s pretty, but good luck trying to grab it. A rainbow is not a physical object stapled to the sky. It’s a product of your reference frame. Which is to say, it’s not real.

An interesting viewpoint, and one I don’t recall coming across before, probably because I don’t do physics that ventures into this area. One of the stumbling points I’ve observed in discussions about what is real is the definition of real — real as in a physical object instead of a concept, or real as in not an illusion, i.e. not fake? Here it looks like the former: a rainbow is not a physical object, hence it’s not real. But it’s not an illusion, not some bit of fakery. The effect (refraction of light) is real.

There are plenty of not-real (not physical object) things in physics; the author concludes that basically all of it falls into this category, but that’s not the point of physics. Physics exists to tell us how nature behaves, not what it is. Electric fields and phonons and lots of denizens of physics models don’t physically exist. There is no claim that they do. It’s that nature behaves as if they existed, and since that lets us predict and retrodict what happens, that’s good enough.