I wish I knew more.
I’m often blown away by statements you sometimes hear that “scientists think they know everything.” A quick google search came up with over 60 pages with that exact quote – admittedly some are put-ons, though not all. It makes for entertaining reading. But I digress.
I’m not a scientist (though I sometimes play one on Youtube, but I digress again). Still, I spend a lot of my time reading and writing about scientists. They know, and are perfectly willing to admit, that they don’t come anywhere near knowing everything. In fact, a good definition of science might be, “we don’t know, but we can use the methods of science to try to find out.”
By the way, this admission doesn’t affect my atheism. I don’t say (and I’m pretty sure no one else says), “we’ve looked in every possible corner and hiding place and there is no God.” Instead I say “let’s see what we can explain without resorting to something that we can’t explain.” That’s what makes me an atheist, nothing else. OK, this digression thing is getting out of hand. My point is . . .
I’m re-reading The Fabric of the Cosmos by Brian Greene. You might have heard of his more famous book, The Elegant Universe. They’re both amazing, but if I had to pick one over the other, I’d pick FoC. I’ve reached the part, early in the book, where he deals with quantum entanglement. It is absolutely jaw-drop-to-the-floor mind-boggling. I find myself wishing I could understand the mathematics behind it all better, because it’s so easy to get tangled in imprecise mental pictures. But since that’s all I’ve got, here goes.
Greene tells the story of Einstein, Podolsky, and Rosen (or EPR), and their attempt to show that quantum mechanics must not be a full description of the world. Even though they weren’t trying to, what these three did, instead, was lead us to something even weirder, much weirder, than ordinary quantum mechanics. To use Einstein’s word, it’s downright spooky.
Here’s the experiment. Suppose you’ve got some process that always produces two particles flying off in opposite directions. Maybe the particles are electrons, maybe they’re photons. Let’s suppose they’re photons. The way they’re created, we can be absolutely certain that one of the photons must have spin in one direction, and the other must have spin in the opposite direction. We don’t know which particle has which spin until we measure it. So great, the photons fly off some distance and we measure the spin of one. Now we know, without even looking, what the spin of the other one must be.
What EPR said was that the photons must have had those spins to start with, but quantum mechanics couldn’t tell us that. Therefore, quantum mechanics is incomplete. Seems reasonable to me.
Quantum mechanics, on the other hand, makes the bizarre claim that photons don’t have properties like spin until we measure them. It seems to work in the mathematics, but is it just a bookkeeping trick? I always kind of felt in my bones that it was. Sure, we say that Schroedinger’s cat is neither alive nor dead until we look in the box, but come on! That can’t really be true, can it?
And this experiment seems to point out the silliness of the whole idea. For according to quantum mechanics, the act of measuring the spin of one of the photons doesn’t just give it a spin. The act of measuring gives its partner a spin, too. But wait a cotton-pickin’ minute! That photon is shooting off in the other direction, at the speed of light (since, after all, it is light!), and by the time we get around to measuring the first photon, the two photons might be very, very far apart. How could the second photon know instantly that the first photon just got measured? EPR’s answer was, well, it couldn’t , so both photons had to have a spin to start with. Quantum mechanics said, well, I don’t know, but that’s the way it has to be.
Things might have remained in the philosophical jumble forever, except for a very smart guy named John Bell. Bell realized that by using statistics you could tease out whether or not a particle has definite properties like spin, or whether those properties are set only at the moment they are measured. Greene tells a great story using TV characters and titanium boxes. The real experiments were less entertaining than that, but amounted to the same thing.
What it comes down to is this. In the experiments to test Bell’s idea, either EPR would have been proven right, and the photons really would have had spin the whole time, or else quantum mechanics would have been proven right, and the spin wouldn’t be there until we measure it.
The outcome of it all is that quantum mechanics is right, and EPR is wrong. The moment you measure the spin of one photon, instantly the other photon’s spin is set. Before that single measurement, neither photon had a definite spin. The cat was neither dead nor alive, in both places.
This is amazing and strange, and shows just how far scientists are from knowing everything. No one knows what this means, except that it means there’s something deep and fundamental that we just don’t get. Yet.
The two photons are entangled. They can be a mile, or a thousand miles, or a thousand light years apart. Yet somehow they are connected. When something happens to one photon, the other “feels” it – not just soon, but instantly. The space between the two is no barrier. The time you’d think it would take to send this information is no barrier. Somehow, despite what we think of as a separation in space, the two photons are still connected.
We still have a lot to learn. And I have more reading to do.