Is Quantum Theory About Reality or What We Know?
POSTED BY JAMES OWEN WEATHERALL ON DEC 04, 2017
Does the quantum state ultimately represent some objective aspect of reality, or is it a way of characterizing something about us, namely, something about what some person knows about reality? Photograph by Ben Scholzen / Flickr
Physicists know how to use quantum theory—your phone and computer give plenty of evidence of that. But knowing how to use it is a far cry from fully understanding the world the theory describes—or even what the various mathematical devices scientists use in the theory are supposed to mean. One such mathematical object, whose status physicists have long debated, is known as the quantum state.
One of the most striking features of quantum theory is that its predictions are, under virtually all circumstances, probabilistic. If you set up an experiment in a laboratory, and then you use quantum theory to predict the outcomes of various measurements you might perform, the best the theory can offer is probabilities—say, a 50 percent chance that you’ll get one outcome, and a 50 percent chance that you’ll get a different one. The role the quantum state plays in the theory is to determine, or at least encode, these probabilities. If you know the quantum state, then you can compute the probability of getting any possible outcome to any possible experiment.
But does the quantum state ultimately represent some objective aspect of reality, or is it a way of characterizing something about us, namely, something about what some person knows about reality? This question stretches back to the earliest history of quantum theory, but has recently become an active topic again, inspiring a slew of new theoretical results and even some experimental tests.
If it is just your knowledge that changes, things don’t seem so strange.
Over the last 15 years or so, physicists have asked whether the quantum state could be epistemic in a similar way. Suppose there is some fact of the matter about the configuration of the world—something like an arrangement of particles in space, or even an actual result in the die game—but you do not know what it is. A quantum state, according to these approaches, is just a way of characterizing your incomplete knowledge about the configuration of the world. Given some physical situation, there might be more than one correct way of assigning a quantum state, depending on what information you have.
It’s appealing to think of the quantum state this way because of how quantum states change when you measure something about a physical system. Measuring a system will generally change its state from one in which each possible outcome has some non-zero probability to one in which only one outcome occurs. That’s a lot like what happens when, in the die game, you learn that the die does, in fact, show a six. It seems strange to think that the world would change simply because you measured something. But if it is just your knowledge that changes, things don’t seem so strange.
Another reason to think the quantum state is epistemic is that, in most cases, there is no way of telling, with a single experiment, what the quantum state actually was before the experiment. This also resembles probabilities in the die game. Suppose that another friend came along to play the game, and insisted that the probability of the die showing a six was only 10 percent, while you still say it is 17 percent. Could a single experiment show who is right? No. The reason is that the actual outcome—of six, say—is compatible with both of your probability assignments (though one may be more accurate in the sense of getting the frequencies right over many rolls). There’s no way of telling if you or your friend is right in any particular case. According to epistemic approaches to quantum theory, the reason you cannot experimentally distinguish most quantum states is just like the die game: There are some possibilities for the actual physical situation that are compatible with multiple quantum states.
To see why the quantum state might represent what someone knows, consider another case where we use probabilities. Before your friend rolls a die, you guess what side will face up. If your friend rolls a standard six-sided die, you’d usually say there is about a 17 percent (or one in six) chance that you’ll be right, whatever you guess. Here the probability represents something about you: your state of knowledge about the die. Let’s say your back is turned while she rolls it, so that she sees the result—a six, say—but not you. As far as you are concerned, the outcome remains uncertain, even though she knows it. Probabilities that represent a person’s uncertainty, even though there is some fact of the matter, are called epistemic, from one of the Greek words for knowledge.
This means that you and your friend could assign very different probabilities, without either of you being wrong. You say the probability of the die showing a six is 17 percent, whereas your friend, who has seen the outcome already, says that it is 100 percent. That is because each of you knows different things, and the probabilities are representations of your respective states of knowledge. The only incorrect assignments, in fact, would be ones that said there was no chance at all that the die showed a six.
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