By David K. Johnson, Ph.D., King’s College
In scientific philosophy, there are two ways to view quantum physics. Realists are those who suggest that a theory’s ability to make accurate predictions justifies the conclusion that the theory is true, so it describes reality. The non-realist, on the other hand, will say that the predictive power of a theory isn’t a reason to think that it describes reality.
The Two Different Views of Quantum Physics
In chemistry, the theory that chemical reactions are the result of interactions between atoms and molecules is seen by realists to be a good reason to think that atoms and molecules exist and interact in the way described by chemical reaction equations. But non-realists don’t think that way.
After all, other theories could be imagined that make the same predictions. They would say that a theory’s predictive power means that it’s an accurate instrument for making predictions, and that’s it. This view is also called instrumentalism.
For example, Einstein’s relativity equations treat space-time as an actual substance in which objects are situated—a substance that is warped and curved by mass and makes objects behave in certain ways.
But an instrumentalist would argue that this doesn’t mean that space-time is really a substance; it just means that pretending that it is could be a useful way to make predictions about the behavior of objects. Indeed, it seems that this is what Einstein thought about relativity.
Because of the weird things that quantum mechanics suggests about the nature of subatomic particles—things that don’t even seem to make sense—many scientists want to say the same thing about quantum mechanics. “Quantum wave functions don’t tell us what particles are really like,” they say. “They’re just an instrument for making accurate predictions. We shouldn’t even wonder what they mean.” As physicist N. David Mermin once summarized this view: “Shut up and calculate!”
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The Problem with Instrumentalism in Quantum Physics
The problem with taking this approach with quantum mechanics, or relativity for that matter, is essentially twofold. First, scientists generally don’t do this with anything else. A pilot’s assumption that the world is round, lets him/her successfully arrive at their destination. Someone would be thought crazy if they said this wasn’t a good reason to think the world was round—that assuming the world was round was just a useful navigational instrument.
Indeed, modern scientists champion science’s ability to describe reality because of its predictive success. It’s completely arbitrary to suddenly abandon this notion because quantum mechanics or relativity states something that someone doesn’t like.
Second, the argument for instrumentalism is predicated on a mistake. The fact that multiple theories can make the same predictions doesn’t mean there is no reason to favor one over the other. The criteria of adequacy—fruitfulness, simplicity, scope, conservatism—can be used to delineate between theories that make the same predictions. It’s called inference to the best explanation.
Indeed, it seems that a theory accurately describing reality is the best explanation for why it’s able to make successful predictions. The reason assuming the world is round works for navigation is because the world is round. The reason treating subatomic particles as waves predicts their behavior so accurately is because they are waves.
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The Measurement Problem
But this still leaves us with a problem. If the realists are right, then subatomic particles are waves until they are measured, at which time they collapse down into particles with particular properties. But how does measurement do this? And why does it collapse a wave function in one way, rather than another? This is famously known as the Measurement Problem.
According to physicist Hugh Everett III, what happens is this: When someone makes a measurement of a wave function, they don’t collapse it, they become part of it. Literally, to account for what their observation does, they have to plug themself in as a variable into the quantum wave equation.
But since, according to realists, quantum wave functions represent the way the world is, that must mean that the person is now in a superposition just like the wave function. So they have multiple contradictory properties at once. They are in a state of having observed the particle both here and there. But how can that be, since they are only aware of one of those outcomes?
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The Realist’s Solution to the Measurement Problem
It must be, Everett argues, that observing wave function splits someone in two so that there is another version of them that observes the other outcome—another one of them, in another universe. So in this interpretation, the measurement of quantum events creates alternate universes.
Of course, wave functions can collapse in the absence of measurements; but anytime they do, every possible outcome of that collapse is realized in a separate universe. When it comes to location, there could actually be more than two.
Bryce DeWitt, who later endorsed and developed this interpretation, once suggested, “Every quantum transition taking place on every star, in every galaxy, in every remote corner of the universe is splitting our local world into myriads of copies.”
Common Questions about How Scientists View Quantum Physics
Realist scientists believe that a theory making correct predictions is proof that the theory is true but non-realists disagree. They believe that just because a theory is used to calculate something accurately doesn’t make the theory true in itself.
If realist scientists are right, subatomic particles are waves until they are measured, at which time they collapse down into particles with particular properties. The Measurement Problem is how measurement does this.
One of the problems is that the argument for instrumentalism is predicated on a mistake. The fact that multiple theories can make the same predictions doesn’t mean there is no reason to favor one over the other.