###### By Don Lincoln**, **Fermilab

## Danish physicist Niels Bohr and German physicist Werner Heisenberg collaborated over the period of 1925 to 1927, trying to unravel the true meaning of the work of Louis de Broglie, Erwin Schrӧdinger, and Max Born. They accepted Born’s proposal that the wave function was related to the probability of finding, for example, an electron in a particular configuration. But they wanted to understand what the wave function was saying about the world.

### Max Born’s Proposal

Born proposed that the wave function was related to the probability that the electron would be detected in a particular configuration. That statement could be a bit confusing, so let’s explore an analogy that will help.

Suppose you’re in your house at night. The house is dark. You’re at the top of a set of carpeted stairs holding your phone, whose battery is dead by the way. You drop the phone and it tumbles down the stairs, landing on some unknown stair. It’s dark, so you can’t see where the phone landed. Now in classical physics, the phone ended up where it ended up. You don’t know where it is, but it’s somewhere. You’ll find out when you walk down the stairs and step on it.

### Wave Function in Quantum Mechanics

In quantum mechanics, the wave function would say that there is one probability that the phone landed on the first stair, a different probability that it landed on the second stair, a third probability it landed on the third stair, and so on.

Fudging things a bit, we would say that the wave function was the probability of landing on stair one times the state of landing on stair one, plus the probability of landing on stair two, times the state of landing on stair two and so on.

Born’s contribution was to say that the numbers in front of the configurations in the equation were related to the probability of finding each configuration.

### Copenhagen Interpretation

The Copenhagen interpretation said that the wave function didn’t describe probabilities in the way we ordinarily think about them.

In the common case, there is a probability that the object will end up in that outcome a certain percentage of the time. But, if we were to watch the object during the shaking and movement and settling, we’d see it move in a complex but fixed path and end up in some configuration. Back to our stair example, the phone would bounce a bit and end up on a particular stair. It’s never on more than one stair at the same time.

However, in the Copenhagen interpretation, things are different. In the Copenhagen interpretation, what happens is that the object doesn’t bounce around into a single outcome.

Instead, what happens is the object simultaneously bounces into all configurations, or at least that’s what the wave function says. It’s as if the phone bounces down the stairs and is simultaneously and literally on every possible stair, some percentage of the time.

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### Collapse of the Wave Function

When we look for our phone, it isn’t everywhere; we find it somewhere. According to Bohr and Heisenberg, it’s when we look, that the phone sort of coalesces into one spot. It instantaneously transforms from being everywhere to being in a single place. In physics, we call this the collapse of the wave function.

Furthermore, according to the Copenhagen interpretation, the thing that collapses the wave function is the intervention of a conscious being. According to their thinking in the late 1920s, it literally took a mind to convert reality from a spread out, diffuse and indeterminate situation to a distinct and unique observation.

### Roots in Eastern Philosophy

That idea—the idea that reality isn’t made real until it is observed by a mind—feeds directly into the ancient philosophical discussion of whether the mind is somehow different from the body—the so-called mind/body problem. People take this intellectual tradition to try to tie quantum mechanics to Eastern philosophy.

There are two books from the 1970s, called the *Dancing Wu Li Masters* and *The* *Tao of Physics*, both of which pander to this connection and both of which staggeringly misrepresent Buddhism, Taoism, quantum mechanics, and common decency.

The claim that the mind has a singular role in quantum mechanics is still common in the public arena, but scientists have long-since abandoned the idea. In fact, even Bohr and Heisenberg abandoned the idea in their later years. However, it’s easy to see why it persists among science enthusiasts.

### Reaction to the Interpretation

The scientific community has long abandoned the original formulation of the Copenhagen interpretation. Even the philosophical community has abandoned it by and large. Most physicists of the era hated the idea and came up with a number of absurd examples. For instance, Einstein famously said that he refused to accept that the moon wasn’t there when nobody was looking at it.

We can sum up the Copenhagen interpretation, which said that quite literally that a quantum object was simultaneously in several distinct states, until a conscious observer looked at the object. At the moment of observation, the wave function collapsed to a single possibility, the thing that was actually observed.

### Common Questions about the Copenhagen Interpretation of Quantum Mechanics

**Q: Who came up with the Copenhagen Interpretation?**

Physicists Niels Bohr and Werner Heisenberg collaborated over the period of 1925 to 1927 and came up with the idea of Copenhagen Interpretation.

**Q: What was Max Born’s theory?**

Max Born proposed that the wave function was related to the probability that the electron would be detected in a particular configuration.

**Q: How was the Copenhagen Interpretation received by the researchers?**

The scientific community has long abandoned the original formulation of the Copenhagen interpretation. Even the philosophical community has abandoned it by and large. Most physicists of the era hated the idea and came up with a number of absurd examples. For instance, Einstein famously said that he refused to accept that the moon wasn’t there when nobody was looking at it.