Quantum mechanics is weird – I’m sure you read this somewhere. And why is it weird? Oh, that’s because there’s that “scary action in the distance,” isn’t it? Einstein said that. Yeah, that guy again. But what’s scary in the distance? What did Einstein really say? And what does that mean? That’s what we’ll talk about today.
The vast majority of sources on the Internet claim that Einstein’s “creepy action at a distance” refers to entanglements. Wikipedia, for example. And here’s an example from Science Magazine. You can also find plenty of videos on YouTube that say the same thing: Einstein’s gruesome act from afar was entanglement. But I don’t think that’s what Einstein meant.
Let’s look at what Einstein actually said. The origin of the expression “creepy long-distance effect” is a letter that Einstein wrote to Max Born in March 1947. In this letter, Einstein Born explains why he doesn’t believe that quantum mechanics really describes how the world works.
First, he assures Born that he knows very well that quantum mechanics is very successful: “I understand, of course, that the statistical formalism with which you pioneered captures an important truth.” But then he explains his problem. Einstein writes:
“I can’t seriously believe it [in quantum mechanics] because the theory is not compatible with the requirement that physics should represent reality in space and time without a creepy long-distance effect … “
There it is, the creepy action in the distance. But what exactly was Einstein referring to? Before we get into that, I need to quickly remind you how quantum mechanics works.
In quantum mechanics, everything is described by a complex-valued wave function, commonly referred to as psi. From the wave function we calculate probabilities for measurement results, for example the probability of finding a particle at a certain location. We do this by taking the absolute square of the wave function.
But we cannot observe the wave function itself. We only observe the result of the measurement. Above all, this means that we suddenly have to “update” the wave function when we perform a measurement for which the result was not one hundred percent certain. This is because the moment we measure the particle, we know that it is either there or not. And this update is available immediately. It happens everywhere at the same time, apparently faster than the speed of light. And I think * Einstein was worried about that because he said this twenty years ago in the discussion of the 1927 Solvay Conference.
In 1927 Einstein used the following example. For example, suppose you aim a beam of electrons at a screen with a tiny hole in it and ask what happens to a single electron. The electron’s wave function is bent at the hole, which means that it spreads symmetrically in all directions. Then measure it a certain distance from the hole. The electron is equally likely to have gone in any direction. But if you measure it, suddenly you will find it at some point.
Einstein argues: “The interpretation according to which [the square of the wave-function] expresses the probability that this particle will be found at a certain point, adopts a completely special mechanism of action in the distance, which prevents the wave, which is continuously distributed in space, from producing an effect in two places on the screen. “
What he’s saying is that the wave function on the left side of the screen must somehow know that the particle was actually detected on the other side of the screen. In 1927 he called this action in the distance not “creepy” but rather “weird”, but I think he was referring to the same thing.
In Einstein’s electron argument, however, it’s pretty unclear what affects what. Because there is only one particle. Because of this, Einstein later studied the measurement of two entangled particles with Podolsky and Rosen, resulting in their famous 1935 EPR paper. This is why there is an entanglement: Because you need at least two particles to show that the measurement on one particle can affect the other particle. But the entanglement itself is not a problem. It’s just a type of correlation, and correlations cannot be local without some “action” taking place some distance away.
To see what I mean, forget all about quantum mechanics for a moment. Suppose I have two socks that are identical except that one is red and the other is blue. I’ll put them in two identical envelopes and mail one to you. As soon as you open the envelope and see that your sock is red, you will know my sock is blue. This is because the information about the color in the envelopes is correlated and that correlation can extend over great distances.
There is no scary action taking place, however, as this correlation was created locally. Such correlations exist everywhere and are constantly being established. For example, imagine you bounce a ball off a wall and it comes back. This transfers the momentum to the wall. You can’t see how much, but you know the total momentum is preserved so the momentum of the wall is now correlated to that of the ball.
Entanglement is a correlation like this, it’s just that you can only create it with quantum particles. Suppose you have a particle with zero total spin that breaks up into two particles that can have either plus or minus one. One particle goes to the left, the other to the right. You don’t know which particle has which spin, but you know that the entire spin is retained. Either the particle that went to the right had spin plus one and the particle that went to the left had minus one, or vice versa.
According to quantum mechanics, there are both possibilities before you have measured either particle. You can then measure the correlations between the spins of both particles with two detectors on the left and right. It turns out that the entanglement correlations can be stronger than non-quantum correlations under certain circumstances. That’s what makes them so interesting. But there is no creepy act in the correlation itself. These correlations were created locally. Instead, Einstein worried that the wave function for the particle on the other side would change as soon as you measure the particle on one side.
But isn’t that the same thing with the two socks? Before you open the envelope there was a 50-50 chance, and when you open it it jumps to 100-0. But there is no scary action going on. It’s just that the likelihood was a proposition about what you knew, not what was really the case. Indeed, which sock was in which envelope had already been decided when I sent it.
Yes, that explains the case for the socks. However, this explanation does not work in quantum mechanics. If you think that it has already been decided which spin was going in which direction when they were emitted, this will not result in sufficiently strong correlations. It’s just not compatible with observations. Einstein didn’t know that. These experiments were carried out only after his death. But he knew that entangled states can show whether creepy acts are real or not.
I’ll admit that I’m a little defensive about good old Albert Einstein because I feel like a lot of people are too gleeful to explain that Einstein was wrong about quantum mechanics. But if you read what Einstein actually wrote, he was extremely careful about expressing himself, and yet most physicists rejected his concerns. In April 1948 he repeated his argument to Born. He writes that a measurement on a part of the wave function is a “physical intervention” and that “such an intervention cannot immediately affect the physical reality in a distant part of space”. Einstein concludes:
“It is for this reason that I tend to believe that quantum mechanics is an incomplete and indirect description of reality that is later replaced by a full and direct one.”
So Einstein did not believe that quantum mechanics was wrong. He found it incomplete that something fundamental was missing from it. And in my reading, the term “creepy long-range effect” referred to the measurement update, not entanglements.