Quantum reality is either strangely different or it is collapsing

Quantum mechanics, examined closely, poses profound questions about reality. These questions often take the form of thought experiments, which are later (usually much later) followed by actual experiences. One of the most difficult and profound of these is a thought experiment proposed by Eugene Wigner in the 1960s, called “Wigner’s Friend” (you don’t want to be Wigner’s Friend). Now, much later, Wigner and his friend have been formalized and extended. The result puts us in contradiction: either the reality is much stranger and less real at the level of quantum mechanics, or quantum states cannot exist on a large scale.

Don’t be Wigner’s friend

To understand why Wigner shouldn’t have friends, we first need to delve into a few details of quantum mechanics. Imagine measuring the spin of a single electron. The spin has an orientation in space, but it is not possible to measure this orientation. Instead, we must to choose an orientation and measure the rotation along that orientation. We can therefore ask an electron whether its spin is vertically up or down. The result (all other things being equal) will be either up or down with a 50% probability.

Let’s say we measure the rotation and find that it is increasing. All subsequent measurements will also confirm that it is in place. The measure has defined the vertical spin component (a process often referred to as the collapse of the wave function). But that doesn’t say anything about the horizontal component of the spin – the horizontal component will remain in a superposition of left and right spins. This means that if we spin our device in such a way as to measure the spin left and right, the result will be random – the electron will either be spinning left or spinning right with a probability of 50%.

It’s a relatively simple experiment to do, so Wigner grabs her friend, Alice, and places her in a sealed lab. Alice measures the spin of a stream of electrons that are prepared in a state of superposition. Wigner is outside the lab and will measure the entire lab. Alice, before passing out, determines that an electron is spin-up. But Wigner didn’t take a measurement, so he sees Alice in an overlay to have measured spin-up or spin-down. When Wigner takes his measurement, hypothetically, he could come up with a result where Alice was measuring the spin-down when in fact she was measuring the spin-up.

Two “facts” contradict each other, but both are based on reality. Wigner’s solution to this problem was that the quantum state cannot exist at the level of the observer: the superposition state must collapse before this can happen.

Wigner extension

The newly released extended version combines the original thought experiment of Wigner’s friend with the thought experiment of John Bell (Bell’s thought experiment is old enough that it is now regularly tested in experimental physics) . Bell’s experiment is a bit more complicated than Wigner’s friend experiment, combining the idea of ​​layering and entanglement. Suppose I have two entangled electrons. This means that they have a combined quantum state, which is only defined by the two electrons together. As an example, the entanglement process could result in two electrons with a total spin of zero.

This means that they are in a top to bottom and bottom to top overlay. If I measure one of these electrons to spin, then the other is automatically spinning down. When done correctly, Bell’s experiment shows that it is impossible for the spin of electrons to be predetermined. They must be in an overlay; when measured, they must randomly choose an orientation.

Bell’s experiments were performed with wide separations between the places where particles (usually photons) are measured, preventing information from traveling between the two ends of the experiments without moving faster than light. Indeed, the last twenty years have seen a systematic closure of possible gaps in Bell’s experiments.

In Wigner’s extended friend experiment, the two friends perform Bell’s experiment on a pair of entangled particles in separate sealed labs. Two Wigners (otherwise known as super-observers) then measure the labs and compare the results. The measurement results reveal the correlations between the entangled pair, filtered by the collapse of the wave function of the two observers.

So what is this reality anyway?

What Wigner and Bell tell us about reality requires reflection. Physicists generally describe reality by a set of mathematically defined conditions. For example, causality tells us that an effect must be preceded in time by a cause. The locality says that causes propagate at the speed of light: if a photon cannot travel between the location of the cause and the location of the effect before the effect occurs, then it violates the locality (and, potentially, causality).

Now we also have to consider things like measurements. Researchers define the absolute character of observed events, which means that what I observe is real and does not depend on anything else. They assume that there is no super-determinism (we make free choices) and that the locality is still operational. Researchers call this trio a local friendliness.

The researchers used mathematical definitions of these statements to calculate the limits of local usability. They show that under the right conditions, correlations that violate these limits will be seen in the extended experience of Wigner’s friends. Their laboratory experiments confirmed that these violations do indeed occur. Local friendliness is not how the Universe works.

If we reject local friendliness, we have to make decisions. We have to accept some of the following possibilities: an observer’s measurements are not necessarily real, reality is not local, super-determinism is real, or quantum mechanics stops working somewhere before macroscopic observers are involved.

Huh?

To extend that last paragraph: the researchers’ version of Wigner’s friend was not actually a macroscopic observer – it was a photon, which is an inherently quantum object. It is possible that the complete thought experiment with macroscopic observers in a quantum state is simply prohibited by some unknown law of physics. On the other hand, it is not beyond the realm of possibility (according to the researchers) that an artificial intelligence on a quantum computer can be considered a macroscopic observer who is still in a quantum state, which means that we could extend testing of this in the future. .

However, the most important point is that reality may depend on erasing quantum effects before they get too great. The alternatives are that we have a lot less free choice than we think, which is a bit of a pain. Beyond that, we have to reject locality and perhaps causation, which could be even more painful for physicists.

Nature Physics, 2020, DOI: 10.1038 / s41567-020-0990-x (About DOIs)