What is the quantum observer effect

"Both cannot be right"

Physicists developed the fundamentals of quantum mechanics as early as the 1920s. This theory can be used to describe the behavior of tiny particles - such as atoms or electrons - very successfully. However, from a certain size, matter composed of these particles no longer obeys the rules of quantum mechanics. In order to better understand the boundary between the quantum and everyday world, physicists have now developed a thought experiment that they recently presented in the journal "Nature Communications". In an interview with Welt der Physik, Renato Renner from ETH Zurich explains how he and his colleagues uncovered a contradiction with this theoretical experiment.

World of physics: How does the familiar everyday world differ from the quantum world?

Renato Renner: This can best be illustrated using the example of a single particle: the state of a particle, which can be described using classical mechanics, is given by its location and its speed. I need these two pieces of information to predict the future behavior of the particle. In quantum mechanics, on the other hand, you need a lot more information to determine the state of a particle. In addition, it is not possible to give the exact location of a particle. Instead, you only know the probability that the particle is in a certain location.

How can you imagine that? Is the particle in several places at the same time or is it just not known any better?

Renato Renner

I don't like the phrase “it's in several places at the same time” because it's nowhere at the same time. And even if I knew everything about the particle, I could only say that there is a certain probability that it is in a certain place. In quantum mechanics, this uncertainty is fundamental, in contrast to classical physics. Because only when I observe the particle - that is, measure it - do I know exactly where it is. Before that, it is in a so-called superposition of different states - in this case of different possible positions.

What happens when I observe the particle?

This is where the so-called measurement problem begins, for which there is still no agreement on how to describe it theoretically. What happens experimentally, on the other hand, is clear: if I measure the position of a particle, I will observe it at a certain location. And if I measure the position a second time, the particle will still be there. That is, in quantum mechanics, the observation influences the state of the particle. Up to the point in time of the measurement, the particle is in a state of superposition and is then set to a result.

So far we have only spoken of one particle. Can several particles be described by quantum theory?

One of the big questions for us is how many particles can still be described by quantum mechanics. It has been experimentally proven that small particles such as electrons, atoms or molecules have to be described with the help of quantum mechanics. One can now argue that this theory should also describe what is made up of these particles. However, this is not so easy to show experimentally. Because in systems made up of many particles there are many interactions with one another, which means that the particles behave like a classic system. But what we cannot verify experimentally can still be investigated in a theoretical experiment. Therefore, we now want to use a thought experiment to check whether quantum mechanics can also describe systems of any size or complex.

What does the thought experiment look like?

In essence, our thought experiment is an extension of an earlier experiment - called "Wigner's friend". There is an observer - Wigner's friend - who flips a coin. If you look at the coin as a quantum system, after the toss - as long as you don't look - it is in a superposition of “heads” and “tails”. Only by observing Wigner's friend is it possible to measure which side the coin has landed on. But now Wigner's friend is in a locked laboratory that is viewed by an outside person - Wigner himself - as a quantum system. And from Wigner's point of view, Wigner's friend and the coin in the laboratory are in a superposition state, although his friend already knows how the coin fell.

Is not that a contradiction?

Not really. Because as soon as Wigner opens the laboratory and takes a look, he will observe exactly one throw result - “heads” or “tails”. And this will be the same result that his friend also observed. The thought experiment does not speak against quantum mechanics because the measurement results of Wigner and his friend do not contradict each other. In our expanded thought experiment, we add two more actors. These can each find out in their own way which side the coin fell on. For this, the four participants can partially communicate with each other. We have now calculated that under certain circumstances two of the observers arrive at a different result of the coin toss. One is sure that the coin shows “heads” and the other observes the result of the throw “tails” at the same time. But of course not both can be right - and with that we have a contradiction.

Why is that?

We have made various assumptions in our thought experiment. Since the overall result leads to a contradiction, one of the assumptions must be wrong. One assumption is, for example, that the quantum mechanics known to us can actually be applied to the entire laboratory, including Wigner's friend and the coin - quantum mechanics is therefore generally applicable. That is the most complex assumption of the thought experiment. Personally, I would be most likely to give up this assumption as it has never been experimentally confirmed.

Is there any way to recreate the thought experiment in the laboratory?

Certainly not in the form in which we present it, since quantum mechanical experiments cannot be carried out with people. An alternative would be conceivable: For the thought experiment it is not necessary that Wigner's friend and the other observers are human. So they could be replaced by machines. These should be perfectly controllable computers that know the rules of quantum mechanics, carry out measurements and can draw conclusions from them. Such computers do not yet exist, but at least that would be a possibility. The purpose of a thought experiment is actually to check the consistency of a physical theory. Without performing the experiment, we can already say with certainty that certain assumptions of quantum theory are inconsistent. With an experiment we could investigate the causes. But I don't need that for the statement itself. That's actually the beauty of a thought experiment.