Schrödinger’s Cat: When Reality Is Ambiguous

Why is Schrödinger’s cat, of all things, considered a good illustrative example of quantum physics?

Martin Schultze: This is probably because a cat is a familiar and vivid example. In classical physics, everything is unambiguous: a cat is either dead or alive. In quantum mechanics, a system can exist in several states at the same time over a certain period of time – in other words, it can be dead or alive at the same time. This so-called superposition is a central feature of quantum physics. Only when we measure it, i.e. look into the box, does the system commit itself to a state. This sounds paradoxical, but it is a real, experimentally confirmed phenomenon that shows that different rules apply in the quantum world than in our macroscopic world of experience. Therefore, experimental confirmation with a real cat wouldn’t work.

Kazuhiro Yabana: Schrödinger’s cat is also so popular because it illustrates the border between our everyday experience and the quantum world. In theoretical physics, it is clear that quantum mechanics is the fundamental theory – classical mechanics, which we associate with Newton’s laws, is only an approximation. It works well in everyday life, but if you really want to understand nature, you have to deal with the quantum level.

Why don’t we notice these quantum phenomena in everyday life?

Schultze: This has to do with the size of the systems. The smaller a system is – the fewer particles are involved – the stronger the quantum mechanical effects become. With large objects, such as a cat or a mobile phone, these effects disappear in the background because countless particles influence each other. But when you look at individual atoms or electrons, our usual, classical descriptions fail. There you can see that nature behaves in a different way to what we would intuitively expect.

Yabana: You could say that classical physics is a special case of quantum physics that only applies when the systems are large enough. In the microscopic world, however, quantum mechanics is indispensable. Without it, we would not be able to explain many phenomena.

We humans want clear answers – yes or no, here or there, dead or alive. However, quantum objects do not need such clarity

What exactly does it mean to say that a particle can be in several places at the same time?

Yabana: In quantum physics, every particle has the character of a wave. An electron is therefore not a tiny sphere, but more like a cloud – a so-called probability distribution. This cloud describes where the electron can be found and with what probability. If you measure it, you “force” it to show up in a certain place. But that is just a single moment. The actual nature of the electron is not point-shaped, but distributed.

Schultze: This cloud is not due to an uncertainty regarding measurement, but describes reality itself. The electron is actually present in a certain area of space at the same time. The act of measuring reduces this complexity to a single position – a snapshot that does not show the whole picture. This is difficult to accept because we are used to things being either here or there. But in the quantum world, things work differently.

So, is this also a problem of our human perception?

Schultze: Absolutely. We humans want clear answers – yes or no, here or there, dead or alive. However, quantum objects do not need such clarity. They can assume several states at the same time without this being a contradiction.

I like to compare this with a very human question: “Are you happy?” There is rarely a simple yes or no answer. If I force you to choose only between these two answers, it will hardly do justice to your actual emotional state. It is the same with a quantum mechanical system. If we ask it whether it is in state A or B, we force it to give an answer that is far too simplistic to really describe its reality.

Yabana: That’s a nice comparison. The quantum world cannot be squeezed into our frameworks of everyday thought. It shows us that nature itself is ambiguous and at the same time more diverse than we are used to.

Many people only associate quantum physics with theories – does it also have practical significance in everyday life?

Yabana: Much more than you might think. For example, the glow of a hot metal. First it lights up red, then blue, then white. This can only be explained by quantum physics. Even why glass is transparent or metals conduct electricity can only be understood with the help of quantum mechanics models.

Schultze: Or think of your smartphone. The semiconductors in the display, the LED lighting, the laser technology in communication – all of this is based on quantum physics. Without this knowledge, there would be no modern electronics. Every LED, every transistor and every solar cell utilises quantum mechanical principles. When light meets matter, when electrons are stimulated or energy is transferred, these are all quantum physical processes. Our entire technology depends on it.

Schrödinger’s cat stands for overlapping states, so-called superposition. What other quantum physical phenomena are important?

Schultze: In addition to superposition, the quantisation of energy is central, i.e. the fact that energy can only be transferred in fixed portions, so-called quanta. Then there is entanglement: two particles can be connected in such a way that they influence each other, no matter how far apart they are. This sounds like science fiction, but it has been experimentally confirmed. New technologies such as quantum computers and quantum communication are based on these principles.

Yabana: A quantum computer basically utilises precisely the property described by Schrödinger’s cat: that a system cannot only be zero or one, but can assume many states simultaneously. This means that a quantum computer can theoretically perform a large number of calculations in parallel – which is what makes it so powerful. In a way, it is the technical application of the cat paradox.

And now to the crucial question: Is Schrödinger’s cat alive or dead in the end?

Schultze: From the point of view of quantum physics, this is a wrong question. As long as we do not “check”, the system is in a superposition of both states. Only observation forces us to commit ourselves. We humans want clear answers, but nature itself seems to have no problem with being ambiguous. Quantum physics shows us that reality is more complex than our language or intuition allow.

Yabana: And perhaps this is precisely the most beautiful lesson to be learnt from Schrödinger’s thought experiment. That our idea of reality is only a small part of what is actually possible.