[This is a transcript of the video embedded below.]

As we all know, the internet was invented so that we could spend our days watching cat videos. That’s why this video is about the most famous of all science cats, Schrödinger’s cat. Is it really both dead and alive? If so, what does that mean? And what does the latest research have to say about it? That’s what we’ll talk about today.

Physicists have found quantum mechanics strange since it was first discovered more than a century ago. A particularly special aspect of quantum mechanics is that you are forced to accept the existence of superpositions. These are systems that can be in two states at the same time, until you carry out a measurement that suddenly “breaks down” the superposition to a certain measurement result.

The system here could be a single particle, like a photon, but it could also be a large object made up of many particles. The thing is that in quantum mechanics, when two states exist separately, like an object that is here and there, the superposition – that is the same object here and there – must also exist. We know this experimentally and I explained the math behind it in an earlier video.

Now one might think that it is something that only tiny particles can do when one is in a quantum superposition. However, these large object overlays cannot simply be ignored as you can take the small objects and amplify them to macroscopic size.

Erwin Schrödinger wanted to illustrate this reinforcement with a hypothetical experiment that he had worked out in 1935. In this experiment, a cat is in a box, along with a poison bottle, a trigger mechanism, and a radioactive atom. The atomic nucleus has a fifty percent chance of decaying in a given time. When it crumbles, the trigger breaks the poison vial, killing the cat.

The decay follows the laws of quantum physics. Before you measure it, the kernel is both decayed and not decayed, and so it appears that the cat is both dead and alive before you open the box. Or is it?

Well, depends on your interpretation of quantum mechanics, which is what you think math means. In the most widely used interpretation, the Copenhagen interpretation, the question of what condition the cat is in before you measure it is simply meaningless. You shouldn’t ask. The same goes for any interpretation that quantum mechanics is a theory of what we know about a system, not the system itself.

In contrast, when interpreting many worlds, every possible measurement result occurs in a separate universe. So there is one universe the cat lives in and one where the cat dies. When someone opens the box, that decides which universe they are in. But as far as the observations are concerned, the result is exactly the same as in the Copenhagen interpretation.

The pilot wave theory, which we talked about earlier, states that the cat is really only in one state at a time. You just don’t know which one it is until you look. The same applies to models with spontaneous collapse. In these models, the collapse of the wave function isn’t just an update when you open the box, it’s a physical process.

It’s no secret that I am committed to superdeterminism myself, which means that the measurement result is determined in part by the measurement settings. In this case, the cat can start in an overlay, but when you measure it, it has reached the state that you are actually observing. So there is no sudden breakdown in superdeterminism, it is a smooth, deterministic and local process.

Now one cannot experimentally distinguish between interpretations of mathematics, but collapse models, superdeterminism and under certain circumstances the pilot wave theory make different predictions than Copenhagen or many worlds. So, of course, you want to do the experiment!

But. As you’ve no doubt noticed, cats are usually either dead or alive, not both. The reason for this is that even tiny interactions with a quantum system have the same effect as a measurement, and large objects like cats are constantly interacting with something like air or the cosmic background radiation. And that’s enough to destroy a cat’s quantum overlay so quickly that we would never observe it. But physicists are trying to push the experimental frontier to get large objects into quantum states.

For example, in 2013 a team of physicists from the University of Calgary in Canada amplified a quantum superposition of a single photon. They first fired the photon at a partially silvered mirror called a beam splitter, so that it became a superposition of two states: It passed the mirror and was reflected off of it as well. Then they used part of that overlay to trigger a laser pulse that contained a fair amount of photons. Finally they showed that the pulse was still superimposed on the single photon. In another experiment in 2019, they amplified both parts of this superposition and again found that the quantum effects survived for up to 100 million photons.

Now a group of 100 million photons is not a cat, but it is larger than your standard quantum particle. In some headlines, this has been referred to as the “Schrödinger’s kitten” experiment.

But just in case you think a laser pulse is a bad approximation for a cat, how about this? In 2017, scientists from the University of Sheffield placed bacteria in a cavity between two mirrors and reflected light between the mirrors. The bacteria absorbed, emitted, and absorbed the light several times. The researchers were able to show that in this way some molecules of the bacteria became entangled with the cavity, which is a special case of quantum overlay.

In an article published the following year by scientists at Oxford University, however, it was argued that the observations on the bacteria could also be explained without quantum effects. That doesn’t mean that this is the correct explanation. In fact, it doesn’t make a lot of sense because we already know that molecules have quantum effects and couple to light in certain quantum ways. However, this criticism shows that proving that something you are observing is really a quantum effect can be difficult, and the bacteria experiment is not quite there yet.

Then let’s talk about a variant of Schrödinger’s cat that Eugene Wigner invented in the sixties. Imagine this guy Wigner is outside the lab where his friend is opening the box with the cat. In this case, not only would the cat be dead and alive before the friend watches it, but it would also be a dead cat and a live cat until Wigner opens the door to the room where the experiment took place.

This sounds both completely crazy and an unnecessary complication, but take a moment for me because this is a really important twist in Schrödinger’s cat experiment. Because if you think that the first measurement, i.e. the friend watching the cat, actually led to a definite result, only that the friend outside the laboratory doesn’t know, then you effectively have one as long as the door is closed – deterministic Hidden variable model for the second measurement. The result is already clear, you just don’t know what it is. We know, however, that deterministic models with hidden variables cannot give quantum mechanics results unless they are also superdeterministic.

Of course, you cannot conduct the experiment with cats and friends, etc. either, as their quantum effects would be destroyed too quickly to observe anything. Recently, a team at Griffith University in Brisbane, Australia created a version of this experiment using multiple devices that measure or observe pairs of photons. As expected, the measurement result agrees with the predictions of quantum mechanics.

This means that one of the following three assumptions must be wrong:

1. No superdeterminism.
2. Measurements have clear results.
3. No scary action in the distance.

The lack of superdeterminism is sometimes referred to as “free choice” or “free will,” but it really has nothing to do with free will. Needless to say, I think what is wrong is the rejection of superdeterminism. But I’m afraid most physicists would prefer to reject objective reality right now. Which one do you want to give up? Let me know in the comments.

As of now, scientists are working hard to unravel the secrets of Schrödinger’s cat. For example, one promising line of research that is still in its infancy is to measure the heat of a large system to see if quantum overlays can affect its behavior. You can find references to it as well as the other articles I mentioned in the information below the video. Incidentally, Schrödinger didn’t have a cat, but a dog. His name was Burschie.