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

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

Quantum mechanics has struck physicists as weird ever since its discovery, more than a century ago. One especially peculiar aspect of quantum mechanics is that it forces you to accept the existence of superpositions. That are systems which can be in two states at the same time, until you make a measurement, which suddenly “collapses” the superposition into one definite measurement outcome.

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

Now, you may think that being in a quantum superposition is something that only tiny particles can do. But these superpositions for large objects can’t be easily ignored, because you can take the tiny ones and amplify them to macroscopic size.

This amplification is what Erwin Schrödinger wanted to illustrate with a hypothetical experiment he came up with in 1935. In this experiment, a cat is in a box, together with a vial of poison, a trigger mechanism, and a radioactive atom. The nucleus of the atom has a fifty percent chance of decaying in a certain amount of time. If it decays, the trigger breaks the vial of poison, which kills the cat.

But the decay follows the laws of quantum physics. Before you measure it, the nucleus is both decayed and not decayed, and so, it seems that before one opens the box, the cat is both dead and alive. Or is it?

Well, depends on your interpretation of quantum mechanics, that is, what you think the mathematics means. In the most widely taught interpretation, the Copenhagen interpretation, the question what state the cat is in before you measure it is just meaningless. You’re not supposed to ask. The same is the case in all interpretations according to which quantum mechanics is a theory about the knowledge we have about a system, and not about the system itself.

In the many-worlds interpretation, in contrast, each possible measurement outcome happens in a separate universe. So, there’s a universe where the cat lives and one where the cat dies. When someone opens the box, that decides which universe they’re in. But for what observations are concerned, the result is exactly the same as in the Copenhagen interpretation.

Pilot wave-theory, which we talked about earlier, says that the cat is really always in only one state, you just don’t know which one it is until you look. The same is the case for spontaneous collapse models. In these models, the collapse of the wave-function is not merely an update when you open the box, but it’s a physical process.

It’s no secret that I myself am signed up to superdeterminism, which means that the measurement outcome is partly determined by the measurement settings. In this case, the cat may start out in a superposition, but by the time you measure it, it has reached the state which you actually observe. So, there is no sudden collapse in superdeterminism, it’s a smooth, deterministic, and local process.

Now, one cannot experimentally tell apart interpretations of mathematics, but collapse models, superdeterminism, and, under certain circumstances, pilot wave theory, make different predictions than Copenhagen or many worlds. So, clearly, one wants to do the experiment!

But. As you have undoubtedly noticed, cats are usually either dead or alive, not both. The reason is that even tiny interactions with a quantum system have the same effect as a measurement, and large objects, like cats, just constantly interact with something, like air or the cosmic background radiation. And that’s already sufficient to destroy a quantum superposition of a cat so quickly we’d never observe it. But physicists are trying to push the experimental boundary for bringing 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 through the mirror and also reflected back off it. Then they used one part of this superposition to trigger a laser pulse, which contains a whole lot of photons. Finally, they showed that the pulse was still in a superposition with the single photon. In another 2019 experiment, they amplified both parts of this superposition, and again they found that the quantum effects survived, for up to about 100 million photons.

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

But just in case you think a laser pulse is a poor approximation for a cat, how about this. In 2017, scientists at the University of Sheffield put bacteria in a cavity between two mirrors and they bounced light between the mirrors. The bacteria absorbed, emitted, and re-absorbed the light multiple times. The researchers could demonstrate that this way, some of the bacterias’ molecules became entangled with the cavity, so that is a special case of a quantum superposition.

However, a paper published the following year by scientists at Oxford University argued that the observations on the bacteria could also be explained without quantum effects. Now, this doesn’t mean that this is the correct explanation. Indeed, it doesn’t make much sense because we already know that molecules have quantum effects and they couple to light in certain quantum ways. However, this criticism demonstrates that it can be difficult to prove that something you observe is really a quantum effect, and the bacteria experiment isn’t quite there yet.

Let us then talk about a variant of Schrödinger’s cat that Eugene Wigner came up with in the nineteen-sixties. Imagine that this guy Wigner is outside the laboratory in which his friend just opens the box with the cat. In this case, not only would the cat be both dead and alive before the friend observes it, the friend would also both see a dead cat and see a live cat, until Wigner opens the door to the room where the experiment took place.

This sounds both completely nuts as well as an unnecessary complication, but bear with me for a moment, because this is a really important twist on Schrödinger’s cat experiment. Because if you think that the first measurement, so the friend observing the cat, actually resulted in a definite outcome, just that the friend outside the lab doesn’t know it, then, as long as the door is closed, you effectively have a deterministic hidden variable model for the second measurement. The result is clear already, you just don’t know what it is. But we know that deterministic hidden variable models cannot produce the results of quantum mechanics, unless they are also superdeterministic.

Now, again, of course, you can’t actually do the experiment with cats and friends and so on because their quantum effects would get destroyed too quickly to observe anything. But recently a team at Griffith University in Brisbane, Australia, created a version of this experiment with several devices that measure, or observe, pairs of photons. As anticipated, the measurement result agrees with the predictions of quantum mechanics.

What this means is that one of the following three assumptions must be wrong:

1. No Superdeterminism.

2. Measurements have definite outcomes.

3. No spooky action at a distance.

The absence of superdeterminism is sometimes called “Free choice” or “Free will”, but really it has nothing to do with free will. Needless to say, I think what’s wrong is rejecting superdeterminism. But I am afraid most physicists presently would rather throw out objective reality. Which one are you willing to give up? Let me know in the comments.

As of now, scientists remain hard at work trying to unravel the mysteries of Schrödinger's cat. For example, a promising line of investigation that’s still in its infancy is to measure the heat of a large system to determine whether quantum superpositions can influence its behavior. You find references to that as well as to the other papers that I mentioned in the info below the video. Schrödinger, by the way, didn’t have a cat, but a dog. His name was Burschie.