Is Schrödinger’s cat really dead AND alive? An Interview with Quantum Physicist Eric Cavalcanti
Is Schrödinger’s cat really dead and alive? This century-old experiment that queries the meaning of the wave function, the central object of quantum mechanics, remains a mystery. Is there one true story, or is human belief in a definitive observer-independent reality an illusion? Here we speak with Dr. Eric Cavalcanti about a recent experiment so illuminating that the discoveries were documented in the New York Times.
Your recent experiments have been making waves the world over. Can you explain in basic terms the importance of your most recent discoveries into the meaning of the wave function?
The wave function is one of the fundamental elements of quantum physics. Physicists use it all the time in their calculations to predict results of experiments. And it’s a very successful theory, of course—without it we wouldn’t have computers, lasers, and any of a number of modern technologies. It’s one of the two pillars of modern physics, the other being Einstein’s theory of general relativity.
But there is a century-old debate about what the wave function means. You see, the wave function describes some apparently bizarre situations. If you take it literally, it says that an object (say, an electron, or an atom) can be in several places at the same time, or distant objects can be somehow instantaneously connected, among other puzzling features. So there is this debate about whether it represents an objective property of a system or just our limited information about it. In other words, is this electron really in two places at the same time, or is it in one or the other place, but we just don’t know which?
With these recent theorems and experiments we have given strong evidence against the second view. In fact we rule out a broad class of theories where the wave function represents only limited information about an objective reality. If you want to have a purely objective description of the world, then the wave function must be real.
Can you give some background about the original Schrödinger’s cat experiment upon which these recent discoveries are based?
Erwin Schrödinger—one of the founders of quantum theory—was puzzled by these questions and considered a situation where you put a cat inside a box with a radioactive atom, a vial of poison and a Geiger counter. If the atom decays, it would emit some radiation that would be detected by the Geiger counter and activate a mechanism to break the vial of poison, killing the cat (I know it’s cruel, but it’s just a thought experiment, OK? Don’t try this at home!).
Now quantum physics describes this radioactive atom by a wave function. After a certain amount of time (that depends on the atom, but it could be a minute, say) this wave function describes this atom in a superposition of having decayed and not decayed. Now if the atom has decayed, the cat dies, so it turns out that if you keep following up the quantum description, you end up with this apparently absurd conclusion that the cat is in a superposition of dead and alive at the same time.
This doesn’t happen just with cats, of course, but with everything around us. All the time there are quantum events happening and according to the theory the whole world is in a very complicated superposition of all these possibilities happening in parallel. Now the question is: does this represent reality or just our limited knowledge of it?
Now you can ask: so if all these possibilities really exist, why don’t we see it? It’s a complicated answer. For small things like atoms we do see it all the time in labs around the world, through interference between these alternative possibilities. But for large things that’s very hard to do in practice because of a process called decoherence. But unless quantum theory fails at some point, we have no reason to think it’s impossible in principle, and there are experiments going on right now trying to push that boundary to larger and larger systems.
Do you see a potential for the development of quantum technology based upon these discoveries?
That’s certainly a possibility. The field of quantum computation and quantum information has origins in foundational questions like these. For example, quantum computers were discovered partly because David Deutsch was thinking about parallel universes, and quantum cryptography and teleportation have roots in questions about the apparent ‘spooky action at a distance’ discovered by Einstein and studied by John Bell in the 60s. Our work, as Bell’s work, can be understood as being about the limitations of describing quantum effects within a classical model. So it’s conceivable that it could have some implications for quantum information processing. There’re some indications in that direction, but it’s a bit early to tell.
What is the next step in expanding upon this research?
There are, of course, the questions about applications, above. Also, the experiments and theorems can be improved to rule out an even broader class of theories than they do now. There are lots of technical improvements to be made.
Given your recent discovery, where do you stand on the age-old quantum physics dichotomy of a fixed reality in addition to our observation of it, or the alternative, that ‘reality’ is nothing more than an ocean of infinite possibilities?
A lot of the debates in quantum physics is framed in terms of two different philosophical perspectives, which are impersonated by Albert Einstein on one side and Niels Bohr on the other. Einstein was a realist. He believed that physics is about a real world out there, independent of observers. Bohr was an anti-realist (or operationalist). He believed that the task of physics is not to find out what nature is, but what we can say about nature.
Now it just happens that both of them thought of the wave function as representing information, but Einstein thought that information was about an objective reality, whereas Bohr thought it was about the possible outcomes of observations we can make. There’s a third perspective, which is realist like Einstein, but takes the wavefunction itself to be real. This is the position of Bell, among others.
So out of these three options our theorems and experiments give strong evidence against Einstein’s view. If you want to take the realist perspective, then we show that the wave function cannot represent just information. It must be a real property itself. But following Schrödinger’s cat argument, this implies the world really is an ocean of infinite possibilities.
On the other hand, there are lots of reasons to think of the quantum state as representing information. This seems to be the message of quantum information theory, which studies future technologies like quantum computers, quantum cryptography, teleportation, etc. It seems to be best understood as a kind of generalisation of classical probability theory.
If you want to take this point of view, then our results say you are forced into an operationalist point of view. Then you can still think of the wavefunction as representing information, but now it’s not about a world out there independent of what we do on it. Our actions matter. Our choices of what to observe matter. And they matter not just in the sense that they change what is already there — they take part in creating what will be. The most precise description of this position in modern times is an interpretation called QBism, proposed by Chris Fuchs and others.
Personally, I tend to think that ultimately both views are complementary. On odd-numbered days it may be useful to think as a realist to solve a certain problem, and on even-numbered days it may be useful to think as an operationalist. As a physicist (and as a philosopher) I’m interested in knowing what are the possible ways we can consistently understand the world, and to rule out the ones that are not consistent.
But out of the ones that are left, which one is really true? I believe that ultimately, that question cannot be decided. In fact, I believe we should be able to prove that we cannot decide it one way or another. And even beyond that, I believe that it might not even have an answer! One of my current research projects is to try to put some meat into those assertions. But this is a much longer story.
Interview and photography by Aurora Jane
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