I recently had a conversation with someone, spurred by the last post, that led to yet another description of the Everett many-worlds interpretation of quantum mechanics, which I think is worth putting in a post. It approaches the interpretation from a different angle than I’ve used before.
As mentioned last time, the central mystery of quantum mechanics is that quantum particles move like ever spreading waves, but when they hit something and leave a mark, they do so like little localized balls. Prior to making the mark, that is, prior to a measurement, we say that the particle is in a superposition of multiple states, with those states perhaps involving different positions, momenta, spin directions, or other properties. This is modeled mathematically by the quantum wave function. In Copenhagen and similar collapse interpretations, measurement causes a wave function collapse. All the possible outcomes are instantly reduced to one.
An interesting aspect of this is that if two particles interact, we can often use properties measured from one of the particles to know things about the other particle, kind of like if I have a physical copy of today’s New York Times, I know what everyone else’s copy says.
What makes the quantum version interesting is that such particles, after the interaction, now share a common wave function, that is, they now exist in the same combined superposition, one where every possible combination of their affected properties is an element in the overall superposition. In collapse interpretations, the value of a particle’s spin, for instance, isn’t set until the measurement. In the case of two particles entangled on their spin, when one is measured and collapses to a definite answer, the value of the other particle is also set, apparently instantly.
The thing about entanglement is that there’s nothing in the mathematics constraining this phenomenon to only pairs of particles. It’s routinely observed in much larger collections. For example, all the elementary particles in an atom are entangled with each other to varying degrees, and entanglement is a central feature of quantum computing. According to the mathematics, it can happen among thousands, millions, or even 1030 particles.
Like perhaps the number of particles in a domestic cat.
This is what led to Erwin Schrödinger’s famous (infamous?) thought experiment. Put a cat in a box, with a radioactive element that has a 50% chance of decaying within a certain time period, along with a device that will detect when the decay has happened and release a poison that will kill the cat. If Schrödinger closes the box and waits the specified time, he has a 50% chance of discovering a live cat, and a 50% chance of discovering a dead one. But it’s more complicated than that.
According to his own equation, the radioactive element goes into a superposition of having decayed and not having decayed. Due to their interactions, this means the element becomes entangled with the atoms in the detector, and those atoms become entangled with the atoms in the poison, which become entangled with the atoms in the cat. In other words, according to quantum theory, the cat is in a superposition of being both alive and dead at the same time.
Following the most common interpretations of quantum mechanics in circulation when Schrödinger came up with this scenario, the cat only takes a definite state when Schrödinger opens the box and observes the result. Schrödinger’s point was that this is clearly absurd and shows that quantum theory has a serious problem.
But there’s a major assumption in Schrödinger’s scenario. Don’t feel bad if you don’t see it. Physicists missed it for decades. The assumption is that quantum physics doesn’t apply to Schrödinger himself, or observers in general, that the mathematical structure of the theory stops working with the observation.
On the face of it, this seems reasonable. We don’t observe cats in superpositions, nor scientists doing quantum measurements. But as Hugh Everett pointed out in 1957, this may be sloppy thinking. It assumes that we ourselves are not quantum systems, and fails to consider what we would see if we were.
So, according to the math, what happens when Schrödinger opens the box? The atoms of the radioactive element, detector, poison, cat, and anything else in the box become entangled with the atoms in Schrödinger’s body. In other words, Schrödinger himself goes into superposition, of seeing a live cat and of seeing a dead one. To be clear, each version of Schrödinger only sees the cat in one state, but every state is observed by a version of Schrödinger.
If Schrödinger himself is in an isolated room and calls up his friend outside, Wigner, and says, “Wigner, here are the results of the experiment,” the phone system and Wigner become entangled with the element-detector-poison-cat-Schrödinger system, with a version of Wigner hearing the cat lived, and a version hearing the cat died.
If Wigner broadcasts the results to the world, a version of the world hears about the cat having lived, and a version of the world hears about it dying. The causal effects of the cat living propagate in tandem with the causal effects of the cat dying, entangling the particles in the entire world. We now have a world where the cat lived, and a world where the cat died.
(Technically, due to electromagnetic interactions, the whole planet would have been entangled much faster, but let’s not be pedantic unless it’s relevant.)
Under this interpretation, there is never a wave function collapse, although there is the appearance of one for an observer on any particular branch of the wave function when they lose access to all the other outcomes. This happens through a process called decoherence. One side of this is interaction with the environment breaking up the wave of the system being measured. But the other side is the causal effects of that system propagating into the environment, with the environment and system becoming entangled with each other.
Is this reality? It depends on how well the wave function itself represents reality and how complete it is. As noted in the last post, I do think it represents reality at least at some level. But I think the question of its completeness remains open. I’m keeping an eye on the experiments that stress the core formalism.
Key questions: will we find evidence for a physical collapse of some type? Or for other additional variables that ensure only one outcome? Or will the universe spring something completely unexpected on us, as it’s done before? Only time will tell.
Unless of course I’m missing something?