It’s time we thought again about quantum theory. There’s nothing actually wrong with the theory itself—it works fantastically well for understanding how atoms and subatomic particles behave.
The problem is how we talk about quantum theory. We keep insisting that it
It’s time we thought again about quantum theory. There’s nothing actually wrong with the theory itself—it works fantastically well for understanding how atoms and subatomic particles behave.
The problem is how we talk about quantum theory. We keep insisting that it’s weird: waves becoming particles, things being in two places (or two states) at once, spooky action at a distance, that sort of thing. Isn’t it perverse to clothe in mystery a theory that scientists use routinely to understand the world?
Part of the issue is that everyday objects are discrete, localized, and unambiguous, and so, very different to quantum objects. But why is that the case? Why is our everyday world always “this or that” and never “this and that”? Why, as things get bigger, does quantum physics turn into classical physics, governed by laws like those that Isaac Newton wrote down over three centuries ago?
This switch is called the quantum-classical transition, and it has puzzled scientists for many decades. We still don’t completely understand it. But over the past two or so decades, new experimental techniques have pushed the transition to ever-larger sizes. Most scientists agree that technical difficulties will prevent us from ever putting a basketball, or even a human, in two places at once. But an emerging understanding of the quantum-classical transition also suggests that there is nothing in principle that prohibits it—no cosmic censorship separates our “normal” world from the “weird” world that lurks beneath it. In other words, the quantum world may not be so weird after all.
Imagine a broken drying machine that spits out pairs of unmatched socks. They come in complementary contrasts: if one is red, the other is green. Or, if one is white, the other is black, and so on. We don’t know which of these options we’ll get until we look—but we do know that if we find one is red, we can be sure the other is green. Whatever the actual colors are, they are correlated with one another.
Now imagine the quantum mechanical version of this same machine. According to the Copenhagen interpretation of quantum mechanics developed in the mid-1920s by Niels Bohr, Werner Heisenberg, and collaborators, quantum socks in a correlated state (where the color of one is linked to the color of the other) don’t actually have any fixed colors until we look. The very act of looking at one quantum sock determines the color of the other. If we look in one way, the first sock might be red (and the other therefore green). If we look in another, the first is white (and the other black).
Crudely, you could say that in these correlated pairs the colors of the socks are characteristics that extend well beyond the socks themselves. The color of a given sock is not local, that is, not contained in the properties of just that one sock. The two colors are said to be entangled with each other.
Decoherence bleeds away discord. Quantum phenomena are converted to ones that obey classical rules.
The physicist Erwin Schrödinger described entanglement as the key to quantum behavior, and used it to construct a famous paradox. It begins with an unfortunate cat that Schrödinger imagined trapped inside a box, into which a lethal poison was released by the outcome of some quantum event. Because the event was quantum, it could be in what physicists call a superposition state: both triggering the poison release, and not triggering it.
These superpositions are not unusual for tiny objects like atoms that are firmly in the quantum realm. But, because Schrödinger entangled the event with a large cat, the result is the paradoxical conclusion that the cat is both killed and not killed.
The conventional resolution to the paradox was to claim that making a measurement on a superposition state, like the live–dead cat, forces a choice, so that the superposition collapses the cat—indeed, in effect the whole universe—into one state or another: The cat is either dead or alive, but not both. In that view, we can never see the live–dead cat.
But what was the state of the cat before we looked? According to the Copenhagen interpretation, the question has no meaning. Reality, it maintains, is what we can observe and measure, and it makes no sense to wonder about what things are really like before we make those observations.
Others, most prominently Albert Einstein, couldn’t accept this. They stuck with the classical “realist” view, which says that everything has particular, objective properties, whether we look or not. Einstein and two young colleagues, Boris Podolsky and Nathan Rosen, came up with a version of the “quantum drying machine” thought experiment to try to demonstrate how quantum theory led to a paradox, in which a measurement in one place instantly affected an object in another place. But in the 1980s, measurements of laser photons showed that entanglement really does work that way—not because of “faster-than-light” communication, but because quantum properties can be genuinely non-local, spread over more than one particle.
Since then, experimentalists have been working on building ever-larger quantum objects, which are big compared with atoms but small compared with real cats. They are often called “Schrödinger’s kittens,” and they are rapidly growing up.
One key to these kittens becoming cats has b