Judgment day for quantum leaps
(First written July ’19 in my pop physics newsletter The Ph-word)
The biggest physics news this month came from the field that folks who are not physicists are usually most curious about: quantum mechanics. Also, it’s the field that physicists are most curious about. Because nobody can really understand it…
…For instance, one of the main pylons of quantum mechanics is the notion of discreteness. (Well, with “discrete” as in separate, not as in cheating.)
At a microscopic level, things can take only well-separated values and they pass from one state to another without going through intermediate transitions. An example is the energies that electrons can have when orbiting around nuclei: they can take only specific energies and they jump from one to the other instantly. This discreteness of states characterizes the quantum world and, honestly, how can anyone come easily to terms with nature being weird like this?
Only that what I just wrote isn’t entirely correct. This is not “one of the main pylons of quantum mechanics”; it’s one of the main pylons of the most popular interpretation of quantum mechanics. Quantum mechanics gave people such a deep insight into the reality of nature that after one century we are still fighting about what it really means, so it is approached through different interpretations. The most popular one, the “Copenhagen interpretation”, claims that particles jump between their different states instantaneously and unpredictably.
But now, in an impressive move, an experiment showed that there is actually a gradual transition between different states — only that we hadn’t looked closely enough to see it.
The experiment at Yale University used a circuit that can have different energy levels and is small enough to behave the quantum way. (Such systems have been in use in labs for some years now and are sometimes called “artificial atoms” since they mimic the energy of atoms.) What was new in this experiment is that it reached a near-perfect observation of everything that was going on in it, and that they managed to take measurements of the energy at very short time intervals (millionths of seconds).
This enabled the good people to see that the system didn’t pass “instantaneously” from one energy state to the other, but that it transitioned gradually. They were even able to prove this more firmly by reversing the transition after it started — which would be unthinkable under quantum orthodoxy.
However, don’t think that all weirdness is now gone. When saying that the system transitioned smoothly from one energy to the other, they don’t mean that it passed through the intermediate energies. They mean that it started by being in a state with energy “a”, then it became a mix of state “a” and state “b”, and eventually it moved to a clean state “b”.
Now this might sound a poco psychedelic but for orthodox quantum mechanics it’s business as usual. “Superposition” of states is another of the baffling main quantum pylons (and it is well-known from the joke about Schrodinger’s cat which is in two states at the same time; by the way, this is a joke; nothing as large as the tiniest cat can be in superposition).
But what is radical in last month’s experiment is that eventually “instantaneous leaps” might not be a part of the quantum world at all.