Leifer’s main target in his talk was the Copenhagen interpretation of quantum mechanics, which he says most physicists subscribe to (though there were doubts expressed about that in the room). I’m wary of attempting to define what it is because a large part of Leifer’s argument is that there is no consistent definition. But it’s the interpretation attributed to a bunch of quantum theory’s founding fathers — and the one that physicists are often taught at school. It says that before you look, a quantum object is described by a wavefunction that encompasses a number of possibilities (a particle being here and there, a cat being dead and alive), and that when you look, this collapses into definiteness. Schrödinger’s equation allows you to calculate the probability of the outcome of a quantum experiment, but you can’t really know, and probably shouldn’t even worry about, what’s happening before you look.Leifer is one of the more sensible quantum theorists, and it is nice to see him acknowledge that Copenhagen is the dominant interpretation.
On top of that, Leifer argues that Copenhagen-like interpretations, rather than being the most sensible option (as is often claimed), are actually just as whacky as, for instance, the Many World’s Interpretation.
He hates Copenhagen, and defines it this way:
Observers observe. Universality - everything can be described by quantum mechanics. ... No deeper description of reality to be had. ... Quantum systems may very well have properties, but they are just ineffable. For some reason, whatever properties they have, they're fundamentally impossible to represent those properties in language, mathematics, physics, pictures, whatever you like.He objects to this philosophically, but admits that it is a reasonable position.
The real problem is his "universality" assumption. To him, this means that a time-reversible Schroedinger equation applies to everything, and it enables an observer to reverse an experiment. He goes on to describe a paradox resulting from this reversibility.
I don't remember Bohr or anyone else saying that observers can reverse experiments.
Time reversibility is a bizarre philosophical belief, as I discussed recently. The reasons for believing in it do not have much to do with quantum mechanics. Much of physics, including quantum mechanics, statistical mechanics, and thermodynamics, teaches that time is not reversible.
Leifer claims to have an argument that Copenhagen is strange, but he really has an argument that time reversibility is strange.
His real problem is that he rejects positivism. To the positivist, a system having ineffable problems is completely acceptable. I do not expect to understand subatomic physics by relating properties to ordinary human experiences of the 5 senses. I think it would be bizarre if an atom had a wave function that perfectly represented reality. Get over it. That is not even what science is all about.
On the subject of time symmetry, Quanta mag article:
“That signifies nothing. For us believing physicists, the distinction between past, present and future is only a stubbornly persistent illusion.”
Einstein’s statement was not merely an attempt at consolation. Many physicists argue that Einstein’s position is implied by the two pillars of modern physics: Einstein’s masterpiece, the general theory of relativity, and the Standard Model of particle physics. The laws that underlie these theories are time-symmetric — that is, the physics they describe is the same, regardless of whether the variable called “time” increases or decreases. Moreover, they say nothing at all about the point we call “now” — a special moment (or so it appears) for us, but seemingly undefined when we talk about the universe at large. The resulting timeless cosmos is sometimes called a “block universe” — a static block of space-time in which any flow of time, or passage through it, must presumably be a mental construct or other illusion.