The Decline and Collapse of the Wavefunction Empire

Quantum mechanics is frustrating for everyone—theorists who don’t understand their own equations, experimentalists that have to jump around paradoxes to try and get a measurement, and students who just have no idea what’s happening. It also has the problem of being wrong. With space and time and everything made of particles, QM believers have to accept the enigma of wavefunction collapse (when the probability of states turns into a single state). Does it collapse when we poke at it? Are we somehow controlling the fate of the universe (even if the fate involves only a single photon)?

Experimentalists have had a notoriously hard time with understanding what the hell is going on, the main example being the famous double-slit experiment where electrons act as both particles and waves. In the wave form, electrons pass through both slits to create an interference pattern on the back wall. But when physicists tried to measure exactly which slit the electron passed through the electron turned back into a particle and the interference pattern was lost. It looked like measuring changed the nature of the electron—that we changed the nature of the electron. Crazy enough, buckyballs, which are large soccer-ball shaped collections of carbon atoms, also show wave-particle duality.

Now in a recent Nature paper, Berkeley scientists (of course) have mapped the slow demise of a wavefunction, in the solid state no less. They put a circuit in a superposition (a collection of energy states, i.e. acting as a wave), shoved it in a box, and shot microwaves at it. The energy states alter the microwaves in subtle ways as they pass through the circuit—something researchers can measure without collapsing their experiment. Over microseconds (horribly slow in quantum time), the researchers measured a “collection of snapshots” as the circuit’s wavefunction collapsed. They go on to suggest this measuring-at-a-distance can be used to manipulate wavefunctions, calling it ‘quantum steering.’

Nature wrote a nice article detailing the experiments in terms of quantum mechanics. But people blew up the comments complaining that the research should have been described in quantum field theory terms.  QFT gets rid of the pesky particles and replaces them with fields (like turning a molecule of water into an entire ocean… kind of), completely negating any paradox of wave-particle duality. One commenter, Timothy Eichfeld, had a good explanation of what was happening:

It is not so much as ‘measurement’ as it is the disruption of the very fragile energy states holding the system in place. The ‘wavefunction’ does not know if a human or a frog or stray muon has interfered with its energy system – it does not have a state of being ‘watched’ or not, the issue is that it is unstable and at the top of it’s energy state. It will be knocked back to it’s lowest state depending on what vibration was absorbed by the interaction of the interference pattern on it’s energy signature.

Also in the comments is everyone pitching the book they’ve written on the subject, because, of course, that’s what comment sections on a prestigious scientific website are for.

Shut up and calculate!

Quantum mechanics is confusing. Little by little you wrap your head around the math, just going with the assumption that objects are simultaneously particles and waves, until you feel like maybe there’s something to all those wavefunctions and probabilities. Then you ask what it all means. The confusion rushes back as your professor shrugs.

In the 1920s, the founders of quantum mechanics gathered together in Copenhagen, Denmark to discuss what their math said about the physical universe. In the end, they decided there are fundamental limits to what we can know. They came up with the Copenhagen Interpretation, which according to physicist David Mermin of Cornell University means “shut up and calculate!”

Everyone accepted the Copenhagen Interpretation—if the inventors of the theory aren’t quite sure what it means, how are you supposed to know?! But a small group of scientists aren’t content with the ambiguity of quantum mechanics. With the right perspective, they argue, the meaning will become clear.

The Perimeter Institute for Theoretical Physics is hoping to find clarity. As Christopher Fuchs, one of the Institute’s researchers, says the right perspective will “write a story—literally a story, all in plain words—so compelling and so masterful in its imagery that the mathematics of quantum mechanics in all its exact technical detail will fall out as a matter of course.”

Fuchs is proposing the probabilities inherent in quantum mechanics come from the viewer, not from the object itself. He’s designed a new approach called QBism that, using a type of statistics called Bayesian inference, gets rid of the wavefunctions, amplitudes, and Hilbert-space operators—all necessary in quantum mechanics—and replaces them with simple probabilities.

All in all it’s pretty exciting. I’ve been interested in quantum mechanics and what it really means since I was old enough to realize that we, as scientists, don’t really know everything about the universe. It will be a good day when we can explain the meaning of quantum mechanics to elementary school students in an interesting and understandable story. Maybe QBism will give us that story.