The most thought-provoking scientific meeting I went to in 2015 was Emergent Quantum Mechanics, organised in Vienna by Gerhard Groessing. This is the go-to place if you’re interested in whether quantum mechanics dooms us to a universe (or multiverse) that can be causal or local but not both, or whether we might just make sense of it after all. The big new theme was emergent global correlation. What is this, and why does it matter?
The core problem of quantum foundations is the Bell tests. In 1935, Einstein, Podolsky and Rosen noted that if you measured one of a pair of particles that shared the same quantum mechanical wave function then this would immediately affect what could be measured about the other, even if it were some distance away. Einstein held that this “spooky action at a distance” was ridiculous, so quantum mechanics must be incomplete. This was the most cited paper in physics for decades. In 1964 the Irish physicist John Bell proved that if particle behavior were explained by hidden local variables, their effects would have to satisfy an inequality that would be broken in some circumstances by quantum mechanical behavior. In 1974, Clauser, Horne, Shimony, and Holt proved a related theorem that limits the correlation between the polarization of two photons, assuming that this polarization is carried entirely by and within them. Freedman and Clauser showed this was violated experimentally, followed by Aspect, Zeilinger and many others. These “Bell tests” convince many physicists that reality must be weird; maybe non-local, non-causal, or even involving multiple universes.
For example, it’s possible to entangle photon A with photon B, then B with C, then C with D, and measure that A and D are correlated, despite the fact that they didn’t exist at the same time. Does this mean that when I measure D, some mysterious influence reaches backward in time to A? The math doesn’t let me use this to send a message backwards in time to order the murder of my great-grandfather (the no-signalling theorem becomes a kind of “no-tardis theorem”) but such experiments are still startlingly counterintuitive.
At EMQM15, a number of people advanced models according to which quantum phenomena emerge from a combination of local action and global correlation. As the Nobel prizewinner Gerard ‘t Hooft put it in his keynote talk, John Bell assumed that spacelike correlations are insignificant, and this isn’t necessarily so. In Gerard’s model, reality is information, processed by a cellular automaton fabric operating at the Planck scale, and fundamental particles are virtual particles—like Conway’s gliders but in three dimensions. In a version he presented at the previous EMQM event in 2013, the fabric is regular and its existence many break gauge invariance just enough to provide the needed long-range correlation. The problem was that the Lorentz group is open, which seemed to prevent the variables in the automata being bitstrings of finite length. In his new version, the automata are randomly distributed. This was inspired by an idea of Steven Hawking’s on balancing the information flows into and out of black holes.
In a second class of emergence models, the long-range order comes from an underlying thermodynamics. Gerhard Groessing has a model in which long-range order emerges from subquantum statistical physics; Ariel Caticha has a model with a similar flavor, which derives quantum mechanics as entropic dynamics. Ana Maria Cetto looks to the zero-point field and sets out to characterise active zero-point field modes that sustain entangled states. Bei-Lok Hu adds a stochastic term to semiclassical gravity, whose effect after renormalisation is nonlocal dissipation with colored noise.
There are others. The quantum crypto pioneer Nicolas Gisin has a new book on quantum chance in which he suggests that the solution might be nonlocal randomness: a random event that can manifest itself at several locations. My own suspicion is that it might be something less colorful; perhaps the quantum vacuum just has an order parameter, like a normal superfluid or superconductor. If you want long-range order that interacts with quantum systems, we have quite a few examples and analogues to play with.
But whether you think the quantum vacuum is God’s computer, God’s bubble bath, or even God’s cryptographic keystream generator, there’s suddenly a sense this year of excitement and progress, of ideas coming together, of the prospect that we might just possibly be able to replace magic with mechanism.
There may be a precedent. For about forty years after Galileo, physics was a free-for-all. The old Ptolemaic certainties had been shot away and philosophers’ imaginations ran wild. Perhaps it would be possible, some said, to fly to America in eight hours in a basket carried by swans? Eventually, Newton wrote the Principia and spoiled all the fun. Theoretical physics has been stuck for the past forty years, and imaginations have been running wild once more. Multiple universes that let stuff travel backwards in time without causing a paradox? Or perhaps it’s time for something new to come along and spoil the fun.