Friday, January 20, 2012

Bell's Bizarre Theorem

The discoveries of quantum physics are about to become even stranger.

For many decades, physicists have known that if you prepare two particles in a particular way such that they are “entangled,” then even if you subsequently separate those particles by many miles, they can seem to influence each other. Measuring one particle will result in the other particle having a predictable value. It’s as if you put two coins in a box, shook that box, and then cut the box in half and drove them to opposite parts of the state. Our intuition tells us the coins should be either heads or tails independently of each other. But, if these coins acted like entangled particles, then determining that one coin was “heads” would guarantee that the other one was “tails,” or vice versa. It’s just another routine example of quantum weirdness; it was predicted from theory, and it’s been experimentally confirmed many times since.

Physicists are divided on the explanation for the entanglement phenomenon. There’s the possibility that particles simply carry information with them that determines their properties, these properties always being opposite in entangled particles. Two “entangled coins,” as it were, are always heads/tails, never heads/heads or tails/tails. The problem with this explanation is that a particle would have to possess an inordinate (perhaps infinite) amount of information for this to work. Physicists can measure various properties along any number of coordinate axes — and with entangled particles, they always come up opposite. Every single time. How can an electron carry with it all of this information? The consensus is that it can’t, at least not in such simple terms.

Another possible explanation is that the entangled particles are able to communicate with each other somehow. As you’re measuring one particle, it “tells” its entangled twin what property is being measured and what the result is. The problem here is that this communication would have to exceed the speed of light — by a factor of at least 10,000 according to one experiment — and Einstein showed that this isn’t possible, at least, not in our familiar space and time. So, some physicists have speculated that there’s an undiscovered “communication backchannel” that allows two far-separated particles to talk to each other.

In the 1960s, the Irish physicist John Bell showed that one of the above two scenarios must be false. Bell’s theorem mathematically proves that either (1) a particle does not intrinsically carry with it a specific value of a specific property, meaning that it must violate an assumed principle known as counterfactual definiteness, or (2) information must be able to travel faster than light — one object can exert an influence that jumps across space to affect another object directly. This is what Einstein called “spooky action at a distance,” the violation of the assumed principle known as locality. Basically, Bell’s theorem shows that there may be counterfactual definiteness, or there may be locality, but not both. One of the assumed principles must be wrong.

Just last week, an experiment was announced that may end this debate once and for all. Researchers will be able to put “nonlocality,” the jumping-across-space of information, to a definitive test. If it fails — and many (including myself) expect it to — it will be another setback for the realists, that faction of quantum physicists who believe that every fundamental particle has specific information encoded into it, as if predetermined by God on the day of Creation, which we human observers (and our instruments) can only passively discover, like looking at a coin in a box.

So what would such a result mean? Since it would prove that entangled particles cannot be communicating with each other, it would bizarrely suggest that properties of particles “pop into existence” when they are measured. Like a microscopic Schrödinger’s cat, a particle could possess the property of intrinsic spin in two different directions at once, but when this property is measured, nature somehow “selects” one of them. And literally at the same time, nature also “selects” the spin of its entangled twin to be in the other direction, which becomes clear when we actually do the measurement. (A YouTube video that I worked on discusses one way to imagine this.)

But if measured properties don’t pre-exist in a particle, and separated objects aren’t communicating nonlocally (i.e., faster than light), there’s still the question of how an entangled twin “knows” what value to take upon measurement. Maybe it’s because the entangled objects aren’t actually separated. As discussed in the recent episode of PBS’s Nova called “What Is Space?” there’s significant debate on what spatial separation really means. Experiments suggest that we may need to think of space not as a fundamental feature of the world, but rather as a phenomenon that emerges from a deeper process.

To me, quantum entanglement makes more sense when you resist the temptation to think of twin particles as being “different” objects, one “here” and one “there.” Instead, they’re opposite versions of a single object of some kind, two sides of the same coin, if you will. We don’t detect this thing directly, and it isn’t located either here or there particularly,* but whatever “it” is, it makes its appearance to us as two separate particles with some mirror-image properties.

The separate-ness between these particles is definitely something we perceive in the world. However, it may be a feature of our world — not the particle pair’s world. Which is about as profound as it gets.

* For example, the holographic principle of string theory and quantum gravity suggests that the entangled twins are encoded as information on a two-dimensional surface, and do not fundamentally reside in three-dimensional space at all.