|Nick meets a Luck Wave (design by August O'Connor)|
I have spent more than a third of my life speculating (with congenial physicist friends) about what quantum mechanics might actually mean -- and have even written a book about it. The gist of the quantum dilemma is that we have a Quantum Theory that successfully predicts the results of every physical measurement. But with this theory comes an utter inability to tell a plausible story about what's really going on in the world -- both before, during and after a measurement. Physicists today possess an essentially perfect Quantum Theory, know how to experimentally produce subtle and delicate Quantum Facts, but cannot convincingly tell their kids a Quantum Reality story that adequately explains both Quantum Theory and Quantum Facts.
Many words concerning quantum reality were exchanged by quantum theory's founders -- especially Albert Einstein and Niels Bohr -- but not much progress was made until the remarkable discovery of Irish physicist John Stewart Bell who formulated an experimental test that could confidently eliminate an entire class of quantum reality models. Not only is it rare for physicists (or anybody else) to TALK ABOUT REALITY, it is even rarer (such was the importance of Bell's discovery) to come up with EXPERIMENTS ON REALITY. Consequently, Bell's original 1964 paper has become one of the most-cited publications in physics.
Bell's original experiment involved TWO ENTANGLED PHOTONS -- one sent to Alice and one to Bob. About this setup, quantum theory says two seemingly contradictory things: 1. that the quantum state of Bob's photon depends instantly (faster-than-light) on Alice's choice of what to measure; 2. that this apparent instantaneous action can never be used for signaling.
The physics jargon for instantaneous voodoo-like connections is the word "non-local". Non-local effects (either in theory or in practice) are as welcome in physics as a corpse at a wedding feast.
So before Bell came along, the theory of entangled systems was manifestly non-local (BAD!), but the same theory also assured that no experiment would ever be able to directly reveal this non-locality (GOOD!).
So quantum theory of entangled systems is NON-LOCAL: But all quantum facts are LOCAL.
What about quantum reality -- the underlying causal dynamics behind both theory and fact? Do we live in a world that's deep-down linked by abominable (to the physicist) non-local connections? Or is quantum reality nicely local, just like the quantum facts?
Bell's surprising conclusion (a powerful mathematical proof, not a mere conjecture) is that no local reality can underlie this everywhere local world.
Quantum reality must be non-local, according to Bell's proof.
To a physicist, Bell's conclusion is preposterous and must certainly be wrong. My first entry into the Bell's Theorem game was an attempt to disprove John Bell. Which ended in my formulating the world's shortest proof for the non-local nature of quantum reality.
Bell's Theorem is so simple that it is difficult to find a flaw. But Bell's greatest weakness can be summed up in the dichotomy: if reality exists, then it must be non-local; but if you DENY REALITY, then you are let off the hook.
But what could "denying reality" possibly mean?
One innocent (but crucial) assumption in Bell's proof is called "contrafactual definiteness" (or CFD, for short).
When you do the Bell experiment on a single pair of entangled photons, both Alice and Bob can set their detectors at only ONE SETTING. But to prove BT, you need to consider the possible results of FOUR SETTINGS. Four settings for the same two-photon event.
CFD assumes that: if we had performed three other measurements -- other than the actual one -- we would have gotten three definite (but unknown) results.
But if the nature of the quantum world is such that CFD is not valid, then you can't prove Bell's theorem. In fact, in a non-CFD world, you cannot even formulate Bell's theorem.
A recent paper by Gerold Gründler from Nürnberg, Germany, (What Does Bell's Inequality Actually Prove?) analyzes a few ways of "denying reality" by postulating (and perhaps even proving -- I am not sure) that we live in a world which does not support CFD. Gründler revisits and revises an earlier work by Israeli physicist, the late Asher Peres, entitled Unperformed Experiments Have No Results (3 page pdf).
Peres's paper suggests that we might live in a world where Unperformed Experiments Have No Results. But what are the details of such a world? How does it actually work?
What would it be like to live in a world where CFD is inconceivable?
Here's one attempt to visualize such a world.
Our thinking about the classical world is dominated by movies. It is easy to imagine rewinding the film, changing only one thing, then doing an (imaginary) retake. Certainly this cinematic model of reality allows us to at least imagine what the results of Unperformed Experiments might look like.
We can even add quantum randomness to the picture by allowing some features of the scene to depend on pure chance. Imagine filming a gambling table -- each reshoot, even of the same game, will give definite but different outcomes -- outcomes that are governed by statistical laws -- analogous to the probabilistic outcomes predicted by quantum mechanics. Bell's Theorem can be proved in a world like this -- a world of definite but statistically determined results.
To move into a non-CFD world, consider the case of a single Uranium atom. Physics considers all Uranium atoms to be EXACTLY THE SAME. Yet this one decayed in one second, while its identical sister is still alive after a million years. The first atom's short life is not due to some defect in its constitution. That's just the way the quantum world works -- identical quantum objects behave differently -- FOR NO REASON AT ALL.
Now consider how the movie analogy might work in a fully quantum world. We rewind the film, KEEP EVERYTHING THE SAME. Then change just one setting and reshoot the experiment.
Keeping everything the same is easy in a quantum world: every photon, electron, quark has always been just the same as every other. But the big problem is that each particle in the universe now behaves differently than in the first take -- giving rise to an entirely different universe -- a universe in which not only does the camera not exist, but the cameraman, the human species, and all life on Earth have vanished. In this second take on our present quantum reality the familiar Earth has no doubt disappeared as well.
As Heraclitus warned, you cannot step twice into the same river, because the river is never the same.
Might an Asher-Peres world in which Unperformed Experiments Have No Results be "hyper-Heraclitean" in the sense that in this kind of quantum reality even the same river is not the same river? In fact this river revisited second time round might not even be a river at all but the insides of a black hole.
Trying to visualize quantum worlds in which CFD is inconceivable might push us to think more deeply not only about fully quantum concepts such as "superposition", "entanglement" and "wave/particle duality" but also to revise our old-fashioned assumptions about everyday classical concepts such as "same", "different", "spontaneity" and even how to correctly use the word "again" when it comes to properly refilming in our imagination the very same event that has already happened once.
We house-broke quantum reality
Trained Schrödinger's Cat to purr
Now daily life's more uncanny
Than atoms ever were.
Asher Peres, who titled his bio "The cat who walks by himself", played a vital part in publicizing the role both he and myself played in the discovery of the quantum "no cloning rule". (How the no-cloning theorem got its name.) which eventually inspired MIT professor David Kaiser to write his popular science history book: How the Hippies Saved Physics. In addition to his pioneering work in quantum information theory, Asher Peres will probably be remembered most as a member of the team of six people who devised the remarkably clever Quantum Teleportation process.
The title of this post was taken from Gerard Manley Hopkins's splendid poem: That Nature is a Heraclitean Fire and of the comfort of the Resurrection.
|Asher Peres (bottom right) and his five buddies who invented Quantum Teleportation|