sometimes the same, but sometimes different, than those asked by Alice.
“Do you like steak?” might well be the first of Bob’s questions— but it also might be “do you like fish?”
As before, the husband and wife are intent on disagreeing with each
other (not always now, but by a certain definite amount). The problem is
that they don’t know how to do it. After all, neither one of them knows the reply the other has given. They don’t even know what question the other
has answered! So how can they synchronize their replies?
Here’s a way. They can phone one another.
That is the loophole. If husband and wife could tell each other what
the questions had been, and what their replies had been, they could synchronize those replies. Some sort of “telephone connection” between them would accomplish this. There is nothing in all of physics that explains
just how this connection might work. Of course it’s not a matter of actual phone calls from one quantum particle to another: there’s no such thing. It would have to be something else: something that has never been thought
of before. But so what? Maybe it’s possible after all.
That loophole is a vulnerability in Clauser’s experiment. So his conclusion was open to attack. Perhaps quantum mechanics was not the right theory after all. Perhaps hidden variables actually did exist.
But several years after Clauser’s experiment, a French physicist named
Alain Aspect found a way to close that loophole. He did this by blocking the phone calls. He rendered those telephones— if they existed at all— irrelevant.
Remarkably Aspect managed to do this even though he knew very little about how those hypothetical phones might possibly work. Of course, that’s a hard job. Normally, in order to defend against an attack, you had better know something about the nature of that attack. How can you defend
against an unknown enemy?
Aspect took advantage of the fact that he did know one little thing about
that enemy: it could not travel faster than light. The signals from one quantum particle to the other, whatever they might possibly be, had to obey that cosmic speed limit.
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Figure 12.2
Alain Aspect. Clauser’s experiment had a potential loophole: that somehow the two entangled particles could communicate with one another. Aspect closed that loophole by randomly changing the “questions” asked of them.
The principle that no signal can travel faster than light is enshrined in
physics. It has been experimentally tested over and over again, and always found valid. Not even the weird quantum world can violate it. Aspect found a way to use this principle in his experiment. He created a situation in which the husband and wife would set forth on their journeys, one to Oregon and
one to Florida— perhaps carrying telephones of some unknown design, and
perhaps talking with one another as they traveled. But because those phone signals were not traveling with infinite velocity— the speed of light is great, but not infinite— there would be a tiny interval of time the transmissions
Experimental … 81
took to travel between husband and wife. And in that tiny interval, Alice and Bob would change their questions.
In such a situation, the husband and wife would find themselves forced
to answer a question before they could exchange information. Even had
they possessed telephones, the transmissions would arrive too late. As a
consequence, each reply would be given in a state of total ignorance. Aspect would have rendered those hypothetical telephones irrelevant.
He built the experiment. Then he ran it. He found that the new twist
made no difference. His data showed that, astonishingly, husband and wife
still managed to synchronize their responses. They disagreed more often
than could be accounted for. They persisted in doing the impossible.
I will say it again: they were doing something for which there is no possible explanation.
So Aspect closed the loophole. Unfortunately, however, there is not just one loophole. There are many.
Here’s another— and this is one that was closed by one of the prettiest
experiments I have seen in years. It is known as the “freedom of choice
loophole” and it proposes that Alice and Bob may think they have free will … but actually they don’t.
What does freedom of choice have to do with hidden variables? In
chapter 9’s discussion of a hidden variable theory, I showed that if Bob
rotates his analyzer, a certain fraction of the detections will show disagreements with Alice’s result. But quantum theory predicts more disagreements than that, and Clauser’s experiment confirmed the quantum prediction. In chapter 9, I tried to alter the hidden variable theory by
having the source avoid emitting particles in a certain direction (into the wedge of figure 9.6) in order to mimic quantum theory. But, as I wrote,
this attempt at a fix would not work since Alice and Bob have freedom
of choice. They are free to act in any way they wish. They might elect to
turn their analyzers not to the right but to the left— or not by this angle but by that. Since there is no way to adjust the source in advance to deal with every possible choice, the conclusion was that the hidden variable
idea is not going to work.
But maybe that conclusion is not so certain. For suppose that Alice and
Bob are not really free to turn their analyzers in just any old which way.
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Suppose their much vaunted free will is actually an illusion. Suppose that yesterday they had been hypnotized, and today they are under the sway
of a posthypnotic suggestion forcing them to swing their analyzers only in certain ways … and suppose that the source knows about these ways.
I’m speaking metaphorically, of course. In real experiments sources don’t
“know” anything. And the analyzers’ orientations are not chosen by
people: they are chosen by machines, components of the experimental apparatus. Experimenters try to make sure that these machines behave randomly.
But what if they are not fully successful? What if their so called “random machines” are not really random? What if those machines are actually
being controlled by some process of which the experimenters are entirely
unaware— a process that connects both the source and the analyzers, and
that deceives us into believing in quantum mechanics?
Just like Aspect’s “phone calls,” there is nothing in all of physics that tells us which this controlling process might be. But, again like Aspect’s situation— so what? Maybe it is possible after all. That would be a loophole too.
The freedom of choice experiment did not entirely close this loophole.
But it did restrict it— dramatically. It showed that this hypothetical controlling influence must not operate in the here and now. Rather it operated centuries ago, and it came from a location thousands of trillions of miles away. The experiment grabbed hold of that control, and it shoved it far off into the depths of time and cosmos.
Anton Zeilinger, a burly, affable man with an infectious sense of humor
and a love of life, is a worldwide leader in work on quantum entanglement. Throughout his career he has conducted numerous groundbreaking experiments probing the many astonishments of quantum mechanics. In
Zeilinger’s lab in downtown Vienna, a source emitted entangled pairs of particles. (Like Aspect’s, the actual experiment worked with photons instead of electrons, and it measured their polarizations instead of spins.) A third of a mile away was a bank. We can call it “Alice’s bank” if we wish. One evening a group of physicists invaded that bank. But they were not there to steal
.
Rather, they were there to assemble two sets of scientific equipment, peering out of two different windows.
One of those windows had a good view of Zeilinger’s lab. Through the
window peered a device capable of revealing the polarization of an incoming photon, a photon shot out from his lab. Out a different window, one facing
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Figure 12.3
Anton Zeilinger. Clauser’s experiment had another potential loophole: that the
“questions” asked of the two particles only seemed random, but were in fact being dictated by some unknown mechanism. Zeilinger’s experiment showed that this mechanism, if it existed at all, lay far off in the universe and operated far back in the past. Photo courtesy of the Mind & Life Institute, © The Mind & Life Institute.
in the opposite direction, peered a telescope. It was peering, not at a lab, not even at any earthbound building, but up into the sky. It was gazing at a star.
That telescope was not one of those mighty instruments so beloved of
astronomers, perched on mountaintops or orbiting the earth, but rather the sort of small, unassuming device that an amateur astronomer might own.
Indeed, the telescope was not the hard part of the experiment. The hard
part was what it was connected to— and this was the sort of stuff no amateur
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could afford. Part of that stuff was an instrument that observed individual photons of the light from the star. At any instant a star— or any other source of light— is emitting a vast flood of photons, all of different colors.
The experiment’s equipment was set to grab those photons one by one …
and measure the color of each. Was it more nearly red, or more nearly blue?
Information about that color was routed across the bank to the analyzer
trained on Zeilinger’s lab. And there it entered the most extraordinary of devices: a device that set the orientation of that analyzer according to the information from the telescope— and that was capable of changing its orientation in a millionth of a second. And that was the key element of the experiment: the orientation of the analyzer was set by the color of the starlight.
All this equipment peering out of the windows in the bank constituted
our “Alice.” As for “Bob,” he was located in a different building— a mile away, on the far side of Zeilinger’s lab and its source of entangled photons. In Bob’s building stood similar equipment, with the analyzer catching the second
member of the entangled pair, and with the telescope pointing to a second
star, whose photon would determine the orientation of Bob’s analyzer.
In this way, the group had devised a setup in which the choice of orientations of the analyzers— the questions to ask of the husband and wife— was determined not by the choice of the experimenters, not by the action of some piece of equipment situated in the lab, but rather by infinitesimal bits of light from two different stars as they twinkled in the evening sky over Vienna.
They ran the experiment. It got results in agreement with quantum theory and opposed to the hidden variable theory.
If we are talking about a loophole involving free will, the “will” we are
talking about is that of those two stars. It was they that were directing the experiment, directing by means of photons launched centuries ago. One of
the stars was 600 light years away, which amounts to thousands of trillions of miles. And the light it emitted had been sent forth on its journey toward Vienna 600 years in the past. The other star was more distant still.
Could our hypothetical “preordaining influence”— our hypnotist— have
intervened in the experiment to invalidate its results? It could have done so only by controlling those bits of starlight. That is to say, only by intervening not in Vienna, but far off in the Milky Way. And it was not even intervening now: it had done so centuries ago … at a time, in the words of one of those experimenters, “back when Joan of Arc’s friends still called her Joanie.”17
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And just like Aspect with his experiment, the freedom of choice group
found that their new twist made no difference. Their data showed that, astonishingly, husband and wife still managed to synchronize their responses.
They disagreed more often than could be accounted for. They persisted in
doing the impossible.
Not too many years ago a graduate student had a wonderful idea. He
decided to invent a game. A metaphysical game.
The student’s name was Carlos Abellán. He worked in a research group
led by Morgan Mitchell, based in Barcelona. For years the two of them had
been batting around the whole idea of randomness.
Figure 12.4
Carlos Abellán (left) and Morgan Mitchell (right). Photo: ICFO.
Are the “questions” asked of the two particles really random? All previous experiments had relied on some physical mechanism to achieve randomness— but mechanisms obey the laws of classical physics, and so are not truly random. In the “the BIG Bell Test experiment” vast numbers of people were enlisted to use their free will to create randomness.
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Figure 12.5
The app they created. Image: Maria Pascual (Kaitos Games).
Randomness is a key element of any experiment aiming to test Bell’s Theorem. It is the only way to ensure that our angry couple, intent on disagreeing with one another, have no way of knowing the questions they are about to be asked. And up to that point all the various Bell test experiments had achieved this randomness by mechanical means. They used marvelous and
elaborate mechanisms that were designed to behave unpredictably as they
dictated the orientation of the analyzers. Even the distant stars in the experiment I have just described were at heart mechanisms— the fact they were natural rather than artificial was irrelevant. But Abellán and Mitchell found themselves wondering: are any mechanisms truly random? Or do they only seem to be?
My smartphone tells me that apps exist that behave randomly. Indeed,
I can buy chance. Just now I checked the App Store, and there I found
all sorts of random number generators. I could download any one of them:
each time I asked, it would give me some unpredictable number.
But are these numbers really unpredictable? No, they are not. We tend
to forget that all the marvelous stuff we find on the Web rests in the last
Experimental … 87
analysis on actual, physical machines. We say that those random number
generators reside “in cyberspace”— but cyberspace is not a real thing. It is a term we use for radio signals traveling this way and that through an elaborate network connecting our smartphones with servers— and each of these servers is a computer, an actual, physical device composed of actual physical parts. Somewhere, deep in the guts of a rack of electronics in some faraway server farm, tiny electrical currents flow this way instead of that, and tiny magnets are orientated one way instead of another … and the working of
this immense collection of electromagnetic parts is the working of cyberspace. If I knew exactly the physical configuration of that server I would find that my vaunted random number generator was only an apparently
random number generator.
Maybe this will become more evident if we consider the act of flipping
a coin. It is the quintessential example of randomness. Can I predict how
that coin will land? Of course not. But is it random? No, it is not.
Suppose I knew exactly how high I had tossed that coin. Then I would be
able to predict how long it would take before landing. And suppose I knew
exactly how much spin my thumb had imparted to it. Then I would know
how rapidly i
t was rotating during its flight, and how many times it had
spun over in that interval of time. And if I knew how hard it landed, and at what angle it had struck the table when it did so, then I could predict how high it would bounce and how many times it would flip over before finally
coming to rest. And if I knew whether that coin was showing heads or tails just before I flipped it … why then, if I knew all these things, I would have been able to predict what that flipped coin would show when it landed. And make a million dollars.
For in truth, a flipped coin does not exhibit randomness. Neither does
my so called random number generator. What they exhibit is complexity.
You might be objecting that my “research projects” into the flight of the
coin or the workings of my app are not something that I could carry out
in practice. I agree— but so what? We are not speaking of “in practice.” We are speaking of “in principle,” and the principle is one of absolute determinism: everything that happens in the large scale world is dictated by the inflexible law of cause and effect. And if it is dictated then it is in principle foreseeable … and in that case what our mythical angry couple is doing
may not be so very mysterious after all.
That’s another loophole.
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Abellán and Mitchell wanted to nail that loophole shut. They wanted
to achieve something no machine could do, and achieve true randomness.
They asked themselves: what were the most erratic things in the universe?
People were, they decided.
You and me. The butcher and the baker and the candlestick maker and
everybody else too. All of humanity, in the messiness and unpredictability of free will. Never mind those coin flips and servers and distant stars: Abellán and Mitchell would assemble a team to build an experiment in which it was not a physical mechanism that chose how to rotate the analyzers to
and fro. It would be people— ordinary people, people from every walk of
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