Quantum Theory Cannot Hurt You (6 page)

The fact that the laws of nature permit something to come out of nothing has not escaped cosmologists, people who think about the origin of the Universe. Could it be, they wonder, that the entire Universe is nothing more than a quantum fluctuation of the vacuum? It’s an extraordinary thought.

A coin is spinning. The coin is in a strong box sitting in the mud at the
bottom of the deepest ocean trench. Don’t ask what has set the coin spinning
or what is keeping it spinning. This isn’t a well-thought-out story!
The point is that there is an identical spinning coin in an identical box
sitting on a cold moon in a distant galaxy on the far side of the Universe.

The first coin comes down heads. Instantaneously, without the merest
split-second of delay, its cousin 10 billion light-years from Earth
comes down tails.

The coin on Earth could equally well have come down tails and its distant cousin heads. This is not important. The significant thing is that the coin on the far side of the Universe
knows
instantly the state of its distant terrestrial cousin—and does the opposite.

But how can it possibly know? The cosmic speed limit in our Universe is the speed of light.
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Since the coins are separated by 10 billion light-years, information about the state of one coin must take
a minimum of 10 billion years to reach the other. Yet they know about each other in a split second.

This kind of “spooky action at a distance” turns out to be one of the most remarkable features of the microscopic world. It so upset Einstein that he declared that quantum theory must be wrong. In fact, Einstein was wrong.

In the past 20 years, physicists have observed the behaviour of coins that are separated by large distances. The coins are quantum coins, and the distances are not of course as large as the width of the Universe.
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Nevertheless, the experiments have successfully demonstrated that atoms and their kin can indeed communicate instantaneously, in total violation of the speed-of-light barrier.

Physicists have christened this weird kind of quantum telepathy nonlocality. The best way to understand it is by considering a peculiar particle property called spin.

Spin is unique to the microscopic world. Particles that possess spin behave as if they are rotating like tiny spinning tops. Only they aren’t actually spinning! Once again, we come up against the fundamental ungraspability of the microscopic world. The spin of particles, like their inherent unpredictability, is something with no direct analogue in the everyday world. Microscopic particles can have different amounts of spin. The electron happens to carry the minimum quantity.
This permits it to spin in two possible ways. Think of it as spinning either clockwise or anticlockwise (although of course it isn’t actually spinning at all!).

If two electrons are created together—the first with clockwise spin, the second with anticlockwise spin—their spins cancel. Physicists say their total spin is zero. Of course, the pair of electrons can also have a total spin of zero if the first electron has an anticlockwise spin and the second a clockwise spin.

Now, there is a law of nature that says the total spin of such a system can never change. (It’s actually called the law of conservation of angular momentum.) So once the pair of electrons has been created with a total spin of zero, the pair’s spin must remain zero as long as the pair remains in existence.

Nothing out of the ordinary here. However, there is another way to create two electrons with a total spin of zero. Recall that, if two states of a microscopic system are possible, then a superposition of the two is also possible. This means it is possible to create a pair of electrons that are simultaneously clockwise-anticlockwise and anticlockwise-clockwise.

So what? Well, remember that such a superposition can exist only as long as the pair of electrons is isolated from its environment. The moment the outside world interacts with it—and that interaction could be someone checking to see what the electrons are doing—the superposition undergoes decoherence and is destroyed. Unable to exist any longer in their schizophrenic state, the electrons plump for being either clockwise-anticlockwise or anticlockwise-clockwise.

Still nothing out of the ordinary (at least for the microscopic world!). However, imagine that, after the electrons are created in their schizophrenic state, they remain isolated and nobody looks at them. Instead, one electron is taken away in a box to a faraway place. Only then does someone finally open the box and observe the spin of the electron.

If the electron at the faraway place turns out to have a clockwise spin, then instantaneously the other electron must stop being in its
schizophrenic state and assume an anticlockwise spin. The total spin, after all, must always remain zero. If, on the other hand, the electron turns out to be spinning anticlockwise, its cousin must instantaneously assume a clockwise spin.

It does not matter if one electron is in a steel box half-buried on the seafloor and the other is in a box on the far side of the Universe. One electron will respond instantaneously to the other’s state. This is not merely some esoteric theory. Instantaneous influence has actually been observed in the laboratory.

In 1982, Alain Aspect and his colleagues at the University of Paris South created pairs of photons and sent members of each pair to detectors separated by a distance of 13 metres. The detectors measured the polarisation of the photons, a property related to their spin. Aspect’s team showed that measuring the polarisation of photons at one detector affected the polarisation measured at the other detector. The influence that travelled between the detectors did so in less than 10 nanoseconds. Crucially, this was a quarter of the time a light beam would have taken to bridge the 13-metre gap.

At the bare minimum, some kind of influence travelled between the detectors at four times the speed of light. If the technology had made it possible to measure an even smaller time interval, Aspect could have shown the ghostly influence to be even faster. Quantum theory was right. And Einstein—bless him—was wrong.

Nonlocality could never happen in the ordinary, nonquantum world. An air mass might split into two tornadoes, one spinning clockwise and the other anticlockwise. But that’s the way they would stay—spinning in opposite directions—until finally they both ran out of steam. The crucial difference in the microscopic, quantum world is that the spins of particles are undetermined until the instant they are observed. And, before the spin of one electron in the pair is observed, it is totally unpredictable. It has a 50 per cent chance of being clockwise and a 50 per cent chance of being anticlockwise (once again we come up against the naked randomness of the microworld). But even though there is no way of knowing the spin of one electron until it is
observed, the spin of the other electron must settle down to being opposite instantaneously—no matter how far away the other particle happens to be.

At the heart of nonlocality is the tendency of particles that interact with each other to become entwined, or “entangled”, so that the properties of one are forever dependent on the properties of the other. In the case of the pair of electrons, it is their spins that become dependent on each other. In a very real sense, entangled particles cease to have a separate existence. Like a much-in-love couple, they become a weird joined-at-the-hip entity. No matter how far apart they are pulled, they remain forever connected.

The weirdest manifestation of entanglement is, without doubt, nonlocality. In fact, it would seem that if we could harness it we could create an instantaneous communications system. With it we could phone the other side of the world with no time delay. In fact, we could phone the other side of the Universe with no time delay! No longer would we need to be inconvenienced by the pesky speed-of-light barrier.

Frustratingly, however, nonlocality cannot be harnessed to create an instantaneous communications system. Attempts to use the spin of particles to send a message across large distances might use one direction of spin to code for a “0” and the other for a “1.” However, to know that you were sending a “0” or a “1,” you would have to check the spin of the particle. But checking kills the superposition, which is essential for the instantaneous effect. If you sent a message without first looking, you could be only 50 per cent sure of sending a “1,” a level of uncertainty that effectively scrambles any meaningful message.

So although instantaneous influence is a fundamental feature of our Universe, it turns out that nature does exactly what is required to make it unusable for sending real information. This is how it permits
the speed-of-light barrier to be broken without actually being broken. What nature gives with one hand it cruelly takes away with the other.

Arguably, the sexiest potential use of entanglement involves taking an object and sending a complete description of the object to a faraway place so that a suitably clever machine at the other end can construct a perfect copy. This is of course the recipe for the
Star Trek
transporter, which routinely “beamed” crew members back and forth between planet and ship.

The technology to reconstruct a solid object merely from the information describing it is of course way beyond our current technological capabilities. But, actually, the idea of creating a perfect copy of an object at a remote location founders on something much more basic than this. According to the Heisenberg uncertainty principle, it is impossible to perfectly describe an object—the positions of all its atoms, the electrons in each of those atoms, and so on. Without such knowledge, however, how can an exact copy ever be assembled?

Entanglement, remarkably, offers a way out. The reason is that entangled particles behave like a single indivisible entity. At some level, they
know
each other’s deepest secrets.

Say we have a particle, P, and we want to make a perfect copy, P*. It stands to reason that in order to do this it is necessary to know P’s properties. However, according to the Heisenberg uncertainty principle, if we measure one particular property of P precisely—say its location—we inevitably lose all knowledge of some other property—in this case, its velocity. Nevertheless, this crippling limitation can be circumvented by an ingenious use of entanglement.

Take another particle, A, which is similar to both P and P*. The important thing is that A and P* are an entangled pair. Now, entangle A with P and make a measurement of the pair together. This will tell us about some property of P. According to the Heisenberg uncertainty
principle, however, the measurement will inevitably involve us losing knowledge of some other property of P.

But all is not lost. Because P* was entangled with A, it retains knowledge about A. And because A was entangled with P, it retains knowledge about P. This means that P*, though it has never been in touch with P, nevertheless knows its secrets. Furthermore, when the measurement was made on A and P together and information about some property of P seemed to be lost, instantaneously it became available to A’s partner, P*. This is the miracle of entanglement.

Since we already know about the other properties of P, obtained from A, we now have all we need to make sure P* has
exactly
the attributes of P.
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Thus we have exploited entanglement to circumvent the restrictions of the Heisenberg uncertainty principle.

The amazing thing is that, although we have exploited entanglement to make a particle P* with the exact properties of P, at no time did we ever possess any information about the missing property of P! It was transmitted out of our sight through the ghostly connections of entanglement.
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Calling this scheme teleportation is a bit of a cheeky exaggeration since it solves only one of the many problems in making a
Star
Trek
transporter. The researchers of course knew this. But they also knew a thing or two about how to grab newspaper headlines!

As it happens, the Achilles’ heel of the
Star Trek
transporter turns out to be neither pinning down the position, and so on, of every atom
in a person’s body nor assembling a copy of the person from that information. It’s actually
transmitting
the sheer volume of information needed to describe a person across space. Billions of times more information is needed than for the reconstruction of a two-dimensional TV image. The obvious way to send the information is as a series of binary “bits”—dots and dashes. If the information is to be sent in a reasonable time, the pulses must obviously be short. But ultrashort pulses are possible only with ultrahigh-energy light. As science fiction writer Arthur C. Clarke has pointed out, beaming up Captain Kirk could easily take more energy than there is in a small galaxy of stars!

Teleportation and nonlocality aside, the most mind-blowing consequence of entanglement is what it means for the Universe as a whole. At one time, all particles in the Universe were in the same state because all particles were together in the Big Bang. Consequently, all particles in the Universe are to some extent entangled with each other.

There is a ghostly web of quantum connections crisscrossing the Universe and coupling you and me to every last bit of matter in the most distant galaxy. We live in a telepathic universe. What this actually means physicists have not yet figured out.

Entanglement may also help explain the outstanding question posed by quantum theory: Where does the everyday world come from?

According to quantum theory, weird superpositions of states are not only possible but guaranteed. An atom can be in many places at once or do many things at once. It is the interference between these possibilities that leads directly to many of the bizarre phenomena observed in the microscopic world. But why is it that, when large numbers of atoms club together to form everyday objects, those objects never display quantum behaviour? For instance, trees never behave as if
they are in two places at once and no animal behaves as if it is a combination of a frog and a giraffe.