The Big Questions: Physics (19 page)

Einstein died before he could see Bell’s experiment performed. The first implementation was by the French physicist Alain Aspect in 1982, but there have been innumerable tests since, and they all confirm Einstein was indeed wrong. Entanglement is indeed a spooky action at a distance, one that denies the objective existence of anything. Bell’s electrons take on their properties only when a measurement is performed, that is, only when someone looks at it.

Even if you already believed that a tree falling in a forest makes no noise, it is still truly remarkable to note that, to stretch the analogy, cutting the trunk of one tree can fell another – even when they are in separate forests. A pair of entangled electrons affect each other instantaneously, and from across the universe. It really is as spooky as Einstein claimed: the standard interpretation of time and space seems to wither to nothing in the light of quantum entanglement.

Entanglement is already being put to use. Quantum cryptography, for example, makes use of the ‘remote control’ function, combined with the fact that entanglement connections are extremely fragile, to give a means of securing information. It is rather like the historical practice of placing a seal on important communications; messages encrypted with entanglement are tamper-proof because any attempt to eavesdrop breaks the connection.

Perhaps more conceptually impressive (but of less practical use) is the quantum version of teleportation that entanglement allows. It is a complex operation, but the basics are that a measurement on one of an entangled pair of particles forces a change in the properties of the other. Done with suitable skill and subtlety, the distant particle can be imbued with all the characteristics of the original without ever being in the same place. It involves other particles too, and the transmission of some information through ‘normal’ channels, so it is, perhaps, more of a telefax than a teleportation. However, it is still an impressive innovation. Although it can only be done on single particles, such as photons, so far, there is no fundamental reason why we can’t extend the technology to transmit more and more quantum objects – perhaps an atom or more.

This will undoubtedly prove useful: although it is unlikely that we will ever achieve
Star Trek
-style human teleportation, moving quantum states around in this way promises to allow information processing on an unprecedented scale. Many research groups around the world are trying to develop ‘quantum computers’ that will perform computations at speeds that are exponentially greater than anything a normal computer can manage. Quantum state teleportation will play a key role in the way these machines work. Our role in shaping the universe needn’t stop with observations that determine the existence and properties of a few particles in quantum experiments, however. According to the late John Wheeler, one of the most respected scientists of the 20th century, each of us can change the very history of the cosmos.

Wheeler’s assertion arose from considerations of quantum measurement. It is now widely accepted that one of the strangest manifestations of quantum theory is the phenomenon where, given the option, something like a photon of light will take all available paths. This ‘superposition’ results from the wave-like character of quantum objects. A single photon fired at a screen scored with two narrow slits will produce a pattern of dark and light bands on a screen positioned on the other side of the slits. This is an ‘interference pattern’, and is associated with wave behaviour. To produce interference, however, the photon must have passed through both slits. How could a single photon do that?

It seems reasonable that we should be able to resolve this by watching the photon. If we look at the slits, we will find which one the photon went through. But any attempt to determine which way the photon went destroys the interference pattern. In this scenario, the photon behaves like a bullet, passing through one slit or the other, and produces no interference pattern.

Bizarrely, the photon seems to behave like a wave when nobody is looking, and a particle when they are. The idea of a photon making some kind of conscious choice related to things in its environment is ridiculous to physicists’ ears. This is why Einstein and others said we must be missing something; there must be some ‘hidden variables’ that determine the photon’s behaviour.

Wheeler suggested a way to test this. What if, he said, we only looked at the photon’s path after the photon had made its ‘choice’ of how to behave? Would that alter the photon’s behaviour? Wheeler’s ‘delayed choice’ experiment is not an easy one to perform, but physicists have managed to do it. In experimental set-ups where a photon takes just 14.5 nanoseconds to traverse the apparatus, researchers have managed to change the set-up after the photon has made its ‘choice’ of whether to behave like a wave or a particle. Nine nanoseconds after the photon entered the apparatus, when the photon has already split like a
wave to go through two slits or, like a bullet, gone straight for one slit, the researchers attached a detector to one of the slits.

What was the result? With a detector in place, there was no interference. With no detector, there was interference. This is exactly what standard quantum theory predicted: the presence of a detector forces the photon to behave as a particle, and particles don’t interfere. If the weird behaviour could be explained by the existence of hidden variables, the photon would already have been ‘committed’ to one behaviour or the other. Before experimenters chose whether or not to detect the photon, it would have manifested as a wave or a particle, with no option to save the choice till after it had passed through the slits. Wheeler referred to the result as revealing quantum processes to be a ‘great smoky dragon’. Its tail – the input – we can know. Its mouth – the outcome – is also clear. But the body of the dragon is a cloud of impenetrable smoke, and ‘we have no right to speak about what is present,’ Wheeler said.

What’s more, Wheeler added, we can say the same about the processes of the universe. The emission of light from stars is a quantum process, after all: the individual photons of starlight have much the same character as the photons of laser light we use in quantum experiments. And a delayed choice experiment carried out on a cosmic scale is the same as one done in the laboratory – but with much deeper implications.

In a provocative thought experiment, Wheeler used the phenomenon of gravitational lensing to illustrate his point. When light from a distant star travels towards Earth, it may pass close to a huge galaxy. The galaxy’s mass bends the light, giving us the illusion of two galaxies where there is only one. Einstein cited this phenomenon as a prediction of general relativity, and the prediction was borne out when the British astronomer Arthur Eddington measured the effect in 1919.

In Wheeler’s view, this lensing is just a double slit experiment on a grand scale. A photon coming from the star has two paths it
can take. If we had a way of observing interference effects as a result of the two paths, we would see an interference pattern. Wheeler’s object of choice was the quasar 0957+561A,B. It is just over 7 billion light years away, and thanks to a galaxy that sits between us and the quasar, we see two images of it in our telescopes. The quasar’s light takes 7 billion years to reach us, then, and a good portion of that journey is after the lensing galaxy. So, Wheeler said, we can take our time, and think about whether we want to measure it with a particle detector or a wave detector. Whatever we decide will have determined whether that photon took one path around the galaxy, or both.

And we can make that decision billions of years after the photon has passed the galaxy in question. ‘This is the sense in which, in a loose way of speaking, we decide what the photon shall have done after it has already done it,’ Wheeler wrote in 1981. Bohr’s idea that nothing has properties until a measurement has been done seems odd. But, with his cosmic thought experiment, Wheeler had out-absurded him. Suddenly, Bohr’s idea seems a lot less difficult to swallow.

Quantum phenomena, Wheeler said in 1992, ‘are neither waves nor particles but are intrinsically undefined until the moment they are measured. So, in a sense, Bishop Berkeley was right when he asserted “to be is to be perceived”.’ But we could equally well turn it around and assert that the fact the delayed-choice experiment has been run successfully in the laboratory, if only in theory in the cosmos, shows that we can participate in the history of the universe – perhaps right back to the very beginning.

‘How did the universe come into being? Is that some strange, far-off process beyond hope of analysis? Or is the mechanism that came into play one which all the time shows itself?’ This quote finds Wheeler wondering if the Big Bang was a quantum event – the universe pulled into existence by something that also governs everyday life.

His ‘participatory’ universe, where the role of observers changes the quantum nature of cosmic history, goes some way to arguing for this. But the Cambridge University cosmologist Stephen Hawking goes even further. The quantum world, he says, allows us to determine the whole history of the universe – from where we are right now. He calls it ‘top-down cosmology’, and thinks it solves that perennial question: what came before the Big Bang?

Many physicists would say that trying to discuss things prior to the Big Bang is as ridiculous as asking what lies north of the North Pole. After all, time came into existence with the Big Bang (see
What is Time?
), and until the clock of time is ticking, we cannot consider concepts of ‘before’ and ‘after’.

Hawking, though, is not willing to stop at the moment of creation. It is, he says, quite reasonable to ask what kicked the universe into being. It is a subtle and difficult argument, as one might expect, but it draws upon well-established ideas in physics. The first of these is an interpretation of quantum theory put forward by Wheeler’s most famous graduate student, Richard Feynman.

It is called the ‘sum over histories’, and suggests that quantum processes follow all possible paths simultaneously. In the double-slit experiment, for example, the interference pattern comes from the photon going not just through both slits, but through every other possible path, such as bouncing off the surface of the moon before it hits the detector. All the various paths have an associated probability, which comes in positive and
negative flavours, rather like a wave. When everything is added together, the sum describes what we tend to observe in an experiment.

‘The quantum world allows us to determine the whole history of the universe’

STEPHEN HAWKING

When Hawking applies the sum over histories idea to the universe, he really does mean histories. It bears some relation to an enormous ‘what-if?’ experiment, pulling together every possible scenario for the story of the universe. In one scenario, our solar system failed to form. In another, gravity is wildly increased. Events change too: Hawking has to consider a history in which Elvis is still alive, for example. Each of these scenarios has an associated probability.

What’s even more disturbing than all these strange possibilities is the fact that they are subject to the measurements we are making today. Just as choosing to measure for a particle or a wave changes the outcome of a quantum double-slit experiment, Hawking reckons the way we gaze out at the universe today can change the way it evolved billions of years ago.

Hawking admits it is a strange idea – but it only seems strange because we are inside the universe in question, he says. Someone looking from outside the universe would see nothing strange, in Hawking’s view. And such an observer could see how the universe came into being from nothing. That is possible because Feynman’s sum over histories – and thus Hawking’s calculations – rely on a notion of ‘imaginary time’.

Although it sounds fantastical to use imaginary time, it is not as much of a stretch as it sounds. Engineers routinely use numbers composed of real and imaginary components to describe and predict the behaviour of electrical circuits.

In Hawking’s top-down cosmology, the sum over histories of the universe, calculated using imaginary time, changes normal
time into a spatial dimension. The result of this is that the problematic ‘beginning’ of the universe disappears. Back when the energy of the universe was packed into the tiniest of volumes, everything ran according to quantum rules, and what we call time was actually a dimension of space.