The Big Questions: Physics (5 page)

In this view the equation was a guide to what we might find out about the quantum system under inspection, but had nothing to say about what the nature of the system actually was. In other words, it did not give us a description of the quantum object, only a description of what we could know about it. Philosophically, this was a nightmare. Einstein hated it, as did Schrödinger.

Niels Bohr, on the other hand, loved it. Bohr was based in Copenhagen, where he ran an institute sponsored by the Carlsberg brewery. He was a ‘positivist’: his philosophy said that it was meaningless to talk about something’s objective properties because you could only ever access knowledge about it through subjective measurements. Those measurements will always impose restrictions on what we can know.

The ultimate reality behind Schrödinger’s wave equation was neither wave nor particle, Bohr felt, and so could not be described in any terms we can deal with. His answer was to assume that nothing exists until it is measured. But once a measurement is made, the type of measurement will determine what we see. If you use an instrument that detects something’s position in space, for instance, you’ll see something that has a definite position in space – the entity that we call a particle.

Einstein would have none of this ‘Copenhagen interpretation’ of quantum theory. His great work, relativity, had been built specifically to create a theory that was independent of the observer. The central theme of relativity was that the laws of physics should be the same, whoever is working them out. The notion that the physical nature of the universe was dependent on how we looked at it offended his sensibilities deeply.

Einstein’s problem lay in the fact that describing quantum objects using a wave equation meant that, like waves, they could interfere with one another. When two waves interact, they produce a ‘superposition’, which is the sum of the waves at any point. Where two crests coincide, the superposition is larger than both. When two troughs coincide, the wave trough deepens. If a crest and a trough coincide, the result is flat.

How does this apply to quantum particles? Schrödinger’s wave equation says that, in the right circumstances, they exist in a superposition of different states. Thus an electron circulating in a ring of metal can be circulating clockwise and anticlockwise at the same time. A photon of light can be polarized – that is, have its electric field oriented – in any number of directions at the same time. A radioactive atom, which decays via a quantum process, can be in a superposition state of ‘decayed’ and ‘not decayed’. Though it seems nonsensical, this is what the theory states.

Which is why Einstein and Schrödinger said there must be something missing from the theory. And, to hammer his point home, Schrödinger came up with the cat. ‘One can even set up quite ridiculous cases,’ Schrödinger wrote in a 1935 journal article. ‘A cat is penned up in a steel chamber …’ Schrödinger went on to describe this ‘ridiculous’ case in some detail, unwittingly creating the touchstone for future interpretations of quantum theory.

In the closed steel chamber with Schrödinger’s cat is a tiny piece of radioactive material and a Geiger counter. At any moment, there is some probability that the radioactive material will emit a particle, thus triggering an electrical current in the Geiger counter. But Schrödinger had the Geiger counter rigged up to release a hammer that, on sensing a radioactive emission, would smash a flask of hydrocyanic acid, releasing vapours that would kill the cat.

According to Schrödinger the quantum description of the entire system, including all the atoms that make up the cat, ‘would express this by having in it the living and dead cat (pardon the
expression) mixed or smeared out in equal parts.’ The logic is sound. The indeterminate nature of the radioactive atom, in a superposition of ‘decayed’ and ‘not decayed’ can also put the cat in a superposition of dead and alive.

The kicker comes when the issue of measurement is brought to bear. Bohr had said that there is no definite reality until a measurement is made, because the choice of measuring instrument determines which facet of the system – wave or particle, for instance – the observer will see. So, in Bohr’s view, the act of opening the box and observing the state of the cat would force it to be alive or dead.

This was what Schrödinger found so ridiculous: how can the act of observation change such a fundamental property of a cat? It must be one thing or the other; Bohr was being fooled in the same way that a blurred photograph can give an impression of fog, he said. ‘There is a difference between a shaky or out-of-focus photograph and a snapshot of clouds and fog banks.’

By this time, though, the interpretation of quantum theory was already a matter of public debate: Einstein and Bohr had a famous exchange in 1927, at the fifth Solvay Conference in Brussels. Einstein challenged Bohr with a series of thought experiments. Imagine such and such a situation, he would say: how can the observation, or the interaction with the apparatus cause a superposition to resolve into one state or the other?

The eventual outcome of this argument was a new version of an old experiment: the famous ‘double slit’ experiment. In 1801, Thomas Young overturned Newton’s particle view of light by shining light at a screen scored with two slits. Young observed an
‘interference’ pattern, which can only be explained through superposition of waves. The quantum version asks what happens when you reduce the light intensity so far that quantum theory kicks in. When there is only one bullet, or ‘photon’ of light in the experiment at any one time, there can be no interference, surely?

In Bohr’s view there could – as long as no one was looking to see which slit the photon travelled through. To Bohr, the light is neither a wave nor a particle – those are names that we give something whose properties we have measured. According to Schrödinger’s wave equation, the photons of light go through both slits. Despite being a single particle, each photon is ‘smeared out’ as a wave, effectively having two independent existences as it passes through the slits. As long as no one measures the path the light takes, it takes all available paths.

You might think that this is all wordplay – abstract thought experiments whose weirdness will disappear once the experiments are carried out in the real world. You would, to Bohr’s delight, be wrong. We didn’t find that out for sure until relatively recently. The first double slit experiment with only one particle in the apparatus at any one time was only carried out in the 1970s. But it worked: despite being faced with two slits, a succession of electrons gradually built up an interference pattern on the screen beyond the slits.

And spookily, when an instrument was placed in the experiment to measure which slit the electron went through, the interference pattern disappeared. In other words, measurement made it manifest as a particle, not a wave. That might seem far removed from Schrödinger’s cat – a cat, after all, is a very different beast from an electron. But subsequent experiments have pushed the quantum particle to ever-larger sizes.

We have carried out the quantum double slit experiment with photons, electrons, atoms, and even 60-atom fullerene molecules. The weird interference effect has never disappeared –
unless we tried to look at which slit the particle went through. Plans are afoot to do it with much larger objects: a virus, and maybe something a million times bigger than the fullerene molecule. Apart from the difficulties of building the experiment, there is no fundamental reason to stop there: there is no cause to suggest why a real cat shouldn’t behave in the same way as an electron, given the right circumstances and a cat-flap-sized double slit.

Except, of course, that it is easy to see a real cat, and thus determine which cat-flap it went through. In Schrödinger’s thought experiment, the box has to remain closed so that no one can see the cat, there is no measurement performed, and the superposition remains intact. This leads us to a difficult question, one that Bohr always evaded. What constitutes a measurement? With Schrödinger’s cat, is it when the box opens? When light photons bounce off the cat relaying to us the information that allows us to tell whether the cat is dead or alive? Or is it when those photons enter our eyes? Or when our conscious minds register the state of the cat? Bohr’s answer to this conundrum was, essentially, that physicists just know when they have made a measurement. Modern versions of the Schrödinger’s cat experiment, however, are shedding much more light on the process – and explaining why a cat can’t really be dead and alive at once.

The boundary between the ‘classical’ world that we inhabit and the quantum world of the atoms comes down to the de Broglie waves that brought this whole story into existence. The de Broglie wavelength of a body, which depends on its momentum, gives a measure of the scale at which it will manifest as a quantum wave.

In the double slit experiment, the fullerene molecule has a de Broglie wavelength of around 10
^–12
metres, or a thousand billionth of a metre. The gap between the slits is around half a million times bigger than that; bigger, but not too different in scale. This means the system is suited to exposing wave behaviour.
This is still in line with Bohr’s claim that the choice of measurement apparatus decides which characteristics will manifest, but it does throw out two explanations for why a cat or a person can’t – unlike the fullerene molecule – seem to be in two places at once.

The first reason is practical. Walking along a wall at a couple of miles per hour, for example, Schrödinger’s cat would have a wavelength of around 10
^–28
metres. Its quantum, wave-like behaviour would only be exposed by a measuring device of a similar scale. Since we have never created such a device, we cannot perceive quantum behaviour. Everyday life is, according to Bohr’s scheme, an experimental situation that will always manifest the particle-like nature of everything around us.

The second reason that we are ‘classical’ is that we are emitting radiation. Anything that has a temperature above absolute zero, –273 celsius, emits photons, packets of energy that carry away heat. Experiments have shown that this radiation can be used to find the location of the object, effectively revealing which slit it passed through. In other words, at a temperature above absolute zero, you can’t close the box on Schrödinger’s cat, invalidating the premise of the thought experiment whenever you translate it into the real world.

These experiments were carried out by firing fullerene molecules at a double slit. The hotter the fullerene molecule was as it approached the slits, the more blurred the interference pattern. The hot molecule emits photons, and the energies of the emitted photons are determined by the temperature. Higher temperature essentially gives higher energy, which translates, in de Broglie’s terms, to a shorter wavelength. And the shorter the wavelength of the emitted radiation, the easier it is to infer the emitting molecule’s position. In other words, a hot body seems to give away more information about which slit it might go through.

The same thing happens if the fullerene molecules collide with air molecules on the way to the slits. Normally the
experiments are done in high vacuum, but if the vacuum is not so good, and the position of the fullerene can be inferred by watching what it does to air molecules, the interference pattern fades away. Again, as it becomes possible to infer which slit the molecule goes through, its ability to go through both at the same time begins to disappear. In a partial vacuum, the fullerene behaves as if someone had left the box half-open on Schrödinger’s cat, forcing it to be alive or dead, but not both.