Science and sensibility

One of the really cool things about being at university is the chance it gives you to hear ideas from some immensely bright people. Last month I was fortunate enough to attend a public lecture given by Nobel Laureate Tony Leggett. Professor Leggett's lecture suggested that recent experiments in superconductivity might help to bridge the gap between the phenomenally successful but bizarre theory of quantum mechanics and our perception of the world. Now Im no physicist, I couldnt follow everything that was said I can assure you there are bloggers out there infinitely better qualified to talk about this than I. Sadly, as far as I know none of them were at the talk so I shall attempt to be your flawed conduit to this knowledge.

Ask a group of physicists which theory they feel best describes what makes the universe tick and the vast majority will say quantum mechanics. They do so with good reason, quantum mechanics is probably the single most successful theory in the history of science. Predictions from this theory are borne out in chemistry, modern electronics and even the nature of the starlight reaching our planet from thousands of light years away. But quantum mechanics is weird. For all its startling accuracy nothing in our everyday perception of the world seems to fit in with the way quantum theory says the world works. Despite this apparent gap almost every physicist youll find will tell you that quantum mechanics is the theory that runs the universe and the everyday events we are used to witnessing are governed by it as much as electrons and atoms are.

Leggett is more sceptical. He holds what he himself describes as the esoteric view that it is at least possible that quantum mechanics for all its subatomic accuracy doesnt govern big, macrorealistic systems at all. Before we look at this idea and just how it might be tested lets take a look at just how strange things are in the quantum world and physicists explain the gap between this weirdness and the comparatively humdrum world in which we live.

By and large the quantum world is the world of the very small particles no bigger than atoms. For an example of just how weird things are down at this level look at the classic double slit experiment first performed by Youngto show light had wave like properties .In this experiment light is shone through a partition with two slits onto a black observing wall as is depicted below.

You might expect that this will generate an even gradient of illumination on the observing wall centred on the gap between the two slits. Youd be wrong. Instead a barcode pattern of light and dark bars with diffuse edges is formed. This is the result of light waves interacting with each other after they leave the slits in the partition. The light waves act just like waves on a surf beach; sometimes two peaks combine to from an even larger wave while sometimes a peak and a trough coincide and cancel each other out. The bright bars on the observation wall are areas in which the light waves peaks are adding up while the dark areas are peaks and troughs effectively cancelling each other out.

Now what happens when you shoot particles at the partition? Common sense suggests particles cant interfere with each other like waves so youll get the nice even gradient like we excepted light to give us until we thought about waves. Common sense and quantum physics dont get on so well. In fact, if you fire a few thousand electrons through a two slit experiment and record their impact on the observation wall youll get a distribution just like the one for light shown through the partition:

Whats going on then? Only waves can generate interference patterns like the ones we see so we have to accept that electrons which are definitively particles are also sometimes waves. This fact called wave-particle duality is weird enough but the two slit experiment has an even weirder surprise. Imagine that you are firing one electron at a time through the partition. Now there is only ever one electron between the partition and the observation wall. Surely, even if the electrons are acting like waves, theres no way they can generate an interference pattern, right? Wrong, even firing one electron at a time trough the experiment generates the light and dark bars. Mostphysicists will tell you that each individual electron effectively interacts with both slits on its way past the partition theninterferes with itself as it moves towards the back wall. If we try an calculate which slit a given electron went through by measuring the trajectory with which it hit the observing wall the interference pattern breaks down and we get the smooth gradient we intuition suggested wed get to start with. Observing the electron changes its nature.

So what does all that meant? The prevailing interpretation is that particles can exist as a superposition of two possible states until they are observed in someway then requires them to take up one possibility. So in this the two slit experiment the electrons exists as a superposition of having passed through the left and right slits until we measure their trajectory at the observation wall forcing them to pick one.

The clash between this vision of the workings of the universe and our everyday understanding of it is pretty obvious. It was famously made clear in 1935 by Erwin Schrödingerin his famous thought experiment. Imagine an unfortunate cat is locked in a box with one radioactive atom and a device capable of detecting radioactive decay. The detector is attached to a machine that will spray a poisonous gas into the box when the radioactive atom decays, killing the cat. There is no way of observing the atom, the gas or the cat without opening the box. Now, if we apply normal quantum reasoning before we observe the atom it exists in a quantum superposition of the states decayed and not decayed and by extension the cat has to exist in quantum limbo - a superposition of the states alive and dead.Only opening the box will collapse the superposition and make the system choose one or other state.

And theres the clash. Our common sense, macrorealistic intuition tells us cats are either alive or dead and quantum mechanics suggests it will be a both at the same time! In fact, Leggett informed us that in his experience most physicists agree. Few actually believe that the cat ever exists as a superposition of the two states alive and dead. For this to be true there be some fundamental difference between tiny systems like electrons flying through slits and big ones like cats. Is that fair? Can you claim at once that quantum mechanics is the theory that runs the universe at the atomic level and at the same time say it breaks down with everyday objects like cats?

Any physicists reading this post are probably yelling something about decoherence at this point. Decoherence is a phenomenon by which quantum effects break down and classical physics take over. In experiments on scales larger than a few atoms different particles and waves in the experimental system interact with each other this interaction requires that each particle take a definite state which is achieved by breaking any superpositions. Once these superpositions are broken down everything behaves in normal way. In other words our perception of the world macrorealism - iswhat happens when all quantum possibilities of a system are collapsed to one reality.

Leggett does agree that decoherence happens and that it would probably save Schrödingers cat from quantum limbo by effectively reducing the chances of a superposition existing to zero. Leggett does notbelieve simply invoking decoherence is enough to explain the gap between quantum weirdness at the subatomic level and straightforward behaviour of macrorealism. He thinks it at least possible that at some stage along the scale from atom to cat quantum mechanics actually breaks down and no longer describes whats going on. Being a scientist he wants to test this hypothesis.

To show that quantum mechanics really is as most physicists believe the only show in town when it comes torunning the universe Leggett would like to see an experiment in which a system with millions of atoms acts in a way that is predicted by quantum mechanics but not classical physics. Until a couple of decades ago no experimental systems that avoided decoherence could be designed. In recent years a few experimental systems which will avoid decoherence even on these scales have been realised. . The system that Leggett focused on in his talk is called a Superconducting Quantum Interference Device or SQUID. A SQUID consists of tiny, superconducting loops in which up to 10^15 electrons flow in circles when exposed to magnetic fields. Because of quirks in the construction of a SQUID and the physics dictating the way electrons in it behave either all the electrons will flow in anti-clockwise direction or all the electrons will flow clockwise. Experiments show that under certain circumstances the electrons in a SQUID actually flow in both directions simultaneously. That is to say the SQUID, a system comprising 10^15 particles exits as not in either of the states electrons flowing anti-clockwise or electron flowing clockwise but a superposition of both of these states!

So, Leggett has got what he wanted a quantum superposition with millions of atoms (actually more than a million million atoms). He is still not completely ridded of his scepticism though. He holds the view that while experiments like the SQUID ones (commonly referred to a Schrödingers SQUID experiments) certainly prove that quantum physics works up to that scale they still dont disprove macrorealism on bigger scales. He very briefly described an experiment that could do just that at the talk but I was utterly incapable of understanding it, I refer the better equipped reader to a paper in which he describes it