Matter, the rigid "stuff" we see all around us, isn't really hard little pellets all stuck together but is mostly waves and empty space. It certainly seems as if matter is pretty hard stuff. For instance, salt crystals are so hard that it takes a grinder to reduce chunky ones to a more palatable size. Salt is an assembly of sodium and chlorine atoms. As such it is reasonably typical of the way atoms stick together, the building blocks of bulk material.
But sodium atoms can do some fancy things, according to David Pritchard of the Massachusetts Institute of Technology in the United States. In his laboratory at MIT, Professor Pritchard sent a beam of sodium atoms towards a thin foil, just a few millionths of a metre thick and placed edge-on. The beam split in two and passed on opposite sides of the foil. But when a detector at the far end started to register the atoms coming through, it showed a remarkable banded pattern, "bright" bands where many atoms were detected separated by "dark" bands where few were arrived.
The pattern is distinctive and well known: it is an interference pattern, the signature not of solid particles but of ethereal waves. The pattern arises typically with "real" waves such as water waves and sound. The crests of two waves reinforce each other to give bright patches, while crests of one wave cancel out troughs in the other to give no waves, resulting in dark patches. The inescapable conclusion is that sodium atoms behave like waves.
That is surprising enough, but what is really difficult to comprehend is that "each atom interferes only with itself", according to Professor Pritchard. He explained that each incoming sodium atom was spaced about a metre apart from the others, so that there was little chance of two atoms overlapping.
In other words, a single atom passed both sides of the foil simultaneously. What emerged then recombined to give an interference pattern. Each atom passed on both sides of an impenetrable barrier. The same thing would happen with lots of spaced sheets of foil, or equivalently a barrier with slits cut in it. It's as though when confronted by a row of supermarket check-outs a shopper passes through all of them at once. It's that bizarre.
This is the quantum nature of our universe revealed. And the reason it happens is that nature is like that: there is no deeper explanation. "Objects really propagate according to a wave-like equation that agrees with classical predictions only if you do not look carefully enough," said Professor Pritchard. "Your intuitive notion that the atom has to have a location at all times is incorrect."
The first MIT sodium atom interference experiment was reported in 1991. In the same journal issue appeared details of a similar experiment by Oliver Carnal and Jurgen Mlynek at the University of Konstanz in Germany using helium. Scientists were pleased but not surprised: they had expected it, based on earlier work with tiny fundamental "particles" that also displayed wave-like properties. But early in 1995, Professor Pritchard's group went one step further, showing that molecules of two sodium atoms also show wave-like properties.
"Our experiments have shown that even 'large' objects like molecules behave like waves," said Professor Pritchard. Christian Borde and his collaborators at the Universite Paris-Nord in France have shown interference effects in experiments using iodine molecules, and a Russian group has done similar experiments with even heavier molecules such as osmium tetrafluoride.
How big can we go? "This is clearly an underlying theme of our research; to push quantum mechanics and the observation of quantum effects toward macroscopic objects. It's just a question of developing gentle technique as far as I can see," said Professor Pritchard.
Recently they have published results on an experiment so difficult the great American physicist Richard Feynman proposed it only as a "thought" experiment, one that demonstrates a principle but which is too hard to do in practice.
What Professor Pritchard's group has done is to watch for sodium atoms as they emerge from above or below the foil divider, using single particles of light called photons. When they do this, they find that the results show each atom suddenly spoils the game by going above or below the divider, and the interference vanishes. Atoms no longer behave like waves.
If quantum mechanics is correct it had to be this way. Quantum mechanics says that as soon as the experimenter has a way of determining where an atom or some other particle has gone, then the wave-like aspect vanishes. In terms of the quantum supermarket, the reality of having to pay a cashier means that the shopper is effectively tracked, and a quantum shopper wouldn't show wave-like behaviour after all.
So there is a limit to how strange the quantum world is. "We showed that shining a single photon of light on a system will destroy its quantum interference," he said. "We also showed that quantum coherence is easier to destroy in bigger systems. Thus suggestions that quantum coherence explains ESP or other strange correlations over large distances fly in the face of our results."
Quantum mechanics as a way of explaining ESP may be dead, but Professor Pritchard has done a simple calculation that should make philosophers sit up and take notice. Imagine if a living organism could show wave-like properties. Then passing it both sides of a thin foil to give an interference pattern would mean that in some sense this living organism is in two places at once.
"I calculated that we could see interference of large bacteria if we could let them spend about a year in our interferometer, and could keep it from vibrating during this time," said Professor Pritchard. There is a catch, however: "Unfortunately they wouldn't really be living - we would have to cool them almost to absolute zero to keep the heat photons they radiate spontaneously from messing up the interference pattern." So philosophers are safe - for the moment.