The central assumption of all of classical physics is that large material things in the universe are set pieces, a fait accompli of manufacture. How can they possibly be changed?

The renowned quantum physicist Anton Zeilinger has examined just this question in his Institute for Experimental Physics lab at the University of Vienna, which is at the very frontier of some of the most exotic research into the nature of quantum properties.

Zeilinger is particularly interested in superposition, and the implications of the Copenhagen Interpretation – that subatomic particles aren’t real yet, but exist only in a state of potential. Could objects, and not simply the subatomic particles that compose them, he wondered, exist in this hall-of-mirrors state?

The central assumption of all of classical physics is that large material things in the universe are set pieces, a fait accompli of manufacture. How can they possibly be changed?

The renowned quantum physicist Anton Zeilinger has examined just this question in his Institute for Experimental Physics lab at the University of Vienna, which is at the very frontier of some of the most exotic research into the nature of quantum properties.

Zeilinger is particularly interested in superposition, and the implications of the Copenhagen Interpretation – that subatomic particles aren’t real yet, but exist only in a state of potential. Could objects, and not simply the subatomic particles that compose them, he wondered, exist in this hall-of-mirrors state?

The famous double-slit experiment

To test this question, Zeilinger employed a piece of equipment called a Talbot Lau interferometer, using a variation on the famous double-slit experiment of Thomas Young, a British physicist of the nineteenth century.

In Young’s experiment, a beam of pure light is sent through a single hole, or slit, in a piece of cardboard, then passes through a second screen with two holes before finally arriving at a third, blank screen.

When two waves are in phase (that is, peaking and troughing at the same time), and bump into each other – technically called ‘interference’ – the combined intensity of the waves is greater than each individual amplitude. The signal gets stronger.

This amounts to an imprinting or exchange of information, called ‘constructive interference’. If one is peaking when the other troughs, they tend to cancel each other out – called ‘destructive interference’. With constructive interference, when all the waves are wiggling in synch, the light will get brighter; destructive interference will cancel out the light and result in complete darkness.

In the experiment, the light passing through the two holes forms a zebra pattern of alternating dark and light bands on the final blank screen. If light were simply a series of particles, two of the brightest patches would appear directly behind the two holes of the second screen. However, the brightest portion of the pattern is halfway between the two holes, caused by the combined amplitude of those waves that most interfere with each other.

From this pattern, Young was the first to realize that light beaming through the two holes spreads out in overlapping waves.

Smeared out photons

A modern variation of the experiment fires off single photons through the double slit. These single photons also produce zebra patterns on the screen, demonstrating that even single units of light travel as a smeared-out wave with a large sphere of influence.

Twentieth-century physicists have gone on to use Young’s experiment with other individual quantum particles, and held it up as proof that quantum physics has Through-the-Looking-Glass properties: quantum entities act wavelike and travel though both slits at once.

Fire a stream of electrons at the triple screens, and you end up with the interference patterns of alternating light and dark patches, just as you do with a beam of light. Since you need at least two waves to create such interference patterns, the implication of the experiment is that the photon is somehow mysteriously able to travel through both slits at the same time and interfere with itself when it reunites.

The double-slit experiment encapsulates the central mystery of quantum physics: the idea that a subatomic particle is not a set something yet, but all possible selves – and all at the same moment.

It also demonstrates the principle that electrons, which exist in a hermetic quantum state, are ultimately unknowable. You cannot identify something about a quantum entity without stopping the particle in its tracks, at which point it will collapse to a single point.

In Zeilinger’s adaptation of the slit experiment, using molecules instead of subatomic particles, the interferometer contained an array of slits in the first screen, and a grating of identical parallel slits in the second one, whose purpose was to deflect the molecules passing by.

The third grating, turned perpendicular to the beam of molecules, acted as a scanning ‘mask’, with the ability to calculate the size of the waves of any of the molecules passing through, by means of a highly sensitive laser detector to locate the positions of the molecules and their interference patterns.

Mammoth molecules

For the initial experiment, Zeilinger and his team carefully chose a batch of fullerene molecules, or ‘buckyballs’ made of 60 carbon atoms. At one nanometre apiece, these are the behemoths of the molecular world. They selected fullerene not only for its size but also for its neat arrangement, with a shape like a tiny symmetrical football.

Zeilinger heated the fullerenes to 900 K so they would create an intense molecular beam, then fired them through the first screen; they then passed through the second screen before making a pattern on the final screen.

The results were unequivocal. Each molecule displayed the ability to create interference patterns with itself. Some of the largest units of physical matter had not ‘localized’ into their final state. Like a subatomic particle, these giant molecules had not yet gelled into anything real.

The Vienna team tested out a batch of other gigantic molecules, which also demonstrated the same magical properties.

Repeatedly Zeilinger’s research group demonstrated that the molecules could be two places at once, which meant that they remained in a state of superposition even at this large scale.

He and his team had proved the unthinkable: the largest components of physical matter and living things exist in a malleable state – a piece of Play-Doh, to my mind, just waiting for consciousness to manipulate it into life.