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25 June 2015updated 09 Sep 2015 4:51pm

What does it mean to see something? Take a look at Schrödinger’s cat

It takes only a few photons to trigger our visual sense. Tantalisingly, a few photons can exist in superposition.

By Michael Brooks

What does it mean to see something? It’s a surprisingly philosophical question, especially where quantum physics is concerned.

Quantum physics describes the world at the level of single atoms and smaller, looking at things such as photons, particles of light. These objects are able to be in two places at once, or to spin clockwise and anticlockwise at the same time, a phenomenon known as superposition. Something happens to stop bigger stuff being in two places at once, even though it is composed of atoms and molecules that can exhibit superposition. In experiments, we have achieved superpositions of molecules composed of about 800 atoms but these collapse very quickly. Something about increasing size (or weight) seems to stop the weirdness in its tracks.

Intriguingly, our eyes may help to unravel this. At a meeting of the American Physical Society in Columbus, Ohio, researchers reported that our eyes can register things at the level where quantum weirdness remains. It takes only a few photons to trigger our visual sense. Tantalisingly, a few photons can exist in superposition. The next step is to see whether we notice anything weird when photons in superposition enter our eyes.

This is ambitious. Our unconscious perception is important even to establish whether we see a few photons. The experiments reported in Ohio put subjects in a blacked-out room. When photons were flashed at their eyes, they had to say whether the flash came from their left or their right. Because of the low light intensity, few of the flashes were registered consciously: the subjects couldn’t remember seeing the flash but had to report their gut instinct. They got it right enough of the time to suggest that their brains had registered the photons.

Conscious perception has long been of interest to those who study quantum physics because observation can disturb quantum weirdness. Fire a photon at a double aperture and it will go into a superposition and pass through both apertures simultaneously – but only if no one is looking. Put a detector on the apparatus and the superposition disappears. This is the source of Erwin Schrödinger’s well-known thought experiment. He conceived a situation in which quantum physics leaves a cat both dead and alive, as long as no one observes it.

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What is particularly interesting about real-world experiments is that the detector’s presence seems to determine whether the photon behaves like a wave or a particle. It does not require that a person consciously register the result. There is a mystery here about the nature of observation and measurement and what effect it has on the way the microscopic world manifests in our experiments.

No one understands how an atom or photon “knows” that it is – or might be – being tracked. That is what makes the new proposal so exciting. If you cut out the detection apparatus and go straight to the human eye, a slew of questions and opportunities arises. Does a future collision with the molecular apparatus of the retina count as a detection? Is there a difference between human and electronic photon detection? Does it make a difference whether we are conscious or unconscious of the photon’s arrival?

We may have to wait a couple of years for answers. These experiments are beyond our present capabilities, but our horizons are expanding. They promise insight into issues that have been bothering quantum philosophers for a century.

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