Quantum entanglement, in which paired particles somehow influence each other at apparently instant, faster-than-light speeds, is maybe physic's most bizarre mystery. Now it might be possible to see entangled photons with the naked human eye.
Although the very notion of quantum entanglement seems to fly in the face of everything we think we know about physics, the basic concept is simple enough. If two subatomic particles are entangled, then they are linked in such a way that it's impossible to describe one without measuring both. It's as though two particles act like one big particle, even if the two are spatially separated. Since the very act of measuring a particle alters it, this means that measuring one entangled particle automatically affects its partner. This effect has been observed at distances of nearly a hundred miles.
One type of particle that physicists often entangle is the photon, the constituent particle of light. Photons are often used in entanglement experiments, where separated pairs are sent through detectors to see how quickly the measurement of one alters the other. In new experiments due to start in the next few months, physicists will simply replace mechanical photon detectors with their natural counterparts: human eyes.
Although human eyes are often thought to be rather unimpressive compared to those of other animals, they are actually surprisingly powerful. Human eyes are wired to detect light as little as seven photons wide, and they very rarely detect false positives - photons the eyes think are there but actually aren't. This makes them perfectly decent photon detectors for an entanglement experiment, as long as physicists can entangle bundles of seven or more photons.
Right now, the protocols being developed call for considerably more than that, as an initial pair of photons will ultimately be entangled with bundles of about a hundred photons. The photon pulse will then be sent to the eyes of the test subject, with the other half of the entangled pair sent to a regular detector. If all goes according to plan, the human observation of the light will lead to a measurable change in the other set that is then picked up by the detector.
Of course, the eyes are no match for these mechanical detectors, but that isn't really the point of these experiments. Rather, those designing the experiments hope to demonstrate that quantum effects are not just some physics freak show, but rather something that tangibly exists in the visible world. That isn't just a public relations achievement; indeed, successfully using eyes will help show that entanglement is a robust enough process that it really can work at a level our eyes can see.