Physicists have now observed light behaving as both a wave and a particle ~ a feat that “pulls back the veil” on quantum reality in a way previously thought impossible.
According to the BBC:
For instance, a well-known rule called the Heisenberg uncertainty principle maintains that for some pairs of measurements, high precision in one necessarily reduces the precision that can be achieved in the other. One embodiment of this idea lies in a "two-slit interferometer", in which light can pass through one of two slits and is viewed on a screen.
… Now, Aephraim Steinberg of the University of Toronto and his colleagues have sidestepped this limitation by undertaking "weak measurements" of the photons' momentum. The team allowed the photons to pass through a thin sliver of the mineral calcite that gave each photon a tiny nudge in its path, with the amount of deviation dependent on which slit it passed through.
By averaging over a great many photons passing through the apparatus, and only measuring the light patterns on a camera, the team was able to infer what paths the photons had taken.
On one level, the experiment appears to violate a central rule of quantum mechanics, but Professor Steinberg said this was not the case. "While the uncertainty principle does indeed forbid one from knowing the position and momentum of a particle exactly at the same time, it turns out that it is possible to ask 'what was the average momentum of the particles which reached this position?'" he told the BBC. "You can't know the exact value for any single particle, but you can talk about the average."
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