Anil Ananthaswamy in New Scientist:
The theory, by physicist David Bohm, has been resurrected after researchers carried out experiments on photons that seemed to support it.
The behaviour of the quantum world has befuddled physicists for nearly a century. “We have had geniuses working on it and we still have a problem,” says Basil Hiley, a quantum physicist at Birkbeck College at the University of London, who worked with Bohm until the latter’s death in 1992.
Unlike the classical world, with its clockwork precision and pleasing predictability, the quantum world is rife with randomness.
The famous illustration is the double-slit experiment: if you fire photons at two slits, our classical intuition expects each to pass through one or the other slit and hit a screen on the other side, making a single mark indicative of its particle nature. But when you try it, the photons create an interference pattern of light and dark bands on the screen, as if each photon behaved like a wave and passed through both slits simultaneously.
The dominant explanation of such behaviour is called the Copenhagen interpretation, which states that the question of whether a photon is a wave or a particle has no meaning until you make a measurement – and then it becomes one or the other depending on which property you measure. The other favoured explanation is the many-worlds interpretation, under which each possible state of the photon becomes manifest in an alternate world.
More here.