The first time I really comprehended this I found it freaky. I was reading a book by the physicist Richard Feynman, who was my hero at the time. There I was being bombarded by waves of all kind carrying heavens knows what.
It helped me begin to understand a very strange experience I had a number of years before. One night my stereo tuner, which was turned off, started broadcasting music. It seemed to be coming from the back---from one of the connections. I thought the house was inhabited by ghosts. Why it happened then and only then is beyond me.
So much for the warm-up---which is actually a bit of priming. Here comes the quantum physics. Our physical experience is dominated by objects that have more or less clear boundaries, that are separate from each other. Also causality reigns. If I so choose, with my arm I can knock the folder next to my computer off my desk. This follows the laws of classical physics.
But at the quantum level, the level of subatomic particles, most physicists think that it is all chance and randomness. Probabilities rather than certainty or causality are supposed to rule. It is only when an observation is made that the function that determines these probabilities, the wave function, is said to collapse into a specific state. Before that all possibilities are said to coexist or are superimposed.
I highly recommend this video. It is a very clear presentation of the famous double slit experiment that helped demonstrate the very strange things that happen at the subtle atomic level. It is also very entertaining--- worth watching just to enjoy Dr. Quantum's facial expressions!
Not everyone was happy with the randomness that the usual interpretation of quantum mechanics enshrines at the core of reality. Einstein famously said, "God does not play dice." He was not willing to give up the elegant determinism of classical physics. So he proposed that there must be hidden factors, what he called hidden variables, which really control events at the quantum level.
In the 1930s, along with some colleagues, Einstein devised a thought experiment to show that the usual understanding of quantum mechanics is incomplete. It pointed out a paradox. If particles are governed by chance, then some of the predictions of quantum theory would also indicate that particles far apart from each other do not always behave independently. This would be like twins halfway around the universe instantaneously affecting each other. The notion that particles widely separated in space could communicate instantly is extremely problematic because it violates Einstien's own dictum that the speed of light is the fastest any information can travel. Einstein argued that this indicated that the irreducible randomness or chance quantum mechanics seemed to suggest at the base of everything also had to be wrong.
A younger colleague, David Bohm, in the 1950s became interested in developing a deterministic understanding of quantum mechanics. He did not like that the usual interpretation had no underlying theoretical framework. A strongly intuitive physicist, he favored models he could picture or experience at some level. Like Einstein, he also thought it impossible that information could travel instantaneously between particles--- or faster than the speed of light.
His answer was a model in which a quantum potential guides the behavior of particles in a deterministic but holistic way. (Bohm's work built on an earlier attempt by Louis de Broglie in the 30's to provide an alternate explaination.) For example the quantum potential tells the electron whether one or two slits is open--- see above video---and guides it so the observed results occur. The quantum potential is able to this because it contains what Bohm called “active information” about the entire system. In effect, it allows the particle to “just know” the big picture.
Meanwhile in the 60s another physicist named John Bell , influenced by Bohm, proved theoretically that to extend determinism to subatomic particles would necessarily imply what has come to be called non-locality---that particles far apart from each other would have to be connected or communicate at faster than the speed of light . (This is what Einstein could not accept, but 3 decades later it didn't bother Bell) In the 1980s a French team led by Alain Aspect demonstrated non-locality by performing an experiment proposed by Bohm and Bell (based on Einstein's initial thought experiment). Non-locality is sometimes called quantum entanglement, and it is now well accepted by physicist. In fact efforts are underway to exploit quantum entanglement technologically.
Bohm eventually proposed another whole realm, what he called the implicate order, as the source of the quantum potential. In the implicate realm, the two twins halfway around the universe from each other are actually connected. The implicate realm is unfolded or smeared out throughout our level of reality, what Bohm called the explicate order---like those radio waves that somehow caused my turned-off radio to broadcast music. He often used the idea of a hologram, in which every part contains an image of the whole to capture the relationship between the implciate and the explicate realm. But a hologram is static, whereas he saw the process of unfolding and enfolding between the realms going on continuously. He called it holomovement. To get a better sense of his ideas, check out this interview with Bohm.
This talk of other realms did not endear Bohm’s work to mainstream physics. To add insult to injury, he worked closely with the Indian teacher Krishnamurti for many years. Nonetheless a small number of physicists preferred his causal or ontological model and have worked to refine and extend it. It is now called Bohmian mechanics.
Even though the quantum potential reinstates causality, it leaves us with a universe very different from the commonsense world we experience. ( It is important to point out that none of this affects the laws of physics at the macroscopic level, but rather our picture of the subatomic realm.) In what we generally call objective reality, distant objects only affect each other when a signal of some sort, a communication, travels between them. To rescue locality as well as causality the way we usually think of them, like Bohm, we have to accept another level of reality where distant particles really are close together.
This is quite a trade-off! At the same time there is something that rings true about this situation. I mean this in the sense that things very often do seem to turn into their opposites. In any case, all us dummies can take comfort in something Richard Feynman said, "I think it is safe to say that no one understands Quantum Mechanics."
So far there has been no way to test Bohmian mechanics against the usual interpretation of quantum mechanics that claims that randomness rules. Just recently some preliminary data about the density of the early microwave radiation left over from the big bang seems to support Bohmian mechanics---according to Antony Valentini (also see first reference below and very end of post). If it is confirmed, it will cause quite a stir!
Additional Web Information:
Written in the skies: why quantum mechanics might be wrong, 2008, Nature On-Line (It is limted access, but important so I've reproduced the critical paragraphs below).
Quantum Randomness May Not be Random, 2008, New Scientist
David Bohm and the Implicate Order, by David Pratt
Written in the skies: why quantum mechanics might be wrong
Published online 15 May 2008 Nature doi:10.1038/news.2008.829
Almost all measurements of the cosmic microwave background seem to fit well with the predictions of quantum mechanics, says Valentini. But intriguingly, a distortion that fits one of Valentini’s proposed signatures for a failure of quantum mechanics was recently detected by Amit Yadav and Ben Wandelt at the University of Illinois at Urbana-Champaign (see 'Deflating inflation?'). That result has yet to be confirmed by independent analyses, but it is tantalizing, Valentini adds.
“It’s far too early to say that this is definite evidence of a breakdown in quantum mechanics — but it is a possibility,” he says.
Hiranya Peiris, an expert on the cosmic microwave background at the University of Cambridge, UK, is impressed by the new work. “This is a pretty cool new idea,” she says. “Nobody has ever thought of using the cosmic microwave background to look into really fundamental quantum questions — cosmologists just assume that quantum mechanics is correct,” she says.
But Peiris adds that Valentini must now come up with more detailed predictions about the types of distortion that will arise in the cosmic microwave background to convince cosmologists that they are really caused by a breakdown of quantum mechanics. “He has thrown some really exciting ideas out there, but now he needs to do the nitty-gritty calculations,” she says.