ANYONE who is not shocked by quantum theory has not understood it.'' So said Niels Bohr, one of the founding fathers of quantum mechanics and a foremost interpreter of its metaphysical implications. The shock comes from the jarring discontinuity between those new implications and the old Newtonian world view - from the strange sense of reality that quantum theory describes. Then why not tinker with the theory until it agrees with our more commonplace views? Because the tinkerer stumbles almost immediately over the hard rock of experiment. Quantum mechanics, after all, is scientific: It's based not on speculation but on observation. The following three experiments show, each in a different way, the fundamental challenge quantum physics poses to the commonplace, classical view of the universe.
The two-slit experiment
Through the ages, the nature of light has puzzled researchers. Most 18th-century physicists, including Newton, held that light was corpuscular - composed of tiny particles. Early in the 20th century that idea was reconfirmed by experiments showing that light consists of quanta of energy now called photons. Photons, it appeared, could knock electrons off atoms - acting somewhat like softballs hurled at dolls in a fairground tent. Moreover, you can count single photons as they strike a sensitized plate. So light, it seems, behaves like pellets shot from a gun.
Back in 1803, however, Thomas Young, a London physician and physicist, proved something else about light: It was also made of waves. His proof was based on the fact that when two separate wave patterns overlap, they don't simply blend, like different colors of paint. Instead, they produce something known as an interference pattern.
There's nothing mysterious about that pattern. It appears, for example, whenever you drop two stones close together in a pond. Knowing this, Young set up his famous two-slit experiment (see next page). If light were like pellets rather than waves, he reasoned, those pellets that got through the two slits would head straight for the far wall, landing in the two obvious target areas.
Instead, he found a peculiar pattern of light and dark stripes - which, he surmised, could be caused only by wave interference. So light is a wave, right? Not quite. Modern variations of the experiment prove something astonishing: that even single photons are affected by the interference pattern. But a single photon ought to be able to go through only one of the two slits: Common sense dictates that it can't go through both, any more than a person can leave a house by both the front and back doors at once. It's almost as though the photon knew when the second slit was open.
How to explain this behavior? It helps to recall that, under the terms of Heisenberg's uncertainty principle, a particle does not always reveal such attributes as momentum and location (which, taken together, determine its path). Their attributes appear only as they are being measured. So each unmeasured particle might take a great number of paths - simultaneously. So until we make a measurement and say, ``Yes, it passed through Slit A,'' each particle seems to take both paths. In technical terms, such a particle is said to exist in a superposition of both states.
Then let's make that measurement: Let's find out which slit it passed through. Why not set up a detector behind each slit?
Unfortunately, it's not that simple. To introduce a detector is to introduce an observer into the experiment. And, according to the Copenhagen interpretation of quantum reality, an observer forces the particle to make a choice. Set up your detectors, and the wave-interference pattern on the wall disappears - since, after all, you're now running a very different experiment. If you set out to prove in this way that light behaves like pellets going through a slit, it will behave like pellets going through a slit.
On the quantum scale, a subatomic particle can apparently exist in a superposition of states. But what about the human scale? Can quantum effects ``leak out'' into the visible realm?
In one of the most famous ``thought experiments'' of quantum mechanics - an imaginary experiment, never carried out - Austrian physicist Erwin Schr"odinger examined that question. In place of a single particle, he substituted a cat (see box). According to the Copenhagen interpretation, in which the observer plays a crucial role in determining the nature of reality, the best that can be said is that Schr"odinger's cat exists in a superposition of two states. It is both dead and alive. Only the act of measurement forces it to declare itself in one or the other state.
One can get around the dead-and-alive paradox, of course, with the many-worlds interpretation. According to that view, there are several parallel universes - some with the cat dead, others with it alive.
Or, if that sounds too bizarre, then maybe quantum effects don't ``leak out'' into everyday reality - that there is one set of laws for the quantum realm and another for everyday reality. That suggests, however, that the fundamental laws of the universe are not as consistent as Newton thought - that the laws operating on the smallest atomic scale are not necessarily valid for larger bodies. That raises serious difficulties in knowing where to locate the imaginary line between the ``micro'' (quantum) realm and the ``macro'' (human-scale) realm.
Finally, the problem may lie not in the state of the cat but in our inability to know the state of the cat. The cat may really be dead, or really be alive, even though we can't make a measurement. Some physicists take the view that our inability to measure is due to ``hidden variables'' - fluctuating phenomena that influence the outcome of our experiments but are not embraced by our theory.
The EPR Paradox
Does looking at Schr"odinger's cat change the cat? The concept of such an observer-created reality troubled Einstein. ``I cannot imagine,'' he asserted, ``that a mouse could drastically change the universe by merely looking at it.''
So in 1935, in collaboration with fellow physicists Boris Podolsky and Nathan Rosen, he devised the Einstein-Podolsky-Rosen experiment (known as the ``EPR paradox,'' although there's nothing paradoxical about it). Like Schr"odinger's cat, this thought experiment was intended to expose the flaws in quantum theory.
Suppose, said the EPR team, that you cause a single particle to break down into two identical particles - call them A and B. They fly away in opposite directions. While you can't measure both the momentum and the location of either, you can measure the total momentum of the two. Then, later, you can measure the momentum of one - B, for example. Subtract B's momentum from the total for both particles, and you find A's momentum.
Now you're free to measure A's location. Doing so, of course, will destroy your capacity to measure its momentum. But that doesn't matter: You already know its momentum. Hence, according to EPR, it ought to be possible to measure accurately a particle's momentum and location. To adhere to quantum theory and assume that you can't, furthermore, suggests that somehow A and B are still interconnected - that A would somehow realize that you already knew its momentum, and would refuse to yield up information about its location. That sort of interconnectedness would require ``superluminal'' signaling - faster than the speed of light. But, as the EPR paper concluded, that would make ``the reality of [position and momentum in the second system] depend upon the process of measurement carried out on the first system, which does not disturb the second system in any way. No reasonable definition of reality could be expected to permit this.''
The EPR paradox stimulated a discussion that lasted decades. Finally, in 1964, CERN physicist John Bell devised a theorem that laid EPR to rest - and hinted at whole new world of weirdness.
Using a variation of the EPR experiment devised by David Bohm, Bell imagined a calcium atom that simultaneously emitted pairs of photons (see box). When, in 1982, French physicist Alain Aspect reported his experimental results in a test of this theorem, Bell's conclusion was verified. In certain circumstances, it seems, two photons at opposite ends of the universe can be interconnected in ways that lets one ``know'' what's happening to the other.
So what? If the universe behaves as Newtonian physics says it should, we should see a world made of particles subject only to so-called ``local'' effects. That means that only such forces as can travel between particles at up to the speed of light can ever have any effects.
But Bell's theorem says something else: that nonlocal effects - apparently operating instantaneously over great distances - tell one particle about the measurement being made on the other. It appears that what happens at one detector influences what happens at the other - more quickly than any signal could travel between them.
Some interpreters of quantum mechanics use Bell's theorem to suggest the possibility of communications at speeds that are superluminal. That, however, would necessitate the astonishing conclusion that a message could be received before it is sent.
Does Bell himself discount that idea?
``Oh, absolutely!'' he says. If he wanted to influence the visitor sitting in his Geneva office, he says, ``there's nothing I can do to change you before light has had time to propagate'' - before, in other words, a signal can be sent. So what about the apparent ``signaling'' between the polarization detectors? It's useless, he says, for sending decipherable messages: Even if a message could be sent, we couldn't decode it.
SCHRODINGER'S CAT Schr"odinger's ``thought experiment'' consists of:
- an imaginary cat inside...
- a sealed box (you can't see inside) which contains...
- a vial of poison and
- a hammer poised to break the vial
- a detector of radioactive particles which, when activated, trips the hammer
- a weak source randomly emitting radioactive particles. If the radioactive source emits particles only at random, you can't know the state of the cat until you look inside. When you open the box, will the cat be dead or alive? The answer (in quantum terms): the cat must exist in BOTH states until someone looks, illustrating the quantum concept of ``superposition'' of states.