Streams of electrons shoot at your monitor's screen from behind, and where an electron hits, it kicks out a packet of light energy. Energy is exchanged in fixed discrete quantities, Einstein's "quanta." So a particular screen material, hit by an electron, releases a light packet with a specific amount of energy.

In the retina of your eye there are molecules in which the links between atoms are under tension, like tiny mousetraps that can be set off by a specific energy. (Different energies will appear to you as different colors, red or blue or green.) When a light packet of the right energy strikes a molecule of the right type, it may trigger the molecule to straighten out. This snap launches reactions that send a signal up a nerve to your brain.

But the molecule is not always triggered. Sometimes the light just goes on through, without transferring its quantum of energy. Bohr held that it is a matter of pure chance whether the interaction will happen in any particular case. What can be calculated is the probability that the energy will be exchanged--say, seven out of ten times that a light packet meets a molecule of a given type. If your eyes were more sensitive, at very low levels of light you would see, instead of a constant image, a sparkling, "grainy" picture made up of random flashes.

Is nature truly random at its foundations? Recent experiments at extremely low light levels have found examples of the strange behavior that Bohr's interpretation predicts. Einstein lost the debate... But Bohr has not won it. Physicists today continue to debate how to explain the intractably weird laws of quanta.