Once upon a time, Newton prevailed, and the world was a safe place for all of us.
When you hit a plain, old-fashioned billiard ball, you could predict how fast it would move and in what direction. And when the billiard ball came to rest, you knew exactly where it was. These simple notions seemed obvious, necessary even. Most people believed that for physics to work, it had to be based on such solid and unshakable foundations.
Then on 19 October 1900, physicist Max Planck made a ground-breaking presentation to the German Physical Society. Planck was a sober man and, at 42, a little long in the tooth for a revolutionary. But his discovery was to turn the classical physics of the billiard ball on its head. What he described was an answer to an old question: Why does the colour of radiation from any glowing body change from red to orange and ultimately to blue as its temperature increases? Planck found he could get the right answer by assuming that radiation, like matter, comes in discrete quantities. And he called his little packets of energy "quanta" from the Latin for amount. At the time, Plank seems to have imagined that some deeper explanation of these quanta would emerge.
But it rapidly became clear that the "quantisation" of energy -- dividing it up into individual pieces -- was actually a new and fundamental rule of nature. The classically trained Planck didn't like this conclusion one bit. He resisted it to his dying day, prompting his famous lament that new scientific theories supplant previous ones not because people change their minds, but simply because old people die.
It's not surprising that Planck was unsettled by the implications of quantum theory. If you accept its conclusions nothing is what it seems, or what common sense and Newtonian physics lead you to believe. Things change when you look at them. Objects behave in unpredictable ways.
Take the uncertainty principle, which emerges inevitably from quantum theory. According to this, you can never measure anything as accurately as you'd like. Or to put it another way, measurements affect the thing you're trying to measure. Then there is the notion of wave-particle duality, which says that an electron, for example, may sometimes act like a wave and sometimes like a particle. What all these ideas seem to suggest is that physical objects -- even reality itself -- are not at all what everyone had supposed.
How do such grandiose and alarming conclusions follow from the seemly innocuous statement that energy is divided up into quanta? The American physicist Richard Feynman liked to use a simple and compelling example. Think of light reflected from a mirror. No mirror is perfect, so perhaps 95 per cent of the light bounces off the mirror's surface, while the other 5 per cent passes through, or is absorbed or otherwise lost.
In pre-quantum days, no problem. When light hit a mirror it was seen as a continuous stream of energy: most bounced off the mirror's surface but a fraction streamed through. But Planck recast light as a torrent of quanta -- called photons. Because each photon is indivisible, it must either be reflected or absorbed in its entirety. You can't have 95 per cent of a photon going one way and the rest going somewhere else. But then, to understand what a mirror does to light, you must conclude that 19 out of every 20 photons bounce off the surface while the rogue photon goes its own way. Who decides what each individual photon should do?
Here lies the revolution. Quantum theory says that what happens to any individual photon is genuinely and inescapably unpredictable. It has a 95 per cent chance of being reflected, and a 5 per cent chance of being transmitted or absorbed, and that's all there is to it. There's nothing about any photon, no secret property or hidden clue, that can tell you any more precisely than that what it will do. The unpredictability is innate.
Here's another example. If you rotate your Polaroid sunglasses in front of your eyes, you'll see changes in the amount of light getting through them. Light (as James Clerk Maxwell had shown in 1864) is a type of electromagnetic wave, and the waves can be polarised, like a skipping rope that's made to go up and down or side-to-side, or anything in between. Sunglasses tend to let through vertically polarised light but block the horizontally sort, the source of most glare and reflections.
But a single photon of light coming at your sunglasses has only two options: to pass through or not. What will it do? Again, the best you can do is to know those probabilities. You can't ever predict exactly what any individual photon is going to do.
In the old days of classical physics, you might have wanted to predict what a billiard ball would do when it ran into another billiard ball or the side of the table. To do that, you would need to know its mass, speed and direction, perhaps also at what rate it is spinning, its hardness or springiness when hit, and so on. You might call this list of properties the classical "state" of the billiard ball, and the better you knew its state the better you could predict its behaviour. But quantum theory throws all that out of the window. You can only describe the "quantum state" of a photon in terms of its probabilities. And the probabilities change, depending on what you plan to do with the photon. A photon headed for a mirror will be reflected or transmitted when it gets there. But if the same photon were heading towards a polarising screen, you would need to describe it in a different way. For the classical billiard ball, one set of properties -- mass, speed and the like -- will tell you everything you need to know about it, under every circumstance. But the quantum state of a photon -- now that's a different matter.
You can see why physicists of the old school found quantum theory confusing, alarming and quite possibly dangerous. It seems as if the photon has no reliable properties of its own, and only reluctantly acquires them as a sort of conspiracy between it and the measuring device. The nature of reality it implies recalls Gertrude Stein's comment about the unremarkable city of Oakland, California: "There's no there there."