Years ago, bored at work, I used to come up with sets of random questions to throw at my co-workers (I was their boss, so they had to live with my eccentricities). They could be about science, history, geography, grammar, music, anything. The rules of the game, as they evolved, required that I couldn’t pose a question I wasn’t able to answer myself off the top of my head, which gave me an opportunity to carry on like the intolerable show-off I so rarely have any means to be. After a couple of years I’d thrown literally hundreds of questions at the poor kids, and, still bored, I eventually wrote out all of the answers and put them together in a set of volumes you could find here on the Needlefish main page, were you interested, which I’m guessing you’re not, but anyway:
Some of the answers might include the odd inaccuracy because they were, you know, written off the top of my head.
It sounds like I was trying to run a half-assed office trivia contest, but the thing is, the questions weren’t what I’d call trivial. To me, the identity of, say, the person who first worked out the laws of genetic inheritance, and how he did it, isn’t trivia. The name of the second actor to play Darren on Bewitched – that’s trivia**. How Einstein changed the scientific explanation of gravity from the Newtonian concept of the action of some nebulous force to the description of gravitational effects as simply the way objects move along the curves impressed upon spacetime by massive objects – not trivia.
Moreover, some of the most simple-sounding questions produce absolutely mind-bending answers that most of us have never heard, and can scarcely believe when we do. I just love stuff like that. I’ve spent a lot of my life trying to get those around me to share my fascination with such things. I never succeed, of course, but I’m game to keep trying, so, OK, here’s a simple question for you, the first in what, despite the title of today’s post, is likely to be a decidedly intermittent and not at all daily series: what is light, anyway? Plain ordinary white light coming off a bulb in your living room – what exactly is its nature? How does it move through space? Is it a stream of little particles, or a series of waves?
Fasten your mental seat belts, because incredibly, in the supremely strange world of quantum physics, it can be proven to be both. We generally think of light as a wave, and the various frequencies of light are characterized by different wavelengths, which is demonstrably a fact. Yet, if you conduct a very clever experiment, you can prove that when light hits something, it’s just a particle, which we call a “photon”, while also demonstrating that it must have been a wave just before it hit.
The proof is provided by the “double slit” experiment. Basically, you shine light through two narrow slits, side by side, at a screen that will detect and display light’s arrival. The light that arrives is measured as discreet particles, sure enough; but the places that the particles strike set up what can only be the product of an interference pattern of the type that two waves would create, as if you dropped two rocks in a pond. But if we’re firing a string of particles through two slits, shouldn’t they basically carve two small impact zones, side by side, as bullets would? Yes, if, when emitted, they actually were particles.
But they weren’t. The light as it travels through the two slits is not a series of bullet-like photons, but (get this) a set of literal waves expressing the mere tendency of those photons to exist across an essentially infinite set of possible, but not equally probable, places along their travelling wave fronts. A stream of photons is thus actually a series of probability waves pouring through the two slits. The waves will only reduce down to particles when they hit the screen, and the photons’ locations are measured and determined – at which point, since each can only be in one place, each wave collapses down to a single particle. Until measured, though, each photon could theoretically have been anywhere in the Universe along its wave, as could the particle/waves emitted right beside it, so the two probability waves interfere with each other, and cancel each other out at points of overlap, just as two waves in a lake will do, and sound waves, too.
It gets even more counter-intuitive. You can produce the same effect when you repeatedly fire just a single photon at the slits. Each photon travels as a probability wave, and, upon hitting the slits, exits to the other side as two emanating waves that create the very same pattern of wave interference. The single photon is, in effect, interfering with itself.
This is astounding, even intuitively unacceptable, but it’s irrefutably true. Until something like a photon (or an electron, or a whole atom) is measured, and is therefore required to be at one place or another, it’s just a probability, a tendency to exist somewhere across an almost infinite set of possibilities, some of which are more likely than others. If this makes you wonder what this means about the nature of what we call “reality”, well, you get the idea.
Lately, I’ve more or less accepted the logical necessity of taking it one step further. I think that the particle/wave duality, and numerous other aspects of both quantum theory and macro-cosmology, force us to consider the simulation hypothesis, and accept that the Universe may in fact be synthetic, an artificial construct composed purely of information. This is an idea I explored a while back in this space:
Everything Real Might Not Be – Sort of
Like quantum pioneer Niels Bohr said, at the fundamental level nothing in the Universe is real, not until some action we take up here at the macro level forces the issue by narrowing down the possibilities. This is the true essence of Heisenberg’s famous uncertainty principle; at the quantum level, he realized, the exact position and velocity of a supposed object like an electron can’t both be measured at the same time, even in theory. The very concepts of exact position and exact velocity together, in fact, have no meaning in nature. In other words, nothing is fixed as a real thing with fixed attributes until we measure it, and by measuring it we change it. Atoms, and the sub-atomic particles that compose them, aren’t actually things at all in their native state, not in the way we conceive of “things” anyway. They’re predictive information based on probabilities, and in a sense only spring into being when we act upon them. What does that tell you?
As I always say when talking about Quantum Physics, the hell of it is that they can prove it. In all the history of experimental science, there’s never been a theory that produced so many verifiable predictions, borne out time and again, over and over, no matter how crazy they seem. It made Einstein mental, and it should drive you ’round the bend too. Or, I guess you could decide not to think about it all…
That was fun, yes? Let’s do this again some time!
** Dick Sargent, who replaced Dick York.
the needlefish 
2 comments on “Your Daily Mind-Bender, Issue #1: The Nature of Light”