Why No One Really Understands What Light Is

In 1904 Albert Einstein proposed that light comes in discrete packets, an idea that took many years to be accepted among scientists.  Everyone knew that light was a wave, so the idea was viewed as ridiculous.  It still is, kind of, even though that concept is now foundational to our modern life.

You may have heard that light is a wave and a particle.  I’m not sure what your reaction is, but that should make you scream.  Waves and particles are completely different things, at least in the world we live in.  Imagine a fishing float bobbing on passing waves or spectators at a stadium doing the wave. I’m sure it is obvious that the fishing float and its bobbing motion are completely different things, as well as a spectator and the stadium wave.  But Einstein says differently.

In the world we live in, particles and waves are mutually exclusive categories.  Particles are individual objects, like a tennis ball or a penny.  Particles are counted using integer numbers, i.e. quantized.  You can have 7 tennis balls or 177 tennis balls, but not 7½.  Half of a tennis ball it not just a lesser amount of a tennis ball; cutting it in half fundamentally changes its nature and behavior.  Particles can’t be in the same place at the same time; they exhibit exclusionary behavior.  The whole point of a game of billiards or marbles is to use one particle to knock other ones around.  Thus particles are things, are quantized and are exclusive.

Waves are not.  Waves are not objects, but rather traveling disturbances.  Think about doing the wave at a stadium.  The wave goes round and round the stadium but the individual spectators are simply standing up and sitting down.  There isn’t any thing that is traveling, just a collective motion of a lot of individuals.  Waves also come in all sizes, from massive waves breaking at surfing beaches to small ripples on a pond from a falling grain of sand.  There is no intrinsic limit on how small a wave could be, or by how much one is bigger than another; waves are not quantized.  Waves do not exclude each other, either.  The spreading ripples from two pebbles dropped into a still pond will cross each other and keep on going; a careful observation of the point where the ridges cross each other will reveal that point to be even higher.  Two interacting waves just add up, or exhibit superposition behavior.  Waves are disturbances, not quantized, and exhibit superposition.  Particles and waves are opposites in important ways.

Particles and waves are mutually exclusive categories in our everyday understanding.  So when Albert Einstein proposed that light is quantized, and electrons were observed producing diffraction patterns (due to superposition), people found this confusing.  How could the same thing belong to two mutually exclusive categories?  But so much of our modern life is based on this very fact, officially called quantum mechanics.  Lasers, LEDs, and silicon chips are all based on this principle, so it must be true.  We just don’t understand it.

For a century now people have wrestled with the philosophical implications of it, resulting in a range of claims, from serious to outlandish.  Whatever the philosophical implications, it also illustrates an important truth about how we learn and know things.

Human knowing and learning is fundamentally about connections.  At the cellular level, knowledge is storied or modified by brain cells making, changing, or removing connections with other brain cells.   At the organism level, we learn things by making connections with previous knowledge.  Children learn addition by building on knowledge of counting, which in turn serves as the starting point for subtraction and multiplication.  Mnemonics work by connecting a set of knowledge one is trying to learn to something else.  Learning something is easy if we can connect it to something we already know; it is hard if we cannot.

But where does our starting knowledge come from?  Humans are not born with a storehouse of instinctive knowledge, so we have to start from scratch.  Infants spend a lot of time observing the world and it may seem it takes them a long time to learn simple things.  However, they have no store of pre-existing knowledge to make sense of the world, and so instead have to build it from scratch, constructing and refining the categories through which we interpret reality.  When quite little, my nephew visited the zoo and declared everything to be either a “dog” or a “duck”—until he saw the giraffe.  Our categories get more sophisticated as we get older, but categorizing things is central to our ability to make sense of the world.

But what happens when we encounter things that don’t fit into any of our categories?  Or worse, things fit two mutually exclusive categories?  That’s what happens with light.  It can exhibit superposition and also exclusive behavior, behaving in quantized and non-quantized ways.  Nothing in our familiar world acts like this; they do not fit into our existing categories.  Learning quantum mechanics is hard and confusing because our existing mental categories are no help.  If we keep at it, we’ll eventually construct new category structures like an infant does.  Fortunately that happened in my quantum course, since my graduate research involved experimental and theoretical applications.  But it was hard, and I still feel I don’t really understand quantum mechanics.  There are simply some areas of reality so different from our everyday world that our categories don’t fit and we’ll never feel that we fully understand.  If you find this confusing, you are not alone.  Richard Feynman, who won the Nobel Prize for developing the current quantum theory of how light and charged particles once declared, “I think I can safely say no one understands quantum mechanics.”