Pick Peat!

People have been trying to figure out the physics of light for a long time: the double slit experiment is over two hundred years old, and the fundamental problem of whether light is a wave or a particle has been argued since 400 BC.

The problem is pretty straight forward.  Consider a baseball player whacking a fly ball into the outfield:  everyone experiences the sound of the bat smacking the ball — that’s the sound wave, spreading through out the stadium.  On the other hand, the ball behaves like a single “thing” — it stays in one piece and follows a graceful arc into the glove of the outfielder, instead of spreading like a wave.

The reason the double slit experiment is so intriguing is that it demonstrates that light behaves like a wave and a particle, which is pretty dang counterintuitive if we think about the problem in terms of sound or baseballs.

Here’s a thought experiment:  If you fire a bunch of identical photons at two small slits cut into a screen, and look at the pattern that emerges on the wall beyond, what do you think you’ll see?

If you think of light as a particle, you’d see two bright dots on the wall.  After all, if you threw a baseball, it would pass through the hole and straight on to the wall behind.  Split the baseballs between the holes, and voila: two piles of baseballs … or in the case of photons, two bright dots on the wall.

If you think of light as a wave, you’d see a smear of light: the light wave would pass through both slits and spread out like ripples in a pond, a process called diffraction.  You might even see some patterns in the smear if you think about wave interference between the two slits.

And what actually happens is … all of the above.

If you send a single photon to the screen, and put a dot at where it hits the wall beyond, it will land in a single spot, just like a baseball.  But unlike the baseball, it probably won’t be in line from where you fired it.

Weird.  Lets fire another photon.

It’ll do the same thing, but land in a different spot.  Possibly quite far away from the first.

Keep on firing photons, and you’ll start to see a pattern emerge on the wall … and the pattern looks exactly like the interference pattern you’d expect to see for waves passing through those slits.

Ok, cool.  So, somehow the photons get redirected while in flight to form what looks to be an interference pattern.  But how is that possible?  We’re only firing one photon at a time, so it’s either passing through one slit or the other, right?  Interference comes from two waves interacting, so that means the light has to be passing through both slits — which is only possible if it’s actually a wave hitting both slits at the same time.

Maybe it’s not an interference pattern?  What happens if you cover up one of the slits?  That pattern disappears — so we can reasonably conclude that it really is an interference pattern produced by a light wave passing through both slits simultaneously.  In fact, we can prove it is interference, based on the frequency of the light and the pattern of bright and dark bands.

But wait, weren’t we firing single photons?  Single photons that pass through both slits in the screen simultaneously?   And end up as single points of light on the wall beyond, but organized into an interference pattern from intersecting waves?

Yes, yes, yes, and yes.

Behold: the strange glory of quantum mechanics, and particle-wave duality.  

It turns out that all of the major subatomic particles behave like this — photons, electrons, neutrons, and protons.  There have even been experiments with molecules that exhibited wave-like properties (like diffraction), including fluorinated fullerine, a relatively huge molecule with 60 carbon atoms and 48 fluorine atoms. 

There are still significant debates about the mechanism that causes particle-wave duality, but no doubt that we can see it in action.

Granted, it’s not something we see every day, but when you think about the fact that it’s real and verifyable, there’s nothing “spooky” about it.  There are no laws being broken, or supernatural powers involved — it’s just not something we expect.

Kind of like a surprise party.

That’s how I like to think of quantum mechanics:  it’s the biggest surprise party ever thrown for the physics world, and everyone’s invited.

(image courtesy of Dr. Tonomura and the Wikimedia Commons)