Fear of Clowns

"Faith may be defined briefly as an illogical belief in the occurrence of the improbable."
- H. L. Mencken

Thursday, June 16, 2005

The double slit experiment: who's watching who? 

For a few weeks, I've contemplated doing a series of posts on the topics of life, the Universe, and epistemology and why I think I only understand this stuff as far as I do. The Scientific faith post was a prologue of sorts, although at first not intended to be. I had also intended to make the first post Yes, the other lane usually does go faster, but that will have to wait until later as I felt like writing about the double-slit experiment tonight. I think most have heard of it, and maybe most understand it, but as understanding it is important in general as well as a necessary understanding to have before I try to explain a less-well known but stranger experiment, here it is.

Imagine yourself standing in the middle of an absolutely still wading pool. Start to jiggle one leg around just a bit. The troughs and crests of your waves would radiate outward like this,

Now imagine the pool has a wall running through the length of it. The wall has a small gap so the waves can flow through it, like this,

The waves you're creating would propagate out from the gap, the gap being the center of the waves on the other side of the gap, just like your jiggling leg is the center of the waves on the side you're on,

If you're lucky enough to be near a still pond with a tree or large branch partly submerged by the shore (or imaginative enough to pull up the scene in your mind now), you can splash around and observe that this is how waves travel through gaps and around corners. If there are two gaps in the wall, an interference pattern would appear between the two waves,

Where the two wave crests meet, they will combine to make a larger crest (the light areas on the other side of the wall). Where a crest from one wave meets a trough from the other, the one's crest will equal out the other's trough (creating the dark areas).

Now, instead of standing in a wading pool divided by a wall with a gap in it, imagine two rooms with a barely cracked door in between them. There is a light bulb in one room, and the other room is entirely dark other than the shaft of light shining through the crack.

A beam of light would be cast on the wall opposite the door, as this diagram illustrates with a view from above,

The edges of the beam are not completely sharp - the light gets diffused a bit as it goes through the crack of the doorway, making the edges of the beam on the wall fuzzy,

You can imagine the particles of light, (called photons) in the beam as being tiny bullets being fired from the location of the light bulb - some of the bullets would glance slightly off the door of the frame, changing their trajectory slightly. The same effect causes fuzzy edges of the beam, the photons act much like tiny bullets.

Now pretend the rooms are very, very tiny - the crack in the door the width of a strand of spider's silk. We can place a photographic plate on the far wall of the tiny room to record the fuzzy beam of light. The image produced look much like the beam we saw on the fall wall of the two rooms,

Now imagine two tiny slits between our very small rooms,

Would the photographic plate on the far wall record two diffused beams just like it would beyond an actual wall with beams going through two cracked doors? No. We see an this,

The diffusion pattern is gone - instead we see an interference pattern, as if light acts not like bullets being fired by a gun, but like waves radiating out from a foot in a wading pool,

To repeat, light acts like tiny bullets - particles - when we give it one slit to pass through, but acts like a wave on the top of water when we provide two slits for it to pass through. Is light made of particles or waves? Does the really depend on whether we cover or leave open a second slit in the same experimental set-up?

If we were the first to discover this bizarre phenomenon, we'd want to look deeper into it. What if we assume light is like tiny bullets and repeat the experiment in such a way that only one photon could be traveling through our experimental set-up at once. We could use a very dim light source, as dim as a candle a mile away from the tiny wall.

First, we perform our one-photon-at-a-time experiment with only one slit open. We wait for enough of the dim light - enough photons - to find it's way through the slit to our photographic plate. We expect to see a diffused shaft of light, and we do,

The pattern on our photographic plate,

Now we perform the experiment with both slits open so each photon from our very dim candle a mile away will go through one of the two. We wait for the photographic plate to accumulate enough hits from photons so we can see where they landed, expecting them to have accumulated over time to show up as fuzzy shafts on the photographic plate on the far wall. Since we're only allowing one photon at a time to go through, we expect to see two* bars of diffused light, just as if we repeated the last experiment without changing the photographic plate, but switching which of our two slits was open.

But behold! This is not what we see. The light is acts like a wave - even though we are sure only one photon of light went through one or the other - ore both, as is the case - of the slits at a time,

The pattern on our photographic plate,

The photons chose to act like waves when we gave them two slits to travel through, but decided to act like particles if we left open only one slit. Whether light is a particle like a bullet going through one slit or like a wave on water traveling through both slits depends on whether there is one or two slits open in our experiment!

Are we examining the light, or is the light examining us? Or both ...

It's important to understand the experiment - or more accurately understand the results of the double-slit experiment to be baffling. Email me (or better, leave a comment) if something is unclear - or if you're not baffled: a sign that I didn't explain it well enough. The next experiment will be a step beyond the double-slit experiment, and therefore be a bit more complex, but much, much stranger.

* in the actual experiment the two slits are so close together that we would expect to see the diffused shafts of light merge into one diffusion pattern. The point being that we would see a diffusion pattern, not an interference pattern. The illustrations are easier to understand if they're drawn as they are - the slits being far apart.

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Very baffled - I will look again after some sleep


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