Quantum Mechanics Rules!

I did not write this, but I feel it’s worth posting again.
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A Tale of Two Slits

Posted on: June 1, 2009 2:05 AM, by Ethan Siegel from scienceblogs.com

(It’s just good, clean physics, I swear!) Last week, my buddy Lucas was watching a BBC documentary about petulant musician Mark Oliver Everett (of the Eels), and his quest to understand his father, the late physicist Hugh Everett.

Hugh Everett is famous as the discoverer of the many-worlds interpretation of quantum mechanics, which is a fascinating — although speculative — idea about how the Universe fundamentally works.

Imagine you have a wall with two slits — very close together but
distinct — with a screen behind it. If I throw very small grains of
sand at this wall, we can predict what’s going to happen. Some of the
grains will go through one slit, some of the grains will go through the
other slit, and the rest of the grains will get blocked by the wall. If
I took a look at the screen, I will find two neat piles of sand stacked
up against it.

But what if, instead of a particle like sand, I shot waves, like
light or waves of water through the two slits? Because they’re waves,
they can interfere with one another. Instead of two neat piles, we get a complex interference pattern. Some places have constructive interference, and the light appears more intense on that part of the screen. In other places, there’s destructive interference, and there’s less light (or even no light at all) in those places.

So, here’s where things get weird. What if — instead of sand or light — we shot electrons through those double slits. Electrons are supposed to be particles, right? So they should make two neat little piles. But they don’t. They interfere with each other, and make the pattern on the screen that only waves are supposed to make.

Well, physicists are clever, so they decided to try this little trick: let’s shoot the electrons one-at-a-time
at these two slits. Most electrons that you fire smack against the
wall, but a few make it through. After a few hundred electrons, you
can’t really tell what’s happening, but after tens of thousands make it
through, and you add up where they landed, here’s what you find:

You still get an interference pattern! Somehow — and this seems nuts — each electron is interfering with itself! How is that possible? Is part of the electron passing through one slit and part passing through the other slit?

Well, we can do one more experiment: this time, we shoot
electrons one-at-a-time at this wall, but at each slit, we shine a bit
of light, and detect which slit the electron goes through. As
each electron is fired, one (but never both) of the detectors goes off,
telling you which slit the electron went through. But — and here’s the
crazy part — the pattern on the screen now shows no interference,
and instead we just get two separate peaks corresponding to the two
“classical”, particle-like paths the electrons could have taken.

What are the possible explanations for this? Well, the standard quantum mechanical interpretation (i.e., Neils Bohr’s Copenhagen interpretation) says that everything
is always a wave, and it’s only when an observation is made that this
wave collapses, and things act like a particle. Everett’s idea, the many worlds interpretation, holds that everything is always a particle, but whenever there are multiple possible outcomes, they all happen, but we don’t know which one happened in our universe until we “look”.

The weirdest part? These two incredibly different
interpretations — one postulating googols of other Universes, one
postulating that the laws of physics are not deterministic — both give rise to the same observations.
In other words, these interpretations are not only both valid, but as
far as we can tell, are indistinguishable in our Universe. Wrap your
minds around that one!

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