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Rambo faces off against a pebble in a pond illustrating interference in particles and waves.
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You'd think physics would make sense...at least to
somebody. You'd think that it wouldn't lead perfectly intelligent and sane people to believe that two opposite things can be true at once, or that a cause can come after an effect. Well, think again. The theory of the atom, quantum mechanics, is so strange, that Erwin Schrödinger, who discovered its central equation, said about it: "I don't like it, and I'm sorry I ever had anything to do with it."
One of the strangest predictions of quantum mechanics came from an idea that another physicist, John Wheeler, once had. In 1978 he imagined a weird experiment that may push the boundaries of common sense more than any other experiment. But until now, that experiment was just a thought, a fantasy—no one ever actually performed it. Now, a team of French and Chinese physicists led by Jean Roch, has announced in
Science that they have done it—and sure enough, the universe is a
crazy place.
A Pebble in a Pond vs. Rambo
Rambo charges at a building. As he runs towards a wall with two windows, his two Uzis fire bullets randomly in all directions. Most of them hit the wall, but some make it through the windows, and hit the inner wall, at the other end of the room.
Looking at the back wall of the room, opposite the windows, where did most of the bullets hit? Where are most of the bullet holes? Where's the deadliest place to stand?
If you guessed "opposite each window," you were right. More bullets land opposite the windows than anywhere else. As you move away from each window, you see fewer and fewer bullet holes. Of course, if the windows are close enough to each other, you may see the most bullets right smack-between the two—bullets from both windows can hit that area.
But no matter—the point is that that's how particles, such as bullets, behave. They don't really interact with each other much, they just go on their separate ways, and pile up next to each other as they hit the wall behind the windows.
FNAL
When bullets—or any other particle—are shot at a wall with two holes, most bullets
will fall right behind the two holes, and fewer will land farther away from the holes.
The particles do not interact with each other, and therefore do not show interference.
Now picture a rather different scene: A placid pond in a meadow. For whatever reason, someone has put two parallel barriers in the pond: one has two slits in it, and the other doesn't. After a child throws a pebble into the pond, the ripples hit the barrier with the slits. Now a strange thing happens. The wave that hit the barrier splits into two waves—one flowing from each slit.
Unlike the bullets, these new waves interact with each other quite a bit, in a phenomenon called interference. When the waves hit each other, their effects add up: if both the waves have a peak when they collide, they compound each other, and become doubly strong. This is called constructive interference. If, on the other hand, one wave has a trough where the other has a peak, they cancel each other out. This is destructive interference.
FNAL
When a wave hits a wall with two slits in it, a new wave emanates from each slit.
These two new waves interfere with each other, sometimes canceling each other out
and sometimes strengthening each other. When they hit the next wall, they produce
an "interference pattern": extra strong when they interfere constructively, and almost
nonexistent when they interfere destructively.
So if you were to stand next to the second barrier, the one without the slits, and measure where the waves hit it hardest, you would notice a weird "interference pattern." At the point where both of the waves lined up perfectly, they would hit the wall with double strength. Alternatively, if the two waves happen to line up oppositely, they would not hit the wall at all—they would have canceled each other out.
The moral of the story is that waves interfere with each other, but particles don't. By looking at where the highest number of particles land, or where the wave is strongest, you can tell whether you're looking at waves or particles.
Partiwave? Wavicle? Huh?
In 1801, physicist Thomas Young used the phenomenon of interference to prove that light is a wave. He shone a light on a barrier with two slits in it, and observed the pattern produced on the wall behind it. Lo and behold, there was an interference pattern. At some spots, the light was bright—this is where the two light waves from the two slits interfered constructively. At other points, there was no light at all—at these points, the waves interfered destructively. So case closed—light is a wave.
Not so fast, said Albert Einstein, 104 years later. He used several mysterious experiments, including one called the photoelectric effect," to argue that light was a particle. Light particles came to be called "photons." One of the central mysteries of modern physics developed—light, somehow, acts like a wave sometimes (it interferes) and like a particle other times (the photoelectric effect). Physicists soon realized, to their amazement, that that's true about everything, including electrons, photons, neutrons and the atoms they make up. In fact, if you send electrons through a barrier with two slits, you'll end up with an interference pattern on the other side—in a sense, they are waves, too!
Physics: the Road to Insanity
Some physicists are troublemakers. They see a paradox like that, and they tickle it until it explodes into something that really doesn't make sense. They ask: "What happens if we perform Young's double-slit experiment, except we send only one particle through at a time?" Normally, when you send light through two slits, it behaves as a wave: a new wave starts from each slit, and the two waves interfere with each other. But what if you only send one photon through at a time? It can only go through one slit at a time, and then what will it interfere with? If there were two photons going through at a time, then they could each go through a different slit, and then behave as waves, and interfere with each other. But with only one photon that can't happen—it has to choose one slit or the other, and can't interfere with itself. You should see the "bullet pattern," not an interference pattern.
Nope. The laws of nature, it seems, don't behave so nicely. When physicists actually performed the experiment, the interference pattern showed up! No one could—or can—really explain why. The best explanation, though, goes like this: The photons were flying out randomly in different directions, and they had an equal probability of flying through each slit. Somehow,
each probability was true: the photon went through both slits!
Sure enough, this phenomenon shows up all over the place in the weird world of quantum mechanics. The photons behave as if each probability were simultaneously true. Then, the two opposing, yet possible, realities interfere with each other. The probability was the reality.
Wait, wait. Why don't you just
look at which slit the photons go through? You could put a small detector next to each slit that will send out a signal when a photon passes by. So you'll find out once and for all which slit the photon went through.
Don't worry—the physicists thought of that. They put these detectors up, and they worked—the photons were only going through one slit at a time! Phew! But then, how does the interference work? They checked—and there was no interference! The photons had begun behaving like bullets! The interference pattern disappeared!
Of course, according to the crazy interpretation of probability, this all makes sense. When you're checking to see which hole the photons pass through, there are no more probabilities to play with—only certainties. The different, probable realities can't interfere with each other, because when we're looking, there
are no different, probable realities. Just by looking, you can change everything—and somehow the photons know this.
Weird.
Mind-Blowing Physics: A Strange Experiment Finally Performed
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