How to See Quantum with the Naked Eye


Turn off the lights. Look at this dot and squint. Keep squinting until you see lines of light
shooting out of the top and bottom of the dot. What you’re seeing is actually quantum mechanics. You may have heard that quantum mechanics
is the physics describing small things that you can never see — and will never see — in
your everyday life. But if you know what to look for, you can
see quantum mechanics all around you. There’s nothing special about this dot. Look at a light — any light around you — and
squint. You’ll see streaks shooting out of the top
and bottom of that light, like this. But what are those streaks and where do they
come from? How do we get light up here when the lamp
is all the way down here? Einstein showed that light is particles: photons. And those photons travel in a straight line. So if the photons of light are coming from
here, then maybe I can block the streak by putting my hand between the streak and my
eye. It doesn’t matter how close you get your
hand to your eye; you can never block out the streak. It seems like the streak is actually jutting
up behind your hand. The only way to block the streak is to block
the light itself. So the streak must be coming from the lamp. But how is that possible if your hand is blocking
the path from the lamp to where the streak appears to be, right behind your hand? Which begs the question: is it possible that
the light from the lamp actually isn’t traveling in a straight line? The answer is yes. And here’s an experiment that you can do
to prove it. Get a laser pen and a piece of cardboard. Then slice a very tiny horizontal slit in
the cardboard. Arrange it so that the laser light is going
through the slit onto a wall. The laser light will actually go straight
until it hits the little slit, and then it spreads out into this vertical pattern on
the wall. Some of the light will travel straight until
it reaches the slit and somehow bends and lands up here. If light can bend like this, then it makes
sense that the streaks we saw earlier were coming from the lamp. But so far, none of this explains what the
streaks — or what the laser dots on the wall — are. If instead of laser light going through the
slit you had water waves going through the slit, then there would be places on the wall
where you would have waves crashing and other places where there are no waves. Those would line up exactly with the bright
and dark places on the wall from our laser experiment. This pattern is called a diffraction pattern,
and it’s always created by some kind of wave. This pattern of light on the wall implies
that light is some kind of wave, a wave that can bend around corners, like water waves. Here’s where it gets even crazier. What if our laser only shot one photon at
a time? The first photon would land, possibly, here. The second photon would land, maybe, down
here. But if we let a lot of photons build up, we’d
see this wave pattern. In this way, photons are actually both a particle
and a wave. That is quantum mechanics. When you squint at the lamp, the lamp acts
like the laser, your eyelids act like the slit, and your retina, the group of light-sensitive
cells at the back of your eye, acts like the wall. Now, if you take your laser and shine it on
a wall and then squint while looking at that laser dot, you’ll actually see a diffraction
pattern. But that diffraction pattern isn’t on the
wall, even though it looks like it is. The diffraction pattern is actually on your
retina. You can also see this pattern by carefully
squinting at some small white lights that you can find around the house. When you squint at a lamp, though, what you’re
seeing is a bunch of these patterns all overlapped so it looks like a streak. These streaks are called diffraction spikes,
and now that you know how to find them, I promise: you will see them everywhere you
go. And just think: every time you see a streak,
that’s actually particles of light, photons, acting like a wave, and you’re seeing quantum
mechanics in action. And hit that bell button so you are the first
to find out when there are new episodes.

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