0:00 A single particle of light can be in two completely different places at the exact same time.0:07 It can.0:08 And not only can it, it HAS to be, for the universe to work the way it does.0:14 Okay, that's just...0:15 that's a lot to start with.0:17 Last week we were talking about how the whole universe stretches, which is already a huge idea.0:25 But at least space is just...0:27 space.0:28 This feels different.0:30 This is about what things are actually made of.0:33 Exactly.0:34 We're going from the biggest scale imaginable to the absolute smallest.0:40 And down here, the rules are...0:42 well, they're not what you'd expect.0:45 So walk me through it.0:47 How can one thing be in two places?0:50 My car keys can't even be in ONE place I can find them.0:54 Right.0:55 So, let's forget about light for a second and think about something simpler.1:01 Imagine you have a wall with two little doorways in it.1:05 And you're throwing marbles at it.1:08 Okay, I'm with you.1:10 Some marbles go through one door, some go through the other.1:14 On the back wall, you'll just see two clumps of hits, right behind each door.1:21 Simple.1:21 That's how particles behave.1:24 They are distinct things, in distinct places.1:27 Makes sense.1:28 Like tiny bullets.1:30 Precisely.1:30 Now, what if instead of marbles, we have a pool of water, and we make waves that go towards those two doorways?1:39 Uh, the waves would go through both doors at once...1:44 and the ripples would kind of spread out and run into each other on the other side.1:50 Exactly!1:51 They interfere.1:52 Where two wave crests meet, you get a bigger wave.1:56 Where a crest and a trough meet, they cancel out.2:00 So on the back wall, you'd get this pattern of high-water marks and low-water marks.2:07 An interference pattern.2:09 That's how waves behave.2:11 Okay, particles are marbles, waves are ripples.2:15 Got it.2:15 So what is light?2:17 Well… that’s the billion-dollar question.2:20 For centuries, scientists fought about it.2:23 Is it a wave?2:24 Is it a particle?2:26 The answer is...2:27 yes.2:27 That's not an answer!2:29 It is!2:29 It really is!2:30 Here's the part that breaks your brain.2:34 If you shoot a single particle of light—a photon—at those two doors, or two slits...2:40 it makes a wave pattern.2:42 Wait.2:43 One particle?2:44 But you just said the wave pattern comes from ripples interfering with each other.2:50 How can one particle interfere with anything?2:54 With itself.2:55 No.2:55 Yes.2:56 It's as if the single photon goes through BOTH slits at the same time as a spread-out wave, interferes with itself, and then...3:06 lands on the back wall as a single particle in one spot.3:10 It behaves like a wave when it's traveling and a particle when it lands.3:16 Okay.3:17 My head hurts a little bit.3:19 So light is just this one reaaally strange thing that gets to play by its own rules.3:26 Ah, but see, that's the real kicker.3:29 It's not just light.3:30 What do you mean?3:31 They tried the same experiment with electrons.3:35 The tiny particles that make up, you know, everything.3:40 Atoms.3:40 Electricity.3:41 And what happened?3:42 Don't tell me.3:44 They acted like waves!3:45 A single electron, fired at two slits, creates an interference pattern.3:51 It goes through both at once.3:53 So electrons are also...3:55 both?3:56 They're particles and waves?3:58 Everything is.3:59 You are.4:00 I am.4:00 Your car keys are.4:02 Everything in the universe has a wave nature and a particle nature.4:07 It's called wave-particle duality.4:10 Hold on.4:11 If I have a wave nature, why don't I, like, ripple through two doorways when I walk into a room?4:18 Because you're huge!4:20 The wavelength of an object gets smaller as the object gets bigger.4:25 For an electron, the wavelength is big enough compared to its size that it matters.4:32 For you, your wavelength is so absurdly, comically small—trillions of trillions of times smaller than an atom—that it has absolutely no effect.4:44 You are, for all practical purposes, just a particle.4:48 A big bag of particles.4:50 Wow.4:50 So this...4:51 this weirdness is just hidden from us because we're too big to see it.4:56 It's not hidden, it's just the foundation.5:00 The solid, predictable world we experience is built on top of this fuzzy, probabilistic, quantum weirdness.5:09 It's not a bug, it's the operating system of the entire universe.5:14 So the world only seems solid because we're not small enough to notice the gaps.5:20 You could put it that way.5:22 Or you could say the world is far more mysterious and interconnected than it looks.5:29 Even a single, lonely particle is aware of all the possible paths it could take.
