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Speaker 1: Welcome to brain stuff from how stuff works, Hey, brain

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Speaker 1: stuff loin Volga bamb here. Light, in addition to being

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Speaker 1: a bright patch of sunshine on your window sill, is

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Speaker 1: a metaphor for enlightenment and exploration, which is a bit

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Speaker 1: paradoxical for a phenomenon that, even after thousands of years

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Speaker 1: of inquiries and endless experiments, scientists still can't quite explain.

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Speaker 1: Is it a particle or a wave or both or neither?

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Speaker 1: Do we need a new word for it? Your eyes

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Speaker 1: tell you a lot about the way light behaves. It

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Speaker 1: travels so fast that it seems instantaneous, about a hundred

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Speaker 1: and eighty six thousand miles or three thousand kilometers per second.

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Speaker 1: It blazes through air and space and laser like straight lines,

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Speaker 1: but it also bounces, reflects, and refracts, and when it

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Speaker 1: interacts with the right medium, like a camera lens, it

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Speaker 1: may curve. We know that it's made up of tiny

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Speaker 1: units that we call photons, and we know that the

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Speaker 1: term waves can describe its movements, but neither of these

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Speaker 1: words really encompass lights oddities. In ancient times, the Greeks

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Speaker 1: used philosophy to attempt to address light's wide range of behaviors.

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Speaker 1: Perhaps they thought light is actually composed of little bits

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Speaker 1: of stuff that bounced to and fro. The idea never

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Speaker 1: really caught on. Then, in the sixteen hundreds, French philosopher

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Speaker 1: Renee de Cart became convinced that light was essentially a wave,

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Speaker 1: one that moved through a mysterious substance that he called plenum.

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Speaker 1: Isaac Newton thought that light was a particle, but he

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Speaker 1: was at a loss for a way to explain many

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Speaker 1: of its properties, like the way it refracted and could

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Speaker 1: be split by a prism from a single beam of

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Speaker 1: white light into a rainbow of many colors of light.

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Speaker 1: This was largely before the rise of empirical studies in science,

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Speaker 1: wherein we attempt to answer questions about the world around

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Speaker 1: us by designing experiments that demonstrate well how stuff works.

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Speaker 1: Back in the day, science was a matter of philosophy,

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Speaker 1: people coming up with ideas about how stuff works and

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Speaker 1: basically arguing about the idea's merit to be fair. Our

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Speaker 1: modern microscopes, computers, and other equipment help. Just for example,

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Speaker 1: light's behavior becomes more evident depending on where you're observing it.

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Speaker 1: In the vacuum of space, light zips along at the

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Speaker 1: aforementioned a hundred and eighty six thousand miles or three

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Speaker 1: thousand kilometers per second. But point a beam of light

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Speaker 1: at a very dense bit of matter, say a diamond,

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Speaker 1: and it can slow to only around seventy seven thousand

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Speaker 1: miles or a hundred twenty four thousand kilometers per second,

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Speaker 1: much easier to observe relatively. To try to explain in

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Speaker 1: these are modern times, what light is, Let's first remember

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Speaker 1: some science basics. Waves are not a thing or a substance.

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Speaker 1: They're a property of a thing. A wave is a

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Speaker 1: compressing and stretching of a particular medium, like an ocean

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Speaker 1: wave that drives towards the shore or the ripple that

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Speaker 1: spreads out across the surface of a pond. When you

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Speaker 1: toss in a rock, you can see the waves with

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Speaker 1: your eyes, feel them with your body, and sometimes when

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Speaker 1: a sound wave happens in the air, you can hear

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Speaker 1: them with your ears. Particles, on the other hand, are

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Speaker 1: not quite so easy to define. A particle can be

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Speaker 1: a tiny bit of matter, a matter broken down on

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Speaker 1: into its smallest and most basic units. Water, for example,

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Speaker 1: is made up of countless particles particles that are affected

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Speaker 1: by waves. What's really happening when you watch a wave

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Speaker 1: in the ocean or a ripple in a pond is

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Speaker 1: that each particle or molecule in this case of water

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Speaker 1: is being moved, and thus the medium of the ocean

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Speaker 1: or pond is being compressed and stretched in sequence, and

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Speaker 1: we see waves. But light, as experiments have proven, also

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Speaker 1: consists of particles that we call photons that behave like waves.

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Speaker 1: Let's unpack that. There was a famous nineteenth century double

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Speaker 1: slit experiment in which researchers beamed light through two slits

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Speaker 1: and observed the way the light struck a screen behind

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Speaker 1: the slits. What they saw was that the streams of

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Speaker 1: light affected each other like two hands splashing water in

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Speaker 1: the same sink, as if they were waves interfering with

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Speaker 1: one another. But then in the twentieth century, scientists began

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Speaker 1: their pioneering explorations into sub atomic particles like neutrons and electrons.

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Speaker 1: Albert Einstein wondered what would happen if you admitted light

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Speaker 1: one photon at a time. In the double slit experiment,

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Speaker 1: what scientists saw dumbfounded them. The single photons went individually

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Speaker 1: through the slits, but the way that they struck the

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Speaker 1: screen over time showed the same interference pattern that occurred

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Speaker 1: with full scale beams of light streaming through both slits.

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Speaker 1: This behavior can't be explained by the physics we use

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Speaker 1: to describe particles and waves in the macro world around us.

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Speaker 1: It's in the realm of quantum mechanics, the physics theories

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Speaker 1: that describe what goes on at the very smallest sub

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Speaker 1: atomic levels and which we humans still don't really understand.

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Speaker 1: So ultimately, if you want to answer the question what

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Speaker 1: is light, you could call it both a particle and

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Speaker 1: a wave and you'd be correct. But as for fully

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Speaker 1: explaining why and how it works, we're still working on it.

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Speaker 1: Today's episode was written by Nathan Chandler and produced by

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Speaker 1: Tyler Clang for i Heeart Media and How Stuff Works.

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Speaker 1: To learn more about the weird behavior of light and

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Speaker 1: the history of how humans have thought about it, check

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Speaker 1: out our sister podcast, Daniel mclae explained the universe. Their episode,

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Speaker 1: Is Light, a Particle or a Wave goes into lots

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Speaker 1: more details, and of course, for more on this and

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Speaker 1: lots of other lighthearted topics, visit our home planet How

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Speaker 1: Stuff Works dot com.