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What I want to do in this video is give ourselves
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a basic introduction to the phenomenon of light.
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And light is, at least to me, mysterious.
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Because on one level it really defines our reality.
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It's maybe the most defining characteristic of our reality.
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Everything we see, how we perceive reality,
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is based on light bouncing off of objects
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or bending around objects or diffracting around objects,
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and then being sensed by our eyes,
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and then sending signals into our brain that
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create models of the world we see around us.
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So it really is, almost, the defining characteristic
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of our reality.
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But at the same time, when you really go down to experiment
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and observe with light, it starts
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to have a bunch of mysterious properties.
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And to a large degree it is not fully understood yet.
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And probably the most amazing thing about light--
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well, actually there's tons of amazing things about light--
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but one of the mysterious things is when you really get down
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to it-- and this is actually not just true of light,
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this is actually true of almost anything
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once you get onto a small enough quantum mechanical level--
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light behaves as both a wave and a particle.
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And this is probably not that intuitive to you,
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because it's not that intuitive to me.
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In my life, I'm used to certain things behaving as waves,
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like sound waves or the waves of an ocean.
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And I'm used to certain things behaving like particles,
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like basketballs or-- I don't know-- my coffee cup.
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I'm not used to things behaving as both.
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And it really depends on what experiment you run
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and how you observe the light.
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So when you observe it as a particle,
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and this comes out of Einstein's work
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with the photoelectric effect-- and I won't go into the details
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here, maybe in a future video when
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we start thinking about quantum mechanics--
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you can view light as a train of particles moving
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at the speed of light, which I'll talk about in a second.
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We call these particles photons.
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If you view light in other ways-- and you see it
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even when you see light being refracted by a prism here--
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it looks like it is a wave.
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And it has the properties of a wave.
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It has a frequency, and it has a wavelength.
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And like other waves, the velocity of that wave
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is the frequency times its wavelength.
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Now even if you ignore this particle aspect of light,
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if you just look at the wave aspect of the light,
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it's still fascinating.
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Because most waves require a medium to travel through.
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So for example, if I think about how sound travels through air--
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so let me draw a bunch of air particles.
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I'll draw a sound wave traveling through the air particles.
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What happens in a sound wave is you compress some of the air
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particles and those compress the ones next to them.
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And so you have points in the air that have higher,
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I guess you could say, higher pressure
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and points that have lower pressure,
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and you could plot that.
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So we have high pressure over here.
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High pressure, low pressure, high pressure, low pressure.
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And as these things bump into each other,
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and this wave essentially travels to the right--
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and if you were to plot that you would see this wave
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form traveling to the right.
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But this is all predicated, or this is all
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based on, this energy traveling through a medium.
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And I'm used to visualizing waves in that way.
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But light needs no medium.
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Light will actually travel fastest through nothing,
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through a vacuum.
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And it will travel at an unimaginably fast speed--
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3 times 10 to the eighth meters per second.
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And just to give you a sense of this,
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this is 300 million meters per second.
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Or another way of thinking about it is it
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would take light less than a seventh of a second
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to travel around the earth.
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Or it would travel around the earth more than seven times
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in one second.
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So unimaginably fast.
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And not only is this just a super fast speed,
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but once again it tells us that light
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is something fundamental to our universe.
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Because it's not just an unimaginable fast speed.
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It is the fastest speed not just known to physics, but possible
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in physics.
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So once again something very unintuitive to us
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in our everyday realm.
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We always imagine that, OK, if something
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is going at some speed, maybe if there was an ant riding on top
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of that something and it was moving in the same direction,
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it would be going even faster.
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But nothing can go faster than the speed of light.
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It's absolutely impossible based on our current understanding
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of physics.
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So it's not just a fast speed, it
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is the fastest speed possible.
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And this right here is an approximation.
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It's actually 2.99 something something times 10
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to the eighth meters per second.
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But 3 times 10 to the eighth meters per second
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is a pretty good approximation.
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Now within the visible light spectrum-- and I'll
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talk about what's beyond the visible light spectrum
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in a second-- you're probably familiar with the colors.
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Maybe you imagine them as the colors of the rainbow.
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And rainbows really happen because the light
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from the sun, the white light, is being refracted
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by these little water particles.
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And you can see that in a clearer way when
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you see light being refracted by a prism right over here.
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And the different wavelengths of light-- so white light
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contains all of the visible wavelengths--
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but the different wavelengths get refracted differently
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by a prism.
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So in this case the higher-frequency wavelengths,
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the violet and the blue, get refracted more.
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Its direction gets bent more than the low-frequency
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wavelengths, than the reds and the oranges right over here.
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And if you want to look at the wavelength of visible light,
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it's between 400 nanometers and 700 nanometers.
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And the higher the frequency, the higher the energy of that
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light.
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And that actually goes into when you
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start talking about the quantum mechanics of it--
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that the higher frequency means that each of these photons
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have higher energy.
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They have a better ability to give kinetic energy
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to knock off electrons or whatever else they need to do.
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So higher frequency-- let me write
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that down-- higher frequency means higher energy.
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Now I keep referring to this idea of the visible light.
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And you might say, what is beyond visible light?
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And what you'll find is that light is just
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part of a much broader phenomenon,
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and it's just the part that we happen to observe.
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And if we want to broaden the discussion a little bit,
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visible light is just really part
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of the electromagnetic spectrum.
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So light is really just electromagnetic radiation.
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And everything that I told you about light just now--
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it has a wave property and it has particle properties--
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this is not just specific to visible light.
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This is true of all of electromagnetic radiation.
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So at very low frequencies or very long wavelengths--
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we're talking about things like radio waves,
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the things that allow a radio to reach your car;
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the things that allow your cellphone
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to communicate with cell towers; microwaves, the things that
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start vibrating water molecules in your food
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so that they heat up; infrared, which
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is what our body releases, and that's
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why you can detect people through walls
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with infrared cameras; visible light; ultraviolet light,
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the UV light coming from the sun that'll give you sunburn;
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X-rays, the radiation that allows
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us to see through the soft material and just visualize
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the bones; gamma rays,
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the super high energy that comes from quasars
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and other certain types of physical phenomena-- these
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are all examples of the exact same thing.
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We just happen to perceive certain frequencies
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of this as visible light.
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And you might say, hey, Sal, how come we only
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perceive certain frequencies of this?
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How can we only see these frequencies?
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Literally we can see those frequencies
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with our unaided eye.
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And the reason, or at least my best guess
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of the reason of that, is that's the frequency where
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the sun dumps out a lot of electromagnetic radiation.
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So it's inundating the Earth.
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And if, as a species, you wanted to observe things
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based on reflected electromagnetic energy,
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it is most useful to be able to perceive the things where there
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is the most electromagnetic radiation.
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So it is possible that in other realities or other planets
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there are species that perceive more
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in the ultraviolet range or the infrared range.
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And even on Earth, there are some
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that perform better at either end of the range.
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But we see really well in the part of the spectrum
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where the sun just happens to dump a lot of radiation on us.
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Now I'll leave you there.
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I think that's a pretty good overview of light.
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And if any of this stuff seems kind
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of unintuitive or daunting, or really
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on some level confusing-- this wave-particle duality,
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this idea of a transfer of energy through nothing--
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and it seems unintuitive, don't worry.
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It seems unintuitive even for the best of physicists.
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So you're already at the leading edge of physics thinking.