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Light is the connection[br]between us and the universe.

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Through light, we can[br]experience distant stars

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and look back at the[br]beginning of existence itself.

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But what is light?

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In a nutshell, light is the[br]smallest quantity of energy

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that can be transported, a photon,[br]an elementary particle without a real size

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that can’t be split,[br]only created or destroyed.

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LIght also has a wave-particle duality,

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being kind of a particle and[br]a wave at the same time,

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although this is a lie.

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Also, when we say “light”,[br]we actually mean visible light,

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which is a tiny part of[br]the electromagnetic spectrum,

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energy in form of[br]electromagnetic radiation.

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Electromagnetic radiation consists of

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an enormous range of[br]wavelengths and frequencies.

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Gamma rays have the smallest wavelengths,[br]because they are highest-energy photons.

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But most gamma rays are[br]just under 10 picometers,

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which is still way smaller[br]than a hydrogen atom.

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For reference, a hydrogen atom[br]compared to a cent

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is about as big as a cent[br]compared to the Moon.

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Visible light is in[br]the middle of the spectrum,

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in the range of about[br]400 nanometers to 700 nanometers,

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about the size of bacteria.

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On the other end of the spectrum, radio[br]waves can be up to 100 km in diameter,

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while the biggest[br]wavelengths we know exist

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can span from 10,000 km[br]to a baffling 100,000 km—

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way larger than Earth!

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From a physics standpoint, all these[br]different waves are the same—

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they all have the wave-particle duality[br]and travel at c, the speed of light,

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just at different frequencies.

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So what makes visible light special, then?

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Well… absolutely nothing.

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We just happen to have evolved[br]eyes that are good at

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registering exactly this part[br]of the electromagnetic spectrum.

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This is not a complete[br]coincidence, though;

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visible light is the only set of[br]electromagnetic radiation

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that propagates well in water,

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which happens to be where most eyes[br]first evolved millions of years ago.

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That was a smart move, because[br]light not only interacts with matter,

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it’s also altered by it and can be used

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to gather information[br]about the world around us

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with almost no delay,

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which is arguably really[br]helpful for survival.

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Okay, where does light come from?

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A vast range of electromagnetic waves[br]are created when atoms or molecules

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drop from a higher state of[br]energy to a lower one.

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They lose energy and emit it[br]in the form of radiation.

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At the microscopic level, visible light is[br]created when an electron within an atom

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that is in an excited state drops[br]to a lower energy state

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and loses this excess energy.

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The same way, incoming light can elevate[br]an electron to a higher state of energy

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by being absorbed by it.

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Macroscopically, the moving charge[br]of the electron

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creates an oscillating magnetic field,

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which creates an oscillating electric[br]field perpendicular to it.

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These two fields move[br]themselves through space,

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transferring energy from[br]one place to another,

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carrying information about its[br]place of creation with it.

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Why, of all the things in the universe, is[br]light the fastest thing there is?

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Let’s change the question:

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what is the fastest way to travel[br]through space in the universe?

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It’s c, exactly 299,792,458 m/s[br]in a vacuum,

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one billion kilometers per hour.

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Electromagnetic radiation just[br]happens to move this fast.

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Any particle that has no mass travels at[br]c, without acceleration or any in-between.

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The light that’s being[br]released from a candle

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does not speed up until[br]it reaches lightspeed;

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at the very instant of its creation,[br]its speed is c.

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So why is c, or the[br]speed of light, finite, then?

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Well… nobody knows.

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Our universe is just built this way.

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We don’t have a smart answer here.

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So light is a part of a spectrum,

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an elementary particle that[br]also behaves like a wave,

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propelled by two perpendicular fields

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traveling at the speed[br]limit of the universe.

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Okay, that’s nice and all, but what[br]about all the crazy stuff about

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traveling at the speed of light and time,

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the twin paradox, quantum stuff,[br]things like that?

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We have to save them for another video.

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For now, let’s just be happy that we[br]evolved eyes that pick up

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waves of information[br]permeating the universe,

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making us see things that help us to put[br]our existence into perspective.