What Is Light?

Light is the connection
between us and the universe.

Through light, we can
experience distant stars

and look back at the
beginning of existence itself.

But what is light?

In a nutshell, light is the
smallest quantity of energy

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

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

LIght also has a wave-particle duality,

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

although this is a lie.

Also, when we say “light”,
we actually mean visible light,

which is a tiny part of
the electromagnetic spectrum,

energy in form of
electromagnetic radiation.

Electromagnetic radiation consists of

an enormous range of
wavelengths and frequencies.

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

But most gamma rays are
just under 10 picometers,

which is still way smaller
than a hydrogen atom.

For reference, a hydrogen atom
compared to a cent

is about as big as a cent
compared to the Moon.

Visible light is in
the middle of the spectrum,

in the range of about
400 nanometers to 700 nanometers,

about the size of bacteria.

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

while the biggest
wavelengths we know exist

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

way larger than Earth!

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

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

just at different frequencies.

So what makes visible light special, then?

Well… absolutely nothing.

We just happen to have evolved
eyes that are good at

registering exactly this part
of the electromagnetic spectrum.

This is not a complete
coincidence, though;

visible light is the only set of
electromagnetic radiation

that propagates well in water,

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

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

it’s also altered by it and can be used

to gather information
about the world around us

with almost no delay,

which is arguably really
helpful for survival.

Okay, where does light come from?

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

drop from a higher state of
energy to a lower one.

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

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

that is in an excited state drops
to a lower energy state

and loses this excess energy.

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

by being absorbed by it.

Macroscopically, the moving charge
of the electron

creates an oscillating magnetic field,

which creates an oscillating electric
field perpendicular to it.

These two fields move
themselves through space,

transferring energy from
one place to another,

carrying information about its
place of creation with it.

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

Let’s change the question:

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

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

one billion kilometers per hour.

Electromagnetic radiation just
happens to move this fast.

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

The light that’s being
released from a candle

does not speed up until
it reaches lightspeed;

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

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

Well… nobody knows.

Our universe is just built this way.

We don’t have a smart answer here.

So light is a part of a spectrum,

an elementary particle that
also behaves like a wave,

propelled by two perpendicular fields

traveling at the speed
limit of the universe.

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

traveling at the speed of light and time,

the twin paradox, quantum stuff,
things like that?

We have to save them for another video.

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

waves of information
permeating the universe,

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