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.