Hey smart people, Joe here.
I’m back with another bright idea.
Heh, get it?
Because this video is about sunlight… (glowing sun appears)
Ok, here’s a freaky thought: If the sun instantly switched off like a light bulb–which
can’t happen, by the way, just in case you’re concerned… that’s not how the sun works–but
let’s say someone with a universal remote clicked off the power and the sun did go dark
right now then we wouldn’t know for almost 8 and a half minutes.
Earth orbits around the sun at an average of 150 million kilometers away (sometimes
a little more, sometimes a little less, because our orbit is actually an ellipse, not a circle).
And light travels at the fastest speed there is, around 300,000 km/s.
Divide that speed by the distance to Earth and you get 500 seconds, or almost 8 and half
minutes.
That’s the average time it takes photons of light to get from the sun to the Earth,
and that’s how long it would take before we knew the sun went dark.
But the most amazing thing about the sunlight we see is it’s actually really old.
SUPER old.
AMAZINGLY OLD!
Tens of thousands of years old.
Ok, hold on.
If it takes 8 minutes for a photon to travel the immense distance from the surface of the
sun to Earth… it should take like a second to travel from the sun’s core and escape
into space.
Yet every photon of sunlight that has ever hit your face was born when wooly mammoths
were still walking the Earth.
So why is sunlight so old?
What did you say?!
OPEN
Light travels in straight lines from the sun to us, which is why we cast a shadow on a
sunny day.
But a photon’s journey out of the sun is not so direct.
Photons are byproducts of powerful nuclear reactions in the sun’s core, as hydrogen
nuclei are fused to make helium nuclei.
The core of the sun is basically billions of hydrogen bombs exploding every second.
The outward pressure of these immense nuclear reactions is held in by the enormous mass
of hot gas–the rest of the sun–collapsing under gravity.
You can think of the sun as either a bomb held in by a gravity shell, or a heavy ball
of gas inflated by a nuclear balloon.
Every star is a balancing act between this urge to collapse and the urge to explode.
And this push and pull is what makes it so hard for sunlight to escape the sun.
So all of these fusion reactions happening in the sun’s core release massive amounts
of energy in the form of gamma rays.
Gamma rays are high energy photons.
(Which is how they power the Incredible Hulk) After a gamma ray photon is born, it travels
in one direction until it collides with some other particle inside the sun and ricochets
in another direction.
As they interact with and ricochet off all the other matter inside the sun, these photons
don’t slow down, because photons always travel at the speed of light.
Instead, they gradually lose energy.
Kind of like a ball loses energy as it bounces.
Along their journey, the photons lose more and more energy, going from Gamma Rays to
x-rays, ultraviolet, infrared, and of course, visible light, until the photon finally escapes
the sun.
But to get to space, each photon has to ricochet its way through a game of “nuclear pinball”.
Like this.
This journey takes a lot longer than the straight path from core to surface.
It’s… kinda random.
A random walk.
Imagine you walked out of a tavern to go on an adventure, but before taking another step,
you roll a four-sided die to choose a direction.
The result of the die roll dictates a step in a different direction.
One is forward, two is left, and so on.
This is a type of mathematical problem called a random walk.
The distance traveled will, on average, equal the step size times the square root of the
number of steps.
To walk a distance of 1 kilometer using our four-sided die method, one step every second,
would take 11 days.
One million steps.
It’s a very inefficient way to take a stroll.
One does not simply random walk to Mordor.
And a photon’s pinball-like journey, as it collides with and bounces off protons on
its path out of the sun, is a random walk like this on a very small and a very big scale.
This grid represents a bunch of protons in the sun’s core.
Let’s say a photon is released by a fusion reaction here.
It could go any direction.
Let’s divide our directions like the 12 hour marks on a clock, and roll a 12-sided
die.
(number) Ok, let’s draw a straight line in that direction.
We hit a proton, and our photon will bounce off in some direction.
Roll again.
(number) Line in that direction.
(Hit.
Roll.
Repeat)
In the sun this would be happening in three dimensions!
But you can see that this is going to take a really long time.
In fact, for the approximately 1057 protons in the sun, spread out like our grid, the
average distance between them ends up being 1x10-10 meters, or about half the size of
a water molecule.
To random walk the 690,000,000 m from the solar core to the surface would require 1037
steps, which, for a photon at light speed, is a journey of A HUNDRED BILLION YEARS.
Wait a sec.
Ah, just like I suspected.
That’s actually way older than the universe.
And older than the sun, which only formed about 4.6 billion years ago.
Photons of sunlight can’t be hundreds of billions of years old.
So where did we go wrong?
Well, the actual random walk a photon takes out of the sun is more complicated than our
example, because, unlike our simplified grid here, the sun isn’t the same density all
the way through.
It’s very dense in its core, less dense in the middle, and even less dense on the
outside.
And also a photon won’t always collide with every proton it meets.
The physics is pretty complex and quantum-y here, but the important thing to realize is
to a photon with lots of energy a proton looks really small, and for a photon with less energy
a proton looks big.
So as a photon loses energy along its pinball path, it changes the odds it’ll kah-pew
off a given proton it meets.
When scientists put all this together, the varying density of the sun and the changing
energy of photons along the journey, and they plug it into big computers, with math that
would break my brain, the time it takes a photon to random walk from the sun’s core
to space is about 170,000 years.
Random walks can describe a lot of things in our universe.
The diffusion of liquids and gases.
How bacteria and even animals move.
How Twitter recommends who you should follow.
Even the smell of coffee drifting from this cup: tiny scent molecules, colliding and bouncing
between vibrating air molecules, eventually making their way to my nose… ahh… is a
special random walk called Brownian motion.
But sunlight is the random walk that is responsible for all life on our planet.
And it is really old… or is it?
One of the strangest things about traveling at the speed of light is that time does not
pass.
From the moment a photon is created to whenever it is absorbed, whether that’s after an
8 minute trip to Earth or a 13 billion year journey from the edge of observable universe,
that photon experiences no time.
From sunlight’s perspective, it reaches Earth as soon as it is born.
So sunlight is old, and not old.
Just like I am old compared to a baby, but compared to a mountain I just got here…
age is something you can put a number on, but it always depends on perspective.
That’s pretty enlightening.
Sun pun.
High five.
Stay curious