Be Smart

Hey, smart people.

Joe here.

I'm back with another bright idea.

Ha!

Get it?

This video's about sunlight.

Okay, here's a freaky thought: If the sun instantly switched off like a light bulb-- which can't happen, by the way, 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 eight and a half minutes.

Earth orbits the sun at an average of 150 million kilometers away-- sometimes more, sometimes less-- because our orbit is actually an ellipse and not a circle, and the light travels at the fastest speed there is, around 300,000 kilometers per second.

Divide the distance to earth by that speed and you get 500 seconds-- almost eight and a 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!

Tens of thousands of years old.

Okay, hold on.

If it takes eight minutes for a photon to travel the immense distance from the surface of the sun to earth, it should take 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 woolly mammoths were still walking the earth.

So why is sunlight so old?

You keep saying... [THEME MUSIC PLAYING] 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 biproducts 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.

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.

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.

You can see that this journey takes a lot longer than the nice, linear path from the core to the surface.

It's kind of random.

A random walk.

Imagine you walk out of a tavern and go on an adventure.

But before you take another step, you're going to roll a four-sided die.

The result of this die roll dictates a step in a different direction.

Let's say 1 is straightforward, 2 is to the right, 3 is backwards, and 4 is left.

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, 1 step every second, would take 11 days-- 1 million steps.

It's a very inefficient way to take a stroll.

One does not simply random walk to Mordor.

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 in any direction.

Let's divide our directions like the 12 hour marks on a clock and then roll a 12-sided die.

Okay, let's draw a straight line in that direction.

We hit a proton, and our photon will bounce off in some other direction.

Roll again, draw a line in that direction, hit.

Let's roll again, go in that direction, and so on, and so on.

In the sun, this would be happening in three dimensions.

You can see that this is going to take a really long time.

In fact, for the approximately 10 the 57 protons in the sun, spread out like our grid, the average distance between them ends up being 10 to the minus 10 meters.

or about half the size of a water molecule.

To random walk the 690 million meters from the solar core to the surface would require 10 to the 37 steps, which, calculating the random walk for a photon at light speed, is a journey of 100 billion years.

Wait a sec.

Let's see here-- okay, Big Bang... it was-- ah!

Just as 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?

The actually random walk that a photon takes out of the sun is more complicated than our example, because unlike our simplified grid, the sun isn't the same density all the way through.

It's very dense in its core, and it's less dense in the middle, and even less dense on the outside.

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 that it'll "kapew!"

off of any given proton that it meets.

When scientists put all of this together-- the varying density of the sun, the changing energy of the photons along the journey-- and they plug it into big computers with math that would break my brain, the time that it takes for 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-- it's true.

Even the smell of coffee drifting from this cup, tiny scent molecules colliding and bouncing between vibrating air molecules and eventually making their way to my nose, that's a special random walk called Brownian motion.

But sunlight is the random walk that's 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's absorbed, whether that's after and 8-minute trip to earth or a 13-billion-year trip from the edge of the observable universe, that photon experiences no time.

From sunlight's perspective, it reaches earth as soon as it's born.

So sunlight is old and not old, just like I'm old compared to a baby, but compared to a mountain, I just got here-- spring chicken.

Age is something you could put a number on, but it always depends on perspective.

And that's pretty enlightening.

Sun pun.

Stay curious.