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
