(This is Part 3 of a series on what would happen if the Sun stopped. Read Part 1 and Part 2 first.)

Imagine you're standing in the middle of a crowded room. Not just any crowded room. A packed one. Shoulder to shoulder. So crowded you can't take more than a single step in any direction before bumping into somebody. And every time you bump into someone, you get spun around to face a brand new random direction. You can't see the walls. You can't see the doors. All you can do is push, bump, spin. Push, bump, spin.

You can feel your blood pressure climbing already. You want out. Now. How long does it take you?

The answer depends on how big the room is, naturally, but it also depends on something subtler. You aren't walking out of the room. You're random walking out of the room. Every step lands in a completely random direction. Half the time you're blundering deeper into the crowd without realizing it. Sometimes you go in circles. Sometimes you make a little progress and then immediately undo it.

This is not an efficient way to travel.

There's some math describing how long this takes, and the math is frustrating, especially if you happen to be in a hurry. It says that to cover a given distance by random walk, you can't just take the number of steps a straight walk would need. You have to take the square of that number. If the door is 4 steps away on a normal walk, it's 16 steps away on a random walk. If it's 10 steps in an empty room, it's 100 in a packed one.

Every photon born in the core of the Sun is in exactly this predicament. Worse, actually. The Sun's interior isn't a gas, it's a plasma, every atom stripped down to bare nuclei and free electrons drifting everywhere. And photons absolutely love to interact with free electrons. A photon born in the core travels about one centimeter before slamming into an electron, scattering off in a completely random direction, traveling another centimeter, slamming into another electron, scattering again. And again. And again.

One. Centimeter. The Sun's radius is 70 billion of them. That's the straight-line, empty-room, normal-walk distance. For a photon actually stuck inside the Sun, it's 70 billion squared steps.

If you tried to count them off at one per second, it would take you longer than the current age of the universe. Several times over.

Each step takes only a fraction of a nanosecond, which is good. But there are a staggering number of them, which is bad. Run the arithmetic, and a photon born in the core of the Sun takes around 100,000 years to claw its way out to the surface.

A hundred thousand years.

If photons could simply stream straight out, the trip would take about two seconds. Instead, bouncing around like the unluckiest pinball in history, the journey takes 100,000 years. The random walk inflates the travel time by a factor of roughly a trillion.

The photon striking your face right now was born around the time anatomically modern humans were first spreading beyond Africa. Neanderthals were still around. Agriculture hadn't been invented. Spoken language as we'd recognize it didn't yet exist. Every civilization, every religion, every memory in all of human history is younger than the trip that photon just finished.

Sunlight is REALLY old.

And by the way, it isn't even the same photon that started the trip. Photons in the solar interior don't merely ricochet around like billiard balls. They are constantly being swallowed by electrons and then re-emitted, in new random directions and at slightly different energies. So the gamma ray born in the core, carrying around a million electronvolts, gets ground down step by patient step into longer, softer, lower-energy light. By the time it escapes the surface it's visible light, about one electronvolt, peaking conveniently in the very wavelengths our eyes evolved to catch. The energy survived the journey. The original photon, not so much.

Most of that century-long crawl happens in what we call the radiative zone, the inner 70 percent of the Sun by radius, where the plasma is dense and hot and the photons are trapped in their pinball nightmare. Above the radiative zone sits the convective zone, where the plasma finally turns cool and opaque enough that radiation can't carry the energy along fast enough anymore. So the Sun gives up on radiation and starts to BOIL. Bulk motion takes over: hot blobs of plasma physically rise to the surface, dump their heat, and sink back down. Once energy reaches the convective zone, it pops out to the surface in just a few months.

The upshot is that anything happening in the core of the Sun stays invisible from the surface for about 100,000 years. The light you see from the Sun today is reporting on conditions in the core during the last ice age. If the fusion rate at the heart of the Sun had been quietly drifting for the past 50,000 years, we would have no idea. As far as light is concerned, the Sun's surface is a 100,000-year delayed broadcast.

The Sun is gigantic. The Sun is crowded. Changes deep inside it take an enormous amount of time to propagate outward. We already knew that fusion is so inefficient that the Sun is basically coasting on stored heat, and that the Kelvin-Helmholtz mechanism could keep the lights on for tens of millions of years all on its own.

Now layer on top of that the fact that the surface itself is broadcasting from a hundred millennia in the past.

You can see where this is going.

In Part 4, we finally pull the trigger, switch off fusion, and trace exactly how, and how slowly, the Sun would actually die.