Light created in the sun takes 4,000 years to reach the Earth

Light created in the sun takes 4,000 years to reach the Earth

Many people are under the impression that light created in the sun takes eight minutes to reach the Earth. Not so. The light we're seeing could be anywhere from 4,000 to a million years old.

Yes, any light streaming out from the surface of the sun does make it to the Earth in under ten minutes, but that light didn't begin its journey at the surface of the sun.

It's deep within the sun that most of its light gets made from the fusion of hydrogen nuclei. Hydrogen nuclei don't fuse unless they're shoved together by immense heat and pressure, and so the inside of the sun is pretty dense. It's so dense that even photons take a while to get out of it. Visible light photons only move at the Googlable speed of light in a vacuum. Anywhere else, they are absorbed and re-emitted, are turned different directions and scattered, and generally interact with the substance through which they are moving. So while photons streaming from the surface of the sun can make an uninterrupted run straight to Earth, photons in the sun have to take baby steps of between one millimeter and one centimeter to get anywhere. If it were just a matter of pausing at each step, the photon wouldn't slow down too much, but each time they interact, they are sent out in a completely random direction. This means that, after taking a single step away from the fused atom that birthed them, the photon's second step can send it right back again.

The radius of the sun is about 696,000 kilometers. That's about 69,600,000,000 centimeters, which then needs to be squared to approximately 5 x 10^21 centimeter-long steps taken at the speed of light. Taking those steps means that lights breaks the surface, at most, an average of 4,000 years after it was created. There are a lot of fudge factors involved. The sun isn't equally dense all the way through. Some people imagine that the steps taken will be shorter - and taking it down to millimeters instead of centimeters changes the time from thousands of years to tens of thousands of years - or that the substance it moves through will involve more steps. And so the values for light to reach the Earth from the sun stretch from the conservative four thousand years and eight minutes, to the rather liberal one million years and eight minutes.