There’s a great thought experiment what would happen if the sun suddenly disappears. It would get dark, of course, and the temperature would drop drastically in the next weeks. The earth would no longer be bound to a source of gravity and would get jolted out of the (former) solar system and end up as a rogue planet (planets that travel in interstellar space with no host star). Not great for the temperatures either.
The typical question that surrounds this experiment: how long would it take for earth to realize that the sun disappeared?
Travel distance of a photon
While the earth travels around the sun on an ellipsis with an aphelion (which is the point in space when the earth is farthest from the sun) at 1.017 AU and a perihelion (when the earth is closest) at 0.983 AU, the mean distance is 1 AU - an abbreviation for astronomical unit which is defined as the distance between the sun and the earth (how convenient). In more usable terms, the distance is around 149.6 million kilometers.
How long does it take a photon the to get to earth? The speed of light is 300,000 kilometers per second, a bit of calculation gets us to 8 minutes and 20 seconds.
And because gravity also travels at the speed of light, it takes 8 minutes and 20 seconds for the described effects to take place after the sun vanished.
Getting from the suns core to its surface
A completely different question would be: how long would it take a photon from the time of its creation to the suns surface?
I personally find this question much more fascinating.
How does a photon get created?
The driving force in the creation of photons is nuclear fusion - Hydrogen
atoms fuse together in the extremely hot (millions of degrees - Celsius or
Fahrenheit does not matter at that scale) and dense core of the sun. It takes
4 protons to create a Helium-4 nucleus, two positrons and two neutrinos.
This also releases energy which is then emitted as photons.
The core of the sun is really densely packed and photons cannot travel freely like they do in the vacuum of space. In fact, they can only travel around 1mm before they hit an atom and get scattered.
Image being in a moshpit in a metal concert (or being drunk and hitting all the telephone poles in your area). So move a bit, bump into another person and switch direction. This happens over and over and over again until you leave the moshpit.
But even a moshpit that might get pretty big in larger venues will never reach the size of the suns core: approximately 150,000km, which is around 25% of the suns diameter. The photon now keeps bumping into atoms, getting redirected all the time and has to travel an unimaginable distance.
How long does it take?
Calculations of people much smarter than me state that it takes a photon from the time of its creation to the surface around 20,000 to 200,000 years. Adding 8 minutes and 20 seconds is like adding a single blade of grass to a vast meadow (what an analogy…).
You now might think, wow, the photons must be really annoyed to spend all this time bumping into other stuff before finally getting to the surface. But special relativity tells us that for photons, which travel at the speed of light, virtually no time has passed between the moment of its creation and the moment it hits your retina.
Now that’s a great fun fact!