Why would it take so long for light to leave the Sun?

22 June 2008


[On a previous podcast] You said it would take a photon about a million years to pass from the centre of the sun outwards, but I thought the speed of light was constant?


Yes. There's a reason for saying that. The reason is that Brian Fulton, who's Professor of Astrophysics at York told me that. So I know it's right! The reason is that the sun is so dense because it's such a huge body with so much pressure in the middle that the photon effectively behaves like a gigantic game of pinball. The photon gets banded around all over the place and finds it very difficult to escape. If you extrapolate back to the reaction, the fusion reaction: four hydrogen atoms fusing together to make one helium and some energy. The energy that came was probably made at least a million years ago to make that photon. It's just taken that long to filter its way to the surface. If you took it to its logical conclusion if the sun suddenly stopped all its activity now you'd still probably have a million years of light locked away inside.

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