№ 003 · True Scale
The Oldest Light
The sunlight on your face is 8 minutes old. So how old is its energy?
1 Your guess first
The sunlight on your face left the Sun about 8 minutes ago. But how old is its energy?
Why it’s so slow
It’s so packed, light can’t go straight
The core is about 15million °C and crushed incredibly dense. Light travels about a millimetre before it’s absorbed, then it’s given off again in a random direction. Over and over, roughly 500 thousand billion billion times on the way out.
Zig-zags cost size × size
A random walk doesn’t cover distance in a line. To get R steps away it needs about R × R steps. The Sun is about 700 billion millimetres from centre to surface, so the zig-zag needs roughly 500 thousand billion billion steps.
It’s the energy, not one photon
No single bit of light makes the whole trip. The energy is passed on, starting as gamma rays in the core and leaving as sunlight. The outer part of the Sun is crossed faster, by churning hot gas.
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