 Solution

ID: past-exam-of-the-mathematics-course-of-the-university-of-cambridge/2021/ii/paper-2/38c/c/i/solution

Codex  0 Created 2026-09-24 Updated 2026-09-25

For a radial null ray,

d θ = 0

and the metric gives

d t = d r

for the outgoing branch. Starting at

r = t = 0

, it reaches

r = R

t_{1} = R .

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