## A bunch of photons all approximately parallel to the x-axis:
as.photon(as.3vel(cbind(0.9,runif(10)/1000,runif(10)/1000)))
## mirror ball:
jj <- matrix(rnorm(30),10,3)
disco <- sweep(matrix(rnorm(30),10,3),1,sqrt(rowSums(jj^2)),`/`)
p <- as.photon(c(1,0,0))
reflect(p,disco)
table(reflect(p,disco)[,2]>0) # should be TRUE with probability sqrt(0.5)
## relativistic disco; mirror ball moves at 0.5c:
B <- boost(as.3vel(c(0.5,0,0)))
p |> tcrossprod(B) |> reflect(disco) |> tcrossprod(solve(B))
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