Talk:1441: Turnabout

Explain xkcd: It's 'cause you're dumb.
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In the few seconds the photons take to get to the moon and back, the earth has moved enough on its axis that the reflected beam from a perfect retroreflector is not gonna hit the protagonist.

The retroreflectors for the Apollo missions were deliberately spoiled so they return six slightly offset beams, angled such that photons from one of them will go back near enough to the source.

Oh, and of course there's also the whole r^4 thing too. ‎ (talk) (please sign your comments with ~~~~)

Citation needed. And did you mean the inverse square law? 07:37, 31 October 2014 (UTC)
Since you acknowledge that the reflectors for the Apollo missions were constructed to take this into consideration and the photons will return near enough to the source, the cartoon is still valid.  Now, whether the photons would retain sufficient energy upon their return to cause harm when they did not have enough power to destroy the reflector in the first place is a subject for another discussion . 07:49, 31 October 2014 (UTC)
The photons will retain the same power (more or less...), but there will be less of them. The laser (e.g. the one from the Apache Point observatory) spreads out on its way to the Moon (perfect collumation into a millimetre-wide laser beam is neither practical nor desirable, given the need to 'flood' the vicinity of the reflector in leiu of impossible accuracy, including to account for asymmetric atmospheric distortion on the beam and its return) so that only a small proportion of the beam hits a reflector unit (one of the largest being 0.6m²) and the returning beam (for reasons already mentioned) is again much wider than the collecting telescope (3.5m diameter, possibly). There's usually no more than a dozen photons (per each short pulse of the laser) that actually make the return trip to be detected, and often it's well down into single-figures, requiring many such pulses to gather enough photons to make a statistically significant analysis.
All this, of course, does render even more ridiculous the concept of manually firing even a combat-strength laser beam across the necessary void and back again to such a precise hit (and, BTW, the What-If someone was mentioning is and shows a possibly less-tightly-collumated-than-Apache-Point laser having a diameter of almost half that of the Moon itself). But what the hey? ;) 10:04, 31 October 2014 (UTC)

Superimposing the 3rd and 5th panels over each another shows the beam does not come back exactly to its source Defaultdotxbe (talk) 08:09, 31 October 2014 (UTC)

Can anyone comment on how likely/unlikely it would be to accidently hit a retroreflector on the moon? The moon takes up only a very small proportion of the sky (when visible at all of course), and so the likelyhood of that wild shot even hitting the moon at all has to be pretty low. (I'm sure there was a What-If on this...) --Pudder (talk) 09:22, 31 October 2014 (UTC)

Randall covered lasers hitting the moon in what-if 13 --Pudder (talk) 09:45, 31 October 2014 (UTC)