Title text: Whenever I miss a shot with a sci-fi weapon, I say 'Apollo retroreflector' really fast, just in case.
In the comic, two people are engaging in a battle with laser guns. One appears to gain the upper hand as he jumps on an obstacle, as the other's shot goes wide. He delivers the classic line "Any last words?" and is answered with the confusing phrase "Apollo retroreflectors". The earlier wild shot, reflected off the Moon, promptly lances down from space and hits him in the back.
A retroreflector is a device or surface that reflects light back towards its source. Several such devices were placed on the Moon during the Apollo missions and have been used ever since by scientists on Earth to measure the distance between the two bodies using laser ranging. Retroreflectors were placed by the American Apollo 11, 14, and 15 missions. The Soviet Lunokhod 1 and 2 rovers also carried such reflectors; attempts to use them for laser ranging were unsuccessful from 1971 to 2010, but were successfully renewed after the rovers' positions were photographed by the Lunar Reconnaissance Orbiter.
The title text may be a reference to the common practice of "calling bank" in the game of basketball. In basketball, the backboard may be used to deflect the ball into the hoop. This is called a "bank shot." In casual games, if the player using the backboard in this way does not indicate that it was intentional by "calling bank" before releasing the ball, the basket may not be counted in order to not give the player credit for a wild shot that happened to go in. When a player releases a shot that they realize is off the mark they sometimes quickly say "bank" to try and fool the other players into thinking that they were intentionally trying to "bank" the ball off the backboard into the hoop. In the title text scenario, "Apollo retroreflector" is used the same way "bank" is in basketball, i.e., the shooter meant to hit the target by reflection rather than directly.
Randall discussed the effect of hitting the Moon with lasers in What If: Laser Pointer and the likelihood of hitting a celestial object with a laser in What If: Into the Blue.
The likelihood of the wild shot being aimed at the Moon is fairly low in itself, and the probability of accidentally hitting a retroreflector on the Moon is lower still. Even if it did, it is highly unlikely that a pistol-sized generator could produce a beam coherent enough to inflict damage after traveling to the Moon and back, as lasers built for the purpose of hitting retroreflectors on the Moon typically get a return around one quadrillionth of the original beam, and a visible light laser would need a very large lens or mirror in order to still be relatively concentrated upon hitting the reflectors.
- [A guy and Cueball are engaged in a fight with laser pistols. The guy is standing behind a small box, firing his gun at Cueball.]
- [Cueball is kneeling behind a larger box, returning fire.]
- [The guy leaps on top of the larger box, knocking Cueball backwards and off balance. Cueball fires wildly into the air.]
- [Now the guy is standing on the box. Cueball is sprawled on the ground, laser pistol out of reach, at gunpoint.]
- Guy: Any last words?
- Cueball: "Apollo retroreflectors."
- Guy: What?
- [The guy gets shot in the back by the returning beam of Cueball's wild shot.]
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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. 184.108.40.206 (talk) (please sign your comments with ~~~~)
- Citation needed. And did you mean the inverse square law? 220.127.116.11 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 .18.104.22.168 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 http://what-if.xkcd.com/13/ 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? ;) 22.214.171.124 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 http://xbehome.com/uploads/retroreflector.png 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)
- And in what-if 109: the chance of hitting the moon is 1/180000 126.96.36.199 20:13, 5 November 2014 (UTC)
What is a 'reflective' palindromic number like mentioned in an (early?) version of this explanation:
- Citation: "It is worth noting that the number of this comic is 1441: a 'reflective' palindromic number."
All I could find about reflective ~palindromes was that you should be able to mirror the number/word and still get the same. But you cannot mirror 4 into 4. So in that way it cannot be reflective... (1 maybe if the font is the correct one, else only 8 and 0 in numbers and some letters like A and X). Of course it is a palindrome as it would read 1 4 4 1, also in reverse. But reflective - not so much? If it is not reflective, then it has no relevance in the explain as it was this reflection that had a reference to the story... Kynde (talk) 10:49, 31 October 2014 (UTC) What
- I agree, the number is a palindrome, but not a reflective palindrome. I'm not quite decided on whether the fact it is a palindrome holds any relevance, or is simply a coincidence. --Pudder (talk) 11:10, 31 October 2014 (UTC)
I've removed a sentence about "continuity problems" and why the beam does not go right through the opponent (or hit the original shooter). No laser tag set that I know of is powerful enough to go right through someone, I imagine that would be regarded as a slight health & safety issue. You could argue that panel 1 shows the beam going through someone, but I think that is in fact behind him. As far as him being knocked over, I would guess that is to emphasise his being hit, rather than physically knocked over. --Pudder (talk) 13:35, 31 October 2014 (UTC)
- IMO, it's not laser-tag at all, but a full-strength offensive laser-blast (of at least 'stun' level). The first 'through the body' shot is actually a miss 'in depth' (rarely for XKCD comics without other explicit perspective cues to the image, like this one, it's in the <TwilightZoneMusic+Reverb>Third! Dimension!</TwilightZoneMusic+Reverb>...), neither harming nor impeded by the attacker. The rebounded shot at the end appears to have at least caused the same attacker to lose grip of his gun, if not caused physical damage to his back. (Although I suppose he could be wearing a "shocker" or vibrating-on-hit style of laser-tag kit, or is reacting to the sound-effect arising from the unexpected "hit detection".) No doubt a combat-level laser would be dialled up to damage your target without necessarily burning straight through.
- Also (regarding another comment), while I imagine it'd be useful to wear armour designed to be reflective (or even retroreflective!) all over, it would probably be impractical, whereas the lunar retroreflector array being used might possibly itself withstand the beam for at least long enough to get a decent amount of bounce-back. Anyhow, with such Improbable Aiming Skills, I suspect at least latent and subconcious Force abilities were in play, which can handwave all such niggling problems. Right guys? (And also opens the doorway for this being a universe with full-on Stormtrooper Armour... you know, the kind that's Reactive Armour, but assembled the wrong way round so it ends up hurting you more than if you were wearing no more than woven and leather fabrics...) 188.8.131.52 14:24, 31 October 2014 (UTC)
As the earth is turning, the reflected beam would hit different spot. In that 2.5 secs it would take the beam to travel back to the earth, earth would have turned 1.160km at the equator. So even if the beam would stay collimated the beam would miss coming back. 184.108.40.206 (talk) (please sign your comments with ~~~~)
Clearly black-hat stole an apollo reteoreflector and mounted it nearby. Panel 4 just happened really really fast 220.127.116.11 18:28, 2 November 2014 (UTC)
Why does the fourth paragraph mention signaling the reflector? You don't have to tell a mirror that there's a laser incoming. He's just claiming to have intentionally aimed at a reflector when it looked like he missed his shot. --18.104.22.168 22:05, 2 November 2014 (UTC)
Yeah, I don't know why the second half of the fourth paragraph is there either as it seems irrelevent to the comic or the retroreflectors. Squirreltape (talk) 12:33, 3 November 2014 (UTC)
+1. Agreed. Don't get that part about 'signaling' at all. Should be elaborated-upon or removed. I added a note with the body of the descrption becauwse I'm a newbie to Wiki-editing.22.214.171.124 21:37, 3 November 2014 (UTC)
- Would this be legal under international treaties concerning the weaponization of space?
Placing weapons in orbit is prohibited. But after something is placed on the moon for peaceful purposes, can it be used for the purpose shown in this comic?126.96.36.199 06:13, 4 November 2014 (UTC)
By saying retroreflectors you make it seem as if you had planned the whole thing. If you don't then it will be assumed that it happened inadvertently and you won't get credit for the hit. (see the part about calling the bank) 188.8.131.52 11:30, 4 November 2014 (UTC)
This last phrase (starting at "in reality") needs further elaboration to be meaningful. Why does one need to signal the photoreflector for any purpose? Why would an information-carrier need to be sent/received? There is an unstated assumption here. I've reworded it to make clear that contributor's intent: signaling the reflector to aim.184.108.40.206 12:28, 4 November 2014 (UTC)
There is a community portal discussion of what to call Cueball and what to do in case with more than one Cueball. I have added this comic to the new Category:Multiple Cueballs. In this case there is no reason to call one Cueball and the other friend. It could easily be the other way. So I have changed to remove Cueball.--Kynde (talk) 14:56, 15 March 2015 (UTC)
- After having looked at several other Multiple Cueballs comics I have regretted this change. It had already been changed partly back so that is great. It is clear (to me now) that the one shouting retroreflector is of course Randall (per the title text). And that guy is thus Cueball as he is the protagonist of the story - i.e. the one with the interesting remark. But I think it is nice that it is still made clear that they both looks like Cueball. --Kynde (talk) 13:41, 24 March 2015 (UTC)