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| Time Machine Conversation |
 Title text: It's possible to do sea navigation without a compass, but you'll have to get some spoilers from the Polynesians. |
Explanation
Cueball has used a time machine to travel to the Iron Age and has a conversation with an ancient-times version of Hairbun (who seems to be a farmer, since she's holding a pretty modern-looking hoe and seems to be particularly knowledgable of the 'latest' plow developments). However, he is very surprised to find that she does not know about the compass (a very common tool in the present day). The magnetic compass was first invented in China around 200 BCE, well after the end of the Iron Age, and it wasn't used for navigation until the 11th century AD. For an Iron Age farmer the concept of a 'weird rock that always points north', as Cueball puts it, would seem quite ridiculous, and the inherent dangers of sea travel might well seem to be insurmountable ones. The subject of how many things that today are seen as perfectly normal and standard could seem very strange to those who lived before they were created has been covered before in xkcd.
Cueball inadvertently starts explaining compasses and then worries about the impact his words might have. According to common time travel tropes, this interaction might cause a chain of events that will lead to Cueball not existing, or worse, which would create a paradox (if it isn't already already a different kind of paradox through being a pre-existing component of Cueball's original timeline). However, rather than the potential radical impact he might have on history by introducing this concept earlier than should have happened, he appears to be concerned that he may have given her a spoiler for upcoming history[1]. Presumably he feels he has deprived her (or humankind more generally) of the joy that would have come with its eventual discovery.
He is then also concerned that he has managed to 'spoilerise' the concept of 'the spoiler'. The modern meaning of "spoiler" didn't arise until the 1970s, which post-date the Iron Age.[citation needed] Spoiler warnings became common on Usenet newsgroups in the late 1980s. Cueball may have created a temporal paradox by introducing the concept thousands of years earlier, although any such 'change' made to that time might easily have been forgotten again in the two or three thousand years since this encounter. In any event, while telling people thousands of years ago that there was a way to make a compass might have changed history significantly, telling them that there are stories that they would enjoy less if they knew the ending before hearing the story seems less likely to have made a significant impact. It's also likely that, even if the term 'spoiler' was adopted by these Iron Age people, it would long have fallen out of use by the time it came to be invented in the late twentieth century.
The title text has Cueball about to unleash another 'spoiler' on how to navigate without a compass, but he stops himself before saying it. However, he does still end up accidentally revealing that Polynesians know about it, though whether this was another unintentional slip or a deliberate clue left for Hairbun is unclear. It is thought that so-called 'Polynesian navigation' used other methods of marine navigation (celestial navigation, observation of birds, ocean swells, and wind patterns). As the Polynesians lived in the Pacific, which would probably be unknown to Hairbun and difficult to reach from her location, the clue is useless. It is unclear where Hairbun is, but it is likely that she is in Europe or the area around the Arabian Peninsula, where the term 'Iron Age' is most relevant, and which are quite far from the Pacific. Revealing the existence of the Pacific and its inhabitants to her may cause its own disruptions to history, though.
Whether or not this was an intentional connection, the stars known by some as "the Plough" (Ursa Major, perhaps more popularly known as "the Big Dipper" in the US) are also useful in finding the northern pole star (not the same star then as now but still in the same constellation), hence potentially linking both of the farmer's initial remarks.
The comic is based on shaky ground, as it's not clear how they're able to communicate so easily, unless it's part of the function of the time-travel technology. While humans did have language for thousands of years by this time, it would be very far removed from modern English, yet somehow they understand each other's speech. It also appears that the very existence of time travel is not considered a spoiler for an Iron Age person, or even in any way remarkable to them โ this might imply that the farmer is already very well aware of such phenomena (or even that Cueball will later have already visited the same society/farmer at an earlier date), which may be one way to explain apparently fluent conversational American English being spoken.
Time travel is a recurring theme on xkcd.
Transcript
- [Cueball is on the left with a ghostly halo around him. Hairbun is on the right, holding a hoe vertically.]
- Cueball: Oh hi! Guess my time machine works. How's life in the Iron Age?
- Hairbun: Not bad. Developing new kinds of plows.
- Cueball: Cool.
- Hairbun: And my brother was just lost at sea.
- [Only Cueball is shown, with Hairbun out of the panel.]
- Cueball: I'm sorry.
- Hairbun [from outside the right side]: It's OK. I think sea navigation is probably impossible.
- [Cueball and Hairbun are both shown again.]
- Cueball: Oh yeah, you don't have the compass, right?
- Hairbun: The what?
- Cueball: The weird rock that always points north?
- Hairbun: What are you talking about?
- [Cueball and Hairbun are both shown. Cueball holds his hand to his chin.]
- Cueball: It does sound ridiculous when I say it out loud. Anyway, spoilers for the magnetic compass. Sorry.
- Hairbun: What's a spoiler?
- Cueball: ...Spoilers for the concept of a spoiler, too.
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