Since this is steel, the ships used typically date back to World War I or World War II. ![]() Low-background steel is the most famous kind of low-background metal, used in real life for highly sensitive particle detectors in physics and medicine, and is salvaged from ships sunk before 1945 (the Trinity nuclear test). In the movie Back to the Future, when Marty tells Doc that the time machine runs on plutonium, Doc exclaims, "I'm sure that in 1985, plutonium is available at every corner drug store, but in 1955, it's a little hard to come by" (from this transcript). In the movie Avengers: Endgame, time travel is used to retrieve minerals important to a future plan. In the book Timeline, time travel is used to record historical events for entertainment purposes. In the Star Trek movie The Voyage Home, time travel is used to retrieve whales and transport them to the present. Frequently, it is done with the goal of making money, but other purposes are used as well. Using time travel to retrieve items from the past that are not available in the present is a frequent trope in time travel-related media. However, while this might be a pragmatic solution, going back in time to sink ships and murder the occupants doesn't seem like a particularly morally acceptable solution, not to mention opening up potential time travel paradoxes such as what if one of the ship occupants killed was an ancestor to one of the protagonists? If this were a real scenario, there would probably be less drastic solutions available, such as purchasing quantities of lead from the time (would need to convincingly impersonate a local and have something that could be used as currency) and dropping them in the ocean from a (rented) non-destroyed ship, which as a bonus eliminates the need to extract it from the charred remains of a ship later. They could also specifically target ships that were in waters that are well-suited for salvage operations. This would both provide for many more shipwrecks to salvage, and give the team a good idea of where those wrecks were, when they returned to modern times. The team realizes (apparently at Black Hat's suggestion), that a solution is to use their single trip to take modern military hardware back to the era of the Roman Empire and use it to sink multiple ships. Megan mentions that they have only enough for a single trip. The number of shipwrecks of that age that can be found and successfully salvaged for metal is quite small, which puts this material in short supply. Because it has spent many centuries continually underwater, it is both shielded from radioactive particles, and has had time for natural radioactivity to fade. When it is extracted, lead is naturally contaminated with the radioactive isotope Pb-210, with a 22 year half-life. The Roman lead was produced before atmospheric nuclear tests occurred and therefore did not have resulting radionuclides in the air used in its manufacture. Lead ingots from Roman cargo have been used in experiments. In order to shield this equipment, "low-background metal" is salvaged from sunken ships. Megan explains that, while delicate equipment is often shielded from radiation by lead, metal produced in modern times is contaminated by nuclear fallout in the atmosphere, which means that the shielding itself has enough radioactivity to interfere with highly delicate equipment. However, Megan and Black Hat's machine requires the use of "low-background" metal, which is in short supply. Time travel is a common trope in science fiction, and specifically here on xkcd, and such a discovery would be likely to change the world as we know it. In this comic, a team including Megan and Black Hat who have invented a time travel machine presents it and their problems to Cueball. ![]() ![]() Title text: The only effect on the history books were a few confusing accounts of something called 'Greek fire.'
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