Low-Background Metal
Low-Background Metal
[Black Hat stands behind Megan who addresses Cueball who stands on the other side of a table with a machine. The machine is a rectangular box with a small dome with one large and two small antennas on top. It seems to point in Cueball's direction as it has a broad protrusion at the back and protrusion at the front that gets smaller towards the tip.  The word "Time" is written on the side, and below that is possibly more illegible text.] Megan: Our time machine works. Megan: But we're almost out of low-background metal. Cueball: What's that?
[Close-up on Megan who lifts her hand palm up.] Megan: Modern metal is contaminated by fallout from nuclear testing, and lead also has natural radioactivity that fades over time. Megan: To shield sensitive equipment, physicists use lead from sunken Roman ships. Megan: But shipwreck lead is hard to find.
[Back to the original setting, Megan has turned to Black Hat, who has his hand on his chin.] Black Hat: How much do we have? Megan: Enough for one trip through time. Black Hat: Hmmm...
[The three are now in a helicopter, with Megan piloting, Cueball as a passenger in the back, and Black Hat firing a flamethrower at a Roman ship beneath them through the window behind the cockpit. Two sailors with Roman type helmets are looking on as the stern of their ship catches fire. One of them throwing his arms out to the side. The intact sail is still up behind them and behind that another sailor jumps into the water, down to a fourth sailor already in the water. Two already-burning ships can also be seen to the left of the ship under attack. One is burning all over, with the mast still up but the sail long gone, and the third ship is almost completely sunk, but the part above the water is aflame. Seven small clouds are around the helicopter in the sky.] Flamethrower: Fwooosh

In this comic, a team including Megan and Black Hat who have invented a time travel machine presents it and their problems to Cueball . 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. However, Megan and Black Hat's machine requires the use of "low-background" metal, which is in short supply.
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.  In order to shield this equipment, "low-background metal" is salvaged from sunken ships.  Lead ingots from Roman cargo have been used in experiments.  The Roman lead was produced before atmospheric nuclear tests occurred [ citation needed ] and therefore did not have resulting radionuclides in the air used in its manufacture. When it is extracted, lead is naturally contaminated with the radioactive isotope Pb-210, with a 22 year half-life. Because it has spent many centuries continually underwater, it is both shielded from radioactive particles, and has had time for natural radioactivity to fade.
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. Megan mentions that they have only enough for a single trip.  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.  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. They could also specifically target ships that were in waters that are well-suited for salvage operations.
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, [ citation needed ] 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.
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. Frequently, it is done with the goal of making money , but other purposes are used as well. In the Star Trek movie The Voyage Home , time travel is used to retrieve whales and transport them to the present. In the book Timeline , time travel is used to record historical events for entertainment purposes. In the movie Avengers: Endgame , time travel is used to retrieve minerals important to a future plan. 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 ).
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 ). Since this is steel, the ships used typically date back to World War I or World War II.  (It should be noted that the vast majority of applications that previously required special low-background steel can now once again use ordinary newly-produced steel, as the concentration of radionuclides in the atmosphere has declined almost to pre-1945 levels in the decades since the cessation of atmospheric nuclear testing, due partly to the shorter-lived of these radionuclides having decayed away and partly to processes such as the carbon cycle having removed most of the still-extant radionuclides from the atmosphere.)
The title text refers to Greek fire , which was an incendiary weapon invented and employed by the Byzantine empire. It was a flammable liquid, famously said to burn on water, that was used in naval combat to set fire to enemy ships. As it was a closely-guarded military secret, many of the details have been lost to time, and modern chemists have only been able to develop educated guesses of what it probably was. Randall proposes a rather outlandish alternative hypothesis: that all records of Greek fire were actually in reference to the modern weapons used by the time travelers. It is also notable that, if the time machine was taken to the time of the classical Roman empire, Greek fire would not yet have been a known term. Perhaps the weapon wielded by the time travelers was later conflated with the Byzantines' weapon, or perhaps the time machine was taken to a period a few centuries later than classical Rome.
In 1063: Kill Hitler a single-use time machine is available. It is also used by Black Hat. However, due to the way the time machine in this comic is used, it must be assumed that they can use it again after the salvage of lead from the sunken ships.
[Black Hat stands behind Megan who addresses Cueball who stands on the other side of a table with a machine. The machine is a rectangular box with a small dome with one large and two small antennas on top. It seems to point in Cueball's direction as it has a broad protrusion at the back and protrusion at the front that gets smaller towards the tip.  The word "Time" is written on the side, and below that is possibly more illegible text.] Megan: Our time machine works. Megan: But we're almost out of low-background metal. Cueball: What's that?
[Close-up on Megan who lifts her hand palm up.] Megan: Modern metal is contaminated by fallout from nuclear testing, and lead also has natural radioactivity that fades over time. Megan: To shield sensitive equipment, physicists use lead from sunken Roman ships. Megan: But shipwreck lead is hard to find.
[Back to the original setting, Megan has turned to Black Hat, who has his hand on his chin.] Black Hat: How much do we have? Megan: Enough for one trip through time. Black Hat: Hmmm...
[The three are now in a helicopter, with Megan piloting, Cueball as a passenger in the back, and Black Hat firing a flamethrower at a Roman ship beneath them through the window behind the cockpit. Two sailors with Roman type helmets are looking on as the stern of their ship catches fire. One of them throwing his arms out to the side. The intact sail is still up behind them and behind that another sailor jumps into the water, down to a fourth sailor already in the water. Two already-burning ships can also be seen to the left of the ship under attack. One is burning all over, with the mast still up but the sail long gone, and the third ship is almost completely sunk, but the part above the water is aflame. Seven small clouds are around the helicopter in the sky.] Flamethrower: Fwooosh
