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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 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.
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.
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.
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. A large word 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?
- Machine: Time
- [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
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Spoiler Alert for Avengers Endgame next comment 184.108.40.206 20:36, 18 June 2020 (UTC)
- I can't help but notice that the basic premise of this comic is very much like the reason for going back to 1970 in Avengers: Endgame, when they needed more Pym particles for time travel. I wonder if Randall re-watched it again recently? — KarMann (talk) 17:10, 17 June 2020 (UTC)
- Whoa! Spoiler alert! Disney Plus won't have Infinity War until next week. I'm not watching them out of order! Mathmannix (talk) 12:16, 18 June 2020 (UTC)
Oh, that's new to me, that they use roman ships to get to higher quantities of lead. For Steel they use German ships. after world war I, the german high seas fleet was captured and put under arrest in scottish waters. To not allow the enemy to utilize the ships, they all sank themselfes. wikisource --Lupo (talk) 05:46, 18 June 2020 (UTC)
Is there any evidence of Roman ships using lead for ballast? I know it was used as a structural metal and utility metal (sheathing, seam sealing, anchors, tools). It was also carried as cargo. But it seems likely that Roman lead was too expensive to be used as ballast, particularly compared to, say, rocks or concrete. (Note that loading cargo low in the hull is not ballast, cargo is there because you want to move it, ballast is there only to keep the boat upright.)220.127.116.11 18:08, 19 June 2020 (UTC)
- The Romans used lead as a sweetener, I think they had plenty. --18.104.22.168 08:06, 20 June 2020 (UTC)
- The references I found to modern use of Roman lead referred to ingots from Roman cargo. Controversy over the use of Roman ingots to investigate dark matter and neutrinosLead Use on Roman Ships and its Environmental Effects 22.214.171.124 21:04, 20 June 2020 (UTC)
There's one leg of the time-machine missing from the 3rd panel. (or is it the side of a base?) --126.96.36.199 19:57, 17 June 2020 (UTC)
Pb-210 (half-life 20.4 years) is a decay product of radon, and thus accumulates everywhere that is exposed to the atmosphere or where radon seeps from the ground. I suspect it could be a contaminant in lead from some lead mines, but wasn't able to find any references ShadwellNH (talk) 20:00, 17 June 2020 (UTC) Paul
- Pb-210 is mentioned in Controversy over the use of Roman ingots to investigate dark matter and neutrinos. 188.8.131.52 21:04, 20 June 2020 (UTC)
One use only?
The way I understand it, the time machine is one-use unless you find other Low-Background Metal. If you find it, you can make more trips. It would appear that the trip is successful.
--184.108.40.206 01:31, 18 June 2020 (UTC)
So you'd say a car is also one-use, unless you find a gas station? 220.127.116.11 08:51, 18 June 2020 (UTC)
- (out of chrono... I am 18.104.22.168): No, I'd say that this is not a one-use time machine, so it's wrong to compare it to one-use time machines. Luckily, someone else changed the text already. --22.214.171.124 13:39, 19 June 2020 (UTC)
- "I am 126.96.36.199" no you're not.
- No, but if the parts it was made of had to be replaced after every trip, I definitely would. 188.8.131.52 16:59, 18 June 2020 (UTC)
- Sure, but the ability to rebuild the car with completely new material doesn't turn it into a multi-use car. 184.108.40.206 04:11, 19 June 2020 (UTC)
Real life use of this lead?
Does anyone know whether there is any truth whatsoever to scientists using lead from sunken ships to shield delicate equipment? Obviously not time machines, but there are some pieces of equipment that might be sensitive to radiation.
Also, would lead that was in the ocean actually be safer from nuclear fallout than lead that was underground and mined after the nuclear testing ended? 220.127.116.11 03:31, 18 June 2020 (UTC)
- Yes. At least it is done with steel.  18.104.22.168 04:50, 18 June 2020 (UTC)
- Yes the water is a better insulator than air. Also the fallout would be partially absorbed by plants/animals before reaching the ocean bottom. 22.214.171.124 16:51, 19 June 2020 (UTC)
- Low Background Lead is also used, mentioned in the Good article. The equipment that need this stuff is mostly radiation sensors, very precise ones that can detect even smallest amounts of radiation. And for the last Question, you can't find pure natural lead, its mostly contaminated with radioactive elements (most lead in the universe results from decay chains). And common lead is made through recycling. Ancient lead from roman ships had enough time for the radioactive elements to decay into stable lead. --126.96.36.199 06:12, 18 June 2020 (UTC)
May be complicated
The 1968 Story Hawk among Sparrows discusses the problems modern war hardware may have when used against old tech. -- 188.8.131.52 07:39, 18 June 2020 (UTC)
- This was also part of the premise of the 1980 movie "Final Countdown", when the aircraft carrier Nimitz shows up in the Pacific Ocean on December 6, 1941. Nutster (talk) 13:38, 18 June 2020 (UTC)
- Biggles: The Movie had a WW1 flying ace take a 1980s helicopter (ostensibly unarmed, except fortuitously/inevitably against the Big Bad Weapon) back to his era, thanks to a Time-Twin plotline. Thus, IIRC it only did well to defend against era-local aicraft by the mythical skill of the eponymous pilot, and was handily lost once the temporal-trickery job was finally accomplished. If you enjoy that era of kitcsh then I'd suggest you not pass up a viewing, even if not actually seek it out. 184.108.40.206 17:11, 18 June 2020 (UTC)
- I am also mildly disappointed that the helicopter is not Airwolf. Nutster (talk) 03:11, 19 June 2020 (UTC)
They could just send a cache of modern lead back in time and wait till it cools down. 220.127.116.11 06:30, 19 June 2020 (UTC)
Naah, that would totally violate causality. Not to mention that you'd now have the exact same atoms existing in two spatial places at the same time. That could quite easily lead to the Earth being engulfed in a giant wormhole. Cellocgw (talk) 10:05, 19 June 2020 (UTC)
Can we Add Topic with impunity now?
WOOOPEEEE!18.104.22.168 10:45, 19 June 2020 (UTC)22.214.171.124 10:48, 19 June 2020 (UTC)