explain xkcd - User contributions [en]

2321: Low-Background Metal

2020-06-20T21:14:23Z

162.158.106.106: /* Explanation */ Pb-210

{{comic
| number = 2321
| date = June 17, 2020
| title = Low-Background Metal
| image = low_background_metal.png
| titletext = The only effect on the history books were a few confusing accounts of something called 'Greek fire.'
}}

==Explanation==
{{incomplete|Created by a TIME TRAVELER. Please mention here why this explanation isn't complete.''Should the similarity of the premise to Avengers Endgame be mentioned?'' Do NOT delete this tag too soon.}}
In this comic, a team including [[Megan]] and [[Black Hat]] who have invented a {{w|time travel}} machine presents it and their problems to [[Cueball]]. Time travel is a common trope in science fiction, and specifically [[:Category:Time travel|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 {{w|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 {{w|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 {{w|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.

{{w|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 {{w|Trinity (nuclear test)|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 {{w|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.

==Transcript==
:[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''

{{comic discussion}}

[[Category:Comics featuring Megan]]
[[Category:Comics featuring Black Hat]]
[[Category:Comics featuring Cueball]]
[[Category:Time travel]]
[[Category:Physics]]

Talk:2321: Low-Background Metal

2020-06-20T21:04:39Z

162.158.106.106: citation to article about use of lead from roman ship cargo in physics

Spoiler Alert for Avengers Endgame next comment [[Special:Contributions/162.158.75.60|162.158.75.60]] 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? — [[User:KarMann|KarMann]] ([[User talk: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! [[User:Mathmannix|Mathmannix]] ([[User talk: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. {{w|Scuttling_of_the_German_fleet_at_Scapa_Flow|wikisource}} --[[User:Lupo|Lupo]] ([[User talk: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.)[[Special:Contributions/173.245.54.187|173.245.54.187]] 18:08, 19 June 2020 (UTC)
:The Romans used lead as a sweetener, I think they had plenty. --[[Special:Contributions/162.158.106.178|162.158.106.178]] 08:06, 20 June 2020 (UTC)
::The references I found to modern use of Roman lead referred to ingots from Roman cargo. [https://phys.org/news/2013-11-controversy-roman-ingots-dark-neutrinos.html Controversy over the use of Roman ingots to investigate dark matter and neutrinos][https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1095-9270.2007.00145.x Lead Use on Roman Ships and its Environmental Effects] [[Special:Contributions/162.158.106.106|162.158.106.106]] 21:04, 20 June 2020 (UTC)

== Drawing ==

There's one leg of the time-machine missing from the 3rd panel. (or is it the side of a base?) --[[Special:Contributions/162.158.155.126|162.158.155.126]] 19:57, 17 June 2020 (UTC)

== Mined lead ==

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 [[User:ShadwellNH|ShadwellNH]] ([[User talk:ShadwellNH|talk]]) 20:00, 17 June 2020 (UTC) Paul
:Pb-210 is mentioned in [https://phys.org/news/2013-11-controversy-roman-ingots-dark-neutrinos.html Controversy over the use of Roman ingots to investigate dark matter and neutrinos]. [[Special:Contributions/162.158.106.106|162.158.106.106]] 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.
--[[Special:Contributions/188.114.103.129|188.114.103.129]] 01:31, 18 June 2020 (UTC)

So you'd say a car is also one-use, unless you find a gas station? [[Special:Contributions/162.158.158.167|162.158.158.167]] 08:51, 18 June 2020 (UTC)

::::(out of chrono... I am 162.158.158.167): 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. --[[Special:Contributions/188.114.102.22|188.114.102.22]] 13:39, 19 June 2020 (UTC)

:::::"I am 162.158.158.167" no you're not.

: No, but if the parts it was made of had to be replaced after every trip, I definitely would. [[Special:Contributions/172.69.70.117|172.69.70.117]] 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. [[Special:Contributions/141.101.98.130|141.101.98.130]] 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? [[Special:Contributions/108.162.216.172|108.162.216.172]] 03:31, 18 June 2020 (UTC)
: Yes. At least it is done with steel. [https://hackaday.com/2017/03/27/low-background-steel-so-hot-right-now/][https://www.good.is/articles/the-search-for-low-background-steel][https://www.stainless-steel-world.net/mobile/webarticles/joanne-mcintyre/disappearing-warships-scavengers-raid-war-graves-for-low-background-steel.html] [[Special:Contributions/172.69.33.129|172.69.33.129]] 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. [[Special:Contributions/162.158.123.155|162.158.123.155]] 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. --[[Special:Contributions/141.101.105.114|141.101.105.114]] 06:12, 18 June 2020 (UTC)

== May be complicated ==

The 1968 Story [https://scifi.stackexchange.com/questions/109243/modern-military-jet-goes-back-to-world-war-i Hawk among Sparrows] discusses the problems modern war hardware may have when used against old tech. -- [[Special:Contributions/141.101.76.82|141.101.76.82]] 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. [[User:Nutster|Nutster]] ([[User talk:Nutster|talk]]) 13:38, 18 June 2020 (UTC)
: {{w|Biggles_(film)|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. [[Special:Contributions/141.101.98.130|141.101.98.130]] 17:11, 18 June 2020 (UTC)
* I am also mildly disappointed that the helicopter is not Airwolf. [[User:Nutster|Nutster]] ([[User talk:Nutster|talk]]) 03:11, 19 June 2020 (UTC)

== Safer alternative? ==

They could just send a cache of modern lead back in time and wait till it cools down. [[Special:Contributions/141.101.76.82|141.101.76.82]] 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. [[User:Cellocgw|Cellocgw]] ([[User talk:Cellocgw|talk]]) 10:05, 19 June 2020 (UTC)

== Add Topic ==

Can we Add Topic with impunity now?
WOOOPEEEE![[Special:Contributions/108.162.216.216|108.162.216.216]] 10:45, 19 June 2020 (UTC)[[Special:Contributions/108.162.216.216|108.162.216.216]] 10:48, 19 June 2020 (UTC)

Talk:2320: Millennium Problems

2020-06-17T09:35:22Z

162.158.106.106: Query

Ironically, Randall misspells Perelman as "Perlman" in the comic but spells it correctly in the alt-text.

[[Special:Contributions/172.69.63.147|172.69.63.147]] 02:56, 16 June 2020 (UTC)

:Perhaps he meant Perlman the Perl-programming superhero? ;) [[Special:Contributions/162.158.123.145|162.158.123.145]] 03:33, 16 June 2020 (UTC)

There has been some controversy over the millennium prizes, given that in mathematics important results are often a product of the work of different mathematicians who are not necessarily close associates. Perelman reportedly believed that his work was a corollary to prior work by Richard S. Hamilton.

I think the idea of this comic is an extension to a question, which I've seen before in this discussion, "what if person A shows that 2 millennium problems are equivalent, and then person B proves one of them?" Should person B get both prizes, or should person A get one of them? It is easy to think of situations where it is hard to know who deserves the credit, and I think this comic takes that
to a logical exteme. [[User:Probably not Douglas Hofstadter|Probably not Douglas Hofstadter]] ([[User talk:Probably not Douglas Hofstadter|talk]]) 03:59, 16 June 2020 (UTC)

The [https://en.wikipedia.org/wiki/Grigori_Perelman Wikipedia article] for Grigori Perelman states the following: "The Clay Institute subsequently used Perelman's prize money to fund the 'Poincaré Chair', a temporary position for young promising mathematicians at the Paris Institut Henri Poincaré.", so no funding would be available for Randall's eighth prize. [[Special:Contributions/162.158.74.61|162.158.74.61]] 04:21, 16 June 2020 (UTC)

By process of elimination, the matrix and the equation should represent Yang-Mills and P=NP, but which is which? The 4x4 matrix could represent the 4D unitary transformation from Yang-Mills? The equation seems to say 'Ar + (squiggles)' but I can't think of any complexity problems that might take this form. --[[User:Quantum7|Quantum7]] ([[User talk:Quantum7|talk]]) 06:35, 16 June 2020 (UTC)

Is "millennium problems" also a pun on "millennial problems", i.e. those issues which seem straightforward to adults but baffle the younger generation (the "millennials")? See for example comic [[2165]]. --[[Special:Contributions/188.114.102.48|188.114.102.48]] 00:48, 17 June 2020 (UTC)

The image is projected by a projector on the ground that Cueball is apparently standing in the way of, but there's no Cueball-shaped shadow on the projected image. {{unsigned ip |108.162.219.192}}
: '''"there's no Cueball-shaped shadow on the projected image."''' - of course not! Cueball is clearly constructed from lines - which (of course) have no width and therefore zero area and as a consequence, cannot obstruct any photons to cause a shadow to form. [[Special:Contributions/172.69.70.213|172.69.70.213]] 02:13, 17 June 2020 (UTC)
::Ha ha. No I think it is easy to see that Randall/Cueball is actually standing to the side of the projectors beam and he is thus not in front of the projector; it is thus not strange that his shadow is not there! --[[User:Kynde|Kynde]] ([[User talk:Kynde|talk]]) 09:32, 17 June 2020 (UTC)

It is clearly Randall that makes this presentation based on the caption. Have added this to the explanation and transcript --[[User:Kynde|Kynde]] ([[User talk:Kynde|talk]]) 09:32, 17 June 2020 (UTC)

Regarding [https://mobile.twitter.com/johncarlosbaez/status/1271577502947741696], could a professional number theorist please opine on the proof? [[Special:Contributions/162.158.106.106|162.158.106.106]] 09:35, 17 June 2020 (UTC)