3085: About 20 Pounds

About 20 Pounds

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Title text: In addition to gravity, burritos interact through the strong, weak, and electromagnetic forces, which is believed to be a major contributor to their popularity.

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Explanation

This is one of 60 incomplete explanations: The article might be off-topic. We should focus on explaining the comic, not everything known to humans about dark matter. The article might be too technical and short, especially the leading paragraph. Don't remove this notice too soon. If you can fix this issue, edit the page!

The nature of dark matter is a significant unsolved problem in physics. We observe that galaxies spin faster than we expect based on the nearby observable matter. Likewise galactic motions indicate additional mass that we have not observed in other ways. Dark matter is the name we give to this mass. In the comic, Cueball and Ponytail consult an oracle to learn about dark matter.

The pentacle and candles suggest that the oracle is supernatural, summoned by an occult ritual; something which would present its own challenges to our understanding of the physical world. There may be a pun here, in that they may be using 'dark magic' to communicate with something from the 'dark realm' on the assumption that it will know about dark matter. However, the word 'dark' in dark matter simply means that we do not know how to observe it; we have no evidence that dark matter is evil or satanic, though Randall may consider it cursed. The oracle is used very similarly to how people have been using and customizing large language models.

In general, not all forces interact with all particles; indeed, gravity is believed to be the only force that interacts with everything we have observed. If a force doesn't interact with a particle, then the particle's existence cannot be directly observed via disturbances in that force. In particular, something that doesn't interact with electromagnetism cannot be 'seen', as photons will pass through it relatively unaffected, and likewise cannot be felt, because collision is a side effect of the Pauli exclusion principle.

Even neutrinos, famous for interacting with almost nothing, still interact via the weak force, allowing them to be detected with sufficiently large tanks of dense material. This is the main reason neutrinos cannot be dark matter: they interact far too much to be a viable option. A particle that interacts with nothing except gravity could only be detected by a gravitational telescope.

We can theorise the presence of dark matter thanks to how something interacts with the matter we can see, i.e. the nature of the rotation of far away galaxies implies significant additional mass that does not emit, absorb, reflect or diffuse light. While there are also other theories which try to account for what we do see, a number of the suggestions are that it is one or other form of unseeable but gravitationally-interactive substance, given the soubriquet of "dark matter".

In trying to determine what form this actually exists in, and on the assumption that if it exists at all then it is also drifting past (and through) our own neighbourhood, experiments here on Earth are trying to detect it using any other means. For example, a popular dark matter candidate is a weakly interacting massive particle, which interacts via the weak nuclear force. Dark matter experiments build large tanks of liquid xenon with a gaseous top, keep them extremely dark, and look for some way in which passing particles might interact and produce electroluminescent scintillations. There are good theoretical reasons to expect some interaction, which would mean the oracle in the comic is lying, but there are no sufficiently conclusive results as of now.

There is one dark matter candidate where the only interaction is overwhelmingly gravitational: black holes formed through collapse in the early Universe. These primordial black holes may not be detectable through any terrestrial experiment. However, even these objects can be found through their lensing effects if they are sufficiently large and common to account for the 'missing mass' we are looking for. Black holes of around 10 kg would also likely quickly evaporate through Hawking radiation, so are not a good dark matter candidate. Black holes of around asteroid mass would be extremely hard to detect and are a good dark matter candidate given current information.

The particle's mass is described vaguely as about twenty pounds, roughly 10 kilograms, in line with how all-knowing oracles legendarily use ambiguous statements. This is a ludicrous amount of energy for particle physics. Any interaction would have to involve an equally ludicrous amount of other particle mass being in exactly the right place and time, a coincidence that might be so rare that one would not expect it to occur ever in the history of the universe. By comparison, the heaviest single particle we have observed, with a mass over a hundred times that of the proton, is around a tenth of a trillionth of a trillionth of a pound.

Under more normal circumstances, we might still hope to observe the properties of the particle via creating it ourselves under controlled laboratory conditions. But again, there is no reasonable way to focus the energy required into a single particle interaction. The most powerful particle accelerator in the world, for example, peaks at about ten thousand times the mass of the protonn (a solid billion times less energy than required) so it's out too.

Despite all this, twenty pounds is also much too small to be detectable via gravitational interaction; its influence on the orbits of planets, say, or the strength of its gravitational lensing effect, would be entirely negligible. In the scenario posed by the comic, then, there is no plausible way to observe more about dark matter while on Earth. Even if we did find some such particles naturally occurring, and had instruments that could measure such small gravitational forces, since it would interact only via gravity, the only properties it could have other than mass would be its decay rates from other particles. Which, again, would all be essentially nil, due to its mass.

The oracle proceeds to break expectations by suggesting that Ponytail and Cueball go out for burritos. When faced with the apparent futility of continuing to try to investigate dark matter, the oracle predicts that going out for burritos is precisely as productive as any other approach. It justifies the suggestion by burritos being "pretty" good, again neither exactly quantifying the oracularity, and likely not even giving the optimal idea.

The title text observes that burritos interact through all four known fundamental interactions, making burritos popular. The electromagnetic force mediates the chemical reactions leading to a burrito's taste, the strong force keeps atomic nuclei together, and gravity gives burritos heft, all of which are helpful for enjoying them. It's hard to see how the weak force, which takes part in radioactive decay, helps with burrito enjoyment or popularity, but the weak force is responsible for the nuclear fusion that allowed the complex elements of the burrito to exist in the first place.

The previous comic 3084: Unstoppable Force and Immovable Object dealt with particles which do not even interact with gravity. In 2035: Dark Matter Candidates these 20 lb dark matter particles fit between magic 8 balls and space cows. The squirrels that make up 2186: Dark Matter near the earth must be pretty chunky. Talking to a floating sphere is becoming a returning subject in xkcd. See more about other instances of this on the page for the Time traveling Sphere series.

Transcript

[Cueball and Ponytail are standing in front of a pentacle with lit candles at the corners. A black sphere, the oracle, is floating above the middle of the pentacle.]

Ponytail: Dear oracle,

Ponytail: What is the nature of dark matter?

Oracle: It's about 20 pounds.

[Close up of oracle]

Off-panel: What?

Oracle: Dark matter is a particle. It weighs about 20 pounds.

Oracle: It only interacts through gravity.

[Same view as first panel]

Cueball: Only gravity, huh?

Cueball: So none of our experiments are really going to tell us any more about it, then.

Oracle: Afraid not.

[Same view as first and third panels, except Cueball lifted his forearm.]

Cueball: So what do we do?

Oracle: You should go out for burritos.

Ponytail: How will that help?

Oracle: Well

Oracle: Burritos are pretty good.

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Discussion

Wow - first here! I can't help thinking 'about 20 pounds' could be exactly 10 kg! 0r even one Newton?! RIIW - Ponder it (talk) 05:50, 6 May 2025 (UTC)

No you weren't. I got here about two hours ago. (sorry if i'm being mean or condencending) 172.69.176.6 (talk) 08:39, 8 May 2025 (please sign your comments with ~~~~)

"One Newton" and "10 kg" are totally different things. "10 kg" would cause 1 Newton of gravitational force if you were in a world with about 1% of Earth's gravity, though. --172.69.109.86 09:53, 6 May 2025 (UTC)

Oops! In my rush I should have checked and put 100 Newtons. I was relying on 10kg being about 22 pounds, or rather the other way around, and then a particle having mass not weight and Science using Metric units. Apologies. RIIW - Ponder it (talk) 11:41, 6 May 2025 (UTC)

(Moved your reply up a bit. You seemed to respond to "20 pounds are...", below, and split their timestamp signature from their message. And forgot to sign properly, at first, so I got edit-conflicted twice whilst trying to post myself and correct your initial error. Please take a bit more care, everybody. 172.70.163.53 11:52, 6 May 2025 (UTC))

20 pounds are approximately 9.072 kg, so not exactly 10 kg (in fact, it rounds to 9). 172.70.134.55 10:02, 6 May 2025 (UTC)

That's the wrong way to think about it. "Exactly 10kg" is "exactly 22.0462lbs", but that (to the nearest single significant figure) is legitimately "about 20lbs". See any given step in 2585: Rounding, especially where that 'disagrees greatly' with an adjacent step.

As with any Oracle (that's worth its omphalos), it may be giving an entirely true answer which nevertheless is deliberately phrased as ambiguous and misinterpretable, the possible supernatural complement to the 'exact words' genie contract. As with the 2741: Wish Interpretation genie, the Oracle may slip into less "unhelpfully helpful" mode immediately after, though for different reasons. However, "burritos are pretty good" also suggests that there's some other thing that is more good, so — again — it's giving a sufficient response to what they (now) should do, but not a perfect one.

As I write, the explanation (probably needs a general rewrite) doesn't mention anything about the burritos except as title text, or I would have ensured the famed exact-words/vague-detail was noted in that bit. (Shorter than here.) 141.101.98.82 11:46, 6 May 2025 (UTC)

Nobody said anything about "exactly 20 pounds". 20 pounds is about 10 KG and about 100 Newtons when considered as a force rather than a mass. The comic says "about 20 pounds". 172.70.230.63 22:36, 6 May 2025 (UTC)

Though I don't think it at all merits being described as a reference, I am minded of the The Usenet Oracle (at least when I knew of it). Though, if it was to be a deleliberate shout-out, I'd expect a few more actual in-jokes. 172.70.86.130 06:10, 6 May 2025 (UTC)

I bet Randall is in some kind of force-interaction-related, What-if-induced rabbit hole right now (or has been at the time of writing). Wondering what the next comic will be about. 172.71.144.175 08:39, 6 May 2025 (UTC)

"Nature of ... 20 pounds" is a reference to the koan "A monk asked Tozan, 'What is the nature of Buddha?' He replied, 'Three pounds of flax.'" Someone can add this to the explanation. 172.70.111.115 08:57, 6 May 2025 (UTC)

There is a similar story in the Principia Discordia. When asked what is the meaning behind POEE, a Discordian cabal, Malaclypse the younger answered "five tons of flax." FlavianusEP (talk) 16:26, 6 May 2025 (UTC)

"something that doesn't interact with electromagnetism cannot be 'seen', as photons will pass through it completely unaffected": is this supposed to be true ? I thought photons interacted with gravity, and even the phrase before states that gravity is believed to affect everything. 172.68.151.93 09:17, 6 May 2025 (UTC)

Indeed, photons do interact with gravity. What I had in mind when writing that were direct interactions -- of course everything interacts with everything else via a second-order interaction, <X> -> gravity -> <X> for any particle/field <X>. I can clarify that if nobody gets to it before I get around to it. Linkhyrule5 (talk) 21:45, 6 May 2025 (UTC)

We can infer Dark Matter (and, for that... *ahem* ...matter, also Dark Energy) from what the photons in the universe are telling us that does not look anything like what 'light(-interacting) matter' should be doing. As with some searches for black holes (most particularly, when the theory is that the unseen mass of the universe is a lot of small black holes drifting in the void, not acreting enough to create secondary visible effects), whether or not light is being gravitationally lensed by things (that we cannot directly see) is part of the way that we're narrowing down what-and-where DM is.

And, I think, currently it seems to be considered that it's residing in a webwork of DM tendrils, at extragalactic (indeed, cosmological) scales, such that where the tendril cross is where they draw 'normal' matter together enough to be any given galaxy. But that's in an "explains all(/many) known facts" way, and might yet be incorrect. e.g. if there's side-dimensions (equally undetectable, at least visually) that change the inverse-square dropoff of gravity at large enough scales to govern galactic rotation rates by just enough to fit observations, or we have some other misunderstanding/scientific blind spot that further study may correct.

Or, in short, think Brownian Motion. We can't see a handful of air molecules (not by normal, even microscope-enhanced, human vision), they might as well be invisible. But, by what we see of more visible particles, suggests that they exist as something. Conversly, the æther, a proposed medium for light, was thought to exist in a similar all-pervasive manner (insofar as trivial human experience, though less physically 'interactive' than wind), but deeper checks (as to whether its effects on light were as they should have been) dismissed it as a possible concept.

Depending upon interpretation of the comic (I originally read it as "all dark-matter particles are ~20lbs in mass WIMPs/nano-MACHOs/whatever", but it seems that others take it as "all of dark-matter particles is a single ~20lbs mass particle"; and that's make the oracle-invokers' attitudes more logical, if not the universe), there actually being Dark Matter, but it being just 20lbs of 'something' somewhere in the whole universe, makes it a needle in a galactic-supercluster-sized haystack.

Detecting that would be difficult in the extreme. Even if it's somehow within a few hundred metres of the experimenters. There are ways to observe the movements of small masses at small distances, but when you don't even have a clue if it exists (or is moving/has moved, and how), it's fairly hopeless. Gravitational lensing of light would be impractical at such distances/masses. LIGO may be very clever, insofar as merging high-mass objects at long distances, but not really for this. Event Horizon Telescope's ability to see a black hole('s accretion disk) via Very Long Baseline Interferometry is also totally useless here.

I think I'd also settle for the burritos, given that certainty that I wasn't going to find what I'm looking for via any obvious route. (Assuming I couldn't ask the Oracle to show me the Dark Matter, rather than just answer questions about it. And noting that, if not for the indicated progression of the conversation, I might have assumed the oracular voice were really from the pentagram (more usual for demonology, not oracularities!) and that the dark blob was the 20lbs of Dark Matter. Which, of course, it does not deny, so maybe my headcan[n]on is that the summoned Oracle is the DM, being deliberately evasive, and successfully so. That would satisfy it being both that which Ponytail seeks, and the entity of which Ponytail summons in order to seek it! Cueball, however, is currently just seeking food, which (one assumes) the DM-slash-Oracle is not.) 172.68.229.25 12:48, 6 May 2025 (UTC)

My physics skills are rusty but 20 pounds is much more than the Planck mass. Doesn't this imply that Randall's dark matter particles would be black holes? 172.68.243.107 10:05, 6 May 2025 (UTC)

Yes, you are right that 9 kg is about 417,000,000 times more than the Planck mass (21.76 μg), but no, that doesn't imply that 9 kg dark matter particles would be black holes, for that particle can be larger than 417,000,000 Planck lengths (1 Planck length is c. 1.616255×10^–35 m, so above 7 rm, this particle would not collapse into a black hole). 172.68.245.81 10:23, 6 May 2025 (UTC)

No, unfortunately that doesn't work - if it's not a fundamental particle then it interacts with forces that aren't gravity, and if it is then its Compton wavelength (which is the relevant size for a fundamental particle) is about 1/417000000 Planck lengths. The Planck mass is the maximum possible mass for a fundamental particle (running coupling constants might change the limit from the low-energy observed Planck mass, but not by nine orders of magnitude up). 172.68.55.40 03:23, 8 May 2025 (UTC)

Since it's Star Wars day and the 20 lbs. reference would be causing a massively large amount of mass, would it be safe to say that they "sense a great disturbance in the force?" 67.84.20.42 10:20, 6 May 2025 (UTC)

Back in 2005, when the kg was an actual object's mass, there was an article about what a five pound (~2.268 kg) electron is, but it was deleted, for it is a "trivial result of special relativity". 172.68.245.81 10:23, 6 May 2025 (UTC)

Since pounds are a measure of weight, and weight is a measure of the gravitational attraction between an object and its "planet", what is the reference planet that is being used to define the weight of the Dark Matter particle?
Should we assume that Earth's surface is being used as the reference, even though we have no measurements that suggest DM particles are around us, and no reason to assume that the particles would even notice that Earth has a "surface"?
If Randall wanted to use mass, then he should have used the imperial unit of slug, but I suppose saying that a DM particle is 0.62162 slugs might not give the readers quite the same impression as using 20 pounds. Galeindfal (talk) 13:38, 6 May 2025 (UTC)

I might be missing some humour here, but the pound is actually a measure of mass, just like the gram, so it doesn't vary from a planet to another. You might have fallen prey to the second paragraph of the wikipedia article about the pound-force, which states: 'Pound-force should not be confused with pound-mass (lb), often simply called "pound"' 172.71.127.160 14:35, 6 May 2025 (UTC)

Indeed, when I was at school in England in the sixties, I was taught the pound was a unit of mass, and the Imperial unit of force was the poundal, the force needed to accelerate one pound mass at one foot per second per second.--172.69.195.4 12:00, 8 May 2025 (UTC)

You'd also have been taught that pounds were divided into 240 pennies, I imagine. ;) 172.70.163.150 13:43, 8 May 2025 (UTC)

Is this, by chance, the Internet Oracle? 104.23.187.126 13:49, 6 May 2025 (UTC)

I don't see anything like the pentagram with candles at its web site. The comic seems more like they're summoning a daemon. Barmar (talk) 14:10, 6 May 2025 (UTC)

Any idea where Randall came up with "20 pounds"? Why not 19 or 21 (blackjack!)? Why not use Newtons (too figgy?)? Only thing I can think of is that, in America at least, many people think they are "about 20 pounds overweight." I think that's too much of a stretch (pants???) to be the answer here. 172.68.27.170 14:07, 6 May 2025 (UTC)

I think it's just humorous, adding to the imprecision / casualness of "about 20". Imperial measurements feel "less scientific" than metric. 162.158.146.124 16:26, 6 May 2025 (UTC)

The amount of people confusing mass and weight/force in this thread is pretty disappointing for an xkcd forum. You can't convert pounds into Newtons. 162.158.172.143 16:38, 6 May 2025 (UTC)

The poster meant Kilograms rather than Newtons I assume. Under the assumption that the oracle is satire for large language models and AI chats, the "20 pounds" shows the kind of tone and data the AI has been tuned to provide. It's a way they behave when outside the core domain they are well-trained for, producing a certain brand of mistakes, awkwardnesses, and uncanny valleys, that may be quite humorous when first encountered. 172.70.111.77 22:33, 6 May 2025 (UTC)

I don't think it's accurate to say (as the explanation does right now) that 20 lbs is too little to detect through gravitational interaction. Throwing some numbers together: a 20lbs-sphere of Osmium, the heaviest stable element, is about 4.5cm in radius. If a 20lbs point mass flies by just above the surface of that sphere, it would generate a gravitational force of about 2.5 micronewtons (hooray for Gauss's theorem). That's the weight of a few grains of salt - small, but definitely detectable. If they're all really really fast, or there's always lots of them around at any given time or something, that might wash out any measurements (someone more knowledgeable about dark matter can probably comment what the expected velocity and flux density of 20lbs-dark-matter-particles would be where we are). But in principle, rather measurable! 162.158.172.142 17:00, 6 May 2025 (UTC)

"A particle that interacts with nothing except gravity, could only be detected by a gravitational telescope." -- Detected by a whatnow? Is that a thing which exists? Google had nothing for "gravitational telescope" when I searched for it.
Additionally, are there any theoretical physicists out there who can weigh in on how plausible the "20 pound particle that doesn't interact with anything else" theory is? MeZimm (talk) 19:33, 6 May 2025 (UTC)

Well, we haven't seen any such thing. But... that is of course the point. So, by logical extension, it must be true! :o 172.70.58.130 21:05, 6 May 2025 (UTC)

There’s a few places online that I’ve seen people frustrated about dark matter because it’s been so hard to detect, even hoping that it turns out to be experimental error or otherwise not real (for example, this collectSPACE thread) just because it has been so “annoying” to readers of science news to not have a solution for so long. I think this comic could be kind of a joke along those lines (“what if it turns out dark matter doesn’t matter at all and we’ve been wasting our time?”) but played out in a ridiculous way because dismissing any scientific research, however frustrating, as “a waste of time” would be very un-XKCD. 104.23.90.225 (talk) 21:32+22:01, 6 May 2025 (please sign your comments with ~~~~)

Sounds an awful lot like a primordial black hole (a genuine dark matter hypothesis), doesn't it? Anyone who knows what the Hawking temperature of a black hole of that mass would be? (A black hole with a Hawking temperature less than 2.7 K would absorb more energy than it gives off in Hawking radiation and would therefore be stable in the current age of the universe.)

EDIT: answering my own question, apparently about 10²² K. 162.158.91.110 23:20, 6 May 2025 (UTC)

Shouldn’t there be a comma after ‘well’ (when it is said by the oracle in the last panel), because there’s a comma after ‘dear oracle’ in the first panel? You can’t say that because there’s a break you don’t need a comma because there’s one in dear oracle Broseph (talk) 07:04, 7 May 2025 (UTC)

Well. Perhaps it may need some punctuation. But... It doesn't necessarily have to represent a mid-sentence pause.

Not that I like "you pause for breath, you add a comma" writing, also often used to justify the Oxford cases. Yes, a "clause-pause" coincides with a clausing comma. But if you're representing various other pauses or switches of tone there are mdashes, ellipses, parenthises of various types, the full-stop-slash-period and, in the comic example, the clear newline (and speechline?) acting as a rhetoric 'hold'. (The "Dear oracle," runs on into the next bit, so maybe that comma indicates symantic joining that was never required or implied with the effective "Well <mystical equivalent of taking a breath>" merely as a filled-pause.)

Also note that it's not unknown for Randall to be inconsistent with whether or not to fully punctuate the 'ends' of speech-blocks (and has occasionally added them in post-publication edits; not entirely sure he's ever solely edited for that, but definitely has done when correcting some bigger mix-up/whatever). House-style seems to be fairly loose on that principle. It's effectively "reported speech", most of the time, so you can perhaps even put it down to the character's voice sort of... Trailing off, or even just halted altogether. 172.68.229.67 09:46, 7 May 2025 (UTC)

"However, even if a particle does interact via a given force, an interaction is possible only if energy is conserved." I am puzzled by this statement, which started the paragraph about 20 lbs being ludicrous for a fundamental particle. I think conservation of mass-energy is one of our most fundamental understandings of the universe. The rest of the paragraph does not develop further on the idea of conservation violation. If you know what was intended by this statement and think it is important, please clarify it and put it back in. I liked the use of ludicrous mass. Brought to mind Space Balls and ludicrous speed. (I haven't thought of a way to make an appropriate tangential reference in the article.) 162.158.41.36 19:34, 7 May 2025 (UTC)

I pride myself on being a physics nerd, but also somewhat a writing nerd. I read the article, and the stuff after the mention of a gravitational telescope, and thinking of the non-physics nerds who might be reading it, and... well, this is the closest I've come to wishing there was an explainexplainxkcd.com. // I feel sheepish for not having any revisions to propose, and grateful to whomever wrote this version, but damn. Blame it on the subject material, I suppose. 172.70.135.206 20:04, 7 May 2025 (UTC)

I thought this was also just a reference to AIs like ChatGPT hallucinating facts and saying them with confidence. 172.68.0.190 01:15, 8 May 2025 (UTC)

Traditionally, any actual Oracles were famed for being mostly true but (in the long run) only actually useful for a fortunate few (like "if you attack now, a great kingdom will fall!", but we aren't saying whose; or "trust in wooden walls!", which you then have to work out whether it might mean your navy at sea, rather than a stockade on land). Pretty much the opposite of our current expectations of AI (can be useful, but not necessarily truthful).

And I read this comic as having the ungarnished truth (in-universe, at least), but that doesn't really help anyone to know (except the local burrito vendor). Typical 'mystical contract' sort of thing. 172.70.163.150 13:56, 8 May 2025 (UTC)

to communicate with something from the 'dark realm'

It's a black hole. 2001:8003:dc65:7e01:4460:4138:3d1a:2b74 (talk) 09:08, 27 December 2025 (please sign your comments with ~~~~)
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