Editing 3085: About 20 Pounds

{{comic
| number    = 3085
| date      = May 5, 2025
| title     = About 20 Pounds
| image     = about_20_pounds_2x.png
| imagesize = 666x278px
| noexpand  = true
| titletext = In addition to gravity, burritos interact through the strong, weak, and electromagnetic forces, which is believed to be a major contributor to their popularity.
}}

==Explanation==
{{incomplete|Created by a £20 20-LB PARTICLE.  In ludicrous units.  Are any categories missing? Don't remove this notice too soon.}}
The nature of {{w|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 {{w|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 [[:Category:Comics with cursed items|cursed]]). The oracle is used very similarly to how people have been using and customizing {{w|large language model|large language models}}.

In general, not all forces interact with all particles; indeed, {{w|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 {{w|Pauli exclusion principle}}.

Even {{w|neutrino}}s — famous for {{what if|73|interacting with ''almost'' nothing}} — still interact via the {{w|weak force}}, allowing them to be detected with sufficiently large tanks of dense material (as most atoms interact with the weak force, however weakly). 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 {{w|LIGO|gravitational telescope}}.

Dark matter is infered from its gravitational interaction with other matter (the hint that it exists is in the apparent rotational profiles of galaxies, for which not enough mass is otherwise visible). While no other form of interaction has yet been observed, terrestrial dark matter experiments are looking for all possible variations of the 'missing mass' that might be detectable by other means. For example, a popular dark matter candidate is a WIMP, or {{w|weakly interacting massive particle}}, which interacts via the {{w|weak nuclear force}}. Dark matter experiments such as XENON 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 (i.e. to expect that the oracle in the comic is lying), but there are no sufficiently conclusive results as yet. The most important is the observational fact that dark matter exists (assuming it does) throughout the universe, and would be six times more abundant than baryons (which make up almost all the weight of known matter in the universe). <!-- This indicates some mediating non-gravitational interaction, strong at sufficiently high energy, that can bring both the dark and light sectors into equilibrium. <=HUH? I THINK I KNOW WHAT YOU'RE TRYING TO SAY, BUT... DAYUM... THAT NEEDS SERIOUS REWRITING. -->

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<!-- (and even if they were, it would be a ''bad idea'' to try){{Actual citation needed|If they exist as ubiquitously as they need to do in order to be discoverable, then they're *already* interacting with us quite a lot, and no feasible terrestrial experiment can make that happen more. Oh, and they exhibit mass, charge and spin (at least, even if not 'hairy'), so actually are capable of more than only gravitational effects.}} -->. 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 (most 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<!-- anywhere near 22.0462 is feasibly "about 20".  The various definitions of pound mass mean that .0462 is using ludicrous precision. -->), in line with how apparently all-knowing oracles legendarily avoid unambiguous statements. This is an absolutely 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 {{w|top quark}}, 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<!-- short scale "trillion", right? ...as if that matters much here --> 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 {{w|Large Hadron Collider|most powerful particle accelerator in the world}}, for example, peaks at about ten thousand times the mass of the proton — 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 {{w|Perturbation (astronomy)|influence on the orbits of planets}}, say, or the strength of its {{w|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<!-- except that because 10kg is roughly a billion Planck masses, the particle must decay by collapsing into a black hole and then exploding in a burst of 10²²K Hawking radiation-->.

The oracle proceeds to break expectations by suggesting that Ponytail and Cueball go out for {{w|burrito}}s, something not considered scientific. 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 — i.e., not at all. It justifies the suggestion by burritos being "pretty" good, again neither exactly quantifying the oracularity, and likely not even giving the optimal idea (for choice of food, or of any other "what now?" diversion that it might give).
 
The title text observes that burritos interact through all four known {{w|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 [[:Category:Time traveling Sphere|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.

{{comic discussion}}<noinclude>
[[Category:Comics featuring Cueball]]
[[Category:Comics featuring Ponytail]]
[[Category:Physics]]
[[Category:Cosmology]]
[[Category:Food]]