Main Page

Explain xkcd: It's 'cause you're dumb.
Revision as of 03:05, 4 September 2013 by Davidy22 (talk | contribs) (New here?)
Jump to: navigation, search

Welcome to the explain xkcd wiki!

We have an explanation for all 2 xkcd comics, and only 0 (0%) are incomplete. Help us finish them!

Latest comic

Go to this comic explanation

Dark Matter Candidates
My theory is that dark matter is actually just a thin patina of grime covering the whole universe, and we don't notice it because we haven't thoroughly cleaned the place in eons.
Title text: My theory is that dark matter is actually just a thin patina of grime covering the whole universe, and we don't notice it because we haven't thoroughly cleaned the place in eons.

Explanation

Ambox notice.png This explanation may be incomplete or incorrect: Every section needs to be filled and explained. Do NOT delete this tag too soon.

Dark matter is a hypothetical form of matter used by the vast majority of astronomers to explain the far too high apparent mass of objects at large scales in our universe. In galaxies, stars are orbiting faster than the gravitational force of the sum of the masses of visible matter in the galaxy could cause, and entire galaxies are observed moving much faster around each other than their visible masses could explain. In galactic collisions, the mass can appear to separate from the visible matter, as if the mass doesn't collide but the visible matter does. A small handful galaxies have been observed to not have this property, suggesting that it is a *thing* that a galaxy can have more or less of and is separable from. At scales of our solar system those effects are too small and can't be measured. In cosmology, dark matter is estimated to account for 85% of the total matter in the universe.

This comic gives a set of possibilities of what dark matter could possibly be, charted by mass from smallest (given in electronvolts) to largest (given in kilograms). Masses in the range 10-15 kg to 10-3 kg are given in grammes.

The joke in this comic is that the range of the mass of the possible particles and objects stretch over 81 powers of ten. Randall filled the gap between real small candidate particles and real large candidate objects with highly absurd suggestions.

Axion

An Axion is a hypothetical elementary particle that might be a component of dark matter.

Sterile neutrino

Sterile neutrinos are hypothetical particles interacting only via gravity. It's an actual candidate for dark matter.

Electrons painted with space camouflage

Electrons are fundamental particles which compose the outer layers of atoms. A large number of electrons in the galaxy would be relatively easy to detect, as they not only interact with light (which dark matter does not appear to), but have a strong electric charge. Presumably, space camouflage is a positively-charged coating which prevents electrons from interacting with light. (Needless to say, this is not an actual candidate for dark matter.)

Neutralino

A Neutralino is a hypothetical particle from Supersymmetry, not something made up by Randall Munroe that sounds vaguely like one. It's an actual candidate for dark matter.

Q-ball

In theoretical physics, a Q-ball is a stable group of particles. It's an actual candidate for dark matter.

In billiards, a cue ball is the white (or yellow) ball hit with the cue in normal play.

Pollen

Pollen is a joke candidate, though people with seasonal allergies may suspect that the universe genuinely is made up entirely of pollen in the springtime.

No-See-Ums

No-See-Ums, also called Ceratopogonidae, a family of small flies (1–4 mm long) who can pass through most window screens. Another joke candidate.

Bees
8-balls

In pool, the 8-ball is a black ball numbered 8. It's a pun with Q-ball/cue ball. Unless undetected aliens have discovered billiards and become addicted to it, 8-balls are found only on Earth and are, hence, unlikely dark matter candidates.

Space Cows

Cows are Bovines extensively farmed on Earth for milk and meat. Although there is folk lore concerning cows acheiving circum-lunar orbits, they have yet to be found elsewhere in the Universe.

Obelisks, Monoliths, Pyramids

While those human constructions are huge on a human scale, they're negligible at universe-scale. It would take a large number of such constructions, distributed through space, to replicate the effects of dark matter; while a scenario could be envisioned where enough such constructs existed, with properties and distribution allowing them to match observations, this is obviously not a likely explanation. They often show up in fiction and pseudo-scientific literature as alien artifacts generating immense unknown power out of nowhere, with the most famous and influential example being the monolith from 2001: A Space Odyssey.

Black Holes ruled out by

Black holes are known in sizes of a few sun masses (about 1030-1031 kg) as remnants of the core of former big stars and the real big ones at the centers of galaxies (millions or even billions of the mass of the sun.) But recent gravitational wave detection indicate that black holes at 50 or 100 sun masses also exist while their origin is still not understood. Randall doesn't mention this but some astronomers hope that these could fill at least a part of the gap.

Except the last item all range below the mass of the sun (2x1030 kg) while the smallest known black hole is about four sun masses.

  • Gamma rays: If dark matter were black holes of this size, the black holes would be evaporating in bursts of Hawking radiation, and we'd see a buzz of gamma rays from every direction.
  • GRB lensing: Gamma-ray bursts (GRBs) are the brightest events in the universe only been observed in distant galaxies. While gravitational microlensing (see below) is an astronomical phenomenon it doesn't make much sense here. GRBs are short (milliseconds to several hours) and often only detected by gamma-ray satellites in space but rarely at any other wavelengths. Measuring lensing effects would be very difficult.
  • Neutron star data: Neutron stars aren't black holes but they're also small high compact objects at about 1.4 and 2.16 solar masses. While black holes can't be observed directly neutron stars are detectable in many wavelengths. The number of them gives a clue about the number of stellar black holes which is far too low.
  • Micro lensing: Gravitational microlensing is a gravitational lens effect. This is a prediction by Einstein's General Theory of Relativity and was first confirmed in 1919 during a solar eclipse when a star nearby the sun was closer to the sun than it should be. Astronomers have found many so called Einstein rings or Einstein crosses where a massive object in front of other galaxies bends the light toward us. Those massive objects may be black holes but the number is far too low to explain the dark matter.
  • Solar system stability: Our solar system is 4.5 billion years old and very stable since then. If not we wouldn't exist. If dark objects at 1024 kg - 1030 kg (mass of Earth until Sun) would resemble the dark matter there should be many of them even in the vicinity of our solar system and the system wouldn't be stable at all.
  • Buzzkill Astronomers: Black holes above a certain size would be impossible to miss, due to the effects they have on nearby matter. But at the mass of some 1030 kg there must be many supernova remnants we still haven't found.
Maybe those orbit lines on space diagrams are real and very heavy

Any diagram of our solar system (or any solar system) will show lines representing the path the planet takes around its sun. Since planets orbit in ellipses, there will be an ellipse for every planet. This lines don't show real objects, though. Astronomers just draw them on pictures of the solar system to show where the planets move. If you draw a line on a map to give someone directions, that line isn't an object in real life; it's just on the map. If these lines were real, they would be huge (Earth's would be 940 million km long (2π AU) and Neptune's would be 28 billion kilometers long. Powers of Ten (1977) gives a good sense of just how large these orbit lines need to be in order to be visible in space diagrams. If these orbit lines were also very dense, they would have a huge mass and could possibly account for the missing 85% of the mass in the universe. But they would also constantly be impaling the inner four planets, including the Earth, which would be a problem. Overall, not a very likely candidate.

Transcript

Ambox notice.png This transcript is incomplete. Please help editing it! Thanks.
Dark matter candidates:
[A line graph is shown and labeled at left quarter in eV and further to the right in g together with some prefixes.]
[The labels read:]
µeV, meV, eV, keV, MeV, GeV, TeV, 10-18kg, ng, µg, mg, g, kg, TON, 106kg, 1012kg, 1018kg, 1024kg, 1030kg
[All items are shown in bars ranging between two approximately values:]
< 1 µeV - 10 meV: Axion
1 eV - 10 keV: Sterile neutrino
1 MeV (exactly): Electrons painted with space camouflage
10 GeV - 10 TeV: Neutralino
100 TeV - 10-17 kg: Q-ball
1 ng - 100 ng: Pollen
0.1 mg - 1 mg: No-See-Ums
10-1 g (exactly): Bees
10 g - 100 g: 8-balls
100 kg - TON: Space cows
TON - 109 kg: Obelisks, monoliths, pyramids
109 kg - 1033 kg: Black holes ruled out by:
109 kg - 1013 kg: Gamma rays
1013 kg - 1017 kg: GRB lensing
1015 kg - 1022 kg: Neutron star data
1021 kg - 1030 kg: Micro lensing
1024 kg - 1030 kg: Solar system stability
1030 kg - 1033 kg: Buzzkill astronomers
1033 kg - >1036 kg: Maybe those orbit lines on space diagrams are real and very heavy


Is this out of date? Clicking here will fix that.

New here?

Last 7 days (Top 10)

Lots of people contribute to make this wiki a success. Many of the recent contributors, listed above, have just joined. You can do it too! Create your account here.

You can read a brief introduction about this wiki at explain xkcd. Feel free to sign up for an account and contribute to the wiki! We need explanations for comics, characters, themes, memes and everything in between. If it is referenced in an xkcd web comic, it should be here.

  • List of all comics contains a table of most recent xkcd comics and links to the rest, and the corresponding explanations. There are incomplete explanations listed here. Feel free to help out by expanding them!
  • If you see that a new comic hasn't been explained yet, you can create it: Here's how.
  • We sell advertising space to pay for our server costs. To learn more, go here.

Rules

Don't be a jerk. There are a lot of comics that don't have set in stone explanations; feel free to put multiple interpretations in the wiki page for each comic.

If you want to talk about a specific comic, use its discussion page.

Please only submit material directly related to —and helping everyone better understand— xkcd... and of course only submit material that can legally be posted (and freely edited). Off-topic or other inappropriate content is subject to removal or modification at admin discretion, and users who repeatedly post such content will be blocked.

If you need assistance from an admin, post a message to the Admin requests board.