Editing 2186: Dark Matter
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==Explanation== | ==Explanation== | ||
| − | + | {{incomplete|Created by a DARK MATTER SQUIRREL ON A SPINNING BIRD FEEDER. Please mention here why this explanation isn't complete. Do NOT delete this tag too soon.}} | |
| − | + | [[Megan]] and [[Cueball]] are talking about {{w|dark matter}}, the mysterious invisible mass observed indirectly in the rate at which galaxies rotate. Megan states that dark matter's density in the solar system is 0.3 GeV/cm<sup>3</sup>, as claimed, for example, by [https://arxiv.org/abs/1205.4033 Bovy and Tremaine (2012) "On the local dark matter density" in ''The Astrophysical Journal''.] Cueball does not understand what that means, so Megan explains that it equates to one squirrel's mass of dark matter in the volume of the {{w|Earth}}. In the final two panels, Cueball humorously misinterprets this as implying dark matter is actually one or more squirrels, and thereby provides the mass which causes [https://www.youtube.com/watch?v=PfHu-UJaK0Q squirrels to spin on bird feeders designed to deter them] while birds, with lower mass, do not. This enrages Megan. | |
| − | [[:Category:Squirrels|Squirrels]] are a recurring topic on xkcd, but are not a serious alternative to WIMPs as a scientific explanation for | + | The {{w|gigaelectronvolt}} (GeV) is a unit of energy that can be converted to a mass using {{w|Mass%E2%80%93energy_equivalence|Einstein's formula}} ''E'' = ''mc''<sup>2</sup>. It is typically used for subatomic particles, such as {{w|weakly interacting massive particles}} (WIMPs), one of {{w|Dark matter#Composition of dark matter: baryonic vs. nonbaryonic|several contending possibilities}} for the still-open question of the composition of dark matter, and one which Megan's uniform density figure implies constitutes most of it. For example, the mass of a proton is 0.938 GeV/''c''<sup>2</sup>. However, it is common to omit the ''c''<sup>2</sup> denominator, representing masses as GeV or MeV. A mass represented as 0.3 GeV is equal to 5.35 × 10<sup>−25</sup> grams [https://www.wolframalpha.com/input/?i=0.3+(GeV%2Fc%5E2)+in+grams]. Since the {{w|Figure_of_the_Earth#Volume|Earth's volume}} is 1.083 × 10<sup>27</sup> cm<sup>3</sup> Megan's figures imply that a {{w|squirrel}} has a mass of 1.083 x 5.35 × 10<sup>27−25</sup> g = 580 g [https://www.wolframalpha.com/input/?i=(0.3+(GeV%2Fc%5E2)%2Fcm%5E3)+*+(volume+of+earth)+in+grams] (about 1.3 lb), a typical weight for several species of common squirrels. |
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| + | [[:Category:Squirrels|Squirrels]] are a recurring topic on xkcd, but are not a serious alternative to WIMPs as a scientific explanation for the composition of dark matter. Since the September 2015 detection by the {{w|LIGO|Laser Interferometer Gravitational-Wave Observatory}} (LIGO) and subsequent confirmation by the {{w|Virgo interferometer}} of gravitational waves from unexpectedly many merging {{w|black hole|black holes}} substantially more massive than those produced by stellar collapse, {{w|primordial black hole| primordial black holes}} (PBHs) have become a popular alternative explanation to WIMPs (or squirrels), attracting [https://arxiv.org/abs/1605.04023 proponents at NASA,] and [http://www.buchaltercosmologyprize.org/#announcements other cosmologists] for [https://arxiv.org/abs/1711.10458 several reasons.] But PBHs remain controversial, because if they constituted more than a very small portion of dark matter, [https://iopscience.iop.org/article/10.1088/2041-8205/720/1/L67 alternative explanations would be almost entirely excluded.] | ||
Other alternative hypotheses for the observations suggesting dark matter, such as theories involving the {{w|Modified Newtonian dynamics|gravitational force varying over different distances}}, often upset cosmologists as much as Megan is shown to be, because they violate the {{w|cosmological principle}} among other issues. Part of this frustration may be due to the fact that even after many decades of careful, tremendously expensive, and often [http://www.allesfoen.de/artinscience/wordpress/?p=236 stunningly beautiful] experiments, none of the many explanations for dark matter or the observations suggesting it have as yet any support from direct empirical observations. | Other alternative hypotheses for the observations suggesting dark matter, such as theories involving the {{w|Modified Newtonian dynamics|gravitational force varying over different distances}}, often upset cosmologists as much as Megan is shown to be, because they violate the {{w|cosmological principle}} among other issues. Part of this frustration may be due to the fact that even after many decades of careful, tremendously expensive, and often [http://www.allesfoen.de/artinscience/wordpress/?p=236 stunningly beautiful] experiments, none of the many explanations for dark matter or the observations suggesting it have as yet any support from direct empirical observations. | ||
| − | To help resolve this mystery, the title text imagines using a spinning bird feeder like a {{w|particle accelerator}}, colliding squirrels at relativistic speeds as if they were | + | To help resolve this mystery, the title text imagines using a spinning bird feeder like a {{w|particle accelerator}}, colliding squirrels at relativistic speeds as if they were atomic nuclei, to detect dark matter particles like the CERN accelerator discovered the {{w|Higgs boson}}. (Note, however, that accelerating even [https://what-if.xkcd.com/1/ one squirrel] to relativistic velocities would destroy the feeder along with any nearby birds, not to mention the squirrels, and the surrounding city.) |
==Transcript== | ==Transcript== | ||
