2186: Dark Matter

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Dark Matter

Title text: To detect dark matter, we just need to build a bird feeder that spins two squirrels around the rim in opposite directions at relativistic speeds and collides them together.

Explanation

This explanation may be incomplete or incorrect: 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. If you can address this issue, please edit the page! Thanks.

In this comic, Megan and Cueball are talking about dark matter. Megan states that dark matter's density across the solar system is 0.3 GeV/cm³, as claimed by, for example, by Bovy and Tremaine (2012) "On the local dark matter density" in Astrophysics of Galaxies. Since Cueball likely does not have a science background, he does not understand what this means, so, to clarify, Megan explains that 0.3 GeV/cm³ equates to one squirrel's worth of dark matter for the volume of the Earth. For the last two panels, Cueball misinterprets this for comedic effect as implying dark matter is actually one or more squirrels, and thereby providing the necessary mass to spin on bird feeders designed to deter them, enraging Megan.

The gigaelectronvolt (GeV) is a unit of energy that can be converted to a mass using Einstein's formula E = mc². It is typically used for subatomic particles, such as weakly interacting massive particles (WIMPs), one of several contending possibilities for the still-open question of the composition of dark matter, and the one which Megan's uniform density figure assumes constitutes most of it. For example, the mass of a proton is 0.938 GeV/c². However it is common to omit the c² denominator and to represent masses in GeV (or MeV). A mass represented as 0.3 GeV is equal to 5.35 × 10⁻²⁵ grams [1]. This implies a squirrel has a mass of 580 g [2] (about 1.3 lb), which is a typical mass among several species of common squirrels.

The title text imagines using a spinning bird feeder like a particle accelerator, colliding atomic nuclei at relativistic speeds to detect new and exciting particles like the Higgs boson. Note, however, that accelerating even one squirrel to relativistic velocities would destroy the feeder and be deadly to any birds near the feeder (not to mention the squirrel).

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 Laser Interferometer Gravitational-Wave Observatory (LIGO) and subsequent confirmation by the Virgo interferometer of gravitational waves from unexpectedly many merging black holes substantially more massive than those produced by stellar collapse, primordial black holes (PBHs) have become a popular alternative explanation to WIMPs (and squirrels), attracting proponents at NASA, and other cosmologists for several reasons. However, PBHs remain controversial, because if they constituted more than a very small portion of dark matter, alternative explanations would be almost entirely excluded.

Transcript

[Megan walks with Cueball. She is holding a hand out while telling Cueball something.]

Megan: Dark matter density in the solar system is around 0.3 GeV/cm³

Cueball: Is... that a lot?

[As they continue to walk and talk she spreads her arms out.]

Megan: In terms of mass, it means the Earth contains one squirrel worth of dark matter at any given time.

Cueball: Wow.

[In a frame-less panel Cueball stops while Megan walks past him. Megan is face-palming herself while looking down.]

Cueball: Is there any way to find out which squirrel it is?

Megan: No, it's not literally-

[Cueball holds his hand with one finger up in front of Megan, while she has turned towards him and is holding both arms up, possible with balled fist, as she shouts back at him, shown both with large fat letters and with small lines emanating above her head.]

Cueball: Oh, that explains why they weigh enough to set off those spinning bird feeders!

Megan: Dark matter isn't squirrels!

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Discussion

If dark matter were squirrels, they wouldn't set off the bird feeders because dark matter can't interact with feeders at all! Especially since it's not to the squirrels' advantage to set them off. Unless, of course, the dark matter squirrels do exist and steal from our feeders freely, but we can't notice because dark matter is unobservable at this scale. In fact, it's entirely plausible that some squirrels evolved to be dark matter for this specific advantage, so I wouldn't rule that out. 172.68.244.174 12:12, 7 August 2019 (UTC)

Well if they can't interact with the feeders, they won't be able to steal the food either.141.101.107.66 13:28, 7 August 2019 (UTC)

Not with that attitude. 162.158.126.4 14:10, 7 August 2019 (UTC)

I have never seen a spinning bird - can they turn peanuts into gold?

Yes, but the gold would be radioactive. The much more impressive reaction is to turn gold into peanuts, other than by paying a farmer for peanuts using the gold. Nutster (talk) 14:36, 7 August 2019 (UTC)

Would I be allowed to pay a merchant for peanuts using the gold? --Lupo (talk) 14:38, 7 August 2019 (UTC)

I had to google what a spinning bird feeder is. I had never seen one of these before (I live in Germany). Maybe a link to one of these funny videos should be added. Frank 172.68.110.64 19:59, 7 August 2019 (UTC)

✓ Done 172.69.22.32 01:25, 8 August 2019 (UTC)

You don't have bird feeders in Germany? I know birds exist all around the world, I would think people like to feed them all around the world... Or maybe you're missing squirrels that jump on them and make them spin? NiceGuy1 (talk) 04:18, 15 August 2019 (UTC)

Okay, clicked the link, never heard of that feature before, LOL! I understood just that when they jump on a hanging bird feeder they tend to make it spin, LOL! NiceGuy1 (talk) 04:29, 15 August 2019 (UTC)

Anyone else who read PBH as PHB, the pointy haired boss from Dilbert? --Lupo (talk) 07:06, 8 August 2019 (UTC)

I always think "peanut butter and ham," from PBJ+BLT I guess. 172.68.141.148 07:20, 8 August 2019 (UTC)

I read it as "Pause Buffered Hitstun". Probably not Douglas Hofstadter (talk) 05:17, 11 August 2019 (UTC)

In the explanation, I think I missed a step of logic. Are Windows, Icons, Mice and Pointers made up of Dark Matter? It might explain a few lockups in Windows I've been having lately.Seebert (talk) 11:47, 9 August 2019 (UTC)

How can Megan use the dark matter density across the whole solar system to calculate the density within the Earth? If dark matter interacts with gravity, shouldn't the distribution of dark matter within the Solar system be similar to the distribution of normal matter (i.e. much higher density near massive objects like the Earth)? 108.162.241.124 17:40, 9 August 2019 (UTC)

It interacts with gravity, but in doing so it falls into and through ordinary matter and then just keeps going -- gravity isn't strong enough to slow it down enough to make it bunch up around normal matter. Only galaxies have discernible halos clumped. Most dark matter particles that fall into our solar system would just fall out the other side at the same speed, tossing them into the gravity of another star before they slow down enough to come back, in most cases. The linked density paper talks about the extent of that effect as the basis of their calculations. 172.68.141.184 01:01, 10 August 2019 (UTC)

For comparison, it would be nice to have the average density of "regular" matter of the earth, the sun, interplanetary space, interstellar space, etc. --141.101.105.246 18:59, 9 August 2019 (UTC)

It's a little less than one fifth overall. However, you can't really have a local density of normal matter because it's clumped into stars and planets. Even at our scale, what's underneath is almost certainly far more dense than what's above you. 172.68.141.184 01:01, 10 August 2019 (UTC)