Explain xkcd: It's 'cause you're dumb.
In physics, the theory of gravity produced by general relativity combined with dark matter are our current best model for explaining the behavior of gravity and galaxies. The evidence supporting this model is extensive. General relativity accurately predicts the orbit of planets, even precise details like the precession of Mercury which Newtonian gravity couldn't fully explain. Dark matter, in turn, explains behaviors of galaxies such as their rotation rates that were not correctly predicted with general relativity alone. Most astrophysicists believe dark matter exists, either in the form of an unknown type of star that is too dim to see, or an undiscovered subatomic particle.
However, because the concept of dark matter posits something unknown and so far undetected, it can be difficult to accept at an intuitive level. It is common to hear objections to dark matter, with a popular alternative idea being that dark matter can be explained away by a modified theory of gravity. One such alternative theory which gets proposed regularly is modified Newtonian dynamics (MOND). In MOND, gravity doesn't simply follow the inverse square law but has more complicated behavior. Usually, the extra behavior is either to say that gravitational force can be affected by the acceleration of the particle, or that it goes from inverse-square to just inverse at large distances. It can be appealing because it's relatively simple—it just changes our understanding of Newton's law of gravitation, rather than requiring entirely new forms of matter or unknown stars to exist—and because it has some nice side-effects, such as explaining why there seems to be a limit on the density of galaxies. Unfortunately, physicists have explored this avenue and cannot reconcile it with all existing data. One famous counterexample is the Bullet Cluster, where two colliding galaxy clusters are ripping through each other. The mass distribution within the cluster can be inferred through gravitational lensing, and appears to show dark matter and ordinary matter being separated to a certain extent which cannot be explained with MOND. Another counterexample is MOND's incompatibility with observations of the motion of galaxies in galaxy clusters. More generally, MOND isn't compatible with general relativity—which has a huge amount of experimental data in its favour—and a MOND-compatible general relativity would be very complicated and ugly.
This comic illustrates physicists' disdain for people who constantly try to challenge the existence of dark matter without realizing all the evidence and theoretical foundation that support it. Apparently members of this department are so tired of hearing the same old ideas being repeated to them, that they have adopted a motto and even erected a sign in an attempt to clear the misconception. The specific impetus for this comic may be the press coverage around this publication by Erik Verlinde (see popular description of the paper here). It was released online three days before the release of this comic and got a lot of coverage exclaiming "this will prove Einstein wrong". While Verlinde's work on entropic gravity is a serious theory derived from thermodynamics and quantum information theory, it is important to keep in mind that it's just a pre-print and hasn't been peer-reviewed or experimentally verified yet. Verlinde's theory also doesn't match all available data - it disagrees with experimental results showing how particles interact with gravity. Thus, it is still a far cry from being a contender for replacing dark matter.
The title text alludes to a similar issue faced by the Department of Neuroscience from popular misconceptions of Mirror neurons. Mirror neurons are brain cells which trigger when watching someone else do something. Experiments claim to have found mirror neurons in humans and apes, and there are theories that make mirror neurons the foundation of learning, empathy, language and consciousness itself. However, the evidence for mirror neurons is still patchy, and even if they exist, it's very simplistic to try to attribute so much of human behavior to a single type of relatively simple cell. In light of this, the motto of the neuroscientists at the department rightfully reflect their frustration. Flipping tables is a common depiction for expressing extreme outrage. It is used here also as a pun because mirrors flip the image in front of them.
Another story of similar press coverage questioning the current established scientific theory was also mentioned two days before the release of this comic, on the YouTube channel Space Time from PBS Digital Studios in their video titled Did Dark Energy Just Disappear?. This one was regarding the paper Marginal evidence for cosmic acceleration from Type Ia supernovae. The video concluded that dark energy is still the best explanation. Note this is about the existence of dark energy rather than dark matter. The two are very distinct concepts.
Science papers with results that supposedly disprove solidly founded theories have been the subject before in 955: Neutrinos.
- [A sign on two posts, in the grass in front of a building with windows and double doors, a window on each door, and bars facing outwards. There is a cement walk leading to the doors. On the sign is the text:]
- Department of Astrophysics
- Yes, everybody has already had the idea, "Maybe there's no dark matter—Gravity just works differently on large scales!" It sounds good but doesn't really fit the data.
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"Two days before the release of this comic the YouTube channel Space Time from PBS Digital Studios released a new video with the title Did Dark Energy Just Disappear?. This was based on the press coverage the paper Marginal evidence for cosmic acceleration from Type Ia supernovae got, which relates to the one referenced in this comic for dark matter." This doesn't seem relevant. Dark energy is totally unrelated to dark matter. Schroduck (talk) 14:33, 11 November 2016 (UTC)
- I agree. I don't see any connection here either.184.108.40.206 16:02, 11 November 2016 (UTC)
- It is the idea that a paper seems to prove a theory wrong and then the press goes out presenting it like a proof instead of asking someone to explain to them why it doesn't fit the data. That is what this comic is about - not dark matter. See the title text. --Kynde (talk) 18:06, 11 November 2016 (UTC)
- In my field (medicine) recent studies have shown that there is a 1-in-3 chance of news coverage being correct (or " mostly correct"). Given the atrocious state of medical research due to drug manufacturers financial influence and author bias, it may be difficult to differentiate news reports from random chance...
"What is the flip the table over reference in title text. To make other do the same through mirror neruons? Still new explanation. Add more if you can" "The title text also uses Mirror neurons as a reference to a joke: it suggests to "flip this table", just as a mirror flips the image in front of it." I too want to think there is a joke here about mirror behavior or something but I just don't get it. Somebody's got to come up with a clearer, and funnier, example!ExternalMonolog (talk) 16:31, 11 November 2016 (UTC)
- Yup, I too had the same thought as your first statement. If someone flips the table, but mirror neurons exist, then they too will flip the tables. So a flip and a flip would result (assuming nothing was on the table, or stuff was bolted into place) in the same orientation as before. Which would be fine, because in that case - mirror neurons would presumably "really" exist, and there wouldn't be any reason to get angry over postulates which state that they do. However, where do you stop the infinite reflections? On an even number, or an odd number? 220.127.116.11
Seems like awkward timing since https://arxiv.org/abs/1611.02269 was posted 3 days ago, a non-MOND non-dark matter theory coming from Prof. Erik Verlinde, and this particular theory starts from first principles yet matches behavior of galaxies. Anon 16:49, 11 November 2016 (UTC)
- The comic doesn't mention MOND that is only in the explanation here. It just say that all data fits with dark matter. The idea is that the department is tired of all the "proofs" that dark matter doesn't exist. Maybe Randall thinks that this new paper is just the next in line and note as explained above this paper has not been peer reviewed. So unless you're and expert and could peer review it then his theory may not fit the data and that is Randall's point. But I'm sure Randall would get your dark matter is still on the table after this paper... --Kynde (talk) 18:13, 11 November 2016 (UTC)
MOND is but one theory among many classical and quantum gravitational theories with differing predictions for galactic rotation and lensing anomalies. There are non-gravitation theories as well. It might behoove some intrepid sole to make a table of theories and dark matter alternatives. Run, you clever boy (talk)
Perhaps Randall is echoing his fellow cartoonist, Scott Adams, when he points out the hypocrisy in science reporting. Recently, Michael S. Kochin exposed government meddling in science reportage among other inconvenient truths. Anyone with an NSF, DoE or EPA grant knows the pressures, as Henry Payne, another cartoonist, points out. FWIW, I side with Bjorn Lomborg, who famously champions a middle way in climate science for the sake of downtrodden peoples around the world. Additionally, Cato provides an IPCC MAGICC climate model simulator for anyone to examine. Should we reconsider this explanation and the explanation for Randall’s Earth Temperature Timeline in this light? Run, you clever boy (talk)
The current explanation doesn't cover the failure of previous experiments to detect dark matter, despite the investment of time, money and effort. Absence of evidence may not be evidence of absence, but it's fair to say that dark matter as an explanation for observations does technically lack direct evidence/detection. – 18.104.22.168 20:44, 11 November 2016 (UTC)
- That's the rub, isn't it? Even LHC and LIGO detections rely on theoretical templates to enhance event rates. Run, you clever boy (talk)
"Of course phlogiston exists. We haven't any observational evidence for it but any idea that combustion works different doesn't fit the data."
"Of course Vulcan exists. We haven't any observational evidence for it but any idea that gravity works different doesn't fit the data."
And here we go again. 22.214.171.124 23:46, 11 November 2016 (UTC)
- But this was the same way that led us to discover Neptune, Uranus, Pluto, and the Kuiper belt. Theme (talk) 06:49, 12 November 2016 (UTC)
- Uranus had been observed on many occasions before eventually being recognised first as a comet then as a planet; no calculation. A calculation did help find Neptune but had also been seen by multiple observers prior to that; there was already real evidence that it existed. Pluto was discovered by accident based on a faulty calculation so you could add it to Vulcan. Nothing in the Kuiper Belt is big enough to cause the perturbations that would allow for their position to be calculated in advance. 126.96.36.199 06:50, 14 November 2016 (UTC)
Since the Bullet Cluster has been brought up again, it should be pointed out that it doesn't provide the iron-clad evidence for dark matter that some appear to think it does. Ask a MOND (or MOG)-sympathetic physicist about it and they'll direct you to Brownstein & Moffat, 2007, which claims to provide a modified-gravity model that fits the data just as well (or perhaps even better) than λCDM (dark matter). I'm not going to pretend to be able to assess the model they present (or even really understand it), and I'm shamelessly parroting a recent blog-post and commentary by Sabine Hossenfelder of the Frankfurt Institute of Advanced Study . But I think the idea that the controversy between λCDM and MOG has been settled is perhaps a distortion of the facts, and those who aren't intimately familiar with the field might be wise to avoid treating it as such. Charleski (talk) 10:19, 12 November 2016 (UTC)
Reminds me of https://xkcd.com/675/. -- Benjaminikuta (talk) (please sign your comments with ~~~~)
Just noticed that this may be the first xkcd comic with a "2x" version for retina displays. astrophysics_2x.png ~Luc [talk] 19:06, 13 November 2016 (UTC)
- What about http://imgs.xkcd.com/comics/old_days_2x.png ? 188.8.131.52 13:26, 14 November 2016 (UTC)
Pet peeve: using brand names to describe generic principles, like "Retina display" instead of "high dpi display". 184.108.40.206 09:47, 15 November 2016 (UTC)
Perhaps this is a comment on the reluctance of established academic departments to support research that challenges the mainstream, accepted theories. Kind of like how quantized light was dismissed for some twenty years after Einstein proposed it. 220.127.116.11 18:59, 16 November 2016 (UTC)