Editing 1758: Astrophysics

{{comic
| number    = 1758
| date      = November 11, 2016
| title     = Astrophysics
| image     = astrophysics.png
| titletext = DEPARTMENT OF NEUROSCIENCE / Motto: "If I hear the phrase 'mirror neurons' I swear to God I will flip this table."
}}


==Explanation==
In physics, the theory of gravity produced by {{w|general relativity}} combined with {{w|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 {{w|Two-body problem in general relativity#Anomalous precession of Mercury|precession of Mercury}} which Newton couldn't fully explain.  Dark matter, in turn, explains behaviors of galaxies such as their {{w|Galaxy rotation curve|rotation rates}} that were not correctly predicted with general relativity alone.  Most astrophysicists believe dark matter exists, either in the form of {{w|Massive compact halo object|an unknown type of star that is too dim to see}}, or {{w|Weakly interacting massive particles|an undiscovered subatomic particle}}.

However, because the concept of dark matter posits something so pervasive yet unknown and so far undetected, it can be difficult to accept, since typically inability to detect something tends to mean non-existence of that thing. One might be reminded of {{w|Aether_(classical_element)|Aether}}, a similar theory that an undetectable substance exists in space to allow light and gravity to travel, although unlike dark matter that has been debunked. Thus, 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 {{w|modified Newtonian dynamics}} (MOND). In MOND, gravity doesn't simply follow the {{w|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 and seemingly more logical — 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 {{w|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 favor — and a MOND-compatible general relativity would be very complicated and ugly.

This comic illustrates physicists' exasperation for people who constantly try to challenge the existence of dark matter without considering 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 dissuasion.  The specific impetus for this comic may be the press coverage around [https://arxiv.org/abs/1611.02269 this publication] by {{w|Erik Verlinde}} (see popular description of the paper [http://phys.org/news/2016-11-theory-gravity-dark.html 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 {{w|entropic gravity}} is a serious theory derived from {{w|thermodynamics}} and {{w|quantum information theory}}, it is important to keep in mind that it's just a {{w|pre-print}} and hasn't been peer-reviewed or experimentally verified yet. Verlinde's theory also doesn't match all available data - it [https://web.archive.org/web/20170115045049/http://motls.blogspot.de/2010/01/erik-verlinde-why-gravity-cant-be.html 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 {{w|Neuroscience}} from popular misconceptions of {{w|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, {{w|mirror neurons#Doubts concerning mirror neurons|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. [http://knowyourmeme.com/memes/flipping-tables 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 [https://www.youtube.com/watch?v=7UNLgPIiWAg Did Dark Energy Just Disappear?].  This one was regarding the paper [http://www.nature.com/articles/srep35596 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]].

==Transcript==
:[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:]
:<big>'''Department of Astrophysics'''</big>
:'''Motto:'''
:''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.''

{{comic discussion}}

[[Category:Cosmology]]
[[Category:Science]]
[[Category:Physics]]
[[Category:Biology]]