1758: Astrophysics

2016-11-11T14:46:55Z

LanguidLegend:

{{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==
{{incomplete|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 theory of gravity produced by {{w|general relativity}} is generally very accurate - it predicts the orbits of planets precisely, even details like the {{w|Two-body problem in general relativity#Anomalous precession of Mercury|precession of Mercury}} which Newton couldn't explain. However, the predictions for the behavior of galaxies are wrong - {{w|Galaxy rotation curve|the galaxies seem to spin at the wrong rates}}.

The standard explanation is that there is something else filling these galaxies, which has mass (and therefore exerts a gravitational pull) but which can't be seen with current telescopes. This is called {{w|dark matter}}.

However, an alternative theory which gets proposed regularly is {{w|modified Newtonian dynamics}} (MOND). In MOND, gravity doesn't simply follow the {{w|inverse square law}} and decay to nothing, 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.

Unfortunately, as the sign says, MOND doesn't fit all the scientific data. One famous counterexample is the {{w|Bullet Cluster}} - two colliding galaxies that are ripping through each other. If MOND were true, then the gravitational force should be the same in all directions (regardless of whether it's attractive or repulsive). However, studies of the bullet cluster show an uneven distribution of matter in the galaxy. This would only make sense if the galaxy has hidden concentrations of dark matter in it.

The specific impetus for this comic may be [https://arxiv.org/abs/1611.02269 this] recent 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 on-line three days before the release of this comic. Verlinde's theory ({{w|entropic gravity}}) isn't MOND - rather, it's derived from {{w|thermodynamics}} and {{w|quantum information theory}} - but it has a lot in common with it. The paper got a lot of "This will prove Einstein wrong" coverage (see [[1206: Einstein]]), even though it's just a {{w|pre-print}} and hasn't been peer-reviewed or experimentally verified yet. Verlinde's theory also doesn't match the data - [http://motls.blogspot.de/2010/01/erik-verlinde-why-gravity-cant-be.html it disagrees with experimental results showing how particles interact with gravity].

Apparently, whoever put up this sign was getting tired of news agencies stating that dark matter has been "disproven".

The title text lists the text on a similar sign standing outside the Department of {{w|Neuroscience}}. Their motto is: "If I hear the phrase 'mirror neurons' I swear to God I will flip this table." {{w|Mirror neurons}} are brain cells which trigger when watching someone else do something. Experiments claim to have found mirror neurons in humans and other 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 and attribute so much of human behavior to a single type of relatively simple cell.

Two days before the release of this comic the YouTube channel Space Time from PBS Digital Studios released a new video with the title [https://www.youtube.com/watch?v=7UNLgPIiWAg Did Dark Energy Just Disappear?]. This was based on the press coverage the paper [http://www.nature.com/articles/srep35596 Marginal evidence for cosmic acceleration from Type Ia supernovae] got, which relates to the one referenced in this comic for dark matter.

==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:Astronomy]]
[[Category:Science]]

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1758: Astrophysics