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Welcome to the explain xkcd wiki! We already have 11 comic explanations!

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Gravitational Waves
"That last LinkedIn request set a new record for the most energetic physical event ever observed. Maybe we should respond." "Nah."
Title text: "That last LinkedIn request set a new record for the most energetic physical event ever observed. Maybe we should respond." "Nah."


Megan, Cueball, and Ponytail are observing the results from a gravitational wave detector (see details below). This comic came out on the day that the first direct observation of gravitational waves was publicly announced on 2016-02-11. The actual event was recorded five months before on 2015-09-14, but it was not reported publicly before they were sure it was a real signal. It seems that Randall new in advance about this announcement because this comic was published on a Thursday, not following the normal publish schedule, to coincide with the announcement, and there were no other comic released Friday that week. (The altered schedule could be viewed as a meta-reference to the warping of spacetime). This is the second time within a month that a new astronomical announcement (of something discovered months before the actual announcement) has resulted in a related comic. The first being 1633: Possible Undiscovered Planets.

From the patterns in the gravitational waves detected by this instrument, it might be possible to guess the nature of the event. (e.g. two bodies with dissimilar masses circling a fixed point, two bodies with equal mass circling each other, collision of two massive bodies, etc.) It might also be possible to triangulate the location of the event. Based on these two facts (the location and nature of the event) we might be able to determine which astronomical bodies caused this event (and the status of those bodies afterwards). Thus, it provides an additional medium to observe the universe in addition to telescopes observing all kinds of electromagnetic radiation. This new medium might enable us to observe properties that we couldn't observe with the rest of our observation instruments.

However, the scientists in this comic appear to be receiving more than the expected signals from black hole collisions, they also receive gravitational spam messages, such as invitations from Linkedin, a mortgage offer, and an announcement of a social meet-up, rather than observing astronomical events. (See table below).

There is also a joke on the social meet-up's use of the word local group because the 'Local Group' is also the technical name for the group of galaxies containing the Milky Way.

It is not clear if these so called "events" are causing gravitational waves to be generated or if something, perhaps an alien civilization, is encoding spam messages in gravitational waves. It is plausible that aliens are using gravity waves to encode their messages, as we do something similar with electromagnetic waves to encode and send our messages. However, it would take an extremely advanced civilization to achieve gravity wave encoding. It requires the controlling of orbits and oscillations of super-massive bodies like the Sun, or more likely bodies ten times more massive than it. For example, the first event detected, both in this comic and in real life, was a merger of two black holes of roughly 30 solar masses each.

The title text makes the speculation, that something is sending spam encoded in gravity waves, seem more plausible, as it follows up with a joke that the message senders have gone to such a length that they caused the most energetic event recorded ever (perhaps on the scale of a few supernovae or black hole collisions). One of the scientist is so impressed with this effort that he suggests that they actually post a reply, but one of the other person declines with a "Nah"! (As you should always do with spam, else you will just encourage the sender by making it clear that there actually is a receiver on this address).

Gravitational waves

A gravitational wave detector is a device used to measure gravitational waves, small distortions of spacetime that were first predicted by Albert Einstein in 1916. Gravitational waves are ripples in the spacetime fabric itself.

In layman terms, a gravitational wave is like moving a stone through water while partly submerged. It will cause waves on the surface of the water as it moves through it. These waves will spread away from the center of disturbance and as they move, they will cause the water molecules to oscillate around their mean positions. Similar waves are created in the space-time fabric when two celestial bodies interact with each other. If you concentrate on an area of the fabric far away from the point of disturbance, it can be observed that if the wave causes compression in one direction, it'll cause expansion of the fabric in the other. See this page for nice animations.

Note that anything with a mass will cause a gravitational wave. Just as waves created by small stones are tiny in comparison to waves created by huge rocks in water, the waves from humans moving around will be tiny compared to the waves created by celestial bodies. Also, the bigger the body, the stronger the wave and the farther away it will travel. That is why we can only detect gravity waves from heavy bodies like black holes or neutron stars but not from us moving around.

Now, let's consider spacetime fabric as a thin rubber sheet. If you mark any two points on this sheet and stretch or compress it along the axis joining those two points, the relative positions of these points with respect to their neighboring points do not change, but the distance between them changes.

LIGO (Laser Interferometer Gravitational-Wave Observatory) is a large-scale physics experiment designed to detect this compression/expansion, and it was LIGO who discovered the signal that caused this comic.

Two facts need to be remembered to easily understand the experiment. First, the speed of light (c) is constant and the speed of an object is the distance moved divided by the time taken to travel that far. Second, gravitational waves cause opposite effects (compression and expansion) in directions perpendicular to each other. At LIGO, an experiment is set up where two perpendicular long tunnels are constructed with apparatus to emit and detect laser beams. The beam from a laser is split into these two tunnels. After going through the tunnel and back again a few times the beams are brought back together. The lengths of the tunnels are set up in such a way that, in the absence of gravity waves, interference between the two combined beams causes them to cancel one another out. When the gravitational wave passes through earth, the lengths of the tunnels are expected to change due to the expansion/compression effect. The incomplete interference means that the lights will not cancel each other out. This observation can be concluded as "detection of the gravitational wave passing through".

Explanation of observed events

Event Explanation
Black hole merger in Carina (30 M, 30 M)

Possibly legitimate result from the gravitational wave detector. M means 1 Solar Mass (1.98892×1030 kg). So the statement means that two black holes, each one 30 times more massive than our Sun were observed merging in the Carina constellation. This observation is similar to the one actually reported in the announcement on the day of this comic, were the two black holes had masses of 36 and 29 solar masses (with a uncertainty that meant they could have had masses as close as 32 and 30 solar masses vs the 30+30 in this event). Also the location of the event matches with Carina. Although at present time it is not possible to pinpoint the location of the event (that will need more detectors spread out across Earth) they still manage to find out that it most likely originated in the Southern Celestial Hemisphere, which is also where the Carina constellation is located.

Zorlax the Mighty would like to connect on Linkedin

A typical LinkedIn request. As this may be a message encoded in gravitational waves, it either means that LinkedIn has now grown outside the Earth, or that the Zorlax person would like to contact Earth. Zorlax is likely a reference to a kid's television gameshow, based on time travel, Zorlax and the Time Travellers. The first sentence in the movie is: Four billions of years ago in the Earths core, destined to be the master of time he is the Mighty Zorlax. It is likely someone with the power over time would be able to create gravitational waves at his own leisure.

Black hole merger in Orion (20 M, 50 M)

Again, a possibly legitimate observation from the gravitational wave detector. It detected a black hole merger of two bodies in the Orion constellation. One of them is 20 times more massive than Sun, the other is 50 times more massive than the Sun. (As Orion is located at the celestial equator and since the masses are far from the one announced on the day of this comic, this would then represent a possible 2nd event to be measured later).

Mortgage offer from Triangulum Galaxy

Triangulum Galaxy, also known as Pinwheel Galaxy, is a spiral galaxy approximately 3 million light-years from Earth. It is the third largest galaxy in the Local Group after the Andromeda Galaxy and the Milky Way, these three being the only spiral galaxies in the group. This is presumably an offer to extend a loan, with a house on Earth serving as collateral. However, unless we develop wormholes or faster-than-light travel technologies, it will be difficult to submit an application, even if the offer is legitimate.

Zorlax the Mighty would like to connect on Linkedin

Same person who sent us LinkedIn invite moments ago. This repeated request may imply that Zorlax is desperate, or may be a jab at LinkedIn's persistence in spamming users with unaccepted connections to view and/or accept them. According to the title text, this was the most energetic physical event ever observed; it may be that Zorlax is demonstrating his capabilities in the hopes that the request would be accepted.

Meet lonely singles in the local group tonight!

The space advertisers are using space-GeoIP technology on a galactic scale to send spam. A Local Group is the technical term for the group of nearby galaxies that also includes the Milky Way, our own galaxy. There are more than 54 galaxies and few other celestial objects in our local group. This Local Group along with several others form the Virgo Supercluster. It would seem that the advertiser is targeting ads to everyone in the Virgo Supercluster. However, finding "lonely singles" in the 54 galaxies within our local group might be easier said than done for humans here on Earth. This kind of spam was previously featured in 713: GeoIP.


[Cueball, with arms up, is standing behind Megan who has her hands at her mouth, and who in turn is standing behind Ponytail, who is sitting in front of a large computer console with a screen, a keyboard, and several items on the side (presumably lights and labels). Three wires lead away from the console out of the image to the right.]
Megan: The gravitational wave detector works! For the first time, we can listen in on the signals carried by ripples in the fabric of space itself!
[Larger panel with the same setting in the middle, but both Cueball and Megan have taken their arms down. More of the wires from the console can be seen to the right. The computer lists six events:]
Computer: Event: Black hole merger in Carina (30 M, 30 M)
Computer: Event: Zorlax the Mighty would like to connect on Linkedin
Computer: Event: Black hole merger in Orion (20 M, 50 M)
Computer: Event: Mortgage offer from Triangulum Galaxy
Computer: Event: Zorlax the Mighty would like to connect on Linkedin
Computer: Event: Meet lonely singles in the local group tonight!

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