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Revision as of 21:16, 24 March 2013

Welcome to the explain xkcd wiki!

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Go to this comic explanation

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."

Explanation

Ambox notice.png This explanation may be incomplete or incorrect: More proofreading to ensure readability is required

Megan, Cueball, and Ponytail are observing the results from a gravitational wave detector (see details below). This comic came out the day after a direct observation of gravitational waves was publicly announced.

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.

There is also a joke on the social meet-ups 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 was a merger of two black holes of roughly 30 solar masses each.

The title text makes the second speculation, that something is sending spam encoded in gravity waves, seem more plausible, as the text 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). One of the receivers is impressed and suggests that they reply. The other person declines with a "Nah".

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. 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 Carina.

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 the Zorlax person would like to contact Earth.

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 Orion. One of them is 20 times more massive than Sun, the other is 50 times more massive than the Sun.

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 not clear if the offer is for a house on Earth or if the advertisers want us to buy a house in the Triangulum Galaxy. Either way, unless we develop Wormhole or Faster-than-light travel technologies, we may not be able to take up the offer, even if it 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 their 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.

Trivia

This comic was published on a Thursday, not following the normal publish schedule, to coincide with the announcement of the discovery of a clear gravitational wave signal on February 11, 2016. Gravitational waves were detected when two black holes collided. The altered schedule could be viewed as a meta-reference to the warping of spacetime.

Transcript

[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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You can read a brief introduction about this wiki at explain xkcd. Feel free to sign up for an account and contribute to the wiki! We need explanations for comics, characters, themes, memes and everything in between. If it is referenced in an xkcd web comic, it should be here.

  • List of all comics contains a complete table of all xkcd comics so far and the corresponding explanations. The red links (like this) are missing explanations. Feel free to help out by creating them! Here's how.

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Don't be a jerk. There are a lot of comics that don't have set in stone explanations; feel free to put multiple interpretations in the wiki page for each comic.

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Please only submit material directly related to —and helping everyone better understand— xkcd... and of course only submit material that can legally be posted (and freely edited.) Off-topic or other inappropriate content is subject to removal or modification at admin discretion, and users who repeatedly post such content will be blocked.

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