Main Page

Explain xkcd: It's 'cause you're dumb.
Revision as of 16:09, 21 November 2013 by Waldir (Talk | contribs)

Jump to: navigation, search

Welcome to the explain xkcd wiki!
We have an explanation for all 1 xkcd comics, and only 30 (2%) are incomplete. Help us finish them!

Latest comic

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."


Ambox notice.png This explanation may be incomplete or incorrect: I've added some detailed explanation. Someone please proofread / review it. Might need some copy-editing.

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 someone (alien civilization?) is encoding spam messages in gravitational waves. It is plausible that aliens are using gravity waves to encode their messages, since we do something similar with electromagnetic waves to encode and send our messages. Although this would take an extremely advanced civilization to achieve gravity wave encoding, since it would require them to control orbits and oscillations of super-massive bodies (think at-least on the scale of the Sun, or typically a factor of ten times bigger than that - the event detected was from two black holes of roughly 30 solar masses each, like the first event listed by the detector).

The title text makes the second conclusion (someone is sending spam encoded in gravity waves) seems more plausible since follows this up with a joke that the message senders have gone such a lengths that they caused the most energetic event recorded ever (maybe they blew up few supernovae). One of the receivers is quite impressed with this and suggests that they have to reply to the spam just because the sender has made such an effort to send the message. The other person is not so impressed, and 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.

The wikipedia article on gravitational waves describes them well, but in simpler/layman's terms, imagine moving a stone through water while it is 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 oscilate around their mean positions. Similar waves are created in space-time fabric when two "heavy" celestial bodies interact with each other. If you concentrate on an area of water-surface (analogous to spacetime fabric) far away from the point of disturbance, you can observe 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 (as well as the gravitational waves wiki page).

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 / neutron stars but not from us moving around.

Now consider spacetime fabric as a thin rubber sheet. If you mark any two points on this sheet and stretch/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, speed of light (c) is a constant and velocity of an object is distance divided by time taken to travel that distance. Second, gravitational waves cause opposite effects (compression and expansion) in directions perpendicular to each other. Putting these two together, 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 those two tunnels, then after going through tunnel and back again (a few times) brought together. Tunnel lengths 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, it is expected that the two tunnel lengths will differ due to the expansion/compression effect described above. The interference will be incomplete, and light will not cancel 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 is a symbol depicting 1 Solar Mass (1.98892×1030 kg). So the statement means that two blackholes, each weighing 30 times our Sun were observed merging in Carina.

Zorlax the Mighty would like to connect on Linkedin

A typical LinkedIn request. Not sure who is Zorlax (the mighty one), but according to title text, he set the record for the most energetic physical event ever observed. He might be looking for a job and this might be him demonstrating (showing-off) his mighty powers instead of simply attaching a resume or filling up his profile. Also, this either means that LinkedIn has now grown outside the Earth, or the Zorlax guy wants to have a job on Earth.

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

Again a possibly legitimate observation from the gravitational wave detector. It detected blackhole merger of two bodies. One of them is 20 times heavier than Sun, the other is 50 times heavier than the Sun. Both of them are located in Orion.

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 humans develop a Wormhole or Faster-than-light travel, 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 guy who sent us LinkedIn invite moments ago. 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.

Meet lonely singles in the local group tonight!

The space advertisers are using space-GeoIP technology on a galactic scale to send spam. Local Group is the technical term for group of close-by galaxies that also includes Milky Way (our galaxy). There are more than 54 galaxies and few other celestial objects in the local group. Local Group along with several other local groups form Virgo Supercluster. So, it seems that the advertiser might be targeting ads to everyone in the Virgo Supercluster. However, finding "lonely singles" in 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.


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 through the collision of 2 black holes. The altered schedule could be viewed as a meta-reference to the warping of spacetime.


[Cueball, with arms up, is standing behind Megan who has her hands at her mouth, is standing behind Ponytail who sits in front of a large computer console with a big screen, a keyboard, and several items on the side (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!

Is this out of date? Clicking here will fix that.

New here?

Last 7 days (Top 10)

Lots of people contribute to make this wiki a success. Many of the recent contributors, listed above, have just joined. You can do it too! Create your account here.

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 table of most recent xkcd comics and links to the rest, and the corresponding explanations. There are incomplete explanations listed here. Feel free to help out by expanding them!
  • If you see that a new comic hasn't been explained yet, you can create it: Here's how.
  • We sell advertising space to pay for our server costs. To learn more, go here.


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.

If you want to talk about a specific comic, use its discussion page.

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.

If you need assistance from an admin, post a message to the Admin requests board.

Personal tools


It seems you are using noscript, which is stopping our project wonderful ads from working. Explain xkcd uses ads to pay for bandwidth, and we manually approve all our advertisers, and our ads are restricted to unobtrusive images and slow animated GIFs. If you found this site helpful, please consider whitelisting us.

Want to advertise with us, or donate to us with Paypal?