Gravitational Waves
Gravitational Waves
[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!

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 knew 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 comics released Friday that week. (The altered schedule could be viewed as a meta-reference to the warping of spacetime .) That scientists knew there might be an announcement on the way, and more details for the interested can be seen in these two videos from Space Time : Have Gravitational Waves Been Discovered?!? and LIGO's First Detection of Gravitational Waves! (See also their follow up The Future of Gravitational Waves ).
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 scientists 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.) Randall may have been referring to the fact that the detected event had a power output equal to 50 times that of the entire visible universe.
In 1365: Inflation gravitational waves are also mentioned.
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 can be detected. 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. (For a very detailed description of what the team at LIGO did and the history behind see this 90 minutes feature Gravitational Waves: A New Era of Astronomy Begins from the 2016 World Science Festival).
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, destructive interference between the two combined beams causes them to cancel one another out, resulting in the detector observing zero light intensity. When the gravitational wave passes through earth, one of the tunnel is expected to expand while the other is expected to compress. Due to the difference in lengths, the destructive interference is incomplete and the detectors will be able to detect the presence of light. This observation can be concluded as "detection of the gravitational wave passing through".
[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!
