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Explain xkcd: It's 'cause you're dumb.
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Revision as of 23:07, 7 August 2012


Welcome to the explain xkcd wiki! We currently have 11 comic explanations. Come and add yours!

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The End of the Rainbow
The retina is the exposed surface of the brain, so if you think about a pot of gold while looking at a rainbow, then there's one at BOTH ends.
Title text: The retina is the exposed surface of the brain, so if you think about a pot of gold while looking at a rainbow, then there's one at BOTH ends.

Explanation

Ambox notice.png This explanation may be incomplete or incorrect: Edited by a leprechaun. We are here. We are watching. The bit about percentages of the mass of the Sun should be made more readable. Also need to explain title text.

Megan appears to reference the myth that at the end of every rainbow lies a leprechaun's pot of gold. Instead of claiming that leprechauns and their gold don't exist, Cueball offers the refutation that, technically, rainbows are circles, so they do not have an end.

However, Megan counters that if one considers the path light takes to form a rainbow, then it forms a two-cone structure, where the Sun (the vertex of the outer cone) emits light rays that move towards the Earth (forming the faces of the outer cone), then reflect off water droplets located at just the right angle (the circular base) to reach our eyes (the vertex of the inner cone). Thus, such a rainbow structure can be said to have "ends", represented by the vertices of the two cones: one at the eye of the viewer, and another at the light source (usually the sun).

Megan then says that the Sun is indeed a pot of gold. The Sun is approximately 1.989 × 10^30 (1 nonillion 989 octillion) kilograms[1], and its abundance of gold is approximately 0.3 parts per trillion[2]. Based on these numbers, the sun contains 5.967 × 10^17 (596 quadrillion 700 trillion) kilograms of gold. This equates to 5.967 × 10^14 (596 trillion 700 billion) metric tons of gold. As such, Megan's statement that the sun contains "quintillions of tons of gold" is off by a factor of roughly 4000. But the amount of gold within the sun is far more than a pot's worth nonetheless.

The amount of water in the oceans is about 1.35 × 10^18 (1 quintillion 350 quadrillion) metric tons[3]. This means that Megan's next statement (that there is more gold in the sun than water in the oceans) is off by a factor of roughly 2300 (though it would have been true if the previous one was).

Cueball then asks about leprechauns. Megan replies that the leprechauns all died when the Sun formed, implying all the gold in the Sun is actually the remnants of leprechaun gold. However, she does not consider the explanation where the leprechaun is on the other end of the rainbow...

The title text suggest that, since the pot of gold exists in the brains of people thinking about it, and the retina is the foremost part of the brain for the light perception, it can be argued that, in addition to existing in the sun as the comic explains, the gold (and leprechauns) also exist at the other end, in the retina and brain of the person seeing the rainbow -- as long as they are thinking about a pot of gold at the time, and then it's gone as soon as they stop thinking about it.

Transcript

[Megan and Cueball are walking.]
Megan: There's a pot of gold at the end of the rainbow.
Cueball: Rainbows are circles. They have no end.
Megan: Not quite!
[In a borderless panel, a multi-part graphic is shown depicting what Megan is describing off-panel: a short cone inside a longer cone, with the longer cone having its point starting at the Sun, the shorter cone having its point at a miniature Cueball's head, and both cones sharing the same circular base. The diagram is repeated from 3 different perspectives to make the structure easier to grasp.]
Megan (off-panel): A rainbow is light leaving the Sun, bouncing off the clouds, and converging on your eye. It's an inside-out two-ended cone.
[Megan and Cueball are still walking.]
Megan: One end of that cone is your retina.
[A wider view of the same scene, with Megan and Cueball walking on a dark ground.]
Megan: The other end is the Sun—which contains quintillions of tons of gold. There's more gold in the Sun than water in the oceans.
Cueball: So there is a pot of gold!
Cueball: What about leprechauns?
Megan: All incinerated as the sun formed. Very sad.

Trivia

As of January 19, 2017, the value of gold is 42,692.98 USD per kilogram. Based on this, all of the gold in the sun is worth 2.5474901 × 10^22 (25 sextillion 474 quintillion 901 quadrillion) USD.


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