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Explain xkcd: It's 'cause you're dumb.
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Types of Approximation
It's not my fault I haven't had a chance to measure the curvature of this particular universe.
Title text: It's not my fault I haven't had a chance to measure the curvature of this particular universe.

Explanation

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In physics and engineering, problem solving typically requires approximations, as physical properties of the universe can be difficult to model. For example, in introductory physics classes, theories are introduced in frictionless environments.

In the comic, Cueball, the physicist, generally dealing with straight math, is introducing a problem with the assumption that the particular curve is a (perfectly) circular arc with a radius represented by R. Megan, the engineer, also assumes that the curve is similar to a circle, with a deviation factor of 1/1000.


The joke arises when Ponytail, the cosmologist, uses the ridiculous approximation of pi equal to 1. In actuality, pi is an irrational number, usually truncated to 3.14. Choosing the value of pi as 1, or 10, as later suggested, completely defeats the purpose of pi for describing a circle. This is a parody of the tendency of cosmology to change the units of measure such that important constants (such as the speed of light or the gravitational constant) are equal to 1, which highly simplifies the formulas without compromising the math. In this case, the number pi is a dimensionless factor, not a directly measured quantity, which means the math will not work.

Transcript

Ambox notice.png This transcript is incomplete. Please help editing it! Thanks.

[There are three panels, each labeled on top]

[Physicist Approximations]

Cueball: We'll assume the curve of this rail is a circular arc with radius R.

[Engineer Approximations]

Megan: Let's assume this curve deviates from a circle by no more than 1 part in 1,000.

[Cosmologist Approximations]

Ponytail: Assume pi is one.
Off-panel voice: Pretty sure it's bigger than that.
Ponytail: OK, we can make it ten. Whatever.


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