Difference between revisions of "849: Complex Conjugate"

Explain xkcd: It's 'cause you're dumb.
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m (Transcript: Completing explainxkcdization of transcript)
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:Cueball: That's right.
 
:Cueball: That's right.
  
:[Dramatic zoom. It appears Cueball is writing on the side of the panel.]
+
:[Dramatic zoom. It appears Cueball is writing on the side of the panel.]
 
:Cueball: Shit just got ''real''.
 
:Cueball: Shit just got ''real''.
  

Revision as of 01:42, 7 January 2014

Complex Conjugate
Fun fact: if you say this every time a professor does something to a complex-number equation that drops the imaginary part, they'll eventually move the class to another room and tell everyone else except you.
Title text: Fun fact: if you say this every time a professor does something to a complex-number equation that drops the imaginary part, they'll eventually move the class to another room and tell everyone else except you.

Explanation

This comic is a joke on the common phrase "Shit just got real", meaning that something has suddenly increased in difficulty so it is now a real challenge. Cueball is standing in front of a board delivering a lesson, and is about to multiply a wavefunction by its complex conjugate. In mathematics, a complex number has a real part (a) and an imaginary part (b*the imaginary unit). The number can be written as a+bi. The complex conjugate of a number (a+bi) is a-bi. When one multiplies a complex number and its conjugate, the result is (a+bi)(a-bi), which is a^2+b^2. This process yields a number that does not have the imaginary part, making it real, which is a play on the aforementioned phrase. Accordingly, Cueball dictates that the people who are not skilled enough should leave.

The title text states that if you joke about this every time it's applicable, that will be annoying enough that the class will ditch you and attempt to take place in another room.

Transcript

[Cueball is standing at a whiteboard covered in equations, one of which is the time dependent Schrodinger equation.]
Cueball: Okay, anyone who's feeling like they can't handle the physics here should probably just leave now.
Cueball: Because I'm multiplying the wavefunction by its complex conjugate.
Cueball: That's right.
[Dramatic zoom. It appears Cueball is writing on the side of the panel.]
Cueball: Shit just got real.


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Discussion

Actually multiplying complex number (x + iy) by its complex conjugate (x - iy) does not "remove" imaginary part, but calculate square of absolute value of complex number, (x^2 + y^2). BTW. in quantum physics the wavefunction is complex valued, and its absolute value is probability density (a real valued function). --JakubNarebski (talk) 00:57, 18 December 2012 (UTC)

I got hit in the face with my complex conjugate and lost an eye. 108.162.238.114 (talk) (please sign your comments with ~~~~)

I procreated with my complex conjugate and lost myself. 108.162.216.114 19:44, 12 August 2014 (UTC)

(a+bi)*(a-bi)= a^2-b^2 not a^2+b^2 108.162.218.142 15:48, 24 June 2015 (UTC)

(a+b)*(a-b) = a^2 - b^2 . However (a+bi)*(a-bi) = a^2 + b^2 since i^2 = -1. 108.162.219.65 (talk) (please sign your comments with ~~~~)

I think that the moving to a title text is a joke about them kicking you out of the room 173.245.50.65 20:54, 1 May 2016 (UTC)

Is it relevant that when he performs the complex conjugate multiplication, we go from a 2d image of the board to a 1d image of the board? My thinking is, that complex numbers are often used to describe a set of coordinates in 2d space. 162.158.158.233 (talk) 06:58, 14 January 2020 (please sign your comments with ~~~~)

It might be a nice coincidence, or happened on purpose. However in my understanding and the usecases I've seen for example in electrical engineering, it was the other way round. Complex Numbers were used to describe something , e.g. phase shift, and then it could be visualized as coordinates where it was visible as a sinus-curve. (I Might be slightly wrong about the details here. It's been some time...) --Lupo (talk) 07:18, 14 January 2020 (UTC)

What's with that red text? Is that MML? ⟨Winter is coming⟩ Marethyu (talk) 21:27, 5 May 2022 (UTC)

Everyone immediately recognised how the product of a complex function with its conjugate "makes shit real" but I think there's another layer of comedy (in a sort of anti-joke way) where the complex conjugate of a real number (a+0i) is (a-0i) in other words the product of an entirely real-valued function and its complex conjugate would just be its square which is an anticlimactic turn on the joke. 172.70.93.13 15:13, 31 August 2022 (UTC)