Why are waves represented as complex numbers?

  • #1

Main Question or Discussion Point

Why do we represent waves as complex numbers? Why wont real suffice? Thanks for any help.
 

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  • #2
arildno
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We COULD represent them only with reals.
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However, by using the fact that the complex exponential is, simply, an expression involving the trigs, using the complex exponential is perfectly valid, but ALSO allows us the particularly simple properties of the exponential when dealing with trig. functions.
 
  • #3
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The equations representing waves generally come from solving second order PDEs/ODEs, and hence can be represented using complex numbers because of Euler's formula.

$$e^{i\theta} = \cos(\theta) + i\sin(\theta)$$

I'm sure somebody else can give a more enlightening answer, but that's the way that I understand it.
 
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  • #4
Philip Wood
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That's it: mathematical ease. However, in wave mechanics, complex numbers are an essential part of the Physics.
 
  • #5
Thanks all,
 
  • #6
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Why do we represent waves as complex numbers? Why wont real suffice? Thanks for any help.
There is not really any difference between a pair of real numbers (with a given relationship) and a single complex number. In the frequency domain you need amplitude and phase, so that is two numbers and so complex numbers is a reasonable mathematical representation.
 
  • #7
WannabeNewton
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We can for example represent any solution to the source-free Maxwell equations (which decouple into two wave equations, one for the magnetic field and one for the electric field) as the real part of a Fourier transform which is, roughly speaking, a continuous linear superposition of plane waves of the form ##\vec{E} = \vec{E}_0 e^{i(\vec{k}\cdot \vec{r} - \omega t)}## which is extremely useful because we can restrict ourselves to analyzing plane waves and then build any other vacuum solution via a Fourier transform. All we have to do is carefully take the real part at the end of a calculation in order to get physical quantities.
 
  • #8
jtbell
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However, in wave mechanics, complex numbers are an essential part of the Physics.
Just to clarify: here "wave mechanics" means specifically "quantum mechanics."
 
  • #9
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All we have to do is carefully take the real part at the end of a calculation in order to get physical quantities.
But there are cases where the physical quantity inherently requires two real numbers to represent it and those two real numbers are related in such a way that representing them as a single complex number is reasonably, both mathematically and physically.

For example, in MRI you detect the amount of magnetization in the plane transverse to the main magnetic field. There is a strength of the magnetization and a direction, requiring two real numbers to describe. In such cases, the physical quantity of interest is actually a complex number.
 
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  • #10
WannabeNewton
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But there are cases where the physical quantity inherently requires two real numbers to represent it and those two real numbers are related in such a way that representing them as a single complex number is reasonably, both mathematically and physically.

For example, in MRI you detect the amount of magnetization in the plane transverse to the main magnetic field. There is a strength of the magnetization and a direction, requiring two real numbers to describe. In such cases, the physical quantity of interest is actually a complex number.
Thanks for the cool example! MRIs are pretty sweet :) but I didn't mean to say that we would need to take the real part to get the physical quantity in all physics problems making use of complex numbers and such; I just meant that we would do that in order to get the physical electric and magnetic fields from the respective plane wave solutions in the above example.
 
  • #11
meBigGuy
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The simple answer is that a wave has amplitude and phase, which requires two numbers. Complex numbers are one of the ways to represent a vector with magnitude and phase (rectangular coordinates).
 

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