Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Playing around with imag. numbers.

  1. Apr 25, 2009 #1
    We define i=Squrt(-1). However, from what math I know, we just use i as a tool in complex exponentials, complex analysis, etc. I curious what happens if you actually, take powers of i. Does this actually mean anything? Is it mathematically interesting. What is i^(1/2) other than just i^(1/2)?
  2. jcsd
  3. Apr 25, 2009 #2


    User Avatar
    Science Advisor

    i (like all complex numbers) has a polar representation. i=ei(pi/2). Roots of i can be obtained by dividing the exponent. For square root divide by 2. To get all the roots use the fact that ei(2npi)=1, for all n. So to get the other square root, use n=1.
    Last edited: Apr 25, 2009
  4. Apr 25, 2009 #3
    Ok. I see how to calculate powers of i now, but is this at all mathematically or physically ineteresting?
  5. Apr 26, 2009 #4


    User Avatar
    Science Advisor

    My answer is yes, but to explain it would require descriptions of lots of physics theory as well as mathematics.
  6. Apr 27, 2009 #5


    User Avatar
    Science Advisor
    Homework Helper

    Extremely interesting. kth roots of unity (of which powers i^(1/n) are a special case) have a wide variety of applications.
  7. Apr 27, 2009 #6


    User Avatar
    Science Advisor

    i^(1/2)= sqrt(2)/2+ i sqrt(2)/2 or sqrt(2)/2- i sqrt(2)/2.
  8. Apr 27, 2009 #7
    "What is i^(1/2) other than just i^(1/2)"

    Do you mean what are physical interpretations of it, or what are other ways to write it? If it's the latter, then knowing the geometric interpretation of complex number multiplication helps a lot. Multiplying two complex numbers corresponds to adding their angles (with the positive x-axis) and multiplying their absolute values.

    Now it's easy to see what two numbers squared give i. They must have magnitude 1, and since i makes an angle of 90 degrees, one root must make an angle of 45 degrees. Another solution would be the number on the unit circle that makes an angle of 225 degrees. According to the fundamental theorem of algebra, those are the only two solutions.
  9. Apr 28, 2009 #8
    This is what I was looking for. A geometrical way to express the analytical expression. I guess I didn't explain that very well.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook