Search results

  1. L

    Angular momentum commutator

    I have been told that L and P^2 do not commute, but I don't see why. It seems like the commutator should be zero. \left[ \vec{L} , P^2 \right] = \left[ L^k , P_i P_i \right] = \left[ L_k , P_i \right] P_i - P_i \left[ L_k , P_i \right] = \left( - i \hbar \epsilon_{i}^{km} P_m \right)...
  2. L

    Simplifying expression

    I'm finding the percent error in a S.R. problem and getting a really small number. How can I find the exact percentage? This is the result that needs to be simplified: 6.7x10^(-16) / (1 + 6.7x10^(-16)) If I do an order of magnitude approximation, then the bottom becomes 1, but that will...
  3. L

    Riemann-Christoffel tensor

    I'm trying to work through getting the Riemann-Christoffel tensor using covariant differentiation and I don't see where two terms cancel. I have the correct result, plus these two terms: d/dx^(sigma) *{alpha nu, tau}*A^(alpha) and d/dx^(nu) *{alpha sigma, tau}*A^(alpha) Sorry, I couldn't...
  4. L

    Would you put off graduating?

    I have two options right now and I would like to hear what you would do in this situation. I don't have any research experience to put on a grad school application. If I take the GRE this fall, I will just be starting QM, so I probably won't be able to solve many of the QM questions on the...
  5. L

    Vector decomposition (Helmholtz)

    I have to show that a generic vector can be decomposed into an irrotational and solenoidal component: V(r) = -Grad[phi(r)] + Curl[A(r)] I'm not sure how to start. Do I need to take the curl or div of V and use a vector identity? Any help would be greatly appreciated!
  6. L

    Causality and Psychics

    I don't know if this belongs in Philosophy or Skepticism/Debunking... I seen the section of the paper with the daily horoscopes and got to wondering: How would physics change if one of these Miss Cleo or Edgar Cayce type people were real (capable of consistently and accurately stating what...
  7. L

    Physicist title

    "Physicist" title I'm not sure what section to post this in, so I guess this one will have to do. I was recently told by an engineer that they have the title "Engineer" with a bachelor degree. I have always assumed the title "Physicist" was reserved for someone with a phd. But that got me to...
  8. L

    Maxwell's electromagnetic wave equation confusion

    I'm not understanding something here. Maxwell's wave equation is: Laplacian of E = (1/c^2) * second partial of E (sorry, I don't know how to write symbols) But the second partial derivative is the Laplacian. So how can you scale the laplacian of E by a number and get the laplacian of E as...
  9. L

    Schools Grad school gpa

    Which do you think would be better for getting into a theoretical physics program: -- a 2.5 cumulative gpa with a 3.5 physics gpa or -- a 3.0 gpa, both cumulative and physics Also, if your gpa is around 3.0 (+- 0.2), what are your chances of actually getting into a phd program?
  10. L

    Implicitly differentiating PDE (multivariable calculus)

    The problem: Find the value of dz/dx at the point (1,1,1) if the equation xy+z3x-2yz=0 defines z as a function of the two independent variables x and y and the partial derivative exists. I don't know how to approach the z3x part. I thought you would use the product rule and get 3(dz/dx)2x +...
  11. L

    Arbitrary Velocity C in Special Relativity

    I am working through Einstein's "On the Electrodynamics of Moving Bodies", and I can't figure out why we are using c instead of a random constant velocity. It seems to me that c is an arbitrary value for the argument. I'm sure there is an explanation... I just can't think of it. Can anyone clear...
  12. L

    Terminal Velocity of Cats

    Thought this was funny and decided to share:
  13. L

    Programs What's the purpose of a minor?

    I am finishing my junior year as a physics major, and I've heard that alot of people get a minor in math or something like that. I plan to start grad school immediately after finishing my undergrad. My question is, what is the purpose of getting a minor? Does it look good on a grad school app...
  14. L

    Distance between atoms

    Is there a way to find the distance between atoms? For example, if you have a pure gold film... how can you find out how far the gold atoms are spaced?