Recent content by mathfeel

  1. M

    Why does gravitational Laplace's equation equal zero?

    It is much easier to demonstrate this using the representation of the Laplace operator in spherical polar coordinate: \Delta = \frac{1}{r^2} \frac{\partial}{\partial r} r^2 \frac{\partial}{\partial r}
  2. M

    True or false? Fundamental particles

    The very successful modern physics theories requires you to NOT distinguish identical particles (the photon that goes in and the photon that goes out) like this. Meaning, there are experiment whose result cannot be explained if we cling to the classical notion of distinguishing all the photons...
  3. M

    Angular momentum from the Lagrangian

    Applying a finite rotation is the same as adding up many many many small rotations. (Infinitesimally) Small rotation is related to the partial derivation: \frac{\partial L}{\partial \phi_j} If the Lagrangian is invariant under rotation, then the above partial derivative is zero. Using...
  4. M

    Elliptical motion of surface buoy.

    Failed at Googling. Can anyone point me to a simple model derivation on elliptical motion of a buoy on surface gravity wave?
  5. M

    I do not understand this solution to this Quantum Mechanics Problem

    Spacial rotation matrices is one representation of the rotation group. Spin operators is another. You need to understand some basic representation theory and how the angular momentum operators are generators of rotation.
  6. M

    Unravelling the Mystery of Light Speed (c)

    SR asserts that there is a invariant speed. One consequences is that this is a maximal speed. Light happens to travel at this speed (for good reason).
  7. M

    Does photon have a rest frame?

    Essentially a photon just came into being (and traveling at c), while the equivalent amount of energy and momentum is taken away from the electron (perhaps dropping down to a lower atomic energy level). As far as we know, this is instantaneous and probabilistic at the single electron-photon level.
  8. M

    Constancy of c - second postulate

    Carry your atomic clock and meter stick to a spaceship that's chasing the light at any speed (subluminal, of course). You will still measure it to that value.
  9. M

    Classical mechanics, motion of a particle over a helix

    You seems to think that the "derivative" of the Lagrangian: \frac{\partial L}{\partial x_j} - \frac{d}{dt} \frac{\partial L}{\partial x} which, if x_j has dimension length has dimension of force, is the force of constrain. NO! You are trying to minimize the Lagrangian again some constrain...
  10. M

    Basic questions on collisions, particles

    There is no such thing as a critical distance. The size of the wavefunction packets is not an intrinsic properties of eletrons/positrons, but what state they are in. And in any states, there is a finite probability that they would annihilate and turn into some other pairs (photon is not the only...
  11. M

    Basic questions on the speed of light

    The numerical value of c is irrelevant. It is just a factor for unit conversion. The universe does not distinguish energy and mass like we do with our language. So historically we invented two units for them, but in fact there is a natural conversion factor between them that we did not discover...
  12. M

    Basic questions on the speed of light

    Both of these statements are not correct. The latter seems to have some experimental support of late, but keep in mind that 1. the experiment has not be independently repeated and verified and 2. we have been observing supernova neutrino for many years now and they are found to be...
  13. M

    Determine the rate of emission of quanta from the station

    In my reply #2, I gave two definitions of the same quantity I. By equating them, you can compute u. u is energy / volume. You also know how much energy is in each photon. So you can get photon / volume.
  14. M

    Classical mechanics, motion of a particle over a helix

    You forgot the right-hand-side of the Euler-Lagrange equation: \frac{\partial L}{\partial x_j} - \frac{d}{dt} \frac{\partial L}{\partial \dot{x}_j} = 0 You first two equations are also wrong (check the dimension of each term, they are not the same). For example: \frac{d}{dt}\frac{\partial...
  15. M

    Determine the rate of emission of quanta from the station

    Didn't check your math, but the argument is correct. Not sure how did you get the "size of the radio". But dividing the power by the area of the sphere of radius d, you get power per area (Intensity): I = P/A Intensity is related to energy density u (energy per volume) and speed v of photon by...
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