# Search results

1. ### NASA NASA's Ephemerides

I was wondering, how does NASA actually calculate their super exact Ephemerides?
2. ### I Vector addition and Newton's law

I know that n-body problem can be complicated, but that's for the dynamics. What about a static case: e.g. if I have the distances of several bodies A, B and C etc. and their distance to a reference mass m, can I just use the vector addition of the Newton's gravitational force to add up all of...
3. ### I Foucault pendulum and a round globe

Imagine we are all cavemen without satellite technology and just discovered the Foucault pendulum! As we know the angle of presession depends on the latitude. This can be used to prove that the earth is rotating, right? Now by putting several Foucault pendula around the Earth, at equidistant...
4. ### Same physics, two theories?

Was just thinking, is it ever possible that the same phenomenon can be fully explained by two theories that are not subset of each other? :rolleyes:

Hey everyone! Can anyone explain, why quadrature sampling works the way it does? i.e. taking 2 samples for the I and 2 samples for the Q. I mean I can understand if one tries to sample one mono frequency signal, say 40 Hz sine wave, on 0, 90, 180, 270 degrees, that is a sampling frequency of...
6. ### Name of a geometrical shape

What is the geometrical shape called, when combining 3 ellipses together, such that each two share one focal point and together form a kind of 60 degree triangular shape? It will look similar to Reuleaux triangle, but it is not formed out of a triangle, but 3 ellipses, as if you are gluing...
7. ### Energy of light vs energy of photon

How is energy of electromagnetic waves (classical electrodynamics → Poynting vector) related to the photon energy (E=hv) ? e.g. what is exactly the difference between photons from two radio stations or two laser devices with the same frequency, but different power? thanks! :cool:
8. ### Is unification really needed?

just came to my mind to ask: Is gravitation really a fundamental force? As far as I recall from my GR lecture, gravitation is a property of space-time. Other forces seem to be well described by the grand unified theories (GUT). So why is there still a need to include gravitation? cheers...
9. ### Question on Coulomb law

It is known from the Coulomb's law (F = q E) that if an electric field is applied on a charge, it will accelerate it, i.e. the position of the particle changes macroscopically. But why mechanical displacement? why not a change in particles internal energy, say for example excitation of an...
10. ### Observation time and its relation to the quantum Zeno effect

Hi. I have a problem to understand the following situation regarding observation of a quantum system: Imagine we have an unstable particle in a box, together with many sensors where each is connected to a lamp. The sensors continuously monitor the particle, the lamp turns on whenever the...
11. ### Fortran Fortran FFT library

I am new to Fortran. Is there an ultimate best library for FFT for Fortran (95)? I found this one FFTPACK. I am not sure if this is the best one. Is there anywhere a simple example how to use it? I have huge time files (260 MB each) that I like to read in Fortran and perform FFT and...
12. ### Confused about autocorrelation and PSD

I am confused a bit :confused:. I read in a paper that the following property holds, but can't find where it comes from. \mathsf{E}\left[X(\omega)X^*(\omega')\right]=2\pi\delta(\omega-\omega')S_{XX}(\omega) it says that the expected value of the Fourier transformed signal is proportional...
13. ### Fortran Fortran complex array assignment

I have a an array of type complex. I am trying to assign a value to it in a loop, but I get a strange error "Unclassifiable statement at (1)". I really don't understand why. do t = 1, count real(in(t)) = cos(2 * pi * f0 * t) aimag(in(t))=sin(2 * pi * f0 * t) end do thanks.
14. ### Energy exchange between a bell and a hammer

Imagine a bell that has only 3 ideal resonant frequencies (no damping) and I am hitting it with a point like ideal hammer, does this mean that all of the energy of the hammer is distributed among those 3 resonances? I think not, i.e. the energy stored in the resonant modes are less than the...
15. ### Landau quantization of cyclotron orbit

I don't know what the speed of light c is doing in the Hamiltonian for the Landau quantization. The term doesn't have dimensions of momentum anymore. :confused: \hat{H}=\dfrac{\hat{p}^2_x}{2m}+\dfrac{1}{2m}\left(\hat{p}_y-\dfrac{q|\vec{B}|}{c}\hat{x}\right)^2 Any ideas? thanks.
16. ### Charged particle and virtual photons

Imagine I am sitting on a lead ion and fly happily through the LHC tube :approve:. Suddenly I feel an strong force pushing me sideways to the left. A physicist sitting in the lab frame tells me later, that I have been going through a dipole magnet yoke with field lines from top to bottom...