# Tag Info

1

The blue shift is actually necessary to conserve energy. The photon has energy, and therefore has potential energy relative to the neutron star. It loses gravitational energy as it approaches the neutron star, and it gains energy in the form of blue shift. Indeed, to the first order in GR, the change in energy of a photon is equal to the change in ...

4

I don't think that would even be considered. If you consider Hubble's law - once we get out of our neighbourhood and into the Hubble flow, then what we see is that identically in every direction, the redshift is proportional to the distance we measure to objects by independent means. An explanation involving roughly stationary objects that emit light with a ...

2

Redshift can be also caused by gravity, in which case it is called gravitational redshift, related of course to gravitational time dilation. In regards to cosmology, some people hypothesized that what is perceived as recessional redshift might be caused by some other mechanism by which light loses its energy when traversing cosmic distances, these theories ...

3

The linewidths come out very naturally from Maxwell's Equations by treating the atom as a tiny classical antenna. I do the calculations for the 2p-1s transition in hydrogen on my blogsite here: The Semi-Classical Calculation The idea is that from the Schroedinger equation, the superposition of the s and p states gives you get a tiny oscillating dipole about ...

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The answer is yes, the atom does absorb radiation that does not exactly match the transistion frequency. This is due to the Doppler effect that everyone knows from an ambulance with siren driving by. The frequency you hear is higher if the ambulance moves towards you and lower if it drives away from you. It's the same with the atom. If the atom moves (and ...

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According to Bohr model, the absorption and emission lines should be infinitely narrow, because there is only one discrete value for the energy. There are few mechanism on broadening the line width - natural line width, Lorentz pressure broadening, Doppler broadening, Stark and Zeeman broadening etc. Only the first one isn't described in Bohr theory - it's ...

1

The relativistic doppler shift of frequency is given by $$f_o = \frac{f_ s}{\gamma (1 + (v/c) \cos \theta)},$$ where $f_o$ is the observed frequency, $f_s$ is the emitted (source) frequency, $\gamma = (1 - v^2/c^2)^{-1/2}$ and $\theta$ is as you define. This is a standard result - e.g. ...

3

Unlike light, sound can only travel through a medium - in most situations, air. The velocities in your equations are relative to a fixed reference frame - that of the body of air in which the sound is travelling (which in a typical physics problem is the same as the ground's reference frame). So there really is a tangible difference between the case where ...

0

In the event of spontaneous emission (the one responsible for Doppler cooling) a photon is emitted in a random direction. If you have 100 % collection efficiency and combine these "recycled" photons with the original beam, then it should be possible. Theoretically it is even possible to correct the polarization and wavelength of such photons depending on the ...

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