21,964 research outputs found
Rotational Doppler effect in left-handed materials
We explain the rotational Doppler effect associated with light beams carrying
with orbital angular momentum in left-handed materials (LHMs). We demonstrate
that the rotational Doppler effect in LHMs is unreversed, which is
significantly different from the linear Doppler effect. The physics underlying
this intriguing effect is the combined contributions of negative phase velocity
and inverse screw of wave-front. In the normal dispersion region, the
rotational Doppler effect induces a upstream energy flow but a downstream
momentum flow. In the anomalous dispersion region, however, the rotational
Doppler effect produces a downstream energy flow but a upstream momentum flow.
We theoretically predict that the rotational Doppler effect can induce a
transfer of angular momentum of the LHM to orbital angular momentum of the
beam.Comment: 6 pages, 3 figure
Visualizing the Doppler Effect
The development of Information and Communication Technologies suggests some
spectacular changes in the methods used for teaching scientific subjects.
Nowadays, the development of software and hardware makes it possible to
simulate processes as close to reality as we want. However, when we are trying
to explain some complex physical processes, it is better to simplify the
problem under study using simplified pictures of the total process by
eliminating some elements that make it difficult to understand this process. In
this work we focus our attention on the Doppler effect which requires the
space-time visualization that is very difficult to obtain using the traditional
teaching resources. We have designed digital simulations as a complement of the
theoretical explanation in order to help students understand this phenomenon.Comment: 16 pages, 8 figure
Note: Axiomatic Derivation of the Doppler Factor and Related Relativistic Laws
The formula for the relativistic Doppler effect is investigated in the
context of two compelling invariance axioms. The axioms are expressed in terms
of an abstract operation generalizing the relativistic addition of velocities.
We prove the following results. (1) If the standard representation for the
operation is not assumed a priori, then each of the two axioms is consistent
with both the relativistic Doppler effect formula and the Lorentz-Fitzgerald
Contraction. (2) If the standard representation for the operation is assumed,
then the two axioms are equivalent to each other and to the relativistic
Doppler effect formula. Thus, the axioms are inconsistent with the
Lorentz-FitzGerald Contraction in this case. (3) If the Lorentz-FitzGerald
Contraction is assumed, then the two axioms are equivalent to each other and to
a different mathematical representation for the operation which applies in the
case of perpendicular motions. The relativistic Doppler effect is derived up to
one positive exponent parameter (replacing the square root). We prove these
facts under regularity and other reasonable background conditions.Comment: 12 page
Reverse Doppler Effect of Sound
We report observation of reverse Doppler effect in a double negative acoustic
metamaterial. The metamaterial exhibited negative phase velocity and positive
group velocity. The dispersion relation is such that the wavelength
corresponding to higher frequency is longer. We observed that the frequency was
down-shifted for the approaching source, and up-shifted when the source
receded
Relativistic Doppler effect in quantum communication
When an electromagnetic signal propagates in vacuo, a polarization detector
cannot be rigorously perpendicular to the wave vector because of diffraction
effects. The vacuum behaves as a noisy channel, even if the detectors are
perfect. The ``noise'' can however be reduced and nearly cancelled by a
relative motion of the observer toward the source. The standard definition of a
reduced density matrix fails for photon polarization, because the
transversality condition behaves like a superselection rule. We can however
define an effective reduced density matrix which corresponds to a restricted
class of positive operator-valued measures. There are no pure photon qubits,
and no exactly orthogonal qubit states.Comment: 10 pages LaTe
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