Hot answers tagged coherence
6
Is the coherence length consistently defined as the length at which
these two waves achieve a phase difference of 1 radian?
No
Coherence length is the maximal difference in way traveled by the to rays
without losing the phase relation which allows interference.
This lenghts can be some µmeters (eg for white light from a glowing body)
or several ...
5
There is a phenomenon called decoherence in quantum mechanics which is largely responsible for this. Basically (the following is a simplification), all the strange behavior that occurs in QM tends to happen when the wavefunctions of different particles are in phase. Decoherence occurs when the phases are randomized, so there's no special correlation between ...
5
The coherence length is just the coherence time multiplied by the
propagation speed.
To understand the coherence time,
say you have a wave described, in complex notation, by
$$
E(t) = A(t) e^{i \omega t}
$$
where $A(t)$ is a slowly varying complex amplitude.
You make this wave interfere with a delayed version of itself
and collect the intensity
$$
|E(t) ...
2
Environmentally induced decoherence makes wave function collapse unnecessary.
As far as I know, that's still a conjecture. I might be wrong about that, though.
But the environment, usually taken to be some heat bath, introduces a preferred frame. (That in which the total (spatial) momentum vanishes.)
It's the rest frame of the system + reservoir, ...
2
This is just the comments formalised into a partial answer. First, it's important to realise that entanglement is a type of quantum correlation between two distinct systems. So, it's not useful to consider a single two-level system, and there is no such thing as entanglement between states. As Lubos Motl points out, you need to consider a system that has a ...
2
No. Coherence means "fixed, definite phase relation", or in a polychromatic context it can mean "definite phase relation at any given frequency". What would be the phase relation of the horizontally- and vertically-polarized waves? $0^\circ$? $90^\circ$? There's no reason for any definite phase relation.
Here's a more specific way to think about it. ...
2
At the time of Young there were no lasers to provide coherent light. Incandescent light is incoherent because it comes from many sources and the same is true for sunlight. By passing the light through the one slit
he created a single coherent wave front . It is worth reading his description "on the nature of light and colors" in the link above.
Edit: I ...
2
Yes, coherent radiation means that the phases of two ( or more ) waves representing the radiation differ by a known constant.
Incoherence means that the phase differences are unknown/random.
Laser radiation is coherent because stimulated emission assures phase differences are constant . Radiation from an incandescent lamp is incoherent because the ...
1
I believe, that the derivation is wrong...
If you assume a translationally invariant state, such that $G^1(r, r') = G^1(r - r')$ then you can get the result. Rewrite the exponential as $p r - p' r' = p( r- r') + r'(p - p')$. Since, in this case, the left-hand-side of Eq. (2.27) can only depend on $r - r'$ it must be such that $p = p'$ from the second term. ...
1
Brandon, the simple truth is that you have just asked one of the hardest and least understood questions in all of physics. So, don't feel bad if you don't understand it very well, because, er... no one else really does either?
It's not that we can't model this stuff mathematically. Shoot, Richard Feynman's version of something called Quantum Electrodynamics ...
1
They are two different but closely related concepts
Given two sine waves of equal frequency once can ask what is their relative phase, and are they in-phase or out of phase. So $\sin(\omega t)$ and $sin(\omega t+\pi)$ have a relative phase $\pi$ or 180 degrees and so we would say they are out of phase.
The question of coherence is: how stable is the ...
1
For two waves to be 'perfectly' coherent (I'm assuming electromagnetic, transverse waves) they have to have the same wavelength, polarization and phase.
To go a bit deeper, the coherence can never be perfect.
Only an infinitely long wave has a wavelength that is 'fully defined', ie. has no uncertainty (or width) associated with it.
One therefore defines a ...
1
The beam of white light is refracted by the prism, but the different wavelengths in the light are refracted by different angles. That means when the light falls on the screen the different wavelengths are spread out over a region of the screen. This is what is meant by dispersion.
The dispersion is linear if the different wavelengths of light are spread ...
1
If I understand your question correctly, you wish to use quantum interference to observe phenomena that otherwise have a low probability of occurring? If so, exploiting this is well known and well established in quantum optics. For example, take phenomena that occur with coherently prepared atomic systems, such as Lasing without inversion, ...
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