B How Can We Resolve Noise from Low Frequency Quanta in Electromagnetic Waves?

[snorkack]

What is the lowest frequency/energy where photons have been observed? That is, electromagnetic waves observed as quanta, not continuous waves? Resolving noise that is not thermal noise, not shot noise from the discrete nature of the electrons in the receiving electronic, nor from the quantization of electron states in the receiver, but specifically the quantization of the low frequency electromagnetic waves being received?

 

[f95toli]

There are single photon detectors that work down to a few GHz (the efficiency isn't great but they do work). Below that we can of course infer that there are single photons via their interaction with say ions etc; but it is more indirect.

 

[snorkack]

So since the observed gravitational waves are with frequencies of hectoherz range, the observations involve absorbing at least millions of gravitons? We are pretty far as yet from observing gravitons, correct? (The gravitational wave detectors are probably not better or more free of noise than the best electromagnetic wave detectors, though of course electromagnetic waves can also be detected by much worse and noisier detectors).

 

[PeterDonis]

since the observed gravitational waves are with frequencies of hectoherz range, the observations involve absorbing at least millions of gravitons?

Are you asking about photons or gravitational waves?

Gravitational wave observations in LIGO do not involve "absorbing gravitons" at all. They involve observing the effects of the time-varying spacetime metric of gravitational waves on test objects.

We are pretty far as yet from observing gravitons, correct?

We are many orders of magnitude away from being able to make observations of any kind of quantum aspect of gravity.

The gravitational wave detectors are probably not better or more free of noise than the best electromagnetic wave detectors

I'm not sure how you would even compare them.

 

[snorkack]

Are you asking about photons or gravitational waves?

Gravitational wave observations in LIGO do not involve "absorbing gravitons" at all. They involve observing the effects of the time-varying spacetime metric of gravitational waves on test objects.

Is it possible to observe time-varying electromagnetic field by a receiver that does not alter the incident wave at all?

 

[PeterDonis]

Is it possible to observe time-varying electromagnetic field by a receiver that does not alter the incident wave at all?

I don't think it's possible to get zero alteration, but it's certainly possible to get alteration that is extremely small compared to the total amplitude of the wave.

 

[snorkack]

I don't think it's possible to get zero alteration, but it's certainly possible to get alteration that is extremely small compared to the total amplitude of the wave.

Is, then, your ability to detect the electromagnetic wave tied to your absorbed power (rather than the total incident power)?
Some source:
https://dcc.ligo.org/public/0099/P1200179/001/energy paper.pdf

 

[PeterDonis]

Is, then, your ability to detect the electromagnetic wave tied to your absorbed power (rather than the total incident power)?

I think it depends on the type of detector.

Some source

Again, are you asking about electromagnetic waves or gravitational waves? This source is discussing detection of gravitational waves.

 

[snorkack]

I think it depends on the type of detector.

So are detectors which make no alteration of incident wave at all theoretically allowed?
True, it does not have to be a change in photon count. Compton scattering does not change photon count but is detectable. Scattering, reflection and refraction do not change photon count and in a suitable reference frame do change energy, but do change momentum. You might also change angular momentum of electromagnetic wave by changing its polarization state.
A fundamental problem, in quantum side, with a detector that does not change light at all is Heisenberg uncertainty. If you could use a detector that leaves the incident light unchanged, you could next use another detector to measure a property which Heisenberg says you could not measure simultaneously.

Again, are you asking about electromagnetic waves or gravitational waves? This source is discussing detection of gravitational waves.

In the context. Note that the source is challenging the claim that Ligo does not draw energy from gravitational waves.
My issue is - can you use classical general relativity to prove that Ligo does not alter gravitational waves at all, or by far less than hω, and therefore any as yet unknown quantum theory of gravity must make an exception into Heisenberg uncertainty for Ligo?
(The much more logical thing would be if you could use the classical limit to derive the absorbed energy, or momentum or angular momentum, and show that the resolution is far from the quantum limit).

 

[PeterDonis]

My issue is - can you use classical general relativity to prove that Ligo does not alter gravitational waves at all

You should have asked this question in the OP of this thread. Fortunately we've only had to spend 9 posts finding out what your actual question was; but that's still 9 posts that could have been spent discussing your actual question instead of wandering around trying to figure out what it was.

As far as I know the so-called "claim" that gravitational wave detectors like LIGO extract exactly zero energy from the waves was never actually claimed by anyone. The only claim that was actually made was that the amount of energy extracted by the detector would be small enough, compared to the total energy carried by the wave, that it could be ignored in the theoretical analysis of the detector. The paper you referenced is simply trying to investigate in more detail the magnitude of the actual energy extracted and the mechanism by which it is extracted.

or by far less than hω, and therefore any as yet unknown quantum theory of gravity must make an exception into Heisenberg uncertainty for Ligo?

This does not follow.

First, the paper you referenced is not analyzing any quantum aspects of gravitational waves. It is analyzing quantum aspects of the light in the interferometer, whose interference effects are used to characterize the gravitational waves detected. It has been known since the original design of LIGO that quantum effects on the light would be significant and would need to be allowed for. This paper appears to me to be analyzing the effects of the light on the motion of the mirrors, and then assuming that such effects on the motion of the mirrors correspond to effects on the amount of energy absorbed by the detector from the gravitational wave being detected. This latter assumption, as far as I can tell, is simply hand-waving.

Second, it is not the case that any gravitational wave detector must, at sufficiently low intensity, detect single gravitons, just as it is not the case that any electromagnetic wave detector must, at sufficiently low intensity, detect single photons. For example, consider an ordinary antenna. The state of the EM field that the antenna is detecting is a coherent state, which is not an eigenstate of photon number, and the observable the antenna is detecting is not photon number; roughly speaking it is detecting the amplitude of the coherent state as a function of time. Similarly, LIGO is not a "graviton number" detector; roughly speaking, the gravitational wave it is detecting is a coherent state, and LIGO is detecting the amplitude of the coherent state as a function of time. These observables do not become "single quantum" detections at low intensity.

 

[PeterDonis]

A fundamental problem, in quantum side, with a detector that does not change light at all is Heisenberg uncertainty. If you could use a detector that leaves the incident light unchanged, you could next use another detector to measure a property which Heisenberg says you could not measure simultaneously.

This is wrong. If a quantum system is in an eigenstate of an observable, measuring that observable does not change the state of the system; but that doesn't mean you can then make another measurement of a non-commuting observable that gives you more information than the HUP allows.

And if a quantum system is not in an eigenstate of an observable, then measuring that observable will change the state of the system.