Literature on photons and observers

[danieltanfh95]

Has anyone in the foundations literature explicitly argued that a single free photon cannot serve as an observer in the SR and QFT sense? The negative claim seems absent when I search. Malament, Halvorson-Clifton, Fleming, Rovelli all discuss localization or relational structure, but I haven't found the observer-role exclusion stated directly. Any help would be great, thanks!

 

[Roberto Pavani]

I am not aware of any specific literature discussing this point, so please take the following as a personal perspective rather than a literature reference.

However, I am not sure that the question "can a single photon be an observer?" is the most useful one.

Consider a photomultiplier tube in a detector such as Super-Kamiokande. A neutrino is not detected directly. Instead, the interaction produces charged particles, which emit Cherenkov photons. A single photon may then strike the photocathode, release a photoelectron, and initiate an avalanche that ultimately produces a measurable voltage pulse.

In that sense, the photon is clearly an active participant in the measurement chain. The neutrino is effectively observed through the photons produced by the interaction and the subsequent amplification process.

For that reason, the more interesting question, at least to me, is not whether the photon itself qualifies as an observer, but rather where information becomes encoded, transferred, amplified, and eventually converted into a stable macroscopic record.

 

[danieltanfh95]

However, I am not sure that the question "can a single photon be an observer?" is the most useful one.

I am trying to construct the following argument:

1. frame equivalence is not the same as "what can hold the frame"
2. photons cannot be observers
3. more generally, fundamental bosons cannot be observers
4. thus observers are fermionic and have matter currents
5. thus matter current participate in decoherence and provide a no-collapse way to settle observations

 

[Roberto Pavani]

If photons cannot play any observer-like role, how would you classify experiments in which essentially all the information about the phenomenon is carried by photons, such as astronomical observations, Cherenkov detectors, photomultipliers, or electron-positron annihilation measurements?

 

[danieltanfh95]

If photons cannot play any observer-like role, how would you classify experiments in which essentially all the information about the phenomenon is carried by photons, such as astronomical observations, Cherenkov detectors, photomultipliers, or electron-positron annihilation measurements?

photons can carry information, they just cannot be observers, so measurements must use fermionic matter, which is something we have always done. Photons already cannot be observers in special relativity.

 

[Roberto Pavani]

It is true that 99.9% of measurements eventually involve electrons, but they also involve photons (more generally, electromagnetic interactions). If you remove either one from the measurement apparatus, it will no longer function.

That is why I am struggling to see a sharp physical distinction. The final readout is often electronic, but the information transfer frequently relies on photons as well. LIGO is an obvious example: remove the photons and the detector stops working.

I would view the observer as the entire measurement chain. For a qubit, information about the state is first encoded in an RF/microwave field, electronics convert that information into electrical signals, an ADC digitizes them, and the final result becomes a classical bit. The observer is the whole process, not a particular particle involved in it.

 

[PeterDonis]

matter current participate in decoherence

Even if this is true...

and provide a no-collapse way to settle observations

...this does not follow. Decoherence doesn't provide single outcomes.

 

[PeterDonis]

I am trying to construct the following argument

Is this an argument that appears in the peer-reviewed literature?

If not, it's personal theory and is off limits here.

 

[danieltanfh95]

Is this an argument that appears in the peer-reviewed literature?

If not, it's personal theory and is off limits here.

I am requesting help specifically on "has anyone in the foundations literature explicitly argued that a single free photon cannot serve as an observer in the SR and QFT sense?", and providing context otherwise. I do not wish to discuss the theory here, and rather discuss the theory elsewhere with better support, but any help on literature would be helpful.

 

[PeterDonis]

I do not wish to discuss the theory here

Unfortunately, this thread has shown that it's not possible to ask your question without discussing your personal theory.

Thread closed.