I Measurement effect ('collapse') question

[kw1]

I routinely read that in quantum theory, measurement 'collapses' the wave function. I am unlikely (a geneticist, not a physicist) to understand this very well, but I think my central question is not entirely naive. Whatever the measurement 'collapse' effect really means, I don't understand what the very idea means.

Yes, we set up a detector to 'measure' some quantum phenomenon. But, particle or wave (or both), it would seem that photons, or whatever, are interacting with the world all the time and everywhere (because there is energy and matter, objects and waves etc, everywhere), so I cannot see why a physicist using a photon detector (or whatever) is doing anything the the target wavicle that isn't happening to it all the time and everywhere? Why is the detector not just one of infinitely many 'obstacles' in the photon's path? What makes a physicist's detector any different from everything else all around all the time? How, in that sense, does any 'thing' (photon, etc) ever go un-interfered with?

 

[hilbert2]

I routinely read that in quantum theory, measurement 'collapses' the wave function. I am unlikely (a geneticist, not a physicist) to understand this very well, but I think my central question is not entirely naive. Whatever the measurement 'collapse' effect really means, I don't understand what the very idea means.

Yes, we set up a detector to 'measure' some quantum phenomenon. But, particle or wave (or both), it would seem that photons, or whatever, are interacting with the world all the time and everywhere (because there is energy and matter, objects and waves etc, everywhere), so I cannot see why a physicist using a photon detector (or whatever) is doing anything the the target wavicle that isn't happening to it all the time and everywhere? Why is the detector not just one of infinitely many 'obstacles' in the photon's path? What makes a physicist's detector any different from everything else all around all the time? How, in that sense, does any 'thing' (photon, etc) ever go un-interfered with?

The system that is prepared for quantum measurement needs to be protected from decoherence, which means any kind of unintended and complex interactions. For instance, if an electron beam with some given electron momentum is needed for a measurement, it has to be created in a good enough vacuum so that the electrons don't collide with gas molecules. Also, the beam needs to have low charge density so that the electrons don't disturb each other.

 

[kw1]

Thanks, hilbert2, thanks for that clarification. But how about all that's going on 'out there' in Nature? How is it that anything escapes 'collapse' for more than tiny fractions of a second (and here, my not being a physicist, I don't even ask how things like that get 'created' in the first place). Assuming the item to be detected can even be created without the creating device immediately collapsing its wave function, out here in the wild blue yonder of nature, I would think every such wavicle would be always collapsing. Clearly, I'm missing something, because if not, then something is missing!

 

[hilbert2]

When doing that kind of experiments, the system is artificially isolated from the surroundings in a way that doesn't occur in natural conditions. For example, it's impossible to have anything like coherent states in a biological system where there's liquid phase water and other molecules colliding all the time. Some kind of interactions don't cause decoherence, though. The electron beam that I mentioned doesn't have to be protected from gravity because it affects all the electrons in the same way and its effect is negligible.

 

[atyy]

I routinely read that in quantum theory, measurement 'collapses' the wave function. I am unlikely (a geneticist, not a physicist) to understand this very well, but I think my central question is not entirely naive. Whatever the measurement 'collapse' effect really means, I don't understand what the very idea means.

Because of this problem, the standard interpretation is agnostic about the reality of the wave function and collapse, and they are just tools to calculate the probabilities of measurement outcomes (which are real).

More broadly, this is part of the "measurement problem" of quantum mechanics. Attempts to solve the measurement problem include Bohmian mechanics and the Many Worlds interpretation.

 

[PeroK]

Yes, we set up a detector to 'measure' some quantum phenomenon. But, particle or wave (or both), it would seem that photons, or whatever, are interacting with the world all the time and everywhere (because there is energy and matter, objects and waves etc, everywhere), so I cannot see why a physicist using a photon detector (or whatever) is doing anything the the target wavicle that isn't happening to it all the time and everywhere? Why is the detector not just one of infinitely many 'obstacles' in the photon's path? What makes a physicist's detector any different from everything else all around all the time? How, in that sense, does any 'thing' (photon, etc) ever go un-interfered with?

I have never imagined that a controlled experiment is fundamentally different from what happens in nature, it terms of how quantum objects behave, except that it is a controlled experiment with the results being recorded.

A hydrogen atom, for example, forms in the same way whether it happens in nature or in a controlled experiment.

And, in spectroscopy, we can measure the energy transitions in an atom, even though that atom was never part of a controlled experiment.

 

[Peter Morgan]

[Moderator's note: invalid reference deleted. There is a spin-off thread on stochastic signal analysis in QM that gives valid references for the basic ideas described in this post. See the link in post #10 of this thread.]

Essentially, it argues that we only observe and record events as happening in places where we place exotic materials that have exotic control circuitry. Where events are not observed and recorded, physics ought to restrain itself from saying too much. I prefer not to talk about "detectors", because we have very little idea what we might be detecting, instead more concretely referring to Avalanche PhotoDiodes (APDs, an acronym that can stand in as a name for any piece of apparatus that exhibits events and records them —Avalanche Production Device, perhaps), which makes a more balanced statement about whether something is "detected". An avalanche on a mountain is caused both by the material of the avalanche being delicately balanced and by events in the wider world, which could equally be an earthquake or an errant footfall, wave or particle; in general we can't predict individual avalanches very well, but we can say that avalanches happen in some places more often than in others.

Your particular question asks "Why is the detector not just one of infinitely many 'obstacles' in the photon's path?" and "I cannot see why a physicist using a photon detector (or whatever) is doing anything [to] the target wavicle that isn't happening to it all the time and everywhere?" An APD is at least different in that it much more affects whatever we might say is causing the events in the APD than does air, say, and is entirely different in that the times at which events happen are recorded, and we then set about systematically modeling the statistics of events when we move the APD from place to place.

I should add that my inclination is to be realist enough to say that perhaps there is something in between a light and the APD, but not realist about either photons or wavicles. This may not be satisfying enough to you, in which case you will have to look elsewhere.

 

[Khashishi]

Thanks, hilbert2, thanks for that clarification. But how about all that's going on 'out there' in Nature? How is it that anything escapes 'collapse' for more than tiny fractions of a second (and here, my not being a physicist, I don't even ask how things like that get 'created' in the first place). Assuming the item to be detected can even be created without the creating device immediately collapsing its wave function, out here in the wild blue yonder of nature, I would think every such wavicle would be always collapsing. Clearly, I'm missing something, because if not, then something is missing!

Yup, that's why it is hard to see quantum effects in anything larger than a molecule or so, over a short timespan. Any macroscopic objects in your everyday experience are constantly being "measured" and you will never observe something in superposition without a very controlled experiment. This is why quantum computing is hard: it requires ultra low temperatures and ultra high vacuums to prevent things from randomly happening.

 

[PeterDonis]

I'll look at spinning off a separate thread on this topic and moving the relevant posts to it.

A new thread on stochastic signal analysis in QM is here:

https://www.physicsforums.com/threads/stochastic-signal-analysis-in-qm.944361/