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In quantum physics, any functional interaction like measurement/observation forces particles down to a single state.

Yet when plants do their photosynthesis it's been discovered that they actual capture the energy from photons as both particle and wave (http://www.ucl.ac.uk/news/news-articles/0114/090114-Quantum-mechanics-explains-efficiency-of-photosynthesis), which means that it doesn't collapse into a single state yet the plant is capable of interacting with both state for photosynthesis.

So what is the difference between a measurement with SEM for example or observation with the double-slit experiment, and a plant or any other objects having a functional interaction with a quantum system yet not forcing it into a single state like it should.

In fact are only humane functional interaction like measurement/observation with a quantum system forcing it into a single state?

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  • $\begingroup$ "In quantum physics, any functional interaction like measurement/observation forces particles down to a single state." Be careful! A system is always in one single state. A measurement of operator $X$ forces the system in an eigenstate of that operator. Is that what you mean? If so, please edit the post. It's important to have as clear a question as possible. $\endgroup$ – DanielSank Jul 31 '17 at 5:05
  • $\begingroup$ First, No. Any interaction will either force the system to collapse to a single state or will put both systems in a superposition. I don't understand your photosynthesis example, light has the same energy regardless of the way you want to describe it. What do you mean by 'capture as a wave'? $\endgroup$ – Nimrod Morag Jul 31 '17 at 5:05
  • $\begingroup$ @NimrodMorag what do you mean "collapse to a single state"? All quantum systems are in a single state. The measurement process forces the state to collapse to an eigenstate of the measured operator. $\endgroup$ – DanielSank Jul 31 '17 at 5:07
  • $\begingroup$ Related: physics.stackexchange.com/q/177433/2451 $\endgroup$ – Qmechanic Jul 31 '17 at 5:46
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    $\begingroup$ @DanielSank OP is treating QM too philosophically and you're treating it too mathematically to be relevant to OP's question. "collapse to a single state" = the measurement will be of one of the stationary states of the system $\endgroup$ – Nimrod Morag Jul 31 '17 at 7:15
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Are quantum measurement of a system only occurring with human interaction? In fact are only humane functional interaction like measurement/observation with a quantum system forcing it into a single state?

No, at least that is not how Heisenberg intended the role of the human observer. Her role is rather to describe the experiments and results in the one language which can be communicated:

The Copenhagen interpretation of quantum theory starts from a paradox. Any experiment in physics, whether it refers to the phenomena of daily life or to atomic events, is to be described in the terms of classical physics. The concepts of classical physics form the language by which we describe the arrangements of our experiments and state the results. We cannot and should not replace these concepts by any others. Still the application of these concepts is limited by the relations of uncertainty. We must keep in mind this limited range of applicability of the classical concepts while using them, but we cannot and should not try to improve them.

However, the measurement itself also has a role, and that role is slightly different from the human observer. A quantum system which gets measured is not closed. But if the system is not closed, then you need boundary conditions. And open boundary conditions are notoriously difficult to implement exactly, not just in quantum mechanics. The collapse postulate can be interpreted as an approximation to this open boundary condition, and the Heisenberg cut is the place where the boundary condition gets applied.

So what is the difference between a measurement with SEM for example or observation with the double-slit experiment, and a plant or any other objects having a functional interaction with a quantum system yet not forcing it into a single state like it should.

As is often the case with approximate open boundary conditions, the farer you move them from the system, the less error they introduce. For the SEM, putting the Heisenberg cut between the sample and the SE/BSE detector would give you more accuracy than you could ever want. For the plant on the other hand, it is less obvious where to put the Heisenberg cut. So the difference is between these two scenarios is that the sample in a SEM is separated from the detector in an obvious way, but a reasonable separation (place for the Heisenberg cut) in the cell is less obvious. Maybe it would be more obvious for somebody with sufficient experience with cell biology and photosynthesis.


Let me try to address the question whether this interpretation of the collapse postulate as an approximation to open boundary conditions was just made up by myself. The long quote above was the introduction of chapter 3 The Copenhagen Interpretation of Quantum Theory pp. 46-57 in Physics and Philosophy (1958) by Werner Heisenberg. The following passage is from the end of that chapter (before Heisenberg connects his his clear words with Bohrs "philosophical reflections"):

We have to add some comments on the actual procedure in the quantum-theoretical interpretation of atomic events. It has been said that we always start with a division of the world into an object, which we are going to study, and the rest of the world, and that this division is to some extent arbitrary. It should indeed not make any difference in the final result if we, e.g., add some part of the measuring device or the whole device to the object and apply the laws of quantum theory to this more complicated object. It can be shown that such an alteration of the theoretical treatment would not alter the predictions concerning a given experiment. This follows mathematically from the fact that the laws of quantum theory are for the phenomena in which Planck's constant can be considered as a very small quantity approximately identical with the classical laws. But it would be a mistake to believe that this application of the quantum-theoretical laws to the measuring device could help to avoid the fundamental paradox of quantum theory.

The measuring device deserves this name only if it is in close contact with the rest of the world, if there is an interaction between the device and the observer. Therefore, the uncertainty with respect to the microscopic behaviour of the world will enter into the quantum-theoretical system here just as well as in the first interpretation. If the measuring device would be isolated from the rest of the world, it would be neither a measuring device nor could it be described in the terms of classical physics at all.

Heisenberg clearly makes the point here that a quantum system is not a closed system, but an open system. And he makes it clear that moving the (open) boundary further away is OK, but that believing it can be removed completely would be a mistake.

However, the observation that open boundary conditions are notorious difficult to implement even outside of quantum mechanics is not from Heisenberg. (At least I didn't learn it from him directly or indirectly.) This observation came up in an exchange with Ajit R. Jadhav about local and global hidden degrees of freedom.

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