In the Copenhagen interpretation, if some device measures the state of a quantum system, finding some property of it to be equal to some value, X, then it is then assumed that all other quantum observers 'know'/'are aware'/'accept', that the value of the property of that system is X, regardless of the fact that they have not yet individually, directly, observed the system or its respective property.

Given that the scientific method is one of direct observation/measurement (post the philosophy of the null hypothesis), is this concept a-scientific?

Is realism - the idea that properties or objects exist independent of observation/measurement, strictly a-scientific? Is there any stream within philosophy of science that argues for scientific truth, as apart from observation/measurement?

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    $\begingroup$ I'm voting to close this question as off-topic because it is a philosophy question, not a physics question. $\endgroup$
    – Chris
    Commented Feb 1, 2018 at 23:02
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    $\begingroup$ Maybe the Copenhagen interpretation is not physics but causality and local realism is on topic. $\endgroup$ Commented Feb 1, 2018 at 23:02
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    $\begingroup$ @BillAlsept Sure. But whether realism is ascientific or not is not on topic. Even if you say it's physics, not philosophy, it would fall firmly under the "primarily opinion based" banner. $\endgroup$
    – Chris
    Commented Feb 2, 2018 at 0:11
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    $\begingroup$ @Chris Local reality and especially causality are discussing physics or physical things. Questioning whether or not causality or local realism really exist would be more on the philosophical side. $\endgroup$ Commented Feb 2, 2018 at 0:38
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    $\begingroup$ @Chris: I am voting for this question. I never understood the motivation of votes to close questions. That there is somebody who asks a question is, to me, sufficient proof that somebody is interested in discussing it. If somebody is not interested in it - then do not deal with it. Why would anyone want to destroy somebody else's virtual discussion space? And it is on topic. The questions of the boundaries of physics is of core importance (unless you "believe" in physics as in an "faith" or "ideology") $\endgroup$ Commented Feb 3, 2018 at 12:35

1 Answer 1


If a quantum system has been measured and property Y turns out to have value X then it is an experimental observation that, measuring Y again, will lead to the same value X. There is a long history on this in spin measurements using Stern-Gerlach type of experiments.

I would therefore not call this concept a-scientific, since it is a reasonable conclusion we make from experiment. We even can formulate this as a hypothesis and test it.

The issue of realism seems different to me. I do not perceive the question, whether properties exist independent of observation/measurement as a scientific question but rather as a philosophical non-sense. There is a long ongoing debate on this in quantum mechanics along the lines "Is the moon there when nobody is looking at it". We may, of course, pose this question. However we have no good method of answering the question and it has no practical implication.

There is an interesting aspect in the context of the Bell inequality. If we assume that a property had a value even if we had not measured it and if we assume a few other things (such as free will of the experimenter and an upper limit to the speed by which physical effects can propagate in spacetime) we end up with Bell inequality, which is in contradiction to experiment. This tells us that something with our way of reasoning about the world is wrong. Some believe that the "realist assumption" is wrong, others believe that the "speed limit" assumption is wrong. Still, this situation does not tell us anything about the world, it only falsifies our reasoning about it. We cannot prove a physical "law" and there are, strictly speaking, no physical "laws", only models in our mind.

While the question on realism seems very appealing to me personally, I see no possibility to settle it and no scientific reason to pose it. Objects and their properties exist only in a sense of an ultimate interaction with an observer. There is no such thing like "interaction free" or "observation free" property. (Well, quantum mechanics tells us - as in the Elitzur-Vaidman bomb tester experiment) that there is something like interaction free observation. However, this (experimentally tested result) relies again on certain theoretical assumptions and models. The interpretation of this experiment needs quantum theory. So while it is perfectly reasonable to use an established theory for interpreting it, we still have no reasonable experiment of proving / disproving realism.

A way out may be to (1) accept that asking questions which cannot be settled experimentally is unhealthy and (2) expecting from science more than numerical predictions of experimental outcomes is equally unhealthy - as it makes one uneasy about the bases of science.

Added in Debate

Tautology: If we take "tautology" as it is defined in logics, then clearly no. "Interact", "property", "object" are terms defined in a framework outside of logics and "tautology" is internal to logical language. If posing the question in a broader sense, we need to know what "property", "object" and "interact" mean.

Example 1: We have a pair of socks. one is red, the other is green. We close our eyes. We put one into a spaceship which we send to Sirius, the other we put into a safe. We open our eyes again. we do not know the color of the sock in the safe. 100 years later we open the safe, shine light into it and by this interaction with the sock find that it is red. We immediately know that the other sock is green. Could I have interacted with the other sock? Hm, it is already 80 lightyears away. Do I know a property of the other sock?

Example 2: I generate a pair of photons. From theory and earlier experimentation I know: If one is horizontally polarized then the other is vertically polarized. I wait until they have moved apart. I measure the polarization of one photon. Assuming quantum physics is right, I will make strong assumptions on the polarization of the other. Do I know it? Have I measured it? Have I interacted with the other one? Does the pair consist of two objects, or one object, or two objects in some magical "entangled" state?

Ontology / Epistemology: Methinks that there are two levels. An ontological one, describing what "is" and an epistemic one, describing what we "know". On an ontological level properties need interaction, ultimately with my own body (and it still leaves some doubts, as in "LSD" and "alcohol") On an epistemic level it heavily depends on the theory in which you grasp and formulate your knowledge. But what are theories if not random productions of our mind?

Questions: Methinks that there are questions which are deeply fascinating but which will never be solvable by science. It may be a matter of taste where we draw the lines (and a dissent on this, already, above, prompted a vote to close this stackexchange question, which I, personally, find fascinating). For example, we will never know if our world is deterministic or non-deterministic. There is simply no possibility to make EXACTLY the same experiment twice, as in the second run you will have known and written down the result of the first experiment. Maybe there is a deterministic model of quantum mechanics (actually there is one, de Broglie - Bohm). Maybe we will continue to work with a non-deterministic one. Maybe de Broglie - Bohm is rejected by some on general grounds, maybe it is falsified by some future experiments. Whatever. The question "Is our world deterministic?" cannot be answered in a satisfactory way. This still leaves pondering over the question very interesting. Some researchers spend their life on experiments trying to find out more about this, discarding some hypotheses, narrowing down others. There are researchers out there as well, who do not care.

If we look at questions close enough, we sooner or later will always feel uneasy. My personal position is that this "uneasiness with science and its so called 'results'" is unavoidable and even characteristic for science. Science is not able to provide long lasting, stable answers. This, of course, contradicts our wish for stability and certainty, it hurts our self-esteem as scientists and it is hardly compatible with the drive of every student in science. Therefore, some scientists solve that by calling all of this "un-scientific" or "a-scientific". Some solve it by calling these questions "unhealthy" (they still may be interesting, even if they will never be answerable).


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