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# Questions tagged [measurement-problem]

DO NOT USE THIS TAG just because your question involves measurements (either quantum or classical). The measurement problem asks how wave function collapse occurs during measurement in quantum mechanics, and how it can be reconciled with unitary evolution.

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### Experimental test of the non-statisticality theorem?

Context: The paper On the reality of the quantum state (Nature Physics 8, 475–478 (2012) or arXiv:1111.3328) shows under suitable assumptions that the quantum state cannot be interpreted as a ...
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### Is the uncertainty principle a property of elementary particles or a result of our measurement tools?

In many physics divulgation books I've read, this seems to be a commonly accepted point of view (I'm making this quote up, as I don't remember the exact words, but this should give you an idea): ...
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### Why aren't particles constantly “measured” by the whole universe?

Let's say we are doing the double slit experiment with electrons. We get an interference pattern, and if we put detectors at slits, then we get two piles pattern because we measure electrons' ...
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### What is the difference between a measurement and any other interaction in quantum mechanics?

We've learned that the wave function of a particle collapses when we measure a particle's location. If it is found, it becomes more probable to find it a again in the same area, and if not the ...
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### Isn't the detector always measuring, and thus always collapsing the state?

I have a radioactive particle in a box, prepared so as to initially be in a pure state $\psi_0 =1\ \theta_U+ 0\ \theta_D$ (U is Undecayed, D is Decayed). I put a Geiger counter in the box. Over ...
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### Extension of Schrödinger's cat thought experiment

My question is quite simple. In the thought experiment of Schroedinger's cat: When the scientist measures the state of the cat, its wavefunction collapses into either the alive or dead state. But ...
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### What is the quantum mechanical definition of a measurement?

I hear the word "measurement" thrown around a lot in quantum mechanics, and I have yet to hear a scientific definition that makes sense. How do we define it?
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### Isn't the uncertainty principle just non-fundamental limitations in our current technology that could be removed in a more advanced civilization?

From what I understand, the uncertainty principle states that there is a fundamental natural limit to how accurately we can measure velocity and momentum at the same time. It's not a limit on ...
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### What is an observer in quantum mechanics?

My question is not about (pseudo) philosophical debate; it concerns mathematical operations and experimental facts. What is an observer? What are the conditions required to be qualified of observer, ...
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### Simple example showing why measurement & interaction are different

Does someone know of a clear (pedagogical) example where one can really see(with the math) where interaction and measurement are not synonymous in quantum mechanics? I know that every measurement ...
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### What constitutes an observation/measurement in QM?

Fundamental notions of QM have to do with observation, a major example being The Uncertainty Principle. What is the technical definition of an observation/measurement? If I look at a QM system, it ...
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### How can Schrödinger's cat be both dead and alive? [closed]

So, this goes to something so fundamental, I can barely express it. The Schrödinger's Cat thought experiment ultimately asserts that, until the box is opened, the cat is both dead AND alive. Now, ...
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### Why can't the outcome of a QM measurement be calculated a-priori?

Quantum Mechanics is very successful in determining the overall statistical distribution of many measurements of the same process. On the other hand, it is completely clueless in determining the ...
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### Would every particle in the universe not have some form of measurement occurring at any given time? [duplicate]

I know this is probably a common point of confusion, but I have a specific question about measurements in Quantum Mechanics. I read an explanation on this, but still have a point of confusion. The ...
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### Is every quantum measurement reducible to measurements of position and time?

I am currently studying Path Integrals and was unable to resolve the following problem. In the famous book Quantum Mechanics and Path Integrals, written by Feynman and Hibbs, it says (at the beginning ...
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### What is the difference between general measurement and projective measurement?

Nielsen and Chuang mention in Quantum Computation and Information that there are two kinds of measurement : general and projective ( and also POVM but that's not what I'm worried about ). General ...
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### Is the uncertainty principle just saying something about what an observer can know or is it a fundamental property of nature?

I ask this question because I have read two different quotes on the uncertainty principle that don't seem to match very well. There are similar questions around here but I would like an explanation ...
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### Is there a difference between observing a particle and hitting it with another particle?

First, let me state that I'm a lot less experienced with physics than most people here. Quantum mechanics was as far as I got and that was about 9 years ago, with no use in the meantime. A lot of ...
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### Uncertainty in Uncertainty?

I was reading about how the Planck's Constant can be measured with LEDs, which made me think about this question. The Heisenberg Uncertainty Principle states that: \Delta x \Delta p \ge {\hbar\...
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### What are the strongest objections to be made against decoherence as an explanation of “collapse?”

When we measure an observable A of a quantum system, we get an eigenvalue of A. Without worrying about connotations of Copenhagen vs. MWI, etc., let's just call this "collapse." Question: Among ...
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### On a measurement level, is quantum mechanics a deterministic theory or a probability theory?

Quantum mechanics is a non-commutative probability theory. As such, it fundamentally behaves differently from classical probability theories. This manifests itself most pronouncedly in the uncertainty ...
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### Does measurement, quantum in particular, always increase the total entropy?

Measurement of a quantum observable (in an appropriate, old-fashioned sense) necessarily involves coupling to a system with a macroscopically large number of degrees of freedom. Entanglement with this ...
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### How is it possible that quantum phenomenons (e.g. superposition) are possible when all quantum particles are being constantly observed?

I don't understand how quantum mechanics (and therefore also quantum computers) can work given that while we work with quantum states, particles that this quantum state consist of cannot be observed, ...
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### Is the collapse of the wave function inherently time asymmetric?

Schroedinger's equation, as we all know, is time symmetric. In quantum field theory, we have to come up with a more sophisticated CPT reversal, but the essential point remains unchanged. However, the ...
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### Are Thomas Breuer's subjective decoherence and Scott Aaronson's freebits with Knightian freedom the same things in essence?

In his remarkable works (1,2 and their recent development 3) Thomas Breuer proves by diagonalization the phenomenon that the observer cannot distinguish all phase space states of a system where he is ...
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### Is quantum entanglement functionally equivalent to a measurement?

I saw the following talk the other day: http://www.youtube.com/watch?v=dEaecUuEqfc&feature=share In it, Dr. Ron Garret posits that entanglement isn't really that "special" of a property. He ...
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### POVMs that do not require enlargement of the Hilbert space

The usual justification for regarding POVMs as fundamental measurements is via Neumark's theorem, i.e., by showing that they can always be realized by a projective measurement in a larger Hilbert ...
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### Born's rule and Schrödinger's equation

In non-relativistic quantum mechanics, the equation of evolution of the quantum state is given by Schrödinger's equation and measurement of a state of particle is itself a physical process. Thus, ...