This tag is for questions about the exact nature of wavefunction collapse.
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A question about quantum measurement and associating a linear self adjoint operator to it
I have a question about the concept of measurement and observable in quantum mechanics. I'd like to fist explain my understanding of it and then ask the question.
First we have a system and its ...
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3answers
68 views
Measurements and simultaneity [duplicate]
Picture a situation where we have two observers, $A$ and $B$, and a system in a certain quantum state. If $B$ makes a measurement of some observable, say energy for example, the state will collapse to ...
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0answers
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Isn't Heisenberg Uncertainty Principle a Kind of Standard Error? [duplicate]
Isn't Heisenberg uncertainty principle due to standard error of quantum expected value? If you think so, then isn't standard error due to limits of human accuracy not quantum world itself?
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0answers
33 views
Particles at the Quantum Mechanical Level with Precise Position [closed]
I would like to pose a thought experiment: As I understand it the position of a particle is not fully determined if the moment can be determined by any measure. So I would like put a coordinate ...
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2answers
74 views
How do you come up with a POVM?
This is a made-up example, just to understand a concept. If changing the probability values aids your explanation, that's fine by me.
Say you have a physical quantity $E$ that can take values 1, 2, 3 ...
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2answers
55 views
Is it possible to determine the state or values of something without measuring it
To give context to this question, I am currently looking into non-locality / hidden variables / Bell's Theorem, EPR / etc.
I've noticed the assertion that the values / state of something when ...
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2answers
111 views
Can we measure “wavefunction” of quantum particles?
We know that there is uncertainty principle, so question: can we ever measure wavefunction of particles? I do not think this is possible, but I am not sure. I guess that everything is probabilistic. ...
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4answers
305 views
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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0answers
21 views
Does quantum mechanics depend solely on electromagnetic waves? [duplicate]
I am beginning to learn quantum mechanics. Since determining the position of an object involves probing by electromagnetic waves and since i have read a simple derivation of Heisenberg's uncertainty ...
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5answers
223 views
How do we know that there isn't a classical solution to the measurement problem/Quantum Mechanical uncertainty?
It was mentioned to me that it can be shown that there is no classical explanation for the uncertainty in Quantum Mechanics -- i.e. that there are no hidden workings that we have just not yet seen, ...
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0answers
43 views
Partial Measure Probability
Let be a
$$|\psi\rangle = \dfrac{3}{5\sqrt{2}}|00 \rangle- \dfrac{3i}{5\sqrt{2}}|01 \rangle+ \dfrac{2\sqrt{2}}{5}|10 \rangle - \dfrac{2\sqrt{2} i}{5}|11 \rangle$$
state with two qubits. ...
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3answers
105 views
Observer effect, do this mean literally someone or just any interaction with other matter?
I am a layman and was wondering, the quantum observer effect. The regular notion to laymen seems to be literally "if you look at it", but as I am coming to understand the world I live in better I feel ...
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4answers
220 views
Are photons deterministic?
I propose the following scenario:
At $t=0$, a photon is emitted from a star. At $t=n$, said photon is received and interpreted by some detector.
My question is whether or not it is accurate to say ...
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2answers
91 views
Does performing a measurement on a system change its internal energy?
I'm studying Quantum Mechanics in my spare time from a general point of view (no technical details) so some fundamental question came into my mind:
How is it possible to detect a single photon ...
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2answers
86 views
About Heisenberg uncertainty principle [duplicate]
What would happen if someone invented a way to measure both position and momentum precisely? If it is impossible why?
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6answers
339 views
Why is Heisenberg's uncertainty principle not an experimental error since it is the error created by photons striking on elementary particles?
Why is Heisenberg's uncertainty principle not an experimental error since it is the error created by photons striking on elementary particles?
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3answers
316 views
Can the path of a charged particle under the influence of a magnetic field be considered piecewise linear?
Ordinarily we consider the path of a charged particle under the influence of a magnetic field to be curved. However, in order for the trajectory of the particle to change, it must emit a photon. ...
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2answers
69 views
Does the unpredictability in the “micro world” means that everything is if we can look at it close enough imperfect?
Does the unpredictability in the "micro world" means that everything is if we can look at it close enough imperfect?
I mean, there is a saying "You will never stand in the same river again" or ...
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1answer
91 views
Does the measurement on a subatomic particle give its mass value?
No doubt I am wondering about the Young's double slit phenomena. If we observe or measure the properties of a subatomic particle, we are able to know its mass, velocity and it comes to existence. So ...
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2answers
222 views
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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2answers
137 views
What does the quantum state of a system tell us about itself?
In quantum mechanics, quantum state refers to the state of a quantum
system. A quantum state is given as a vector in a vector space, called
the state vector. The state vector theoretically ...
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1answer
66 views
Is the time of collapse of the wave function empirical?
Is the time of the collapse of the wave function empirical?
Suppose there is a very long von Neumann chain of observations of a quantum system. Suppose also practically irreversible decoherence ...
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1answer
176 views
If nothing is “objectively real” prior to “measurement”, what exactly is a “measurement”?
If nothing is "objectively real" prior to "measurement", what exactly is a "measurement"?
Is there any "objective" criteria to demarcate a process as being a "measurement" or not? If "measurements" ...
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1answer
132 views
Who are “we”, and what counts as a “question” in consistent histories?
If the preferred basis in quantum mechanics and/or choice of consistent histories in consistent histories is arbitrary, and can only be determined by the "questions we ask", just who exactly is this ...
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1answer
573 views
Application of Heisenberg's uncertainty principle
I've the following application of Heisenberg's uncertainty principle.
If a beam of particles in localised in the $x$-direction by a long slit, what is the uncertainty in position?
Firstly, I ...
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2answers
240 views
Why for a spin half particle, possible outcomes of measuring spin projection along any direction are the same?
If one measures the projection of spin of a spin half particle along the x axis one will always get plus or minus half $\hbar$
Measuring it along the y axis one will always get plus or minus half ...
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3answers
265 views
What is the experiment where subatomic particles appear to foresee the future?
I've seen a documentary, whose name I don't remember but I'm curious because it suggests that subatomic particles are able to "foresee the future".
I'll try to describe it here:
Some particles are ...
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2answers
179 views
Can we determine whether or not a particle is entangled?
Suppose Shaniqua and Tyrone have four pairs, a, b, c, and d, of entangled particles. They take their particles and go very far apart. If Tyrone can determine whether or not a particle is still ...
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1answer
184 views
In QM, does random data “come from anywhere”? Also, what are the properties of the data?
I have only taken a basic quantum mechanics course (this book, so you know where I'm coming from), but I've been wondering about something.
If we set up a quantum system in a known state and take a ...
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0answers
59 views
How to explain Tsirelson's inequality using extended probabilities?
How to explain Tsirelson's inequality using extended probabilities?
Some people have tried explaining the Bell inequalities using extended probabilities.
For instance, a pair of entangled photons ...
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1answer
85 views
Practical meaning of making a measurement/observation in QM?
When an argument like 'measure the spin along the $x$ axis', 'observe the position of a particle' and so on is made, what is the implied experimental procedure? Since laboratory equipment is ...
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2answers
202 views
Does every measurement correspond to an eigenstate of an observable?
In the postulates of quantum mechanics, physical observables are described by Hermitian matrices on the state space of a system.
In another of my questions, the measurements of Rydberg-Ritz spectral ...
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6answers
652 views
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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1answer
229 views
Controlling the outcome of a quantum measurement through translational entanglement
According to the paper:
A. S. Parkins and H. J. Kimble, Phys. Rev. A 61, 52104 (2000).
http://pra.aps.org/abstract/PRA/v61/i5/e052104
You can entangle position and momenta of two atoms by using ...
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0answers
38 views
Free Energy and quantum measurement
Free Energy must be expended to reset the state of an measurement apparatus. Is this statement valid in all situations? Is there a Definitive mathematical exposition?
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0answers
46 views
An idea about the simultaneous observation of wave and particle property of light [closed]
It is well known that in a double-slit experiment with single photon source, one can not obtain the which-way info and interference pattern at the same time, since a detector placed behind one slit ...
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2answers
129 views
Measurement of the energy of an atom using a cold substance
An atom was prepared in a superposition of ground state and excited states.I propose to measure the state by coupling the system to a cold enough substance.
By cold enough I mean $$kT\ll E_1,$$ where ...
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4answers
393 views
What is the meaning of uncertainty in Heisenberg's uncertainty principle?
The Heisenberg's uncertainty principle states the following:
$$\Delta p \cdot \Delta x \ge \frac{h}{4\pi}.$$
While studying for my high school physics exams, I fooled myself into believing that I ...
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1answer
219 views
Work done by introducing a spin in supersposition into a Magnetic Field
A spin is created in a superposition of up and down states. A magnet is moved very slowly, towards the spin. What is the work done by the magnet. It may be helpful to imagine that the magnet is ...
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3answers
252 views
Decoherence and collapse
It is said that the decoherence does not solve the problem of measurement and/or the emergence of classicality, can somebody explain it with simple analogies or in a manner accessible to a ...
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2answers
448 views
Is the expectation value always an eigenvalue?
Does the expectation value of an observable must be equal to an eigenvalue of the corresponding operator?
I already know that 0 is not an eigenvalue, but is there any other examples?
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1answer
104 views
Spike when wavefunction collapses
So, when wavefunction collapses, there is a spike occuring. Does this mean that there are parts with the continuous probability of 0? (For example, x position from -9 to -3 has probability of 0, while ...
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3answers
161 views
In Copenhagen, can this idea preserve locality for Bell inequalities?
Generate an entangled pair of qubits. Send to Alice and Bob far away from each other. Both measure along basis in one of two possible orientations. The result is sent to Charlie at some later time, ...
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2answers
611 views
Why is the Heisenberg uncertainty principle stated the way it is?
I spent a long time being confused by the Heisenberg uncertainty principle in my quantum chemistry class.
It is frequently stated that the "position and momentum of a particle cannot be ...
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3answers
306 views
Is it the act of measuring a quantum particle that causes it to lose its uncertainty?
I have designed an experiment. Without going into detail it resolves around the double slit quantum eraser experiments. If we can infer the location of a particle without actually measuring it, does ...
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0answers
72 views
Non-simultaneous measurement of the location and momentum of an electron
I know the uncertainty principle tells us that we cannot simultaneously measure both the location and momentum of a particle with a high accuracy. The usual example we find in lectures is to imagine a ...
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3answers
313 views
Triple slit experiment
If the experiment is done with single photons and a "detector" at one of the slits the interference pattern breaks down.
What happens if three slits are used with single photons and a "detector" at, ...
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2answers
513 views
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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0answers
46 views
Books on the general notions of measurements, observables, states, etc.? [closed]
I am reading the intro chapter in Huzihiro Araki's Mathematical Theory of Quantum Fields, which discusses the general notions of states, measurements, and observables (e.g. the topology on the sets of ...
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3answers
247 views
What is the physical meaning of weak expectation values?
In the two-state formalism of Yakir Aharanov, the weak expectation value of an operator $A$ is $\frac{\langle \chi | A | \psi \rangle}{\langle \chi | \psi \rangle}$. This can have bizarre properties. ...
