This tag is for questions about the exact nature of wavefunction collapse.
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10answers
892 views
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 ...
16
votes
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 ...
15
votes
10answers
878 views
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 ...
15
votes
5answers
673 views
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 ...
11
votes
3answers
136 views
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 ...
10
votes
3answers
315 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. ...
10
votes
3answers
475 views
Bell's theorem and why nonlocality is problematic
I generally hear it assumed that Bell's inequality implies violation of counterfactual definiteness, because locality is considered sacrosanct. I understand of course that measurable violations of ...
9
votes
5answers
612 views
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 ...
9
votes
1answer
47 views
Accurate quantum state estimation via “Keeping the experimentalist honest”
Bob has a black-box, with the label "V-Wade", which he has been promised prepares a qubit which he would like to know the state of. He asks Alice, who happens also to be an experimental physicist, to ...
9
votes
1answer
67 views
Quasiparticles in Bohmian mechanics
My questions are about de Broglie-Bohm "pilot wave" interpretation of quantum mechanics (a.k.a. Bohmian mechanics).
Do quasiparticles have any meaning in Bohmian mechanics, or not? Specifically, is ...
9
votes
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 ...
8
votes
1answer
450 views
The measure problem in the anthropic principle
The anthropic principle is based upon Bayesian reasoning applied to the ensemble of universes, or parts thereof, conditioned upon the existence of conscious observers. That still leaves us with the ...
7
votes
2answers
338 views
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 ...
7
votes
2answers
610 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 ...
7
votes
1answer
159 views
Any link between decoherence and renormalization?
I have been studying decoherence in quantum mechanics (not in qft, and don't know how it is described there) and renormalization in QFT and statistical field theory, I found at first a similarity ...
6
votes
3answers
559 views
Measuring the spin of a single electron
Is it possible to measure the spin of a single electron? What papers have been published on answering this question? Would the measurement require a super sensitive SQUID, Superconductive Quantum ...
6
votes
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. ...
6
votes
1answer
174 views
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 ...
6
votes
9answers
554 views
What exactly is the 'observer' in physics and/or quantum mechanics? [duplicate]
Possible Duplicate:
nature of an observer
For instance, in the double slit experiment, what is exactly defined as an observer?
I remember from somewhere, light is also an observer?
5
votes
8answers
677 views
Schrodinger's dead cat when the evidence is destroyed
In the classic (and morbid) Schrodinger's Cat thought experiment, we imagine putting a cat into a box with a vial of poison which will be triggered by a quantum detectors. We set up a radioactive ...
5
votes
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 ...
5
votes
2answers
447 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?
5
votes
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, ...
5
votes
3answers
564 views
Electrons - What is Waving?
If an electron is a wave, what is waving?
So many answers on the internet say "the probability that a particle will be at a particular location"... so... the electron is a physical manifestation of ...
5
votes
3answers
693 views
Can randomness exist?
Considering every cause has an action, how can anything be random? For something to happen, it must have a cause and through that definition it can't be random.
Considering this why are many quantum ...
5
votes
3answers
320 views
Information conservation during quantum measurement
Consider the following experiment. I take a spin-$\frac{1}{2}$ particle and make a $\sigma_x$ measurement (measure the spin in the $x$ direction), then make a $\sigma_y$ measurement, then another ...
5
votes
2answers
122 views
Decoherence inside black holes
I have a question about decoherence. Assume there is a macroscopic black hole floating around and you have some macroscopic object with you with a huge number of internal degrees of freedom. ...
5
votes
1answer
138 views
Maximum probability of success for distinguishing between two pure states with one measurement
Suppose you have the states such that $\langle \phi | \theta \rangle = cos(x)$ and you have one measurement to distinguish between the two. It is claimed that the probability of success at guessing ...
4
votes
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 ...
4
votes
5answers
479 views
If the size of universe doubled
My question is silly formulated, but I want to know if there is some sensible physical question buried in it:
Suppose an exact copy of our universe is made, but where spatial distances and sizes are ...
4
votes
3answers
348 views
How could a particle be isolated to avoid decoherence?
The question aims to this issue : if there is some technological arrangement (or action) to take over the particle/system in order to keep it in a coherent state, then the field, (force or whatever) ...
4
votes
4answers
392 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 ...
4
votes
5answers
494 views
Eigenvalue Postulate and Experiment Outcomes in QM
In Nielsen and Chuang's text on Quantum Information and Computation, the measurement postulate is stated by using a collection of measurement operators and the outcomes are the indices of the ...
4
votes
3answers
389 views
Is Schrodinger's Cat a real conceptual problem or just a problem with approximations?
In this thought-experiment a cat is placed in a box set with a bottle poison that will release and kill it depending on whether or not a certain radioactive particle decays. The box is kept closed and ...
4
votes
2answers
410 views
Again about all-win lottery
I suggest the following thought experiment that describes a machine which makes everybody happy.
Suppose a lottery is conducted. The winner is awarded a billion dollars plus the title of eternal ...
4
votes
2answers
148 views
What counts as a measurement?
In quantum mechanics, an elementary particle does not have a well defined position until a measurement is performed on it (right?). Such a "measurement" is any sort of interaction with other ...
4
votes
1answer
49 views
Spekkens Toy Model, Internal Comonoids
I have been thinking about Spekkens Toy model in terms of interfaces. The Spekkens paper concerns a physics based on only being able to receive answers to half the number of questions necessary to ...
4
votes
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 ...
4
votes
1answer
928 views
Compatible Observables
My QM book says that when two observables are compatible, then the order in which we carry out measurements is irrelevant.
When you carry out a measurement corresponding to an operator $A$, the ...
4
votes
2answers
72 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 ...
4
votes
5answers
389 views
How isolated must a system be for it's wave function to be considered not collapsed?
As an undergrad I was often confused over people's bafflement with Schodinger's cat thought experiment. It seemed obvious to me that the term "observation" referred to the Geiger counter, not the ...
4
votes
5answers
259 views
Can decoherence work when the environment itself is in a superposition without invoking collapse or splitting?
Decoherence is often presented as a program to solve the measurement problem using only the bare bones quantum mechanics framework of a Hilbert space and unitary Schrödinger time evolution. As ...
4
votes
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 ...
3
votes
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?
3
votes
3answers
208 views
Knowing when wavefunction collapses
So I learned that after a measurement of, lets say the position of the wavefunction of a particle is made, if another measurement of the position of the particle is made right away, you should get the ...
3
votes
3answers
483 views
Time travel and nuclear decay
Reading a previous closed question an interesting variation has come to my mind.
Suppose that time travel to the past was possible:
I wait for an atom to decay and measure the time, $t_{1a}$
I ...
3
votes
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" ...
3
votes
5answers
407 views
Trying to understand the EPR paradox
So I keep reading all these articles on the EPR paradox, and I follow them pretty easily right up until it gets to the most important matter.
Assuming you are trying to measure x and y spin,
...
3
votes
2answers
512 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 ...
3
votes
3answers
422 views
Solution to the quantum measurement problem?
What can be a scientific solution to the Q-measurement problem (other than many worlds idea)? Can it be somehow verified through experiment?
