This tag is for questions about the exact nature of wavefunction collapse.
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2answers
144 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 ...
7
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1answer
157 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 ...
0
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1answer
172 views
Change In Momentum In Uncertainty Principle
The most basic explanation for the Heisenberg Uncertainty Principle is that the momentum and position of a quantum particle is not very distinct when an attempt is made to measure them together. But ...
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1answer
161 views
Quantum Entanglement - Measuring Twice
In the answer here and on the wiki article and many other articles it is mentioned that if one of 2 entangled particles is measured their state collapses according to the Copenhagen interpretation.
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7
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2answers
324 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 ...
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0answers
29 views
Information conservation during quantum measurement in $\psi$-epistemic interpretations [duplicate]
Possible Duplicate:
Information conservation during quantum measurement
I asked a version of the following quesiton previously on Physics.stackexchange, where it didn't get a lot of ...
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3answers
664 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 ...
4
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2answers
388 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 ...
8
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5answers
570 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 ...
4
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1answer
135 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 ...
5
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3answers
312 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 ...
3
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3answers
408 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?
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3answers
463 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 ...
3
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2answers
210 views
Decoherence when no one is looking?
I understand that in the single-electron-at-a-time double slit experiment, if a detector is placed before the slit, the interference pattern vanishes.
Suppose I left the detector on, but put a bag ...
0
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1answer
68 views
Are a measured object always part of the theory?
Is there a notion of measurement, which doesn't correspond to a yes/no question or with the idea of the comparison of two real world objects, which produces a real number?
And does at least one of ...
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 ...
1
vote
3answers
238 views
Double slit experiment and indirect measurements
In the classic Young double slit experiment, with slits labeled as "A" and "B" and the detector screen "C", we put a detector with 100% accuracy (no particle can pass through the slit without the ...
3
votes
1answer
123 views
What does the appearance of a classical particle fundamentally reduce to?
I've been reading an article that describes what seems to be a classical particle as a regularity in the global wavefunction over a quantum configuration space:
When you actually see an electron ...
5
votes
3answers
533 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 ...
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vote
4answers
130 views
Is a macroscopic pair where I observe one of them quantum entanglement?
Of what little I know/understand about quantum entanglement can somebody confirm if the following experiment is a good analogy to quantum entanglement of pair of particles? PS: please don't laugh as ...
2
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1answer
46 views
Unknown quantum state with promise of classical data
I am trying to solve a problem in the measurement and identification of quantum states with a promise as to what states it could be. Here is the problem. Imagine a system that produces qubits in ...
4
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5answers
383 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
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1answer
861 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 ...
-2
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1answer
249 views
Can God collapse the wavefunction? [closed]
This is a semi-theological question which I hope isn't out of place in this forum.
The Copenhagen interpretation requires observers to collapse the wavefunction. Certainly, the question of whether or ...
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3answers
133 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 ...
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4answers
318 views
How can indeterminacy in quantum mechanics be derived from lack of ability to observe a cause?
I don't get this part of quantum mechanics.
I get the part that you can't observe particles and not affect their behavior because you are shooting photons to them while you are observing them, but ...
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0answers
70 views
Identifying fragments when there is a superposition of fragments in quantum Darwinism
In Zurek's theory of quantum Darwinism, information about the pointer states of a system imprint themselves upon fragments of the environment carrying records about the state of the system. ...
2
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6answers
351 views
Do we really know which slit the photon passed through in Afshar's experiment?
The plain old double slit experiment displays interference when we don't measure which slit the photon passed through, and no interference when it is measured. Let's turn our attention to the case ...
1
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1answer
78 views
Do interaction-free measurements require a physical collapse or splitting in order to be truly interaction free?
Interaction-free quantum experiments like Renninger's experiment or the Elitzur-Vaidman bomb tester are often taken to be examples of interaction-free measurements of a system. Unfortunately, such ...
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10answers
527 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?
4
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5answers
257 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
1answer
48 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 ...
5
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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. ...
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8answers
503 views
Does the wave function/density state actually exist?
I have been reading with interest the debates here on whether the wave function/density state actually collapses or not, or whether it is subjective Bayesian or objective with actual complex numbered ...
9
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1answer
45 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 ...
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0answers
132 views
Can the time direction of wave function collapse be reversed?
The laws of physics are invariant under CPT transformations reversing time, inverting space and flipping charges. Almost so. The collapse of the wave function is the odd man out. Can the time ...
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votes
8answers
671 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 ...
9
votes
1answer
61 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 ...
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5answers
387 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,
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3answers
442 views
Defining Measurement in Quantum Mechanics
I should begin by saying that I am a total newbie when it comes to Quantum Mechanics. Therefore my question might sound metaphysical to people who know their stuff. So please forgive.
What I am ...
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3answers
347 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) ...
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1answer
446 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 ...
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1answer
295 views
Measuring the magnitude of the magnetic field of a single electron due to its spin
Is it possible to measure the magnitude of the magnetic field of a single electron due to its spin? The electron's intrinsic magnetic field is not dependent upon the amount of energy it has does it? ...
3
votes
3answers
206 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 ...
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3answers
517 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 ...
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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 ...
3
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3answers
481 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 ...
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6answers
641 views
Measurement problem in the orthodox interpretation
Let's look at the measurement problem in the orthodox interpretation of quantum mechanics as an inconsistency between inner and outer treatment of the measurement apparatus. You can always push your ...
15
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5answers
663 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 ...
4
votes
5answers
476 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 ...
