This tag is for questions about the quantum mechanical measurement problem, such as e.g. the wavefunction collapse/decoherence. DO NOT USE THIS TAG for non-quantum measurements.

learn more… | top users | synonyms

1
vote
3answers
96 views

Why diaphragm in diffraction experiment using electrons is quantum object?

In the book Quantum Mechanics - Volume 1 written by Albert Messiah, page no. 142-143, author says: ...But the diaphragm is a quantum object, just like the electron. Its momentum is not defined to ...
10
votes
2answers
376 views

QM: why is reflection of a photon not a measurement?

Many experiments with entangled photons are sending them through different glass fiber cables (e.g. in opposite directions for spatial separation). The photons will inevitably be reflected many times ...
2
votes
0answers
62 views

Do restrictions on quantum mechanical measurement always just work out to avoid contradictions?

Classically it was said that measurement leads to a collapse of the wave function. However, if there wouldn't be any limit on the process on measurement itself, strange things can happen, e.g. a ...
8
votes
4answers
991 views

How can things be chaotic on a quantum level, yet tangible on a classical level?

This may seem basic, but I am wondering if anyone has any input on this topic. It doesn't make any sense to me (I mean I don't need to use the Schrödinger equation to find my cell phone...). I just do ...
47
votes
9answers
3k views

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): ...
3
votes
4answers
843 views

Double slit experiment and single particles. Is the wave function just a mathematical model?

I really do want to apologize in advance, I know this question has been 'answered' before. I have this 'problem' of feeling negatively toward much of today's 'mystical' interpretations of physics, ...
0
votes
2answers
7k views

How to determine Young's modulus of a wire by Searle's method?

The question is: A student performs an experiment to determine the Young's modulus of a wire, exactly 2m long, by Searle's method. In a particular reading, the student measures the extension in ...
1
vote
2answers
280 views

What does this quantum experiment says about quantum world?

I am complete noob so please bear with me. I always read that in quantum world things exists as probability and only become one when they are observed...or wave collapses into particle. But there was ...
1
vote
1answer
373 views

Understanding wave functions

I'm currently writing an essay on the measurement problem, and I'm not quite certain that I've fully understood the purpose of the wave function, in that does the following sentence make sense with ...
3
votes
0answers
748 views

“Consciousness causes collapse”-interpretation or free will excluded by quantum zeno effect? [closed]

This question is if the von Neumann–Wigner interpretation, aka "consciousness cause collapse" (see e.g. John von Neumann's 1932 book The Mathematical Foundations of Quantum Mechanics) is compatible ...
2
votes
0answers
166 views

What are you studying when you study a Harmonic Oscillator in QM?

This probably is a naive question - so please forgive a self-studier. In the text I am studying, one builds a HO by placing a particle in a potential that increases quadratically from the origin. The ...
3
votes
2answers
307 views

Understanding Well Defined States

I am self-studying from a text in QM. Well defined states are mentioned several times. By and large these are consistent and seem to be readily apparent: states of well defined energy are basis ...
1
vote
2answers
225 views

Can we describe Quantum Mechanics using filters and matrices? [closed]

Can mathematical filters or ultrafilters be used to predict quantum physics 'events' as accurately as using matrices like Schrodinger did? Is there a way to explain some of the predictive power of ...
1
vote
1answer
86 views

Position and potential Energy

Why are the position and potential energy of a particle able to be measured precisely in Quantum Mechanics? I mean why do they commute with each other?
1
vote
1answer
219 views

Collapse of the Free Particle Wave Function

The time evolution of the one-dimensional quantum mechanical free particle ($V(x) = 0$ $\forall x$) is described by the following Schroedinger equation $ -\frac{\hbar^2}{2m}\frac{\partial^2 ...
3
votes
1answer
324 views

The Physical Meaning behind a Commutator [duplicate]

I've just been introduced to the idea of commutators and I'm aware that it's not a trivial thing if two operators $A$ and $B$ commute, i.e. if two Hermitian operators commute then the eigenvalues of ...
10
votes
6answers
2k views

If I drop a leaf twice from the height of a tree in a completely controlled environment, will the trajectory in each case be the same?

Putting my question in other words, can earth form again if a similar initial universe condition is given? The uncertainty principle says that we cannot tell with certainty the position of a particle ...
13
votes
4answers
312 views

Why is QM maximally predictive?

Let's suppose I'm in the lab and I claim that I can predict more than QM can, specifically, I can predict exactly at which moment in time a particle decays. You don't believe me (naturally) so I set ...
4
votes
4answers
394 views

Is commutation relation an equivalence relation?

I'm now learning quantum mechanics with Liboff. In the book it deals with "a compete set of mutually compatible observables" in order to make a state maximally informative. How can one find such set? ...
1
vote
1answer
236 views

Are we so sure about superposition?

Apparently particles can be anywhere when not observed. How strong is this theory really? Okay the wave-function can be collapsed through observation but how are we so sure that when an object is not ...
2
votes
2answers
62 views

Does measuring the exact position of a ball destroy the ball

If you have a macroscopic ball (say, a tennis ball) and you (hypothetically) try to measure the exact position of the center of that ball by measuring the exact positions of the atoms making up the ...
3
votes
0answers
105 views

Does quantum mechanics require classical measurement apparatus?

I am trying to learn quantum mechanics and I have a question. Landau, in his quantum mechanics book says that it is in principle impossible to formulate basic concepts of quantum mechanics without ...
-2
votes
1answer
109 views

If the wave function can be collapsed, can we collapse the particle function? [closed]

No doubt it's been questioned before so what is the theory or experiment? If there is an experiment.
2
votes
4answers
250 views

Measurement of quantum state

Consider a particle in a box system. Assume its state to be a superposition of the ground and the first excited energy states. Consider two observers A and B (rest of the world). A made the ...
1
vote
4answers
806 views

How does wave function collapse when I measure position?

Text books say that when you measure a particle's position, its wave function collapses to one eigenstate, which is a delta function at that location. I'm confused here. A measurement always have ...
13
votes
2answers
658 views

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 ...
2
votes
4answers
249 views

What does quantum phenomena exist as prior to observation?

It's been said that according to the Schrodinger equation, independent of observation, particles exist in a state of a wave function, which is a series of potentialities rather than actual objects. ...
1
vote
1answer
243 views

Is it true that Macrorealism can be shown false?

http://en.wikipedia.org/wiki/Leggett-Garg_inequality As you can see by the link above, it claims that if the violation of the Leggett–Garg inequality can be demonstrated on the macroscopic scale, it ...
2
votes
2answers
681 views

If only one slit is observed in the Double Slit experiment, will the unobserved slit produce an interference pattern?

I am having a difficult time solving this. Say that electrons are emitted from a source S at a very slow rate. If both slits S1 and S2 are observed, we would have roughly 50% probability of detecting ...
8
votes
3answers
351 views

Is the Copenhagen interpretation merely an approximation to quantum mechanics?

So, I'm reading Max Tegmark's Our Mathematical Universe (Knopf edition, p. 229). He's discussing Everett/MWI for a bit and I'm not really paying attention and then I wake up to this: [I]t's time ...
15
votes
4answers
2k views

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, ...
3
votes
1answer
266 views

Is kinetic energy in QM a state-property or is it distributed?

Suppose we have a quantum mechanical system, which is well described by its wave function in r-representation $\Psi$. We are interested in the properties of an observable, say the kinetic energy $T$. ...
3
votes
5answers
596 views

The quantum state just after a position measurement

The wave function of a free particle is given as, $$\psi(x) ~=~ e^{-{ x }^{ 2 }/{ a }^{ 2 }}.$$ Then a position measurement is made and the position of the particle is found to be at $x=a$. My ...
1
vote
0answers
128 views

Why are cosmic conspiracies considered undesirable in quantum interpretations?

Quantum interpretations like superdeterminism, nonlocal hidden variables, etc. are regularly dismissed by the vast majority of physicists because they require "cosmic conspiracies" which can ...
2
votes
3answers
262 views

How can the reduction postulate be removed with the other postulates of QM still leading to correct predictions?

In the axiomatic presentation of QM, I've seen it stated many times that the reduction postulate is not needed and/or incorrect, and could be gotten rid of. However, without the reduction postulate, ...
4
votes
2answers
180 views

Without apparatus can we say that the system is measured(decohered) by the environment?

"Einselection" and "tridecompositional uniqueness theorem" seem to resolve the preferred basis problem. But the premise is that there are three parts in discussion.(system, apparatus, environment) ...
2
votes
0answers
134 views

How can the pre-measurement be fulfilled?

In the decoherence program, the pre-measurement refers to the evolution in which the system and apparatus form a Schmidt state. In Maximilian Schlosshauer's review article(2005), I read "the linearity ...
0
votes
3answers
228 views

Why does a wavefunction collapse when observation takes place?

Why does a wavefunction collapse when observation takes place? Can this question be explained in non mathematical terms? I have tried finding the answer but couldn't find a clear explanation.
1
vote
1answer
118 views

Wigner friend experiment

Let's supposed we take the Wigner's friend experiment from the metaphysical arena and try to implement it as an actual physical experiment Assuming Wigner's lab friend is kept as a coherent ...
0
votes
3answers
195 views

Discrete movement vs wave function collapse

I remember once, as a child, thinking that objects do not really "move," but that at a very small scale they would have to "disappear" and then "appear" again at their newly shifted position, just the ...
0
votes
1answer
115 views

What if Correspondence principle in QM happening in classical sense? [closed]

I assume a simple set up (hope to generalize it later)... suppose there is a position pdf (need not be position but any) (probability density function) which is the magnitude of a $\Psi(x)$ predicted ...
1
vote
0answers
138 views

Quantum Eraser under Lorentz Boost

Suppose I am conducting the Quantum Eraser experiment. The results of this experiment are easy to understand with the traditional quantum mechanical interpretation of a pair of entangled photons. ...
0
votes
0answers
114 views

The status of unobservable quantum mechanical predictions

Orthodox quantum mechanics intrinsically requires an observer - since the only connection from the mathematical formalism to physical reality is through the measurement axiom (probability of observing ...
5
votes
5answers
1k views

Is the wave function of a particle re-created after a measurement stops?

Yeah, I haven't quite understood, or been told, what happens to, for example an electron and its wavefunction, when you stop to measure it. I mean, an electron has a wave function describing its ...
1
vote
1answer
84 views

Wave function collapse in system with many coordinates

Consider general state of a system with spin-$1/2$ $$ \psi = \frac{1}{\sqrt{2}}\left[\phi_{+1/2}(x) \left( \begin{array}{cc} 1 \\ 0 \\ \end{array} \right) + \phi_{-1/2}(x) \left( \begin{array}{cc} 0 ...
1
vote
0answers
60 views

What exactly constitutes a measurement? [duplicate]

As far as I understand it, quantum mechanics only predicts the probabilities of finding a system in some state of some observable when we make a measurement. So, what exactly constitutes a ...
4
votes
3answers
316 views

Does the uncertainty principle make simulation of systems impossible?

Is it possible to fully define a system, then be incapable of simulating or calculating its future states due to the Uncertainty Principle? If it can be done, how?
6
votes
4answers
830 views

Is uncertainty principle a technical difficulty in measurement? [duplicate]

Is the uncertainty principle a technical difficulty in measurement or is it an intrinsic concept in quantum mechanics irrelevant of any measurement? Everyone knows the thought experiment of measuring ...
1
vote
0answers
69 views

How does linearity of a measurement imply that the commutator of all measured observables are $c$-numbers?

I really don't understand with the linearity conditions I have where this comes from.
10
votes
5answers
1k views

Does an electron move from one excitation state to another, or jump?

I'm wondering, when an electron changes state, does it move from one state to another over some (very small) time period? Or does it change from one state to another in no time? If the former, what ...