This tag is for questions relating to what, if anything, the quantum mechanical formalism and experimental results say about the way the world works.

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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
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1answer
653 views

Does this new quantum experiment rule out the possibility of a many-worlds interpretation?

This brand new published result (nature): Experimental non-classicality of an indivisible quantum system by Radek Lapkiewicz, Peizhe Li, Christoph Schaeff, Nathan K. Langford, Sven ...
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5answers
611 views

Schrodinger's cat experiment

What's wrong in taking the cat as an observer in Schrodinger's experiment? Plz kindly elaborate! And if possible also describe about possible logics if the question bears the answer No.
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2answers
2k views

How should I interpret the expectation value $\langle x p\rangle$ in quantum mechanics?

$xp$ is not a hermitian operator and hence doesn't represent an observable. Then, how can we interpret the expression $$ \langle x p \rangle \text{,} $$ the expectation value of position times ...
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6answers
446 views

Is quantum entanglement an objective or subjective property?

Imagine the following gedankenexperiment. Observer Alice is right here on Earth. Observer Bob is at say Alpha Centauri. A pair of maximally entangled qubits is formed with one qubit handed over to ...
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1answer
125 views

(thought) experiment re: Bell's Theorem and Schrodinger's cat

I apologize if this question is naive. I am wondering about what would happen with the following experiment. Start with a standard Bell's Theorem setup: We have two quibits entangled in a particular ...
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2answers
239 views

Are probability-preserving variations of QT with respect to the Born rule mathematically possible?

Is it possible to create (m)any theoretically workable framework(s) - that do(es) produce probabilities - by taking QM and replacing the Born(-like) rule(s) with something that is not equivalent to it ...
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2answers
519 views

What is the physical interpretation of the density matrix in a double continuous basis $|\alpha\rangle$, $|\beta\rangle$?

(a) Any textbook gives the interpretation of the density matrix in a single continuous basis $|\alpha\rangle$: The diagonal elements $\rho(\alpha, \alpha) = \langle \alpha |\hat{\rho}| \alpha ...
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2answers
218 views

Can the Heisenberg interpretation or path integrals apply to open quantum systems?

It has been claimed by some people that Schrödinger's picture is more misleading compared to the Heisenberg principle or path integrals, and that we would be better off abandoning the ...
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3answers
589 views

Wigner's friend and intersubjectivity in quantum mechanics

Suppose there's a radioactive material and a 1/2 quantum probability of detecting it by a Geiger counter. This puts the system in a superposition. Also suppose you are in the same room, and the walls ...
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1answer
92 views

Is there an absolute minimum scale to the universe? If so, why?

Based on my rather circumscribed understanding of modern physics, one of the key insights of quantum mechanics over previous scientific theories is the prediction that there exists an absolute limit ...
3
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2answers
291 views

Determinism loophole?

I was thinking about the question I posted yesterday, and I thought of a better way to ask it. I'm trying to figure out why QM necessitates "pure randomness". Assume you have a photon that has a ...
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2answers
351 views

Weak measurements rule out Many Worlds Interpretation?

I came across a paper that claims to prove that the Many Worlds interpretation is invalid by applying weak measurements. The paper can be found here: ...
3
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2answers
234 views

Interpretation of $e|\psi|^2$ as electron density

In solid state physics the electron density is often equated to $e|\psi|^2$. However, the Sakurai says (Chapter 2.4, Interpretation of the Wave Function, p. 101) that adopting such a view leads "to ...
3
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1answer
212 views

Question about Wigner's friend

The Wigner's friend thought experiment can be used to understand non-realism in quantum mechanics. For anyone not familiar, the thought experiment involves two researchers observing an experiment at ...
3
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1answer
97 views

Does quantum fingerprinting really argue for the exponential size of wavefunctions?

Does quantum fingerprinting really argue for the exponential size of wavefunctions? Quantum fingerprinting is the idea that an exponentially long classical string can be encoded in a linear number of ...
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0answers
88 views

Are frequency and likelihood the same across the multiverse?

My probability text distinguishes between two interpretations of probability values: the frequency of occurrence "as percentage of success in a moderately large number of similar situations" (coin ...
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3answers
356 views

Are quantum mechanics and determinism actually irreconcilable? [closed]

As a preface, I am not a physicist. I'm simply interested in abstract physics and fundamental principles of the universe and such. As such, if you can provide an answer for the layman (as ...
3
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3answers
275 views

How would a realist interpretation of the Mermin-Peres square look like?

How would a realist interpretation of the Mermin-Peres square with counterfactual definiteness and the existence of states prior to measurements look like?
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0answers
335 views

Does the extended probability ensemble interpretation of quantum mechanics make any sense?

Gell-Mann and Hartle came up with the extended probability ensemble interpretation over here. Basically, they extended probability theory to include real numbers which may be negative or greater than ...
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2answers
120 views

What's wrong with this experiment showing that either FTL communication is possible or complementarity doesn't hold?

The assumptions are: Alice and Bob have perfectly synchronized clocks Alice and Bob have successfully exchanged a pair of entangled photons The idea is simply to have Alice and Bob perform the ...
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6answers
474 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 ...
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3answers
210 views

Are there any QM effects where charged particles are not intimately involved?

Are there any QM effects that have been/could be measured from interactions involving non-charged particles? Elementary QM is all about the electron energy levels in the atom, photon - atom ...
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3answers
251 views

What's the worst thing about the tomographic approach to QM?

I saw a paper on arXiv that referenced this approach to an ontology of QM: Phys.Lett. A213 (1996) 1, S. Mancini, V. I. Man'ko, P.Tombesi Symplectic tomography as classical approach to quantum ...
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1answer
704 views

Can silicon droplets bouncing on a vibrating surface be a model for Quantum Mechanics?

In this video on youtube it is claimed that silicon droplets bouncing on a vibrating surface show behaviour in analogy to particle/wave duality in Quantum Mechanics. Is this true? Did they ...
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4answers
274 views

How can new interpretations of QM help?

There is some current work on interpretations of quantum mechanics. How do you think can interesting results in that area help physics? Can it change quantum physics or make it easier? Which ...
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4answers
281 views

Why should the observed probability distributions in quantum mechanics always align with the pointer basis of decoherence?

It has always been claimed decoherence solves the problem of the preferred-basis for observed probability distributions, but why should this be the case? If there is only one world, and there are ...
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4answers
297 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, ...
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2answers
293 views

Interpretation of de Broglie wave

Until what point can the de Broglie wave be thought as a real wave? I mean, is it made of something? What amplitude does it have? Is it a sine wave? How can it be related to the wavefunction of the ...
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4answers
196 views

Why do we consider the evolution (usually in time) of a wave function?

Why do we consider evolution of a wave function and why is the evolution parameter taken as time, in QM. If we look at a simple wave function $\psi(x,t) = e^{kx - \omega t}$, $x$ is a point in ...
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2answers
237 views

Is there a manifestly Lorentz invariant formulation of the many worlds interpretation?

First, I give an absurd example. A conscious observer lives on Earth at time t. A light-year away, at a space-like separation, a nuclear bomb chain reaction goes off. A nuclear bomb chain reaction is ...
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2answers
74 views

Logic of the 'imaginary wave function collapse' argument in Double Slit experiment

My question is in regards to the stance that the 'wave function collapse' is not an actual physical occurrence. That is, you are not, by observation, changing the particles position from a wave to a ...
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2answers
195 views

Was TP Singh right to say that a theory of quantum gravity necessitates the Copenhagen Interpretation?

http://iopscience.iop.org/1742-6596/174/1/012024 In the above link we see TP Singh arguing that only Copenhagen will work for a theory of quantum gravity. Some of his key points are "quantum theory ...
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2answers
208 views

What does the Copenhagen interpretation say about the position of a particle before measurement?

Suppose there is a particle in space. When we measure the position of that particle, we get a particular value with a probability that can be calculated from the wave function. But, according to the ...
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2answers
1k views

What is the difference between parallel universe and multiverse?

What is the difference between parallel universe and multiverse? Is it parallel universe or universes?
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1answer
367 views

M-theory and many-worlds interpretation

I am getting some confusion on whether M-theory accepts many-worlds interpretation. Can anyone show me the reasons or rebuttals for the possibility of the many-worlds interpretation in M-theory? ...
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2answers
155 views

Does quantum randomness predicate an infinite number of realities?

I am a layman when it comes to physics and especially quantum mechanics. I have seen many documentaries on the subject, and often in these productions there is a physicist featured explaining the ...
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1answer
154 views

What is the difference between realism in locality, and counterfactual definiteness?

I understand the EPR-experiment and the Bell inequalities. I can see how dropping 'locality' solves the issue, and how dropping 'realism' solves the issue (e.g. there are really no hidden variables ...
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1answer
106 views

Product of position eigenvectors at different times

I've been thinking about this, and it might sound like a stupid question, but I can't seem to find an answer anywhere, here goes: Whenever we calculate expecation-values between two position ...
2
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1answer
195 views

Many-worlds interpretation vs 'just' randomness?

I have this question about MWI I always wanted to ask but never dared to! It could be that I just don't know enough physics to understand the answer, or the question! Anyway, here goes: What is it ...
2
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1answer
73 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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4answers
355 views

Is there any quantum interpretation which isn't “crazy” at all? [closed]

Is there any quantum interpretation which isn't "crazy" at all? Exponentially many parallel worlds in MWI, superdeterministic conspiracies, and/or nonlocality in hidden variables, idealism and the ...
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4answers
236 views

Are Everettian branchings global or local?

Everett's theory of quantum mechanics is about the wavefunction of the whole universe holistically. If a branching occurs very far away at the Andromeda galaxy, do I also branch? Are branchings global ...
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1answer
80 views

Decoherence in Everett quantum mechanics

Take an initial state and its environment, $E$, as follows, $$ |\psi\rangle_i = |0\rangle |E\rangle + \sqrt{2}|1\rangle |E\rangle. $$ Suppose that I've written it already in the basis in which the ...
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2answers
144 views

Can the quantum eraser experiment result indicate a 'computed universe'?

The quantum eraser experiment tells us that a photon shot at two slits is a wave, unless you measure which slit is taken and you do not destroy the measurement result. I've found this very similar to ...
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1answer
258 views

If there is no collapse of the wave-function does this mean that the many worlds interpretation of QM must be wrong?

If as some people suggest, there is no collapse of the wave function (is there a standard name for this position), then must one rule out the many-worlds interpretation of QM ?
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2answers
483 views

What happens after the collapse of a wavefunction?

If I have a quantum system which I prepare in a certain state, this state then evolves unitarily via a Hamiltonian. Suppose an observer provokes a collapse of the wave function by a certain ...
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5answers
306 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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2answers
161 views

Cardinality of the Universes Set

No expert by any means, but sometimes, in different contexts the term multiverse used. In quantum mechanics, some say that it is possible that there are actually many universes where all the possible ...
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1answer
432 views

What is the spatial mode of light or the spatial mode of a massive particle?

I'm extremely confused by what physicists mean by the spatial mode of light. I am also equally if not more confused by what the spatial mode of a massive particle is. Can anyone help me out by ...