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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871 views

Are many-worlds and the multiverse really the same thing?

Are many-worlds and the multiverse really the same thing? Not too long ago, Susskind and Bousso uploaded the article "The Multiverse Interpretation of Quantum Mechanics" with the thesis that the ...
6
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2answers
142 views

Can an observer be the observed?

As a supplement to this question as to whether particles can be observers, supposing that the answer is yes. One could suppose a setup where particle A is observing particle B, but what to stop us ...
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5answers
529 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 ...
6
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2answers
202 views

Bell's Theorem graph

My friends and I got into an argument about determinism, and I brought up that quantum events are random. But I couldn't prove it. I found the Wikipedia page on Bell's theorem, which seems to imply ...
6
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1answer
383 views

Residues in QFT propagator

It is a well known fact that the location of the pole of a propagator (in QFT) can be interpreted as the physical mass. Is there an interpretation for the residue of the propagator? Note: I´m ...
6
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3answers
318 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
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1answer
165 views

If a fundamental theory exibits e.g. a mirror symmetry, in what sense it the underlying geometry real?

Are the more recently discovered symmetries in string theory such that the theories based on mirroring geometries are absolutely the same from an observable point of view? I have mirror symmetry ...
6
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3answers
207 views

How can the quantum state of the universe decohere

Decoherence explains how a classical state appears once quantum information in a quantum state leaks out. But presumably that environment has its own quantum state which then leaks out to a larger ...
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5answers
2k views

Connection between quantum physics and consciousness

Can someone explain the quantum physics-consciousness connection? In the double slit or quantum eraser experiments, the system behaves as a whole, with some apparent time independent traits. Invoking ...
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2answers
426 views

Why is quantum mechanics based on probability theory? [duplicate]

What makes us formulate quantum mechanics based on probability theory? Isn't the real quantum world based on unknown laws to us? Is it possible that results of an experiment will be measurable in ...
5
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3answers
370 views

Could quantum mechanics work without the Born rule?

Slightly inspired by this question about the historical origins of the Born rule, I wondered whether quantum mechanics could still work without the Born rule. I realize it's one of the most ...
5
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3answers
442 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
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1answer
726 views

What is the meaning of the Fourier transform of Feynman propagator?

I know $K(a,b,t)$ is the probability amplitude of find a particle that starts at point a in b in a time t later. There is also an expression that sometimes is called green function: ...
5
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2answers
146 views

Basic Interpretation of Compostion of Observables and their Measurement

Given two (or more) observables $A, B$ which commute one can construct a third observable $C= A \circ B$. If $\psi$ is a common eigenvector of $A, B$ with eigenvalues $\lambda_1, \lambda_2$ then it is ...
5
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1answer
483 views

't Hooft for laypersons

I have looked at some of 't Hooft's recent papers and, unfortunately, they are well beyond my current level of comprehension. The same holds for the discussions that took place on this website. (See, ...
5
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2answers
712 views

Does the Hilbert space of the universe have to be infinite dimensional to make sense of quantum mechanics?

Does the Hilbert space of the universe have to be infinite dimensional to make sense of quantum mechanics? Otherwise, decoherence can never become exact. Does interpreting quantum mechanics require ...
5
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4answers
654 views

Quantum Physics and the Law of Large Numbers

On page 1 of this recent paper by Bousso and Susskind we read. This question is not about philosophy. Without a precise form of decoherence, one cannot claim that anything really "happened", ...
5
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1answer
620 views

Bohmian loophole in PBR-like theorems

I'm reviving and expanding this question, because of the new paper today, by Aaronson et al. The more general question is: How does quantum-potential Bohmian mechanics relate to no-go theorems for ...
5
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5answers
300 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 ...
5
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1answer
275 views

Uniqueness of quasiclassical consistent histories

The current zeitgeist here is on interpretations of quantum mechanics, so let me add my own two cents here. As you may know, consistent histories is an alternative interpretation proposed in a series ...
5
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2answers
413 views

What happens when particle-antiparticle pairs annihilate in MWI?

The many-worlds interpretation of quantum physics is built around a configuration space, where the position of a particle is three components of the position of that universe. What happens with ...
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0answers
116 views

Is it possible to reproduce the energy spectrum of quantum chaos using classical cellular automata?

Is it possible to reproduce the energy spectrum of quantum chaos using classical cellular automata? It's hardly impressive to reproduce harmonic oscillators.
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0answers
140 views

Does it make sense to speak of amplitudes of finite closed boundaries in QFT?

A example of amplitude in Relativistic Quantum Mechanics or specifically in QFT is the amplitude of a field configuration on a space-like hyper-surface of space-time to "lead" to another field ...
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3answers
347 views

Many-worlds: Where does the energy come from?

With regard to the theory that each time a wave function collapses the universe splits so that each possible outcome really exists - where does all the energy required to create all the new universes ...
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3answers
1k views

Canonical Commutation Relations

Is it logically sound to accept the canonical commutation relation (CCR) $$[x,p]~=~i\hbar$$ as a postulate of quantum mechanics? Or is it more correct to derive it given some form for $p$ in the ...
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4answers
669 views

Retrocausal interpretations of quantum mechanics

Can retrocausality resolve the paradoxes of quantum mechanics? The Copenhagen interpretation presumes something has no property until it is measured, but retrocausal interpretations explain that away ...
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3answers
225 views

What's wrong with this faster-than-light gedankenexperiment?

It is common wisdom - and mathematically proven - that quantum entanglement cannot be used to bypass the relativistic speed limit and transfer information faster than light. So there must be something ...
4
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2answers
485 views

Shor's algorithm and Bohmian Mechanics

Do quantum computer's tell us anything about the foundations of quantum theory? In particular Shor argued in the famous thread by 't Hooft Why do people categorically dismiss some simple quantum ...
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3answers
611 views

Assumptions in Bell's Theorem

It is often Stated that Bell's Theorem is equivalent to the statement: No theory of Local Hidden Variables can reproduce all of the predictions of quantum mechanics. I see nowhere in Bell's Theorem ...
4
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4answers
557 views

Is the wave function objective or subjective?

Here is a question I am curious about. Is the wave function objective or subjective, or is such a question meaningless? Conventionally, subjectivity is as follows: if a quantity is subjective then ...
4
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3answers
754 views

Is contextuality required in quantum mechanics?

I still don't really understand what contextuality means in reference to quantum mechanics. If someone could give a clear definition that would be great. It sounds like it means you can't always ...
4
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2answers
121 views

Books about alternative interpretations of quantum mechanics

Recently I've heard that there exist alternative interpretations of quantum mechanics which, while not as widespread as the Copenhagen intepretation (or so it would seem), are equally valid in the ...
4
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1answer
195 views

Temporal part of Quantum Wavefunction

I was hoping that someone could give me the more fundamental reason that we take as the temporal part of a quantum wavefunction the function $e^{-i\omega t}$ and not $e^{+i\omega t}$? Clearly ...
4
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2answers
206 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 ...
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1answer
70 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 ...
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1answer
143 views

Motivation for Wigner Phase Space Distribution

Most sources say that Wigner distribution acts like a joint phase-space distribution in quantum mechanics and this is justified by the formula ...
4
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1answer
160 views

Quantum Mechanics - Hidden Variables

In Steven Weinberg's Lecture on Quantum Mechanics (p. 342), he writes: The correlation between the spins of the two particles can be expressed as the average value of the product of the ...
4
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2answers
412 views

Many-worlds: how often is the split how many are the universes? (And how do you model this mathematically.)

When I read descriptions of the many-worlds interpretation of quantum mechanics, they say things like "every possible outcome of every event defines or exists in its own history or world", but is this ...
4
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7answers
722 views

Which physically acceptable quantum interpretations do not require the existence of any observer at all?

Some interpretations of quantum mechanics — like the Copenhagen interpretation in particular — require the existence of an observer. The role of the observer is a bit mysterious. After ...
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2answers
2k views

What is the meaning of Wheeler's delayed choice experiment?

Wheeler's delayed choice experiment is a variant of the classic double slit experiment for photons in which the detecting screen may or may not be removed after the photons had passed through the ...
4
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1answer
65 views

Is frequency or Bayesian interpretation used in quantum mechanics?

In quantum mechanics, we discussed about probability. There are two kinds of interpretations: frequency and Bayesian. Which one is actually used in quantum mechanics? My impression is, it doesn't ...
4
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1answer
169 views

Event-by-Event Simulation of Quantum Phenomena

I just recently stumbled over http://rugth30.phys.rug.nl/dlm/ and http://www.sbfisica.org.br/bjp/files/v38_26.pdf As the title suggests these are presentations of mathematical models capable of ...
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0answers
78 views

Free Will Theorem question

The Kochen-Specker Theorem says, if I understand it correctly, that the results of spin measurements cannot be predetermined independent of measurement. They get to this conclusion by describing 33 ...
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3answers
717 views

Is it possible that QM is just GR?

The more I learn about General Relativity, the more it seems like it isn't fully understood. It seems that before it's full consequences were exhaustively understood, not 10 years after its discovery, ...
3
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3answers
453 views

If the multiverse and many-worlds don't exist, how should we interpret probabilities?

If the multiverse of eternal inflation and the many-worlds of quantum mechanics don't exist, how should we interpret the meaning of probabilities? If there is only one copy of the universe out there, ...
3
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3answers
923 views

Can superdeterminism resolve contextuality, entanglement and Shor's algorithm in quantum mechanics?

Superdeterminism is the idea that the apparent freedom for the choice of experimental apparatuses and their settings are nothing but an illusion. Contextuality is the dependence of the properties of a ...
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1answer
245 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
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1answer
631 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
585 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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4answers
466 views

How do we show that no hidden variable theories can replace QM?

I've always hit two big stumbling blocks in conceiving of the proof or disproof of hidden variable theories as being even valid idea, let alone an answerable question... I feel I must be ...