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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48
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5answers
22k views

Is the universe fundamentally deterministic?

I'm not sure if this is the right place to ask this question. I realise that this maybe a borderline philosophical question at this point in time, therefore feel free to close this question if you ...
22
votes
7answers
4k views

Why do people still talk about bohmian mechanics/hidden variables [closed]

I was reading the Feynman lectures in physics and after thinking about it for a while it seems particularly unreasonable to talk about hidden variables. Let us say that the electron has some internal ...
10
votes
6answers
2k 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 ...
11
votes
5answers
2k views

Consequences of the new theorem in QM?

It seems there is a new theorem that changes the rules of the game in the interpretational debate on QM: http://www.nature.com/news/quantum-theorem-shakes-foundations-1.9392 Does this only leave ...
7
votes
4answers
2k views

On the nature of the collapse of the wave function

The collapse of the wave function by measurements is one of the most mysterious properties of quantum mechanics. At what scale does the wave function collapse? What are the conditions for a ...
8
votes
2answers
444 views

Should it be obvious that independent quantum states are composed by taking the tensor product?

My text introduces multi-quibt quantum states with the example of a state that can be "factored" into two (non-entangled) substates. It then goes on to suggest that it should be obvious1 that the ...
6
votes
2answers
327 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 ...
2
votes
2answers
918 views

EPR-type experiments and faster-than-light communication using interference effects as signaling mechanism

I understand that faster-than-light communication is impossible when making single measurements, because the outcome of each measurement is random. However, shouldn't measurement on one side collapse ...
2
votes
5answers
396 views

Wave/particle duality

Apologies if this has been asked before (I did check and I believe it wasn't). I have a question about the particle/wave duality of photons (or other particles). Depending on what and how we measure ...
18
votes
6answers
769 views

Simple example showing why measurement & interaction are different

Does someone know of a clear (pedagogical) example where one can really see(with the math) where interaction and measurement are not synonymous in quantum mechanics? I know that every measurement ...
9
votes
6answers
1k views

Will Determinism be ever possible?

What are the main problems that we need to solve to prove Laplace's determinism correct and overcome the Uncertainty principle?
7
votes
5answers
590 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 ...
21
votes
5answers
4k views

Why do people rule out local hidden variables?

I bet the automatic response to my question would be "Bell's theorem" and of course I am not disputing Bell's proof. I am however uncertain of one of his assumptions. The so called "no conspiracy" ...
15
votes
5answers
1k views

What combinations of realism, non-locality, and contextuality are ruled out in quantum theory?

Bell's inequality theorem, along with experimental evidence, shows that we cannot have both realism and locality. While I don't fully understand it, Leggett's inequality takes this a step further and ...
14
votes
5answers
2k views

The transactional interpretation of quantum mechanics

John Cramer’s transactional interpretation of quantum mechanics (TIQM) is billed as resolving the fuzzy agnosticism of the Copenhagen interpretation while avoiding the alleged ontological excesses of ...
6
votes
2answers
516 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 ...
5
votes
2answers
923 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 ...
3
votes
0answers
156 views

Testing Many-Worlds Interpretation (MWI) with a causality-violating configuration of “superluminal cables”

Suppose we managed to arrange a causality-violating transmission of data with hypothetical “superluminal cables” (SLC; see both links for respective descriptions) and expect, similarly to ideas ...
0
votes
2answers
3k views

What is wrong with the De Broglie–Bohm theory a.k.a “Causal Interpretation” of quantum theory? [duplicate]

Possible Duplicate: Why do people still talk about bohmian mechanics/hidden variables I've heard of De Broglie–Bohm theory a.k.a causal interpretation of quantum theory. The predictions ...
2
votes
3answers
240 views

What happens with a tunneling particle when its momentum is imaginary in QM?

In classical mechanics the motion of a particle is bounded if it is trapped in a potential well. In quantum mechanics this is no longer the case and there is a non zero probability of the particle to ...
14
votes
13answers
3k views

Arguments for and against Many Worlds?

I would like to hear the best arguments for and against the Many Worlds interpretation of QM.
10
votes
4answers
817 views

Can quantum mechanics really be the same as underlying deterministic theory?

I am perplexed by recent papers by 't Hooft giving an explicit construction for an underlying deterministic theory based on integers that is indistinguishable from quantum mechanics at experimentally ...
5
votes
5answers
691 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 ...
15
votes
3answers
487 views

Classical and quantum probabilities in density matrices

In textbooks, it is sometimes written that a mixed state can be represented as mixture of $N$ (I assume here $N<+\infty$) quantum pure states $|\psi_i\rangle$ with classical probabilities $p_i$: ...
8
votes
3answers
1k views

How does Bell's theorem rule out the possibility of local hidden variables?

It seems to be common consensus that the world is non-deterministic and this is proved by Bell's theorem. But even though Bell's experiments proved that the theory of quantum mechanics work, How does ...
7
votes
6answers
942 views

Maxim Raykin's “solution to the measurement problem” using infinitely many derivatives

Recently I was made aware of the following arXiv preprint by Maxim Raykin: Analytical Quantum Dynamics in Infinite Phase Space. As far as I understand it, Raykin's idea is to reinterpret quantum ...
10
votes
3answers
859 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 ...
8
votes
8answers
734 views

Given entanglement, why is it permissible to consider the quantum state of subsystems?

Quantum entanglement is the norm, is it not? All that exists in reality is the wave function of the whole universe, true? So how come we can blithely talk about the quantum state of subsystems if ...
6
votes
6answers
985 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 ...
5
votes
3answers
496 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
votes
5answers
336 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, ...
2
votes
1answer
83 views

Do electrons have a location before they are observed/measured? [closed]

Is this in all interpretations of QM? What about interpretations that are realist (MWI, penrose, ect)?
10
votes
4answers
523 views

What is an interpretation of quantum mechanics?

In the sense of "Copenhagen Interpretation", what exactly is an interpretation? What purpose does an interpretation serve? Can an interpretation be tested or even be correct or incorrect independent ...
4
votes
3answers
648 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 ...
3
votes
2answers
249 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 ...
3
votes
3answers
1k 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 ...
0
votes
3answers
118 views

EPR Paradox resolution: the spin is fixed at creation but its measurement isn't?

The Wikipedia article on the EPR paradox uses the example of an electron and positron created from a common source, each moving in an opposite direction to the other. Detector A is used to measure the ...
1
vote
2answers
162 views

Non-locality and Bell's theory

Non-Locality – (just ) one more question? I have read comments that Bell’s theory proves quantum mechanics is non-local, and also comments that it does not. I have read a comment by a very eminent ...
0
votes
1answer
147 views

Uncertainty and wave-trains

My textbook and the following extract from feynman's lectures present the same idea regarding wavetrains and uncertainty in their wavelengths. Why is it that a wavetrain confined to some space has an ...
0
votes
1answer
173 views

The System and the Measuring Gadget

In Quantum Mechanics the value of an observable results from the interaction between the "system" with the "Measuring gadget". But when the experimenter[or the technologist concerned] is ...
81
votes
3answers
14k views

How come some people are claiming that the Big Bang never happened?

A news story is going viral on social media networks claiming that two physicists have found a way to eliminate the Big Bang singularity, or in layman's terms (as claimed by many sensationalist news ...
21
votes
4answers
506 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 ...
11
votes
3answers
858 views

Why am I wrong about how to view gauge theory?

Edit: I know there have been some similar questions but I don't think any had quite articulated my particular confusion. If gauge symmetries are really just redundancies in our description accounting ...
9
votes
2answers
2k views

Is the “consistent histories” interpretation of QM a “many worlds interpretation” in disguise?

The so called consistent histories interpretation is claimed to be a correction of the Copenhagen Interpretation. One of its aim, as much as I can see is to show that observers don't have any special ...
14
votes
6answers
2k views

Is Shor's algorithm a demonstration of the many worlds interpretation?

David Deutsch is very fond of pointing out Shor's integer factorization algorithm is a demonstration of the many worlds interpretation. As he often asked, where else did all the exponentially many ...
11
votes
3answers
455 views

Is it possible to define a “it went through two slits” observable?

This concerns the famous two-slit experiment. Electrons or photons or your favorite particle, doesn't matter. As we all know, the attempt to detect which slit the quanta pass through leads to loss ...
4
votes
1answer
159 views

Does Hardy's paradox represent a proof against Bohm's interpretation of the quantum mechanics?

This is a thought-experiment, see "Quantum Mechanics, Local Realistic Theories, and Lorentz-Invariant Realistic Theories", Phys. Rev. Lett., Vol. 68, No. 20, page 2981, year 1992, that rules out local ...
13
votes
3answers
1k views

How much is quantum computation changing the interpretation of quantum theory, and, if at all, how?

At the beginning of quantum computation, David Deutsch made a strong claim that the Many Worlds interpretation of quantum theory was at the foundation of his ability to do what he did. There was a lot ...
8
votes
3answers
880 views

Determinism, classical probabilities, and/or quantum mechanics?

[I]f you want a universe with certain very generic properties, you seem forced to one of three choices: (1) determinism, (2) classical probabilities, or (3) quantum mechanics. [My emphasis.] ...
6
votes
5answers
3k 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 ...