Linked Questions

54
votes
5answers
6k views

Why aren't particles constantly “measured” by the whole universe?

Let's say we are doing the double slit experiment with electrons. We get an interference pattern, and if we put detectors at slits, then we get two piles pattern because we measure electrons' ...
38
votes
4answers
1k views

Why doesn't a typical beam splitter cause a photon to decohere?

In many experiments in quantum mechanics, a single photon is sent to a mirror which it passes through or bounces off with 50% probability, then the same for some more similar mirrors, and at the end ...
13
votes
3answers
573 views

Why doesn't superselection forbid almost every superposition?

A superselection rule is a rule that forbids superposition of quantum states. As stated by Lubos here, one cannot superpose states with different charges because of the conservation of charge: An ...
9
votes
3answers
1k views

When light reflects off a mirror, does the wave function collapse?

This question is specific to the Copenhagen interpretation, which states that the wave function collapses on interaction. If we have a beam of light reflected off a mirror, whether you see this light ...
3
votes
3answers
173 views

How do photons interact with a very fine edge?

Suppose you have some material which has a low reflectance and is opaque to an incomming photon when the angle of incidence is small. Now take that material and make a narrow (20:1 width:length ratio ...
4
votes
2answers
124 views

Are superpositions contagious?

Does quantum mechanics really predict that a particle prepared in a state of superposition of spin will result, after being measured by an appropriate instrument (Stern-Gerlach device), in a ...
4
votes
6answers
160 views

How do “many worlds” theorists explain particle double-slit interference patterns?

In the past couple of weeks I’ve re-watched Sidney Coleman’s wonderful 1994 lecture “Quantum Mechanics” in Your Face” and Sean Carroll’s 2019 Google talk “Something Deeply Hidden: Quantum Worlds & ...
0
votes
2answers
298 views

Is an atom's photon recoil on absorption affected by a beam splitter?

Consider a photon with momentum p approaching a half-reflective mirror. After encountering the mirror it is in a superposition of momentum +p and -p and the mirror (we presume) is in a superposition ...
6
votes
1answer
214 views

When does Quantum Mechanics Measurement lead to Mixed State

Consider the context of the Stern-Gerlach experiment. As is stated on numerous sources (e.g. Feynman Lectures, MIT Lecture), a silver atom in the state of $\vert+z\rangle$ that is put through a Stern-...
1
vote
3answers
401 views

Meaning of phase relationship for a superposition of states

I have studied an introductory course in quantum mechanics, and yet I still do not understand the significance of a phase difference between quantum states that a system is in a superposition of. In ...
5
votes
1answer
303 views

How does a reflected photon keep its entanglement [duplicate]

I'm curious how a photon from an entangled pair, travelling through say a delayed choice quantum eraser experiment, can bounce around between mirrors but remain entangled with its partner photon, when ...
3
votes
2answers
140 views

Why can’t macroscopic objects like apples be isolated but a photon can?

I read that the reason that macroscopic superpositions decohere is because it’s difficult/impossible to isolate macroscopic objects from the environment. Why is this so difficult/impossible and why ...
3
votes
2answers
138 views

In the double-slit experiment, why don't slits destroy interference pattern?

In the double-slit experiment with electrons, if a detector is placed in one of the paths to show where the electron is passing, the wave nature of the particle is lost and you have no interference. ...
-2
votes
1answer
109 views

Double slit experiment: why does the barrier between the slits not collapse the wave function?

During the double slit experiment the electron passes through both slits, how come the material between the two slits does not collapse the wave function and we get an interference pattern? On the ...
0
votes
2answers
96 views

The wavefunction's knowledge of its surroundings

This seems to me it must be trivial, but I have not been able to grasp it. As I understand it, the wavefunction crucially depends on its immediate surroundings, whether it be a nucleus, a box, etc. ...

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