44 votes
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Practically, how does an 'observer' collapse a wave function?

The other answers here, while technically correct, might not be presented at a level appropriate to your apparent background. When the electron interacts with any other system in such a way that the ...
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  • 735
44 votes
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When will a wave function collapse if the observer was only a camera and the video was watched later in time?

The collapse of the wave function happens whenever the quantum system initially described by the wave function becomes entangled with environment — the part of the Universe that wasn't tracked by the ...
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33 votes

I'm not seeing any measurement/wave function collapse issue in quantum mechanics

The collapse becomes `mysterious' once you realise that: All things, including lab equipment is arguably composed of atoms that should satisfy quantum mechanics It is impossible to design an ...
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29 votes

When will a wave function collapse if the observer was only a camera and the video was watched later in time?

I agree with the answers given by Ruslan and Xcheckr. I would like, however, to caution against a common mistake of confusing of what an observer means in physics and philosophy: in philosophy it ...
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21 votes
Accepted

Why are wavefunction collapses instantaneous?

It’s important to remember that quantum mechanics is a tool that we use to describe the world — it is not the same as the world. For all that we love to talk about wavefunctions, it’s not clear at ...
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20 votes
Accepted

Isn't the detector always measuring, and thus always collapsing the state?

Good question. The textbook formalism in Quantum Mechanics & QFT just doesn't deal with this problem (as well as a few others). It deals with cases where there is a well-defined moment of ...
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  • 4,271
19 votes
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What are the strongest objections to be made against decoherence as an explanation of "collapse?"

I think most arguments in the literature can be boiled down to the point that decoherence does in no way touch the linearity of the Schrödinger equation, and thus cannot make an "or" from an "and". ...
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  • 1,479
19 votes
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How can two electrons repel if it's impossible for free electrons to absorb or emit energy?

It is true that the reactions $$e + \gamma \to e, \quad e \to e + \gamma$$ cannot occur without violating energy or momentum conservation. But that doesn't mean that electrons can't interact with ...
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18 votes

Which basis does the wavefunction collapse to?

The system doesn't "know" anything. The only uncontroversial statement one can make about the (strong) measurement of a quantum system is that you will make the correct predictions if you assume that ...
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  • 108k
18 votes
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Does the collapse of the wave function depend on the observer?

The measurement problem is one of the most relevant open problems of quantum mechanics. What is a measurement? What constitutes an observer and what doesn't? Is the wavefunction a physical object (...
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  • 2,644
17 votes
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Would every particle in the universe not have some form of measurement occurring at any given time?

What you describe is the process known as decoherence: any interaction of a quantum system with its environment (e.g. with photons or other particles passing by, and, yes, most likely interacting ...
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  • 1,972
16 votes

Does the collapse of the wave function happen immediately everywhere?

That isn't really the right question to ask. We never measure wave functions. We measure properties like position, momentum, energy of an electron. Whether the electron is spin up or spin down. The ...
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  • 27.7k
15 votes
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Why is wave-function collapse still being taught in quantum mechanics?

There are many interpretations, and while there are good arguments in favor of one or another, they are currently not distinguished experimentally. Therefore it is often considered prudent to leave ...
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13 votes
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Is a photon always in a state of superposition while traveling through space?

It's tempting to think of the light as a little ball (the photon), and since little balls have a definite position the little ball has to be in a superposition of a state where it goes through one ...
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13 votes

If a wave function collapses into one state, does it ever go back to a superposition of states?

Unless the wavefunction collapses to an eigenstate of the Hamiltonian, the subsequent time-evolution will produce a superposition. The postulates clearly state that, if you measure the observable $\...
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  • 39.5k
13 votes

If a wave function collapses into one state, does it ever go back to a superposition of states?

The way I like to understand this is the following: suppose you have one observable $A$ with spectrum $\sigma(A) = \{ a_n : n \in \mathbb{N}\}$ which we will assume discrete and non-degenerate for ...
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  • 30.1k
13 votes
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Does the collapse of the wave function happen immediately everywhere?

What is a wave function? It is a mathematical function depending on energy and momentum or space and time,$Ψ(p_x,p_y,p_z)$ or $Ψ(x,y,z,t)$ ( in its simple form). This function is a solution of a ...
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  • 223k
13 votes

When will a wave function collapse if the observer was only a camera and the video was watched later in time?

Collapse is not a physical phenomenon! It may be real in a metaphysical sense, but this is a physics website. Collapse is a numerical tool. Here is how we use it: We divide the "universe" ...
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12 votes
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Does $\sigma_x\sigma_p = 0 \cdot \infty$ after a measurement of particle position?

When position is measured, the uncertainty of the resulting delta spike's position is 0 This notion is the root of the problem. Quantum states which are actually eigenstates of the position operator ...
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12 votes

Double slit experiment; evidence of wavefunction collapse

The answer by Craig Gidney is quite adequate for the question, but I want to address the word "collapse" in the title, since search engines will be homing in on it. From webster.com 1: to ...
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12 votes
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Is the recent Nature paper by Minev et al. evidence of new physics?

The trick here in "observing a quantum jump" is that this is not equivalent to doing a strong measurement that "collapses" the wavefunction. The archetypal quantum jump is a two ...
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  • 108k
12 votes

Does the collapse of the wave function happen immediately everywhere?

There is no way to tell if wavefunction collapse is immediately everywhere (whatever that might mean in a relativistic universe), because wavefunction collapse has no observable consequences. The ...
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11 votes

How does a Wavefunction collapse?

I agree in full with Marty Green except the explanations of chemistry in which I was unable to follow so well (that doesn't say that I disagree with them). But, let me put the things in short. The ...
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  • 6,564
11 votes
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Does collapse of wave function to a momentum eigenstate violate speed of light restriction?

There are two answers to this question. The first is that, yes, in non-relativistic quantum mechanics, you can have things going faster than the speed of light, because relativity is never taken into ...
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  • 95.5k
11 votes

Doesn't the Schrödinger's cat inside the box cause the probability wave function to collapse long before a human opens the box?

A theorem of von Neumann says that it doesn't make a bit of difference whether you model the cat (or anything else along the causal chain between closing the box and opening it to observe the cat) as ...
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  • 12.7k
10 votes

Why is wave-function collapse still being taught in quantum mechanics?

So we are taught collapse at school, although there is no experimental evidence of collapse. You don't think this is good. You suggest that we are taught many-worlds instead, although there is no ...
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  • 24.1k
10 votes

Isn't the detector always measuring, and thus always collapsing the state?

No, the detector is not always collapsing the state. When the particle is in an undecayed state its wave function is physically localised with a vanishingly small amplitude in the region of the ...
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  • 19.7k
10 votes

Isn't the detector always measuring, and thus always collapsing the state?

My take on this is that in the original thought experiment, you don't get to monitor the detector. When the detector detects, it kills the cat. But it doesn't tell you then. You only find out when you ...
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