38
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How can Schrödinger's cat be both dead and alive?
Before reading this answer (and to those who are downvoting), I am addressing if the cat is both alive and dead. I don't think the question is asking for a complete explanation of the Schrodinger's ...
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36
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Extension of Schrödinger's cat thought experiment
This is is known as the Wigner's friend thought experiment. According to the many World's interpretation, the superpositions are not a problem. The whole universe ends up in a superposition where all ...
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34
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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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32
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Extension of Schrödinger's cat thought experiment
In a bubble chamber experiment, film was the detecting medium, but film was taken automatically, by the thousands of frames. These bobbins of film went to the various laboratories involved in the ...
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30
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Why aren't particles constantly "measured" by the whole universe?
Seems like the whole universe is receiving information about the electron's position.
Yes, the influence that an electron exerts on the rest of the universe does depend on the location of the ...
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28
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Uncertainty in Uncertainty?
The objects on the l.h.s. of the position-momentum uncertainty relation
$$ \Delta x \Delta p \geq \frac{\hbar}{2}$$
are standard deviations of quantum mechanical operators, defined for any operator $A$...
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26
votes
Accepted
What is the quantum mechanical definition of a measurement?
Until we have an accepted solution of the Measurement Problem there is no definitive definition of quantum measurement, since we don't know exactly what happens at measurement.
In the meanwhile, ...
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26
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Why aren't particles constantly "measured" by the whole universe?
There are time-scales related to interactions, or, equivalently, interaction rates. These interaction rates are often calculated in lowest order based on Fermi’s Golden Rule. An experiment that ...
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20
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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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20
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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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17
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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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Is the Born rule indeed wrong?
As StephenG mentioned in a comment, the paper you're asking about is the subject of a commentary in arXiv:quant-ph/0509130, by Markus Bier; Li and Li attempt a rebuttal of that comment in Appendix C ...
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How can Schrödinger's cat be both dead and alive?
I feel like all the answers here are missing the point.
The cat is not both alive and dead at the same time. That would be, as you put it, ludicrous. The truth is that the cat is in a superposition ...
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Does the particle interfere with itself, or the observer?
The term 'observe' does not mean watching the experiments from a camouflaged hideout so that no one notices you are there. 'Observe' here means 'making a measurement' and hence interacting with the ...
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16
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On a measurement level, is quantum mechanics a deterministic theory or a probability theory?
Is quantum mechanics on a measurement level a deterministic theory or a probability theory?
Probability theory. Evidence: when physicists do quantum measurements they find the results of individual ...
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15
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What is a 'wavicle?'
Imagination has nothing to do with it. Or everything to do with it.
The harsh reality is that electrons are neither particles nor waves. Light is neither particles nor waves. Both electrons and ...
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15
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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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Is uncertainity a postulate?
Your example is probably not a good one to understand Heisenberg uncertainty with, because it mixes two uncertainty phenomena together:
The observer effect (See Wikipedia page of same name);
...
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13
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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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13
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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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13
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What is the quantum mechanical definition of a measurement?
The many-worlds interpretation defines measurement as any physical procedure in which the observer gets entangled with a quantum system. Before the measurement, the universe containing the observer ...
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What problem is the Many-Worlds Interpretation actually solving? Is it a reframing of the measurement problem?
I’ll try to be brief. The Copenhagen interpretation can basically be reduced to two axioms:
Most of the time, quantum states evolve in time according to the Schrodinger equation.
When measurements ...
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Understanding the quantum eraser from a quantum information stand point
The answer is structured as follows:
I will first give the quantum circuit corresponding to a normal double slit (or interferometer),
then the circuit where the which-way information has been ...
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11
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Is it possible to determine the slit a photon went through in the double slit experiment by measuring its flight time?
But knowing the time it took for the photon to go from the source to the observing screen, you can deduct the distance of the photon path and so which slit it passes through
... and indeed such ...
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11
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Measurement in 't Hooft Cellular Automation Interpretation (CAI)
The previous answers are basically correct. In the CA interpretation you use the rules for computing something exactly as in “real” quantum mechanics; you may do exactly the same unitary ...
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What does it mean "not to have a definite trajectory"?
A photon is a name given to a lump in an electromagnetic field that can cause a single electron to change from one energy level to another. The size of the lump in a given region tells you the ...
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10
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Accepted
Does Heisenberg's Uncertainty Principle describe real uncertainty or measured uncertainty?
The Heisenberg Uncertainty principle does not follow from a measurement uncertainty. It arises from the probabilistic nature of quantum mechanics. In quantum mechanics a state describes some sort of ...
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10
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Accepted
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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10
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How do we know that Quantum Mechanics isn't simply a theory of approximations?
We don't. It could well be the case that there is a deeper theory than quantum mechanics which makes all or most of the weirdness go away. There's a lot of people looking for those kinds of theories ...
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