48 votes
Accepted

What is the difference between general measurement and projective measurement?

Note: There is a short summary at the bottom. This is actually also described in Nielsen&Chuang: You don't learn about general measurements, because they are completely equivalent to projective ...
user avatar
  • 14.9k
38 votes
Accepted

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 ...
user avatar
  • 53.8k
36 votes

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 ...
user avatar
  • 9,698
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 ...
user avatar
  • 4,929
32 votes

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 ...
user avatar
  • 223k
30 votes
Accepted

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 ...
user avatar
28 votes

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 $A$...
user avatar
  • 108k
26 votes
Accepted

Uncertainty principle and measurement

There are many steps: Step 1, select a state $\Psi$. Step 2, prepare many systems in same state $\Psi$ Step 3, select two operators A and B Step 4a, for some of the systems prepared in state $\Psi$...
user avatar
  • 24.7k
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, ...
user avatar
25 votes

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 ...
user avatar
  • 2,079
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 ...
user avatar
  • 4,281
19 votes
Accepted

Simple example showing why measurement & interaction are different

This is not a settled question. Just as it is still debated whether or not there is wavefunction collapse, so is it debated what exactly we should understand by a measurement. In the following, we ...
user avatar
  • 108k
19 votes

Entanglement, real or just math?

Entanglement is a real property that can be shown by the violation of the Bell inequalities. How this is commonly done is that a pair of particles are created with entangled spin states in a ...
user avatar
  • 231
19 votes
Accepted

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". ...
user avatar
  • 1,479
17 votes
Accepted

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 ...
user avatar
  • 1,972
17 votes

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 ...
user avatar
17 votes

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 ...
user avatar
  • 271
17 votes

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 ...
user avatar
  • 23k
15 votes

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 ...
user avatar
  • 2,070
15 votes

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 ...
user avatar
  • 43.1k
15 votes
Accepted

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 ...
user avatar
  • 7,120
14 votes

Can I steal your electron?

Well, the wave function of the electron in the ground state of a hydrogen atom (and very similarly in other atoms) behaves like $$ R(r) \sim \exp(-r / a) $$ where $a$ is the Bohr radius, effectively ...
user avatar
14 votes
Accepted

Entanglement and simultaneity

It doesn't really matter, because the phrase "simultaneously affects the other particle" is misleading. Let's suppose you have a pair of totally anticorrelated photons. You measure one of them, then ...
user avatar
  • 14.9k
13 votes
Accepted

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 ...
user avatar
13 votes
Accepted

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); ...
user avatar
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 $\...
user avatar
  • 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 ...
user avatar
  • 30.1k
13 votes

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 ...
user avatar
12 votes
Accepted

Double slit experiment with animals as observers

The claim that during the experiment they let the detector on but did not stored the data so it showed waves, only when they stored the data it showed as particle. is inaccurate and inconsistent ...
user avatar

Only top scored, non community-wiki answers of a minimum length are eligible