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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 ...
BioPhysicist's user avatar
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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 ...
Count Iblis's user avatar
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34 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 ...
Andrea's user avatar
  • 5,220
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 ...
anna v's user avatar
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31 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 ...
Chiral Anomaly's user avatar
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, ...
Selene Routley's user avatar
26 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 ...
flaudemus's user avatar
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21 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". ...
Luke's user avatar
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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 ...
don't train ai on me's user avatar
18 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 ...
Vincent's user avatar
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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 ...
Luzanne's user avatar
  • 2,052
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 ...
Emilio Pisanty's user avatar
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 ...
SuperCiocia's user avatar
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16 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 ...
isometry's user avatar
  • 2,100
16 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 ...
user1247's user avatar
  • 7,398
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 ...
Cort Ammon's user avatar
  • 50.2k
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 $\...
ZeroTheHero's user avatar
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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 ...
Gold's user avatar
  • 36.4k
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 ...
Zhuoran He's user avatar
13 votes
Accepted

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 ...
sasquires's user avatar
  • 340
12 votes

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 ...
G. 't Hooft's user avatar
  • 5,421
11 votes
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 ...
knzhou's user avatar
  • 103k
11 votes

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 ...
Emilio Pisanty's user avatar
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 ...
akhmeteli's user avatar
  • 27.1k
10 votes

How can Schrödinger's cat be both dead and alive?

Forgive the length. I find Schrödinger's cat is much easier to make sense of as a journey through QM, rather than just a few equations that someone says "solves your problems." Schrödinger's cat was ...
Cort Ammon's user avatar
  • 50.2k
10 votes

How can Schrödinger's cat be both dead and alive?

Schrodinger's cat is an exercise in seeing how nonsensical the Copenhagen interpretation is, so answers that attempt to clarify it in terms of CI are not very helpful, in my opinion. As a framework ...
R.. GitHub STOP HELPING ICE's user avatar
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 ...
Marco Ocram's user avatar
  • 26.4k
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 ...
J Thomas's user avatar
  • 2,988
10 votes

Does the particle interfere with itself, or the observer?

Neither the particle not the observer "interferes with itself" . The parts of the wave function passing through each slit interfere. The resulting interference pattern gives the probability ...
my2cts's user avatar
  • 25.3k

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