# Tag Info

5

I don't intend to repeat the points already made in the other answers, consider this as a small addition to those, with the intention to give a more practical description (reminding some of the basic ideas) without getting into observer-philosophies. Quite clearly the act of observing, i.e. measuring a quantum system can be done via many different ...

4

When does an interaction drop the system into an eigenstate? (i.e. when is a measurement=) This is an ill-posed question because, first of all, the system $S$ doesn't drop into any state but each observer $O$ has a state about it, as a state $\rho$ is nothing but the coding of past measurements (so it should be named with reference to being dependent on ...

0

Yes. Bohmian mechanics is superdeterministic. You see, the pilot wave and the "particle" covers everything in the universe, including the experimentalist and the measuring apparatus. The location of the "particle" fixes the experimentalist's will and the detector settings superdeterministically. Neither the experimentalist nor the detector has any free will ...

0

According to the ER=EPR relation, both the system and its double would be connected by an Einstein-Rosen bridge provided the relative phases are chosen just right. The preferred basis would be the preferred basis for the ER bridge, whatever it might be. But it would only be the preferred basis for any suicidal observer who jumps into the bridge, and not any ...

0

Quantum optics demonstrates the existence of interaction-free measurements: the detection of objects without light—or anything else—ever hitting them. Paul Kwiat, Harald Weinfurter and Anton Zeilinger SciAm November 1996 http://www.arturekert.org/sandvox/quantum-seeing-in-the-dark.pdf

10

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 will go through the ideas behind the von Neumann measurement scheme, which is one way to try and talk about measurement in quantum mechanics. An interaction ...

5

All measurements are based on interactions , but not all interactions are measurable. The "set of measurements" is a subset of the "set of interactions". The simplest mathematical way to see this is by Feynman diagrams. Feynman diagrams have one to one correspondence with integrals, and when they describe a measurement, the crossection or lifetime can be ...

1

If you ask whether something went to better since the Copenhagen Interpretation, the answer is discouraging. 1) The collapse remained, nobody found a mechanism that would explain single experiments. I mean, when one prepares many quantum particles in some state in which a certain variable takes values from a given set, we don't know until today why the ...

2

I see two possible answers depending on what you mean by "idealist rather than realist". You suggest an idealism conceived as an epistemological view, where states of a system are relative to an observer or another system. There are such interpretations with several names, I wonder why, as they seem to be the same : Relational quantum mechanics ; quantum ...

1

This answer is not about QM but about a classical system that simulates many aspects of QM very well, and might be of some ineterestto you. There are experiments using oil dropplets that bounce up and down a vibrating fluid, the horizontal motion of these droplets is due to bouncing on the waves that move on the fluid's surface, which in turn are influenced ...

1

I read about Wheelers choice experiment from Wikipedia, not from some article. There is no end to trials as that of Wheelers. What is wrong with his attempt is that the photon is a WAVE. And as any wave, it takes both paths through the interferometer. I saw in Wikipedia the statement "since in the first case the photon is said to "decide" to travel as a ...

10

It's not, rather your assumption that it's always impossible to know which slit a photon went through is incorrect. It's only impossible to know which slit it went through in an experiment where you get a double-slit interference pattern--if you wish you can set up an experiment to find out which slit the photon goes through, but the result will then be no ...

1

This alleged problem falls apart as soon as you do a rigorous analysis. It should be clear that with such random accidents there is no causation. If event A really causes event B, then that's reflected in the state of the system. You'll have a state of the form 1/sqrt(1+|u|^2)[|A B> + u |not(A) not(B)>], so an entangled state containing information about the ...

0

Many-world interpretation can be consistent with the second law of thermodynamics. It is not a problem. Now, about this hypothetical time travel: If you went back in time to tell Shakespeare about his work and he published it after, it looks consistent but it is ultimately a paradox because where does that knowledge come from? Information, or strictly ...

0

Your suggested explanation is that the electron and positron have opposite but definite spins along an axis, but we can only use probabilities when predicting the outcome of measuring it So the idea is that the particles have a single value of spin of the sort that you see when you do a measurement. What happens in the experiment is that reglardless ...

0

Take two balls from different color and put them into a box with two traps. In the period between the balls are trapped and one open the box there is a mixed state for each trap with 50% probability that in the trap is the white ball or the black ball. This state collapses in the moment one open the box. Does that mean that until one open the box, the balls ...

1

Opposite spins yes, definite no. They are in a superposition of states. In one state, particle A is up while particle B is down, while in the other state, particle A is down and particle B is up. When the pair is created, they are in an entangled state, and only after measurement do they assume definite outcomes. The crux of the matter is that this ...

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