4 ms·

you have it wrong, QM is the only physical theory that has randomness as an inherent part, as compared to e.g thermodynamics where randomness is due to lack of

you have it wrong, QM is the only physical theory that has randomness as an inherent part, as compared to e.g thermodynamics where randomness is due to lack of information. It is proven (see Bell inequalities) that randomness in QM isn't due to lack of information. You can't just wave away the collapse mechanism, what do you make of the double alit experiment? isn't the target "real" enough?

konschubert 6y ago
The problem with the double slit experiment is that the target isn't treated as a quantum system. That's why the wave function "collapses": Because it collides with the non-quantum target. It's a useful approximation, but of course in reality, there is no such thing as a non-quantum thing. And if you evolve the target's wave function with the wave function of the particle, then there is no stochastic collapse.
- choeger 6y ago
  What you are saying is that whenever we, the observer, leave the QM model we use the collapse as a computational trapdoor function? Sounds like an interesting point of view. But would that not also imply that we should be able to measure the quantum world with quantum devices? Say we have a quantum property that is extremely close to p=0.5. If we could invent a device to replicate that property perfectly and measure it repetitively we could then estimate ever more accurate boundaries for the "true" value of p, no?
  - Diggsey 6y ago
    You can't "copy" data in QM: operations can neither destroy nor create information.
H8crilA 6y ago
> QM is the only physical theory that has randomness as an inherent part It's either inherent randomness or just a deep hole in the whole thing (similar to the alien chess thought experiment problem). Personally I choose to believe that the theory is just incomplete because nobody can even define what a "measurement" really is, meaning in which cases what we do is a "measurement" and in which cases it is not a "measurement". I also think that this is what people like Feynman refer to when they say things like "nobody understands QM", it's actually "nobody understands the wave function collapse", the rest is just maths.
- konschubert 6y ago
  Agreed. The measurement is the theoretical duct tape between the "quantum world" and the "classical world". But there is no such thing as a "classical world", it's just a useful approximation. And therefore, there is also no such thing as a "measurement", it's also an approximation. (Maybe not even an approximation, but maybe more like a projection...)
- 8note 6y ago
  Once you design a turing machine that can solve the halting problem, I'm sure it'll be able to break Bell's inequalities too Bell's inequalities show that there isn't a local state that can be there.
  - konschubert 6y ago
    Bell's inequalities talk about "measurements". But measurement's aren't real.
  - H8crilA 6y ago
    Turing machine as an abstract concept is known to not hold a solution to the halting problem. There's no question about that, the only question is how the abstract concept assumptions pertain to the real world (infinite tape? maybe a problem, maybe not?; is the execution speed bounded? or maybe we can somehow count to infinity by exponentially increasing the speed? things like that). On the other hand nobody has any clue what the quantum measurement / wave function collapse actually is. There are theories/interpretations but no truly satisfying answers in the same way as for example Newton's equations were a satisfying answer to the elliptical movement of planets, even though we later found out in the 20th century that F ~ 1/r^2 was actually an approximation. We simply don't know, and we have no idea when shall we know.
- the8472 6y ago
  > nobody can even define what a "measurement" really is Isn't that the observer becoming entangled with the measured system?
  - 6y ago
ibab 6y ago
In the Everett/Many Worlds interpretation the appearance of randomness can be explained as an emergent phenomenon resulting from not being able to predict which part of the wave function we will end up in before running an experiment.
- Jweb_Guru 6y ago
  The Everett/Many Worlds interpretation cannot reproduce the predictions of quantum mechanics without extra assumptions (e.g. the Born rule) that don't have any physical basis within the context of MWI.
  - ibab 6y ago
    Yes, you are right to point this out. There are some important details that are still being debated. Personally my impression is that the debate has advanced enough to the point where MWI can’t be outright dismissed based on this argument. There are multiple plausible explanations and the remaining difficulties have more to do with philosophy than physics. Edit: To give one example of an approach that I think is promising: We start by describing the observer and environment through a density matrix (a probability distribution over possible wave functions) and introduce an interaction with a quantum system (e.g. a spin). Given a reasonable interaction, you can show that the entanglement in the combined state (observer, environment and spin) leads to the system approaching a state that is a probability distribution of entangled states where each probability corresponds to the Born rule. Interestingly in this case the probabilities emerge from our lack of knowledge about the microstate of the observer/environment, so it’s actually thermodynamic uncertainty.
    - Jweb_Guru 6y ago
      I'm not dismissing MWI, I'm just saying that current formulations either don't reproduce quantum mechanics or don't really address the existence of randomness within quantum mechanics.
    - Jweb_Guru 6y ago
      While I intuitively like the statistical approach you mention, under it the Born rule holds only approximately, so it should be theoretically possible to observe entangled states which we have never done--i.e. it produces different predictions from the Copenhagen interpretation of quantum mechanics, which means it's not strictly a different interpretation, but its own falsifiable theory. Like I said in another comment, if we ever do observe entangled states directly, people will jump on board one of these alternate explanations like lightning. But until we do, the question of why we never ever observe anything that doesn't look like collapse still needs mathematical justification.
GistNoesis 6y ago
What is usually misunderstood about QM is that QM isn't a model of the world. QM is a model of "what we can observe from the world" based on "what we can observe from the world". QM is a model of "accessible" information. What laymen usually wants is to understand how the world evolve. What QM physicists tell them is there exist some inaccessible information, but using accessible information we have, we know how to predict all the accessible information (albeit stochastic-ally). The typical example to help computer scientists to understand is the seed of a pseudo-random generator in an online casino. The players will never be able to access the seed, therefore the best they can do is make decision based on the value of the generated random numbers they observe and their probabilities. Bell inequalities are a consequence of this modelisation. They are a refurbishing of Boole's inequalities, a theorem about probability which only bound those who use probability. The usual fallacy forward is telling QM is a non-local theory, classical local theory can't violate Bell inequalities, Bell inequalities violations are observed in the real world, therefore the world is not local...

konschubert 6y ago

The problem with the double slit experiment is that the target isn't treated as a quantum system. That's why the wave function "collapses": Because it collides with the non-quantum target. It's a useful approximation, but of course in reality, there is no such thing as a non-quantum thing. And if you evolve the target's wave function with the wave function of the particle, then there is no stochastic collapse.

choeger 6y ago
What you are saying is that whenever we, the observer, leave the QM model we use the collapse as a computational trapdoor function? Sounds like an interesting point of view. But would that not also imply that we should be able to measure the quantum world with quantum devices? Say we have a quantum property that is extremely close to p=0.5. If we could invent a device to replicate that property perfectly and measure it repetitively we could then estimate ever more accurate boundaries for the "true" value of p, no?
- Diggsey 6y ago
  You can't "copy" data in QM: operations can neither destroy nor create information.

H8crilA 6y ago

> QM is the only physical theory that has randomness as an inherent part It's either inherent randomness or just a deep hole in the whole thing (similar to the alien chess thought experiment problem). Personally I choose to believe that the theory is just incomplete because nobody can even define what a "measurement" really is, meaning in which cases what we do is a "measurement" and in which cases it is not a "measurement". I also think that this is what people like Feynman refer to when they say things like "nobody understands QM", it's actually "nobody understands the wave function collapse", the rest is just maths.

konschubert 6y ago
Agreed. The measurement is the theoretical duct tape between the "quantum world" and the "classical world". But there is no such thing as a "classical world", it's just a useful approximation. And therefore, there is also no such thing as a "measurement", it's also an approximation. (Maybe not even an approximation, but maybe more like a projection...)
8note 6y ago
Once you design a turing machine that can solve the halting problem, I'm sure it'll be able to break Bell's inequalities too Bell's inequalities show that there isn't a local state that can be there.
- konschubert 6y ago
  Bell's inequalities talk about "measurements". But measurement's aren't real.
- H8crilA 6y ago
  Turing machine as an abstract concept is known to not hold a solution to the halting problem. There's no question about that, the only question is how the abstract concept assumptions pertain to the real world (infinite tape? maybe a problem, maybe not?; is the execution speed bounded? or maybe we can somehow count to infinity by exponentially increasing the speed? things like that). On the other hand nobody has any clue what the quantum measurement / wave function collapse actually is. There are theories/interpretations but no truly satisfying answers in the same way as for example Newton's equations were a satisfying answer to the elliptical movement of planets, even though we later found out in the 20th century that F ~ 1/r^2 was actually an approximation. We simply don't know, and we have no idea when shall we know.
the8472 6y ago
> nobody can even define what a "measurement" really is Isn't that the observer becoming entangled with the measured system?
- 6y ago

ibab 6y ago

In the Everett/Many Worlds interpretation the appearance of randomness can be explained as an emergent phenomenon resulting from not being able to predict which part of the wave function we will end up in before running an experiment.

Jweb_Guru 6y ago
The Everett/Many Worlds interpretation cannot reproduce the predictions of quantum mechanics without extra assumptions (e.g. the Born rule) that don't have any physical basis within the context of MWI.
- ibab 6y ago
  Yes, you are right to point this out. There are some important details that are still being debated. Personally my impression is that the debate has advanced enough to the point where MWI can’t be outright dismissed based on this argument. There are multiple plausible explanations and the remaining difficulties have more to do with philosophy than physics. Edit: To give one example of an approach that I think is promising: We start by describing the observer and environment through a density matrix (a probability distribution over possible wave functions) and introduce an interaction with a quantum system (e.g. a spin). Given a reasonable interaction, you can show that the entanglement in the combined state (observer, environment and spin) leads to the system approaching a state that is a probability distribution of entangled states where each probability corresponds to the Born rule. Interestingly in this case the probabilities emerge from our lack of knowledge about the microstate of the observer/environment, so it’s actually thermodynamic uncertainty.
  - Jweb_Guru 6y ago
    I'm not dismissing MWI, I'm just saying that current formulations either don't reproduce quantum mechanics or don't really address the existence of randomness within quantum mechanics.
  - Jweb_Guru 6y ago
    While I intuitively like the statistical approach you mention, under it the Born rule holds only approximately, so it should be theoretically possible to observe entangled states which we have never done--i.e. it produces different predictions from the Copenhagen interpretation of quantum mechanics, which means it's not strictly a different interpretation, but its own falsifiable theory. Like I said in another comment, if we ever do observe entangled states directly, people will jump on board one of these alternate explanations like lightning. But until we do, the question of why we never ever observe anything that doesn't look like collapse still needs mathematical justification.

GistNoesis 6y ago

What is usually misunderstood about QM is that QM isn't a model of the world. QM is a model of "what we can observe from the world" based on "what we can observe from the world". QM is a model of "accessible" information. What laymen usually wants is to understand how the world evolve. What QM physicists tell them is there exist some inaccessible information, but using accessible information we have, we know how to predict all the accessible information (albeit stochastic-ally). The typical example to help computer scientists to understand is the seed of a pseudo-random generator in an online casino. The players will never be able to access the seed, therefore the best they can do is make decision based on the value of the generated random numbers they observe and their probabilities. Bell inequalities are a consequence of this modelisation. They are a refurbishing of Boole's inequalities, a theorem about probability which only bound those who use probability. The usual fallacy forward is telling QM is a non-local theory, classical local theory can't violate Bell inequalities, Bell inequalities violations are observed in the real world, therefore the world is not local...