# B Entanglement & Wave Function Collapse

#### Lynch101

No. It's one of:
1. No Mechanism
2. A Mechanism that is nonlocal and will violate relativity in general
3. Retrocausal Mechanism
4. Superdeterministic Mechanism
5. Acausal Mechanism
6. Multiple Worlds
That is excellent! thank you!!

#### DarMM

Gold Member
I would take realism to mean something along the lines of the idea espoused by Lee Smolin that there is something going on in the individual experiments of QM which the statistical model doesn't capture
Everybody thinks there is something going on. "Realism" is a technical term in the philosophy of physics referring to whether you can capture that "something" in mathematical terms that describe what it is actually like.

As I said though I prefer the term "Representational" as opposed to "Realist" because Realist causes the exact confusion you are having here.

#### Lynch101

Everybody thinks there is something going on. "Realism" is a technical term in the philosophy of physics referring to whether you can capture that "something" in mathematical terms that describe what it is actually like.

As I said though I prefer the term "Representational" as opposed to "Realist" because Realist causes the exact confusion you are having here.
The idea that there is no mechanism of wave function colllapse or the notion that unmeasured quantities don't exist somewhat implies that there is "nothing to see" i.e. noting going on.

#### Lynch101

With regard to the question from the OP, where only one of the entangled particles is measured, does this measurement produce correlations in the other particle which suggest that the wave function has collapsed?

#### DarMM

Gold Member
The idea that there is no mechanism of wave function colllapse or the notion that unmeasured quantities don't exist somewhat implies that there is "nothing to see" i.e. noting going on.
No it doesn't. Wave function collapse can be viewed epistemically and measurement quantities can "not exist" prior to measurement because they are created by the experimental context. None of these imply nothing is going on.

#### Lynch101

No it doesn't. Wave function collapse can be viewed epistemically and measurement quantities can "not exist" prior to measurement because they are created by the experimental context. None of these imply nothing is going on.
I would agree with that, but it appears to be tautological that measurement quantities don't exist until they are measured. I think we can reasonably ask the question though, what is the state of the system prior to being measured?

#### DarMM

Gold Member
I would agree with that, but it appears to be tautological that measurement quantities don't exist until they are measured.
It's not tautological because in classical theories the values of measured quantities exist prior to their measurement.

#### PrashantGokaraju

I would agree with that, but it appears to be tautological that measurement quantities don't exist until they are measured. I think we can reasonably ask the question though, what is the state of the system prior to being measured?
In quantum theory, the measurements (interactions with the apparatus) are what define the behavior of the system. There is no sharp separation between these interactions and an "independent behavior" of the system. So there is no question of the measured values existing or not existing before these interactionscht.

#### Lynch101

It's not tautological because in classical theories the values of measured quantities exist prior to their measurement.
Apologies, could you ELI5? It's not an issue I have given any consideration to before, so I'm clear on how that is i.e. I would not be able to explain why that is the case. In what sense do they have values before being measured? Is it that we can predict their values?

I'm probably misinterpreting what is being said, but the system must be in some state prior to being measured. The idea that measured quantities do not exist prior to their measurement seems tautological in this sense, or at least doesn't address the question of the state of the system prior to measurement.

#### DarMM

Gold Member
could you ELI5?
In what sense do they have values before being measured?
In classical mechanics a particle might have some momentum $p$ as it moves along. When you measure it you get a value $p^{'}$ close to the actual momentum of the particle. So the measured result is a recording, up to experimental noise, of the momentum value the particle had before it was measured. Thus there was a value for momentum before you measured it.

This is not the case in Quantum Theory.

#### Lynch101

In quantum theory, the measurements (interactions with the apparatus) are what define the behavior of the system. There is no separation between these interactions and an "independent behavior" of the system. So there is no question of the measured values existing or not existing before these interactions.
Again this appears to say that the measured values don't exist until they are measured.

But we can talk about the state of the system prior to being measured. We might not be able to speak in detail about it, but we can say that we know that there must be something there. There must be something there which can be measured.

It can't be the case that there is nothing there and in measuring this nothing, then something comes into existence.

#### Lynch101

In classical mechanics a particle might have some momentum $p$ as it moves along. When you measure it you get a value $p^{'}$ close to the actual momentum of the particle. So the measured result is a recording, up to experimental noise, of the momentum value the particle had before it was measured. Thus there was a value for momentum before you measured it.

This is not the case in Quantum Theory.
I see! I mean, I don't really see, but I see that this is just another thing I need to try and wrap my head around.

#### PrashantGokaraju

Again this appears to say that the measured values don't exist until they are measured.

But we can talk about the state of the system prior to being measured. We might not be able to speak in detail about it, but we can say that we know that there must be something there. There must be something there which can be measured.

It can't be the case that there is nothing there and in measuring this nothing, then something comes into existence.
chThe idea of 'state' in quantum theory is very different from the one in classical physics. In fact, this is the fundamental confusion which is behind the EPR paradox. Einstein wants to say that you can choose to predict either x or p without "interacting with the system", so the uncertainty principle must be wrong. What Einstein did not realize is that, even if you are not "interacting with the system" or "disturbing the system", when you choose make different experimental procedures to predict either x or p, you are disturbing the state, because the idea of 'state' in quantum theory cannot be seperated from the experimental arrangement. You can disturb the state by disturbing the experimental conditions, even if you are not "disturbing the particle".

#### Lynch101

chThe idea of 'state' in quantum theory is very different from the one in classical physics. In fact, this is the fundamental confusion which is behind the EPR paradox. Einstein wants to say that you can choose to predict either x or p without "interacting with the system", so the uncertainty principle must be wrong. What Einstein did not realize is that, even if you are not "interacting with the system" or "disturbing the system", when you choose make different experimental procedures to predict either x or p, you are disturbing the state, because the idea of 'state' in quantum theory cannot be seperated from the experimental arrangement. You can disturb the state by disturbing the experimental conditions, even if you are not "disturbing the particle".
Would this would apply at the classical level too, just on an imperceptible level?

What if we talk about future experiments for which we haven't arranged the experimental set-up? We can talk about the state of a system then, prior to measurement then, can't we?

Just thinking out loud here, but it could nearly be argued that given the chain of causality, the state of a future system is disturbed by interactions in the distant past. A scientists decision to enter the field of physics and the unbroken chain of causlity disturbs the future system that they meausre.

#### Lynch101

No. It's one of:
1. No Mechanism
2. A Mechanism that is nonlocal and will violate relativity in general
3. Retrocausal Mechanism
4. Superdeterministic Mechanism
5. Acausal Mechanism
6. Multiple Worlds
Do all but #2 comply with relativity?

#### DarMM

Gold Member
It can't be the case that there is nothing there and in measuring this nothing, then something comes into existence
No interpretation says this.

Do all but #2 comply with relativity?
Yes.

#### PrashantGokaraju

Would this would apply at the classical level too, just on an imperceptible level?

What if we talk about future experiments for which we haven't arranged the experimental set-up? We can talk about the state of a system then, prior to measurement then, can't we?

Just thinking out loud here, but it could nearly be argued that given the chain of causality, the state of a future system is disturbed by interactions in the distant past. A scientists decision to enter the field of physics and the unbroken chain of causlity disturbs the future system that they meausre.
This is best illustrated by what is called "Einstein's box"

A box lets out a photon. Now the photon has left the box, so what you do to the box "does not disturb the particle". By different experimental procedures, you can fix either the time when the particle leaves the box, or the energy of the photon. So, without "disturbing the particle", you are free either to say it has an energy, or a particular time of leaving. So Einstein wants to say that, both these properties exist at the same time, since there is no "disturbance".

What is wrong with this argument is that, if you choose to measure the time, you lose the capability to observe any phenomenon to which the idea of conservation of energy can be applied. The apparatus and the system are "entangled", although i dont like this word. There is no "state of the system" and "state of the apparatus" as independent things. they are inseperably connected.

#### DrChinese

Gold Member
In a spacelike seperated case yes you can't argue for causality in either case. And then I assume you are bringing this over to even timelike separations.

I would have thought that neither A or B cause each other's results was a fairly standard view. That's what it is in Copenhagen for example.
The OP asked about causality, so I passed on my comment on that. We have had a lot of recent debate about causality and QM/QFT, and there are those that think causality is present. I don't see any physical (or theoretical) support for that, but that's just me (or maybe not). Those debates centered on the same subject matter as this thread, hence my qualified answer in an attempt to avoid a debate re-hash.

#### Nugatory

Mentor
But we can talk about the state of the system prior to being measured. We might not be able to speak in detail about it, but we can say that we know that there must be something there. There must be something there which can be measured.

It can't be the case that there is nothing there and in measuring this nothing, then something comes into existence.
That’s your classical intuition speaking. In classical physics, a complete specification of the state of a particle includes a statement about its position and momentum as a function of time. The theory can’t even be formulated without assuming that at time $t$ the particle is at position $\vec{x}(t)$ and has momentum $\vec{p}(t)$.

But quantum mechanics doesn’t work that way. The state function does not specify the value of observables, it just gives the probability of observing a particular result if you choose to measure that observable - and the probabilities are such that they cannot be be consistent with unmeasured observables having definite values as they do classically. As well as Bell’s theorem, you might also want to take a look at the Kochen-Specker theorem and also the “Spooky Socks” section of https://www.science20.com/hammock_physicist/einstein_got_it_wrong_can_you_do_better-85544 (I’m not necessarily endorsing the rest of that blog post, just that one section).

#### DarMM

Gold Member
The OP asked about causality, so I passed on my comment on that. We have had a lot of recent debate about causality and QM/QFT, and there are those that think causality is present. I don't see any physical (or theoretical) support for that, but that's just me (or maybe not). Those debates centered on the same subject matter as this thread, hence my qualified answer in an attempt to avoid a debate re-hash.
From reading those debates it seemed (to me) to be caused by talking past each other due to different notions of causality, i.e. Microcausality vs Reichenbach style common cause causality. QFT obeys the former but not the latter.

Same as how I realised many discussions between myself (and @atyy ) and @vanhees71 were due to talking past each other since we meant different things by collapse.

#### Nugatory

Mentor
Just thinking out loud here, but it could nearly be argued that given the chain of causality, the state of a future system is disturbed by interactions in the distant past. A scientists decision to enter the field of physics and the unbroken chain of causlity disturbs the future system that they meausre.
Follow that line of thought to the end, and you will arrive at the position known as “superdeterminism”. (Google for “superderminism t’Hooft” for more).

On the one hand, superdeterminism is about the only way of reconciling the experimentally confirmed predictions of quantum mechanics with the classical intuition that you so emphatically assert in post #36 of this thread.

On the other hand, superdeterminism is a purely philosophical position that cannot be empirically tested and indeed makes a mockery of the entire notion of empirical science. It’s also not a topic that we can discuss here.

#### DarMM

Gold Member
On the other hand, superdeterminism is a purely philosophical position that cannot be empirically tested and indeed makes a mockery of the entire notion of empirical science. It’s also not a topic that we can discuss here
Just to be clear a thread on 't Hooft's model would not be allowed?

#### Nugatory

Mentor
It's not explicitly on our forbidden list and we have a number of past discussions that search (either forum or Google with a site: qualifier) will find. They've generally been fairly inconclusive, which isn't surprising for an unfalsifiable proposition. I'm not sure that this thread will be improved by opening that can of worms, but I'm willing to listen to arguments otherwise.

#### DarMM

Gold Member
It's not explicitly on our forbidden list and we have a number of past discussions that search (either forum or Google with a site: qualifier) will find. They've generally been fairly inconclusive, which isn't surprising for an unfalsifiable proposition. I'm not sure that this thread will be improved by opening that can of worms, but I'm willing to listen to arguments otherwise.
I saw them, I was just wondering if I had missed some discussion of superdeterminism so horrific that it rendered the topic forbidden. I was reading 't Hooft's texts recently.

#### Lynch101

This is best illustrated by what is called "Einstein's box"

A box lets out a photon. Now the photon has left the box, so what you do to the box "does not disturb the particle". By different experimental procedures, you can fix either the time when the particle leaves the box, or the energy of the photon. So, without "disturbing the particle", you are free either to say it has an energy, or a particular time of leaving. So Einstein wants to say that, both these properties exist at the same time, since there is no "disturbance".
This is just an aside, what I was thinking out loud: but it could be argued that everything up to the establishment of the box "disturbs the particle", or at least the chain of causality which establishes the box, disturbs the particle. That's probably not a necessary distinction though.

What is wrong with this argument is that, if you choose to measure the time, you lose the capability to observe any phenomenon to which the idea of conservation of energy can be applied.
My intuitive response to this - which I have learned to expect to be wrong - is that just because we lose the capability of measuring it, surely doesn't mean that it doesn't have that property? The fact that we have the option to measure it in the first place would, to me, imply that it has this property to begin with. The argument would be that we cannot measure a property that it does not have, so the fact that we have the option to measure it implies that it has that property before we measure it. We might not be able to ascribe a value to it, but that represents a limit to our investigative abilities as opposed to the absence of that property.

The apparatus and the system are "entangled", although i dont like this word. There is no "state of the system" and "state of the apparatus" as independent things. they are inseperably connected.
I get that point. There is a broader philosophical argument that could be made that points to the idea that the universe is continuous as opposed to discrete, but that would be a separate issue.

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