[PeteJ (OP)]

Hi folks. My first post here. I have a question, most probably a naïve one...

In QM, whether we favour MW or CI as an interpretation, there is the idea that an observation causes an event, either a 'collapse' or a splitting of worlds.

Does the observation preceed the event, or is it simultaneous with the event?   

Or something else..?

Thanks.

[alancalverd]

An observation cannot logically precede the observed event, and given the finite speed of light (which defines the limiting speed for transmission of information) it cannot even be simultaneous.

It is important to distinguish between the process of observation, which could indeed precipitate an event (say by a photonuclear interaction), and the resulting observation itself (an emitted electron or whatever).  Bouncing hypothetical photons off particles does indeed lead to Heisenberg's indeterminacy equation but isn't the cause of indeterminacy.

Beware of confusing a mathematical model (wave function collapse, world splitting, whatever) with reality. The model is merely our best shot at predicting the outcome of a future event.

[PeteJ (OP)]

Thanks Alan, very clear.

It seems to me that you're right. But it doesn't quite answer my question.

If the observation is not prior to the event and not simultaneous, then what?

How can the process of observation be different from the observation?

[alancalverd]

Process of observation: kick it

Observation: landmine explodes

[PeteJ (OP)]

Hmm. That does not help. The landmine explodes well before we observe the explosion, and if we sense the kick connecting then that is an observation.

I know my question is naïve but it's proving difficult to find an answer. I've tried elsewhere but had no luck.

[David Cooper]

The following is speculation:-

What I suspect happens is that the wavefunction is only able to be sustained up to a certain level of complexity beyond which it must burst (simplify). By creating data in a brain or computer to represent what is observed, the complexity increases because the computer has to be maintained in multiple states to maintain the data in multiple states, so that is where the simplification is most likely to be forced to take place, after which a matching simplification will be forced to take place in the original object being observed to match up to the data which describes it.

[lightarrow]

If the observation is not prior to the event and not simultaneous, then what?

Usually with "event" in this context, in the sense of "measure event", we mean the physical interaction with a measure device. "Observation" is the subjective act of take awareness of the event from a sentient being.
The first can come without the (unnecessary) second.
Someone has speculated on the possibility that the act of observation could modify the wavefunction, but it's not the main idea between physicists.
Physics, in my opinion, shouldn't be affected by subjective acts, so only the physical measure has a meaning.

--
lightarrow

[chiralSPO]

I agree with lightarrow here--I do not think (at least it hasn't been shown) that there is any requirement for a human or computer, or any other entity to "observe" a system for decoherence (wavefunction collapse) to occur. Only an interaction (often called measurement, but doesn't have to involve any type of measurement) is required.

[David Cooper]

When are measurements made without the data ever being processed by a computer or mind? Up until the point where they are processed, the wavefunction could continue to represent multiple possible realities without simplifying. If you then look at the results and that forces a simplification, how are you going to know if you caused that collapse or if it happened back at the time of the measurement? Indeed, it may be that a computer or brain isn't always adding sufficient complexity to cause the collapse, so it might maintain two possible states of that computer/brain which both believe there has been a collapse of the wavefunction when it has not actually occurred yet.

[PeteJ (OP)]

Great. This is getting interesting.

David - I like your idea and haven't come across it before,. Are you suggesting that there is a law of conservation of complexity? .

Lightarrow - I get the picture and do not see it differently. But I don't think the question of whether only the measurement has meaning is a free decision for physicists. To say that only unobserved events have meaning and then they lose it when they are observed would be a bit odd, and not the usual meaning of 'meaning'.

The thing is, your description leaves a finite time between the event and the observation, and this what I'm exploring. But then...

chiralSPO - You say that an observation is unnecessary, that the event (the detection by the detector) is enough for decoherence. This is not my understanding of things. It would answer my question if it is true, but is it true? It doesn't seem to match with the mainstream ideas that I read about. 

Is it widely agreed that no sentient observation is necessary for decoherence? 
   

   

[chiralSPO]

When are measurements made without the data ever being processed by a computer or mind? Up until the point where they are processed, the wavefunction could continue to represent multiple possible realities without simplifying. If you then look at the results and that forces a simplification, how are you going to know if you caused that collapse or if it happened back at the time of the measurement? Indeed, it may be that a computer or brain isn't always adding sufficient complexity to cause the collapse, so it might maintain two possible states of that computer/brain which both believe there has been a collapse of the wavefunction when it has not actually occurred yet.

I have to dig around to find it, but I'm pretty sure that there have been experiments that show, very explicitly, that a wavefunction has collapsed by looking at successive interactions of a particle--for the first few, it behaves as a wave, and then, after a sufficiently disruptive interaction, it behaves as a particle...

Also, recall that A) nuclear magnetic resonance spectroscopy relies on the half life of coherent quantum states of populations of particles; and B) one of the most difficult obstacles to practical quantum computing is spontaneous decoherence/collapse.

[PmbPhy]

Hi folks. My first post here. I have a question, most probably a naïve one...

Welcome to the forum.

In QM, whether we favour MW or CI as an interpretation, there is the idea that an observation causes an event, either a 'collapse' or a splitting of worlds.

Where did you get that notion from? When you make a measurement of a system which is in an eigenstate of the system. E.g. if the system is in an eigenstate of energy and you measure the energy then all you get is a measurement of the energy, period. Nothing happens to the system. If the system is not in an eigenstate of the observable you are measuring then when you measure that observable you'll get one of the eigenvalues of the observable. That's all. You didn't cause anything physical to happen. If you write down the expression for the system in an arbitrary quantum state and then measure an observable then the mathematical expression for the system will change to an eigenstate of the observable that you measured. But again, nothing physical happens to the system other than it now being in an eigenstate. But you can't observe the quantum state. It's simply a mathematical expression which represents the system. It's a misconception to think that something physical happens when the system collapses. That's merely mathematical lingo to describe what's going on with the mathematical description of the probabilities of measuring observables.

Does the observation preceed the event, or is it simultaneous with the event?

Carefully study what I wrote above and you'll understand that this question is meaningless.

[alancalverd]

If you could obtain information about an event before it occurred, you could prevent or modify it, thus invalidating the information you used, ad infintum.....

[evan_au]

the event (the detection by the detector) is enough for decoherence.

This is not my understanding of things.

I understand that one of the barriers to quantum communication is tiny imperfections in the optical fiber, which result in the entangled photons (rarely) interacting with one of these imperfections.

Nobody is observing all the imperfections in the fiber, and nobody (and no test equipment) has seen that the photon has struck a particular imperfection, but the photon is scattered, and coherence is lost.

So decoherence can be triggered by interaction with the environment.

[PeteJ (OP)]

Okay. Thanks. Things are becoming more clear. I'm confused as to how the subject changed to a question that isn't mine but no matter.

PmbPhy - You may be giving the answer to my question but I must admit I don't fully understand it. You seem to be saying that the measurement has no effect on anything except our knowledge of the system.
Is that it?   

Evan-au - says that decoherence can be triggered by the environment, but if decoherence is only about  our knowledge of the system then how can this be?   

[PmbPhy]

PmbPhy - You may be giving the answer to my question but I must admit I don't fully understand it. You seem to be saying that the measurement has no effect on anything except our knowledge of the system.
Is that it?

Sort of. If you're confused then I'd have to say - Join the club! []

Here's what is referred to as the orthodox position: Suppose you have a system which is an particular quantum state which isn't an eigenstate of any observable. Before a measurement of, for example, the position of a particle, is made then The particle wasn't really anywhere.

As Pascual Jordan put it: “Observations not only disturb what has to be measured, they produce it….We compel [the electron]
to assume a definite position…. We ourselves produce the results of measurements.”

If a particle assumes a specific position in space then it might be said that the measurement process "caused" it to happen. If you wish to look at it that way then, in this particular example, cause and effect are simultaneous.

The quote above was taken from the article called Is the Moon There When Nobody is Looking? by N. David Mermin, Physics Today. It's available online at: http://maltoni.web.cern.ch/maltoni/PHY1222/mermin_moon.pdf

You might want to read it. It might help you to understand what you're seeking to learn.

[David Cooper]

David - I like your idea and haven't come across it before,. Are you suggesting that there is a law of conservation of complexity? .

I remind you that what I said is pure speculation, and I'm not suggesting anything about conserving complexity beyond the idea that there may be some limit to how much complexity a wavefunction can maintain before it collapses, but that detecting where that limit is reached may not be possible. If we take Schrödinger's cat as an example, it has not been ruled out that the wavefunction of the cat may be compatible with it being dead and alive. A collapse of the wavefunction could simplify it to eliminate one of those two possibilities, and it may be that the cat is sufficiently complex to force that collapse all by itself, long before anyone looks in the box to see if the cat's dead or alive. When a person looks in the box, again there's the question of whether that person can force the collapse of the wavefunction - it may be that (s)he can't, in which case the wavefunction of the person may have to remain compatible with two contradictory outcomes, one of which registers the cat as alive while the other registers it as dead. At some later point, maintaining the two possibilities may no longer be possible due to there being more complexity than the universe can handle, at which point one of the possibilities must be junked and the cat is then either dead or alive, no longer maintained in an uncertain state where it may yet become either, and at that point the wavefunction of the human will simplify to match (if it didn't simplify first, which is probably more likely), leaving us with a human which knows whether the cat is alive or dead rather than incorrectly thinking (s)he knows while actually maintaining both contradictory beliefs. What we certainly don't see is a human opening the box and finding a cat that (s)he determines is both alive and dead at the same time, but that does not guarantee that the wavefunction has collapsed at that point.

What we do see though are experiments where matter can be seen to be maintaining two contradictory states at the same time, though they aren't really contradictory as the matter hasn't actually been forced to simplify what it's doing. That really is like looking at a cat and seeing it as both dead and alive at the same time, only it hasn't yet been done with anything as complex as a cat. The particular experiment I have in mind involved an item made out of several hundred atoms which vibrated from side to side, but it did so by moving from left to right while at the same time moving from right to left, etc. That experiment showed the effect in something much more complex than a single particle and it opened up the possibility of doing the same thing with bigger, more complex items which are doing more complex things. Perhaps it can be extended to a cat, but perhaps not, and we may never know.

[chiralSPO]

Schrödinger's cat is a terrible thought experiment, and I wish it were not so overused. Schrödinger originally proposed this experiment to expose how ridiculous QM logic is on a macroscopic scale.

Forget about whether or not the cat is complex enough to disrupt the wavefunction, whatever device measures whether or not the quantum event has happened and tells the poison to be released or not has already collapsed the wavefunction.

There are different interpretations of what the wavefunction actually represents, if it is a real thing, or just a useful model etc. Ultimately QM tells us what is knowable and what is unknowable (given our current understanding, but it has yet to fail us)--but this does not mean that we can actually measure anything that is theoretically knowable... Unfortunately the question of whether QM mechanisms are actually deterministic or probabilistic is fundamentally unknowable and cannot be probed.

[PeteJ (OP)]

I wonder why the thread question has been changed. The OP said nothing about the event preceeding the cause. Not sure I'm happy about this. 

PmbPhy - You may be giving the answer to my question but I must admit I don't fully understand it. You seem to be saying that the measurement has no effect on anything except our knowledge of the system.
Is that it?

Sort of. If you're confused then I'd have to say - Join the club! []

Here's what is referred to as the orthodox position: Suppose you have a system which is an particular quantum state which isn't an eigenstate of any observable. Before a measurement of, for example, the position of a particle, is made then The particle wasn't really anywhere.

As Pascual Jordan put it: “Observations not only disturb what has to be measured, they produce it….We compel [the electron]
to assume a definite position…. We ourselves produce the results of measurements.”

If a particle assumes a specific position in space then it might be said that the measurement process "caused" it to happen. If you wish to look at it that way then, in this particular example, cause and effect are simultaneous.

The quote above was taken from the article called Is the Moon There When Nobody is Looking? by N. David Mermin, Physics Today. It's available online at: newbielink:http://maltoni.web.cern.ch/maltoni/PHY1222/mermin_moon.pdf [nonactive]

You might want to read it. It might help you to understand what you're seeking to learn.

Your summary here is just fine and very helpful. I'm not really trying to learn QM but, rather, what physicists think about QM. I think you've answered my question. If it is allowable that the measured particle is not really anywhere until the measurement and is 'reified' by the measurement then the problem I was investigating evaporates. I had thought that physics rejected this idea since it would lend credence to idealism of some sort and also mad ideas like David's  []

... Which I suspect has some truth in it. It is not quite the same idea, but it seems to make some sense that stable complexity requires a material substrate. I.e that mind requires matter. It's a pet theory. 

No need to go there though. Many thanks for all the excellent replies. On one science forum I know this discussion would have descended into a raging argument within five posts.     
 

[PmbPhy]

Your summary here is just fine and very helpful.

Wonderful. I'm so glad to hear that you've got your question answered to your satisfaction.  []

On one science forum I know this discussion would have descended into a raging argument within five posts.     

That's why when I opened my own company I added physics forums to my companies website. Membership is by invitation only. I wait until I get to know a member in one of the forums I frequent and if they have what it takes to be in my forum I send them an invitation. It's simple. I do my best to try to predict whether the person will be able to respect and follow the forum rules. If the person objects to the rules then they're not invited. If they're invited and they join then we can discuss whether any changes should be made to the forum rules. I'm very good about that and am always looking for input from members. Problems sometimes arise when a member doesn't like a rule and then quits because of it. The problem with that is that it was their responsibility to discuss their objections to the rules until we're both satisfied. It's an ongoing process to, not a one time thing. So if I enforce a rule and the member convinces me and other members of the forum that the rule was wrong or didn't apply then I undo the enforcement. The rules are what one typically finds in all forums except that I strictly enforce them.