Why time is not an observable in quantum theory?

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Discussion Overview

The discussion revolves around the question of why time is not considered an observable in quantum theory. Participants explore theoretical implications, definitions of observables, and the nature of time in the context of quantum mechanics.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Exploratory

Main Points Raised

  • Some participants suggest that observables are defined by their interaction with measuring devices, arguing that time does not fit this definition because clocks do not interact with the systems being measured.
  • Others propose that time is relative and could be considered a basic observable in quantum systems, potentially represented by a linear, unbounded operator.
  • A participant questions the standard argument related to Pauli's assertion that a time observable would lead to the Hamiltonian not being bounded from below.
  • There is a discussion about the need to redefine what is meant by an "observable" to include time, with some arguing that this would complicate the understanding of quantum mechanics.
  • Some participants express skepticism about Pauli's argument, suggesting it lacks sufficient mathematical justification and may not hold under careful formulation.
  • Participants discuss the implications of defining properties of physical systems and the circularity in defining quantities in quantum mechanics.
  • One participant mentions that time cannot be localized in a physical wave function, which contributes to the argument against time being treated as an observable.
  • There is a suggestion that the concept of "time of events" could be made into an observable with specific qualifications, such as tunneling time.
  • Several participants engage in a broader philosophical discussion about the nature of existence and time in relation to theories like the Big Bang, with differing views on what "emptiness" means in this context.

Areas of Agreement / Disagreement

Participants express multiple competing views regarding the status of time as an observable in quantum theory. There is no consensus on the definitions or implications of observables, and the discussion remains unresolved.

Contextual Notes

Participants highlight limitations in defining observables and properties, indicating that the discussion is influenced by various interpretations and formulations within quantum mechanics. The complexity of the topic is acknowledged, with references to the need for rigorous mathematical frameworks.

  • #91


Demystifier said:
Oh, now I see your point. And I completely agree with you. In general, the number of "clicks" does not need to be equal to the number of Bohmian trajectories. But the first paper I mentioned
http://xxx.lanl.gov/abs/0904.2287 [Int. J. Mod. Phys. A25:1477-1505, 2010]
discusses that issue as well. Even a short discussion of the Unruh effect is presented. See the discussion around Eqs. (11)-(14) and page 22.
But I don't see how you solve it (btw I think the Unruh effect is plain wrong *physically*, so don't talk to me about that :wink:). I mean you cannot really claim -in my opinion- that you have a single real point like particle which gives two detector clicks at spacelike separated points. That kind of reality is even more perverse than saying there is no particle reality at all: so the sane thing to do in my opinion, would be to limit the class of observables and make predictions which distinguish your theory from ordinary QFT.

That's what I said to Arkajad, it appears to me that your extra level of reality will force you to make such constraints (if you want to remain reasonable), and that is what I would like to see.
 
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  • #92


Careful said:
I mean you cannot really claim -in my opinion- that you have a single real point like particle which gives two detector clicks at spacelike separated points. That kind of reality is even more perverse than saying there is no particle reality at all
But if the theory is nonlocal (as the Bell theorem shows that ANY hidden variable theory MUST be), then it should not be surprising at all. In any case, your argument against it (that this kind of reality is "too perverse") seems rather subjective to me.

One additional comment. As you seem to be aware, it is very difficult to make such an experiment (with two spacelike separated clicks caused by a 1-particle state in a superposition of two spacelike separated wave packets) in practice. In fact, to do this in practice, it seems to me that one would need to prepare an entangled state of two DETECTORS. Since detectors are macroscopic objects, it is practically impossible to really do that in practice. Nevertheless, if one would still do that, it would be a demonstration of quantum nonlocality at the MACROSCOPIC level. (The existing experiments demonstrate quantum nonlocality at the microscopic level only.)
 
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  • #93


Demystifier said:
But if the theory is nonlocal (as the Bell theorem shows that ANY hidden variable theory MUST be), then it should not be surprising at all. In any case, your argument against it (that this kind of reality is "too perverse") seems rather subjective to me.

No, it really isn't. You mix up two things here: nonlocality of interaction in hidden variable theories with nolocality of being of point particle events. The very POINT of hidden variable theories is to dispose of the latter (as the Bohmian approach does), so there is a huge difference between unmeasurable nonlocal signalling and *measurable* nonlocality. I would hope you understand that this distinction is the very core of the idea of Bohm-de Broglie.

Demystifier said:
One additional comment. As you seem to be aware, it is very difficult to make such an experiment (with two spacelike separated clicks caused by a 1-particle state in a superposition of two spacelike separated wave packets) in practice. In fact, to do this in practice, one would need to prepare an entangled state of two DETECTORS. Since detectors are macroscopic objects, it is practically impossible to really do that in practice. Nevertheless, if one would still do that, it would be a demonstration of quantum nonlocality at the MACROSCOPIC level. (The existing experiments demonstrate quantum nonlocality at the microscopic level only.)
Yes but I was talking about very tiny detectors of a micron size or so- that should be feasible even with current technology I believe. The point however is that it is more a principled debate and I guess it is time for people in your sector of ideas to put their money on the table and see if you win or lose.
 
  • #94


Careful said:
No, it really isn't. You mix up two things here: nonlocality of interaction in hidden variable theories with nolocality of being of point particle events. The very POINT of hidden variable theories is to dispose of the latter (as the Bohmian approach does), so there is a huge difference between unmeasurable nonlocal signalling and *measurable* nonlocality. I would hope you understand that this distinction is the very core of the idea of Bohm-de Broglie.
You are right. But as I said, such an experiment requires also an entangled state of two DETECTORS. Therefore, we really must deal with Bell-type nonlocality, i.e., nonlocality of interaction in hidden-variable theories.

By the way, your interesting example (suggesting that the Bohmian theory is "too perverse") is quite similar to an example which is already known: the "surreal" particle trajectories. In the case of "surreal" trajectories, the Bohmian trajectory is completely different from the measured trace which is supposed to represent the "actual" particle trajectory. Yet, Bohmian mechanics easily resolves this "paradox" through quantum nonlocality.
 
  • #95


Careful said:
Yes but I was talking about very tiny detectors of a micron size or so- that should be feasible even with current technology I believe. The point however is that it is more a principled debate and I guess it is time for people in your sector of ideas to put their money on the table and see if you win or lose.
I don't see how could I win or lose. Both standard theory and Bohmian mechanics make the same measurable predictions, even in this case. So if the experiment would show that Bohmian mechanics is wrong, then it would also show that standard QM (including QFT) is wrong as well. It would be very interesting, but Bohmian and standard theory would lose together.
 
  • #96


Demystifier said:
You are right. But as I said, such an experiment requires also an entangled state of two DETECTORS. Therefore, we really must deal with Bell-type nonlocality, i.e., nonlocality of interaction in hidden-variable theories.
Yeh so what, that was never a dispute, was it?

Demystifier said:
By the way, your interesting example (suggesting that the Bohmian theory is "too perverse") is quite similar to an example which is already known: the "surreal" particle trajectories. In the case of "surreal" trajectories, the Bohmian trajectory is completely different from the measured trace which is supposed to represent the "actual" particle trajectory. Yet, Bohmian mechanics easily resolves this "paradox" through quantum nonlocality.
But it is not a paradox, you see, it just shows that the ontology is wrong. I have no doubt that you can try to be clever and talk your way out of it, but where lies the point that you have to admit that you are becoming *unreasonable*? To use your own words: perhaps you should not think like an intellectual here and try to get out of the sh*t, but more act like a genius and avoid the brown liquid all together (my translation :wink:).
 
  • #97


Demystifier said:
I don't see how could I win or lose. Both standard theory and Bohmian mechanics make the same measurable predictions, even in this case. So if the experiment would show that Bohmian mechanics is wrong, then it would also show that standard QM (including QFT) is wrong as well. It would be very interesting, but Bohmian and standard theory would lose together.
You see, that is where I disagree, because what you call a paradox, I call an ontological inconsistency ! And it seems to me that such experiments as alluded to above should be forbidden in your theory.

I mean, you may try to be clever now and look for a different reason why the second detector clicks or so, but the point is that this is a well defined prediction even within FREE QFT and I don't need to consider QUANTUM detectors here. So, I don't have to even talk about entanglement between detectors. All I need are just two different local particle bases corresponding to commuting operators whose product is nonvanishing (which is common stuff in free QFT).
 
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  • #98


Careful said:
But it is not a paradox, you see, it just shows that the ontology is wrong. I have no doubt that you can try to be clever and talk your way out of it, but where lies the point that you have to admit that you are becoming *unreasonable*? To use your own words: perhaps you should not think like an intellectual here and try to get out of the sh*t, but more act like a genius and avoid the brown liquid all together (my translation :wink:).
I don't think that this makes the ontology wrong. Counterintuitive yes, but not necessarily wrong.

A better negative characterization of such an ontology could be "unreasonable", as you suggested. But what does it mean "unreasonable"? If there is a general ontological theory written in terms of very simple general equations, and if this theory agrees with all experiments, and if no other simple ontological theory with these properties is known - then this theory IS REASONABLE for me. (And it will remain look reasonable to me unless someone finds an even better candidate ontology.) Even if, in some special cases, it looks counterintuitive.
 
  • #99


Demystifier said:
I don't think that this makes the ontology wrong. Counterintuitive yes, but not necessarily wrong.

A better negative characterization of such an ontology could be "unreasonable", as you suggested.
Ok, I am not going to discuss about whether drinking pure alcohol is bad for ones health (sorry I could not resist) :wink:. But you know, we have already such theory with a realist ontology, and that's the one I explained to you: QFT. You may dislike the ''reality'' of creation operators, but it is not leading to such ''paradoxes'' as you face. That's what I mean, perhaps you are trying to solve the right problem (measurement) in the wrong way. Actually, in my view, you did not solve the issue of awareness yet in the Bohmian approach: suppose all we have are position measurements, how can one speak then about something like music, painting, love and all that. That may sound philosophical to you, but it really isn't.
 
  • #100


Careful said:
I mean, you may try to be clever now and look for a different reason why the second detector clicks or so, but the point is that this is a well defined prediction even within FREE QFT and I don't need to consider QUANTUM detectors here. So, I don't have to even talk about entanglement between detectors. All I need are just two different local particle bases corresponding to commuting operators whose product is nonvanishing (which is common stuff in free QFT).
I disagree. You cannot say that standard QFT predicts two clicks unless you specify HOW EXACTLY you could measure them. And if you try to specify it, you will find out that you need to know something about detectors on the QUANTUM level. Try it, it could be very illuminating!
 
  • #101


Careful said:
But you know, we have already such theory with a realist ontology, and that's the one I explained to you: QFT. You may dislike the ''reality'' of creation operators, but it is not leading to such ''paradoxes'' as you face.
Are you sure that you really HAVE such a realist ontology? Let me test you: Do creation operators exist before you perform a measurement? If not, then what DOES exist before you perform a measurement?

My point is: QFT in its standard form is NOT a theory with a realist ontology. It is merely a (very successfull) tool for calculating probabilities of different measurement outcomes.
 
  • #102


Demystifier said:
I disagree. You cannot say that standard QFT predicts two clicks unless you specify HOW EXACTLY you could measure them. And if you try to specify it, you will find out that you need to know something about detectors on the QUANTUM level. Try it, it could be very illuminating!
I agree with that (if you followed my discussion with arkadiusz) but I just wanted to point out that QFT does not *require* such view.
 
  • #103


Demystifier said:
Are you sure that you really HAVE such a realist ontology? Let me test you: Do creation operators exist before you perform a measurement? If not, then what DOES exist before you perform a measurement?
Sure creation operators do exist but in a timeless and space-less way. Point is, that there is no ontology in space-time prior to measurement.
 
  • #104


Careful said:
Point is, that there is no ontology in space-time prior to measurement.
Fine. And then what exactly happens during the measurement? And what exactly IS a measurement? And can there be a measurement without a conscious observer?
 
  • #105


Demystifier said:
Fine. And then what exactly happens during the measurement? And what exactly IS a measurement? And can there be a measurement without a conscious observer?
No, a theory of consciousness is a mandatory extension of the theory also in your line of thought actually.
 
  • #106


Careful said:
Sure creation operators do exist but in a timeless and space-less way.
If Bohmian ontology is "unreasonable" because it somewhat differs from the observed stuff (surreal trajectories and your example), then, by the SAME criteria, your ontology is even MUCH MUCH MORE unreasonable.
 
  • #107


Demystifier said:
If Bohmian ontology is "unreasonable" because it somewhat differs from the observed stuff (surreal trajectories and your example), then, by the SAME criteria, your ontology is even MUCH MUCH MORE unreasonable.
No, it really is not since my ontology is not in CONFLICT with observation! My ontology doesn't say something about particle trajectories which do not fit observed tracks in a bubble chamber. In my ontology particle paths do not even exist, and neither do particles by the way.
 
  • #108


Careful said:
No, a theory of consciousness is a mandatory extension of the theory also in your line of thought actually.
First, in the Bohmian theory it is clear that measurement does not require consciousness. Second, you haven't answered the first and the second question (that you quoted).
 
  • #109


Demystifier said:
First, in the Bohmian theory it is clear that measurement does not require consciousness. Second, you haven't answered the first and the second question (that you quoted).

Sure Bohmian theory requires consciousness :-) That's what I just said, how can you interpret things like music, love and so on in terms of position measurements. And yes I answered both questions: it is part of a theory of consciousness which I am not going to explain here before I publish it.
 
  • #110


Careful said:
No, it really is not since my ontology is not in CONFLICT with observation!
Your ontology is in conflict with observation in the sense that the ontological stuff is not the observed stuff.
 
  • #111


Demystifier said:
Your ontology is in conflict with observation in the sense that the ontological stuff is not the observed stuff.
So what, isn't that the hidden variable idea ? :smile: I mean nobody says we have to be extreme positivists huh? Now I got you :-) Just kidding...
 
  • #112


Careful said:
Sure Bohmian theory requires consciousness :-) That's what I just said, how can you interpret things like music, love and so on in terms of position measurements.
You are right that Bohmian theory cannot explain consciousness, but that can be said for ALL known theories as well. (However, this does not change the fact that the notion of measurement in the Bohmian theory is well-defined even without consciousness.)

Careful said:
And yes I answered both questions: it is part of a theory of consciousness which I am not going to explain here before I publish it.
Are you saying that you are going to publish it? Or at least that you are seriously working on it and that you already have some promising results? If so, I am really looking forward to see it when you finish it. :smile:
 
  • #113


Demystifier said:
Are you saying that you are going to publish it? Or at least that you are seriously working on it and that you already have some promising results? If so, I am really looking forward to see it when you finish it. :smile:
I will answer you by private mail.
 
  • #114


By the way, WHERE are you? (As somebody already mentioned, I am in Croatia.)
 
  • #115


Careful said:
I will answer you by private mail.
Thank you in advance! :smile:
 
  • #116


Demystifier said:
By the way, WHERE are you? (As somebody already mentioned, I am in Croatia.)
Belgium.
 
  • #117


Careful said:
So what, isn't that the hidden variable idea ? :smile: I mean nobody says we have to be extreme positivists huh? Now I got you :-) Just kidding...


OMG!(Oh my Hidden variable)! Joking aside, I am sure there are other people curious to read the details of your paper.
 
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  • #118


Careful said:
Belgium.
Do you know Thomas Durt?
 
  • #119


Demystifier said:
Do you know Thomas Durt?
No, where is he? Ah I see, free university of Brussels. No, never met him.
 
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  • #120


Careful said:
Sure, the state of the universe exists and the creation operators do too. But if you would ask me whether ''particles'' (ie. detector clicks, let's call the hypothetical elementary particles ''atoms'') exist and whether we know the number of them, my answer would be a resounding no.
Careful said:
Sure creation operators do exist but in a timeless and space-less way. Point is, that there is no ontology in space-time prior to measurement.

Thanks for your answer. This is interesting. My "special interest" is EPRB and I’ve seen you & Demystifier touched the subject.

My question: In what 'camp' are you, Locality or Realism? (i.e. nonlocality/nonseparability)



I might have missed it, but I think you didn’t answer Demystifier’s question, and I also think it’s interesting:
Demystifier said:
Fine. And then what exactly happens during the measurement? And what exactly IS a measurement?
 

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