Why time is not an observable in quantum theory?

[arkajad]

Ok, I see what you mean, but such observables are all nonlocal...

You would have to define "observable". You would have to define "nonlocal". A measurement never takes place at one point and at one time. The measuring device always occupies some finite region of space, and it reacts only after a certain amount of time when coupled to any system outside. If this is what you call "nonlocal", then all real measurements are nonlocal. So what? Physics can deal with such kind of "nonlocalities". But they are called local. Nonlocality comes into being only when spacelike separated systems can influence one another in a statistically significant way.

 

[Careful]

You would have to define "observable". You would have to define "nonlocal". A measurement never takes place at one point and at one time. The measuring device always occupies some finite region of space, and it reacts only after a certain amount of time when coupled to any system outside. If this is what you call "nonlocal", then all real measurements are nonlocal. So what? Physics can deal with such kind of "nonlocalities". But they are called local. Nonlocality comes into being only when spacelike separated systems can influence one another in a statistically significant way.

Right, so my question is do you guys define an observable from first principles when discussing this? About your interpretation of measurement: well we really don't know that, do we? We have no idea what a measurement even *is*, all we know is what it does: it reveals a number. Anyway, this is a deep question which I feel should not be adressed on a forum. So yeh, I do not know an answer to that issue right away, it seems to me there are distinct possibilities here. Also, we are not going to debate about something as silly as a definition. But you did not answer my question: why would such observables be of fundamental importance to quantum mechanics? So, if you don't like the word observable: you measure either arrival times or time differences, so what should this tell me about the fundamental laws?

 

[arkajad]

Well, we do know what a measurement is. We measure things all the time. We know aht events are. They happen all the time.

Why some type of measurements are of fundamental importance to QM? We do not know whether they will be of fundamental importance in the future or not. And we do know that they were for a long time neglected, theoretically and experimentally. That is why they are interesting. Perhaps something new is lurking there and perhaps not. We will never know if we do not research. That is why more an more people are looking into this business, when new techniques of making measurements become available.

 

[Fredrik]

How can it be an axiom?
We didn't start there. We ended up there based on the theory we developed.

That's not what we did. I described the steps involved in finding a spacetime that's consistent with general relativity. Then I said that if we add another point to this spacetime, it wouldn't be a manifold. That would make it inconsistent with general relativity, which says that spacetime is a manifold (more precisely: a smooth 3+1-dimensional Lorentzian manifold with a metric that solves Einstein's equation). So to include that additional point in the theory, you have to make it an additional axiom. (That would make it a different theory, by my definitions). Since it doesn't lead to any new predictions, or change any of the predictions of general relativity, it's just like an invisible blue giraffe.

So is it inccorrect to say that its a use of the theory to predict (understand) a condition where measurements cannot be made due to physical limitaions as you mentioned in one of your previous replies.

I don't understand that sentence. You're saying something about the time before atoms had formed and no measuring devices could exist. What about it?

 

[Careful]

Well, we do know what a measurement is. We measure things all the time. We know aht events are. They happen all the time.

Why some type of measurements are of fundamental importance to QM? We do not know whether they will be of fundamental importance in the future or not. And we do know that they were for a long time neglected, theoretically and experimentally. That is why they are interesting. Perhaps something new is lurking there and perhaps not. We will never know if we do not research. That is why more an more people are looking into this business, when new techniques of making measurements become available.

Of course we don't : it is not because we do something all the time that we know what it is. It would be like saying that we know God to exist because we have given him a name. We also do not know what physical events are: actually this is an open question in noncommutative spacetime approaches. It is not because we idealize an event to a point that we have understood it. I thought that as a mathematician, you surely would appreciate those points.

Yeh, perhaps there is ... my ''guess'' is that QM might fail for more advanced double slit experiments with massive particles.

 

[dextercioby]

No -- that's why I said "it seems to me...". :-) I looked at quant-ph/9908033 (assuming that's one of the papers you meant?),
but that was a while ago and I forget what I concluded about it.
I'll go take another look...

Cheers.

To add more confusion to this thread when it comes to its coherence, I would add (or probably repeat myself when writing) that there's a real pitty that the approach to QM based on RHS hasn't been pushed significantly deeper. And here I don't mean some new results for a theory that's 85 years old, but I mean at least translating all very well known textbook examples (take Fluegge's problem book) and applications into this rigorous distributional language. The simplest model I could come with right now would be the 1D finite potential. Or was it done and I don't know. Then I would have loved to see models in 2D and in 3D, of course...

EDIT: I just realized that I'm not the only one diverting from the original topic...

 

[rpt]

That's not what we did. I described the steps involved in finding a spacetime that's consistent with general relativity. Then I said that if we add another point to this spacetime, it wouldn't be a manifold. That would make it inconsistent with general relativity, which says that spacetime is a manifold (more precisely: a smooth 3+1-dimensional Lorentzian manifold with a metric that solves Einstein's equation). So to include that additional point in the theory, you have to make it an additional axiom. (That would make it a different theory, by my definitions). Since it doesn't lead to any new predictions, or change any of the predictions of general relativity, it's just like an invisible blue giraffe.

Fredrik,
I know, that I shouldn't say things without fully dunderstanding general relativity and related mathematics. I am telling things based on what you explained to me, to further clarify things for me. I will explain why I said so.

You said FLRW solution gives (+ve curvature) family of 3-sphere as a solution to space-time. Then you said requiring that Einsteins relation satisfing this spacetime results in a relation that relates the parameter defining the 3-sphere and its radius.

I thought the 3-sphere that results at t=0 which is a point does not violate what you said above. I interpreted it as the limiting case of the solution. If we talk about gradients of surfaces in the model (which also exist in limiting sense) why do not include this point as a one which complies with the theory without having additional axioms. At this point there is no space so it does not need any coordinates to describe anything. It just that we include this point in the solution space becase it does't violate the theory. Why does it violate the definition of space-time manifold? Is it not continuous and homogenous?

 

[strangerep]

[...] I looked at quant-ph/9908033 [...] but that was a while ago
and I forget what I concluded about it.

Now I remember... the paper is full of heavy functional-analytic
argument and requires lengthy (re-)reading and sustained concentration.
The author works with unbounded operators in ordinary Hilbert space,
and therefore has to be very careful about domains of definition for
(powers of) the unbounded operators. I doubt that I'll sort it all
out properly in my own mind anytime soon. :-(

there's a real pitty that the approach to QM based on RHS hasn't been pushed
significantly deeper. And here I don't mean some new results for a theory
that's 85 years old, but I mean at least translating all very well known
textbook examples (take Fluegge's problem book) and applications into this
rigorous distributional language. The simplest model I could come with right
now would be the 1D finite potential. Or was it done and I don't know. Then I
would have loved to see models in 2D and in 3D, of course...

Well, any example that uses Dirac bra-ket formalism with
distribution-valued inner product, etc, is secretly using RHS.
But going "deeper" requires significantly heavier math.

In his textbook, Ballentine presents RHS early as the "natural" setting
for QM, but it is rarely mentioned in most of the subsequent chapters, iirc.

Rafael de la Madrid has made some attempts to expose RHS to a
larger readership -- though he limits the heavy math to keep
the papers accessible to that readership. If you look through
his papers via Google Scholar, you find a couple where he treats
the 1D potential example.

I just realized that I'm not the only one diverting from the original topic...

Yeah, we should stop.

 

[rpt]

If what I am saying makes any sense,
The credit should go to Fredrik who explained me beautifully what is the mathematical representation of space-time in general relativity in few lines in very simple language.
I only tried to make an interpretation of what the model may be telling us.

 

[DevilsAvocado]

"Why "time" is not an observable in quantum theory?"

rpt, you did observe time outside quantum theory?? What does it look like!? I’m dying of curiosity!

(joking friendly )

 

[arkajad]

You do not observe time, even outside quantum theory (what color does it have?, what extension?). You observe events. You arrange them causally in your mind. You count small events between bigger events.

 

[Demystifier]

The inadequacy of Bohm de Broglie theory for QFT (there really is no acceptable scheme for particle creation) really shows that such ideas belong to the stone age.

I guess you haven't seen recent papers such as:
http://xxx.lanl.gov/abs/0904.2287 [Int. J. Mod. Phys. A25:1477-1505, 2010]
http://xxx.lanl.gov/abs/1007.4946
Would that be "acceptable" enough?

 

[Careful]

I guess you haven't seen recent papers such as:
http://xxx.lanl.gov/abs/0904.2287 [Int. J. Mod. Phys. A25:1477-1505, 2010]
http://xxx.lanl.gov/abs/1007.4946
Would that be "acceptable" enough?

Ohw I have, but that did not change my conclusion

 

[rpt]

DevilsAvocado,

My understanding is that "time" is not real.
(Therefore unphysical - sorry about this comment)

 

[Demystifier]

Ohw I have, but that did not change my conclusion

So, in your opinion, why is that not acceptable?

And please, don't sound like dishonest people that you refer to in
https://www.physicsforums.com/showpost.php?p=3007820&postcount=37
(I liked that post very much!)

 

[Careful]

So, in your opinion, why is that not acceptable?

And please, don't sound like dishonest people that you refer to in
https://www.physicsforums.com/showpost.php?p=3007820&postcount=37
(I liked that post very much!)

Fair enough... like arkajad you will have to wait for a moment (I really have to limit my time here for one hour a day and I just wrote a long reply to Alkmetheli - if I spelled that right ). I will do it a last wednesday evening, ok?

 

[Demystifier]

OK!

 

[Careful]

OK!

Ok, before, I read it, let me ''test'' you a bit to see if you really understood the difficulty here. If I would take in QFT a simple particle state like

(a ^{*}(f_k) + a^{*}(g_l) ) | 0 > where f_k and g_l are well normalized and centralized wave packages let's say with rougly momentum k and l which fly in distinct directions. Suppose both packages start at A and f_k reaches B and g_k reaches C. Now B *and* C perform a *local* measurement at the *same* time. What would be the right conclusion: (a) nothing is observed since it is not a two particle state or (b) both detectors at B and C could click? So in other words, does this state represent one particle or two particles?

PS: this is not a dumb question.

 

[Demystifier]

Ok, before, I read it, let me ''test'' you a bit to see if you really understood the difficulty here. If I would take in QFT a simple particle state like

(a ^{*}(f_k) + a^{*}(g_l) ) | 0 > where f_k and g_l are well normalized and centralized wave packages let's say with rougly momentum k and l which fly in distinct directions. Suppose both packages start at A and f_k reaches B and g_k reaches C. Now B *and* C perform a *local* measurement at the *same* time. What would be the right conclusion: (a) nothing is observed since it is not a two particle state or (b) both detectors at B and C could click? So in other words, does this state represent one particle or two particles?

PS: this is not a dumb question.

This is a 1-particle state, so one and only one of the detectors will click: either B or C.

(If the efficiency of the realistic detectors is not 100%, then it is also possible that none of them will click.)

 

[Careful]

This is a 1-particle state, so one and only one of the detectors will click: either B or C.

Wrong answer ! Think about it better. For example, in QFT the reality would depend on what all observers measure, so you know suppose I have two apparati away from one and another and both are set to measure the position of the incoming particle on the detector screen, then such operator does not allow for *no measurement*, so what to do here? The hands on-prescription for QFT would be to ignore the operator where no measurement has taken place, but that is not really what has been going on, we ''activated'' both operators. So this is what Rafael Sorkin might call an impossible measurement. Anyway, in practice only one detector will click, but the point I wanted to make is that QFT is ignorant about the ''reality'' of this prediction. So think about this now in the context of the original state - the answer is much more subtle than this.

 

[Demystifier]

Wrong answer ! ... Anyway, in practice only one detector will click

But that's what I said; that only one detector will click. So how can my answer be wrong? At worst it may be incomplete, but not wrong.

 

[Careful]

But that's what I said; that only one detector will click. So how can my answer be wrong? At worst it may be incomplete, but not wrong.

You don't get it, I shoud have phrased it in the following way: if you believe one particle to be present and you ignore this subtle issue, then you might conclude that only one particle will be measured. Now, you should not ignore this issue and think why it could lead to the conclusion that (dependent upon the circumstances) both detectors click !

Hint: construct two local observables which are not diagonal in the particle basis.

 

[arkajad]

But that's what I said; that only one detector will click.

Indeed, that is what happens, and that is what can be easily modeled with an appropriate extension of QT. And there is certainly more than one such extension.

 

[Careful]

Indeed, that is what happens, and that is what can be easily modeled with an appropriate extension of QT. And there is certainly more than one such extension.

Again, ''wrong'' answer ... it depends upon how you construct the detector and what you are going to measure. There is no single reality in a single creation operator! There are good reasons though why in scattering experiments we may ignore these subtleties in practice but because this is a fundamental issue about *reality* we cannot be that blunt. Just make the excercise where you construct two local nondiagonal projection operators, take their product and see what happens in case you let both points approach one and another.

 

[arkajad]

Just make the excercise where you construct two local nondiagonal projection operators, take their product and see what happens in case you let both points approach one and another.

I've made my exercise. Now it is time for you to make yours - and publish it, so that other people can examine it and take it apart.

 

[Careful]

I've made my exercise. Now it is time for you to make yours - and publish it, so that other people can see and take it apart.

Phew, that's a bit on the defensive side no? It was actually Demystifier who asked me to give my reasons why I think a Bohm-de Broglie type of reality fails (I simply said that his paper did not convince me to change my mind). He has to understand that if he does that, he declares lots of theoretically possible experiments in QFT as impossible, so he should take this from the good side and go and study precisely those experiments where disagreement in the interpretation may arise.

 

[DevilsAvocado]

You do not observe time, even outside quantum theory

Well, that was kind of my "hidden message"...

 

[arkajad]

He has to understand that if he does that, he declares lots of theoretically possible experiments in QFT as impossible...

I have studied some of QFT, certainly not all, but somehow I have forgotten which part of QFT tells you unquestionably which experiments are theoretically possible and which are not. Can you be so kind and remind how this can be unquestionably deduced from QFT and from which particular version of it?

Perhaps wrongly but I had an impression that that this a debatable subject that does not belong strictly to QFT but to its rather fuzzy meta-structure that differs from one author to another and that is being constantly checked by experiments.

 

[DevilsAvocado]

DevilsAvocado,

My understanding is that "time" is not real.
(Therefore unphysical - sorry about this comment)

Don’t be sorry, we are all more or less finding our way through the purple haze here. These things are not 'finally' settled.

(Caution: I’m only a layman. Everything I say MIGHT be wrong.)

I think I know what you are asking and why. This is my personal view on the "problem":

In our everyday human life we have a very strong perception that time 'flows', forward. We are depending of a past, present and future, for our human brains to work properly. You can’t remember the future, and hopefully you do not make predictions about the past.

In this macroscopic world, that we spend our life in, physics has four-dimensional coordinate system to form a 4D manifold, representing Spacetime, consisting of the three spatial dimensions (length, width, height) and the temporal dimension (time), giving 3+1 = 4. This is called the Minkowski Space after the mathematician Hermann Minkowski.

In General Relativity, Spacetime is assumed to be SMOOTH and CONTINUOUS – not just in the mathematical sense.

There are several 'reasons' time has a direction forward in the macroscopic world, and maybe the strongest reason (scientifically) is the thermodynamic arrow of time, which could maybe simplest be described as – things are always getting worse (more disordered). Or; if you see two pictures of an egg – one when the egg is whole and one when it’s broken into pieces on the floor – you know for sure which picture was taken first in time.

As you see time, length, width and height, are only DIMENSIONS in the macroscopic spacetime. And naturally – it’s impossible to measure or observe "1 Kilometer" in spacetime as an 'independent' object. What we do is to use the metric system to measure the spatial length between objects, e.g. the Sun and Earth, and we get that the distance is 150 million kilometer. If France didn’t have their way, we could have used any other human-made scale, and get that distance is 75 billion turtles (and then you could have observed "1 turtle"! ). The actual distance between the Sun and Earth would still be exactly the same in both cases though.

To make observations in the macroscopic world, we need objects and events, which cannot 'exist' without spacetime, and vice versa. No objects, no space. No space, no objects. No events, no time. No time, no events.

According to Relativity, space and time are 'flexible', and the speed of light is the only fixed constant. This means that at extreme gravity, like inside a Black hole, time stops = no events. Even light 'freezes' and can’t escape the Black hole.


Now to the microscopic quantum world.

First problem:

As given by the name, QM uses distinct quanta, discrete values, to describe microscopic world. E.g. the energy levels of electrons in atoms or molecules, is said to be quantized. And when an electron 'jumps' to a higher or lower energy level, it does this INSTANTANEOUSLY, not SMOOTH and CONTINUOUS. To apply spacetime to QM, many scientists believe that spacetime should be quantized at the very smallest scale (which is not the easiest task in the world as far as I know).

Anyhow, the current state is that microscopic QM and macroscopic Relativity is not 100% compatible at extreme scales and temperatures, like the Big Bang, and the calculations breakdown in endless infinities. To get full compatibility, we need a solution to quantum gravity, and that is definitely one of the toughest current problems in physics = instant NP + $$$$$$$ ...when solved.​

Second problem:

If we want to observe the exact speed and position of a cannonball at different moments in time, we can do that fairly easy, without expecting the Nobel Prize in Physics for the achievement. If one were to repeat that with an object in the QM world – one would surely get Nobel Prize in Physics, since it’s considered impossible due to the Heisenberg uncertainty principle. We cannot do complete observations of objects in the QM world, and this is not an "apparatus problem", it’s the fundamental nature of the QM world.​

Third problem:

QM is mathematical construction that works perfectly well. On that all agrees. Not everyone agrees on what this mathematical construction describes, there are different interpretations. Some say the QM world doesn’t exist – "Shut up and calculate!". And some say it does. I have absolutely no clue what’s true...​

Fourth problem:

Historically, the founding fathers of QM removed space and time from any underlying reality in QM, when Werner Heisenberg in 1925 introduced matrix equations. The next 'blow' for Einstein came in 1926 when Max Born proposed that QM was to be understood as probabilities, without any causal explanation. No space, no time and no causality... This was too much for Einstein and made him "go haywire". Einstein he used the rest of his life trying to find "another solution", without success.

My guess is that most of the work that was made by the founding fathers in the 1920s, is still valid in today’s QM – No space, no time, no causality.​

Fifth problem:

As far as I understand, if you do try to measure time in the QM world, you will find out that it’s perfectly symmetric. There is NO difference between the past and future! If you 'inspect' two different 'pictures' of a "QM egg", it’s IMPOSSIBLE to tell which is before and after! (You have to ask the experts, who knows what they are talking about , for more info on this).​

Hope this spread some light on QM and reality of time.

Watch this little movie to see what happens to a human that has only 7 to 30 sec "now":

https://www.youtube.com/watch?v=<object width="480" height="385">
<param name="movie" value="http://www.youtube.com/v/wDNDRDJy-vo&fs=1&amp;hl=en_US&amp;rel=0&amp;color1=0x402061&amp;color2=0x9461ca"></param>
<param name="allowFullScreen" value="true"></param>
<param name="allowscriptaccess" value="always"></param>
<embed src="http://www.youtube.com/v/wDNDRDJy-vo&fs=1&amp;hl=en_US&amp;rel=0&amp;color1=0x402061&amp;color2=0x9461ca" type="application/x-shockwave-flash" allowscriptaccess="always" allowfullscreen="true" width="480" height="385"></embed>
</object>

This definitely changed my view on the human need of time...

 

[Careful]

I have studied some of QFT, certainly not all, but somehow I have forgotten which part of QFT tells you unquestionably which experiments are theoretically possible and which are not. Can you be so kind and remind how this can be unquestionably deduced from QFT and from which particular version of it?

Perhaps wrongly but I had an impression that that this a debatable subject that does not belong strictly to QFT but to its rather fuzzy meta-structure that differs from one author to another and that is being constantly checked by experiments.

Sure, you are entirely correct here, but the point is that demystifier does not seem to have grasped this issue. That is also why I told you many posts ago that I appreciated what you tried to do: by constructing measurement devices, you try to get a grip on this question. However, I do feel that more questions can be asked about nature than just position observations (such as the Bohmians do) and this issue complicates severy once one goes over to QFT. I have never seen actually a realistic construction of ''localized'' position observables within QFT made by ardent proponents of this approach. What Demystifier tells in the abstract is yeh, ''not adressing this core issue'' to put it mildly. Once you turn on gravity, this issue gets even exponentially more complicated than it is in flat spacetime, so I definately feel this is not the right way to go, although I certainly do not have a proof against it.

 

[Careful]

To stimulate the discussion in that direction, one should first declare the beables in QFT (according to Bell) and how those beables correspond to observables. Next one should delimit the category (I don't say algebra because I guess it won't be an algebra anymore) of observables which correspond to realistic experiments and measurement apparati so that no interpretational conflicts arise with the reality given by the beables.

For example, in QFT we idealize and construct a number operator, but this is really not an observable and I would not promote it to a beable either. It appears unwise to do so, for me the only beables in QFT are the creation operators: you may have trouble visualizing this or thinking about it in ''physical'' terms, but that is what the formalism actually tells you. In my view, QFT tells us to forget about particles but instead to talk in terms of measurements of energy, momentum and angular momentum (or position if you insist) - or tracks in a bubble chamber as a matter of fact. We notice that those observations can all be derived from elementary quantum numbers and those define what we call particles. So our measurements are derived from an atomic notion, but I would not say at all that we ''measure atoms'' all the time.

So, any theory which denies the reality of the creation operators, has to go through all those steps: and that is what I would like to see, since it will lead to novel predictions.

 

[arkajad]

I tend to agree that creations of particles are "events", perhaps the only real and fundamental events that we can have at this point of our knowledge about functioning of the Nature. Yet they are classical events in the sense that either a particle has been created or not. They are 0-1 Boolean beables. As particles can be created at any place at any time - there should be a mechanism of coupling such a 0-1 field (well, there are different kinds of particles, but let us leave it alone for a while) to the field states. I may have one idea about how it can be done in detail. I would like to see how it can be done in a different way than by a non-Hamiltonian, Lindblad-type coupling.

 

[DevilsAvocado]

Careful & arkajad, I’m just curios – what’s your opinion: Does anything in QM exist before measurement, "creation", or tracks in a bubble chamber, etc?

 

[Careful]

I tend to agree that creations of particles are "events", perhaps the only real and fundamental events that we can have at this point of our knowledge about functioning of the Nature. Yet they are classical events in the sense that either a particle has been created or not. They are 0-1 Boolean beables. As particles can be created at any place at any time - there should be a mechanism of coupling such a 0-1 field (well, there are different kinds of particles, but let us leave it alone for a while) to the field states. I may have one idea about how it can be done in detail. I would like to see how it can be done in a different way than by a non-Hamiltonian, Lindblad-type coupling.

I agree with what you say except that I would not associate these creation operators to particles per se as I stressed before. They correspond to pure particles for some observables, but for others, they don't.

 

[Careful]

Careful & arkajad, I’m just curios – what’s your opinion: Does anything in QM exist before measurement, "creation", or tracks in a bubble chamber, etc?

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. Hypothetically, we have a number operator for the ''atoms'', but that does not translate directly (under all circumstances, in practice it works rather well if the ''atoms'' are sufficiently far away from one and another) into a number of ''particles''. That is my objection against the Bohmian approach (where ''atoms'' and ''particles'' are one and the same thing). I hope demystifier shows up from the vacuum and joins the party :-)

For example, I say it rather abstractly here, but in ordinary QFT one can construct bound states out of different ''atom'' creation operators and those are to be regarded as a single ''particle'' (in terms of representation theory of the Poincare algebra). So instead of say, 3 clicks, you will just get one. A bound state therefore is nothing but an eigenstate of a complicated observable, this observable will not be diagonal in the ordinary free particle basis and can lead to such effects as I explained before. For example, one apparatus may look for a pure ''atom'' and the other one for a bound state. Then you could have two registrations even for just a single ''atom''. I don't say it is likely, but it is possible in theory.

 

[DevilsAvocado]

... I hope demystifier shows up from the vacuum and joins the party :-)

The position of the vacuum is normally Croatia and the time over there is 01:20 AM, so I guess it’s going to take awhile...

I’m basically at the same longitude and I’m having severe troubles with my eyelids right now... neeed tooo... :zzz:

Thanks for your answers! I get back to you tomorrow, promise!

 

[Careful]

The position of the vacuum is normally Croatia and the time over there is 01:20 AM, so I guess it’s going to take awhile...

I’m basically at the same longitude and I’m having severe troubles with my eyelids right now... neeed tooo... :zzz:

Thanks for your answers! I get back to you tomorrow, promise!

Then we are in the same timezone, I work better at night so I will stay up still for a while.

 

[arkajad]

So instead of say, 3 clicks, you will just get one.

Only now I understand what were you talking about. Of course within QFT there are detectors that will act like that. But there are also other that will react differently.

As to "what exists before" - this question belongs, in my opinion, to the Philosophy section, not to Quantum Physics. But, roughly, I think John Archibald Wheeler may have been right when he speculated that Nature somehow "observes (or 'measures') itself all the time" - this is http://en.wikipedia.org/wiki/Ouroboros" .

 

[Careful]

Only now I understand what were you talking about. Of course within QFT there are detectors that will act like that. But there are also other that will react differently.

As to "what exists before" - this question belongs, in my opinion, to the Philosophy section, not to Quantum Physics. But, roughly, I think John Archibald Wheeler may have been right when he speculated that Nature somehow "observes (or 'measures') itself all the time" - this is http://en.wikipedia.org/wiki/Ouroboros" .

It isn't philosophy anymore when you take the Bohmian idea seriously! That's my point (and objection at the same time). Cute picture btw :-)

I deliberately talked about it in this way to make people think about it: by making it more abstract and not referring to particular examples, one gets a deeper insight into the nature of this debate.

 

[Demystifier]

... the conclusion that (dependent upon the circumstances) both detectors click !

Hint: construct two local observables which are not diagonal in the particle basis.

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.

 

[Careful]

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 ). 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.

 

[Demystifier]

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.)

 

[Careful]

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.

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.

 

[Demystifier]

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.

 

[Demystifier]

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.

 

[Careful]

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?

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 ).

 

[Careful]

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).

 

[Demystifier]

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 ).

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.

 

[Careful]

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) . 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.

 

[Demystifier]

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!