Why time is not an observable in quantum theory?

[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.