B Meaning of the word 'instantaneous'

[Deepak K Kapur]

" an electron emits a photon instantaneously" or consider other instantaneous reactions.

What is meant by instantaneous here?

Does it mean there is no time lag between the emission? Does it mean that the emission takes place at a speed greater than the speed of light?

 

[vanhees71]

I've no clue. Of course, in nature nothing happens instantaneously, and a photon needs a (however very short time) to form. Where does this statement come from? I hope, it's not from a serious textbook but from a popular-science book. Note that there are almost no good popular-science books on physics. There are some exceptions: Feynman's books like QED, Weinberg, The first three minutes, Ledermann and Teresi, The God Particle.

 

[Nugatory]

Does it mean there is no time lag between the emission? Does it mean that the emission takes place at a speed greater than the speed of light?

Questions like this are why we have to be cautious about natural language descriptions of phenomena that are more precisely described mathematically - words like "instantaneous" may not be as precise as the speaker had hoped.

You haven't provided the source of the quotation so we have no context and can only guess at what was intended. However, there's a fair chance that they were trying to say that we start in a state with no photon and end up in a state with a photon - but that there are no observable in-between states in which the photon is only partly emitted.

 

[Deepak K Kapur]

Questions like this are why we have to be cautious about natural language descriptions of phenomena that are more precisely described mathematically - words like "instantaneous" may not be as precise as the speaker had hoped.

You haven't provided the source of the quotation so we have no context and can only guess at what was intended. However, there's a fair chance that they were trying to say that we start in a state with no photon and end up in a state with a photon - but that there are no observable in-between states in which the photon is only partly emitted.

This is from a debate/discussion on the nature of reality on you tube in which 9-10 scientists participated.

The actual point is 'is there really no time lag when an electron emits a photon.'

 

[Deepak K Kapur]

I think people are not viewing this thread (especially the expert ones)

OK. I ask in a different way.

What does instantaneous mean?

1. No time lag.

2. A very-very-very small time lag.

This would be easy, hopefully...

 

[vanhees71]

Again, experts are not too interested in pseudo-science. You also have not given your source, where this at best inaccurate statement comes from. It starts with the fact that for itself a single electron cannot emit a photon. You either need it to scatter, leading to bremsstrahlung, or it's bound in an atom and changes from an excited energy level within the atom to a lower-lying energy level (either by stimulated or spontaneous emission).

As I already stated according to quantum theory there is no instanteneous and also no jumps. However, within the here applicable quantum field theory, it is impossible to interpret the transient states during the time evolution where interactions are relevant, in a particle-like fashion. All that's possible to calculate from QFT (in this case QED) are S-matrix elements, which describe the transition-probability rates from an asymptotic free initial (here an electron plus some other particle it scatters from or an electron bound within an atom) to another asymptotic free final state (here an electron + other particles + a photon, or an electron bound in another lower-energy atomic state + a photon).

 

[ftr]

according to quantum theory there is no instanteneous .

What about EPR?

 

[vanhees71]

By construction of relativistic QFT there are no instantaneous interactions at a distance either. This has been discussed countless times in this forum.

 

[ftr]

By construction of relativistic QFT there are no instantaneous interactions at a distance either. This has been discussed countless times in this forum.

You seem to have made a sweeping statement. EPR is considered to be an instantaneous phenomenon(not interaction).

 

[Paul Colby]

EPR is considered to be an instantaneous phenomenon(not interaction).

One could construct a classical version of EPR using two counter rotating gyroscopes. A small explosive device separates the two. Sometime later you measure one gyroscope and, as if by magic, in an instant, you predict the direction of the unmeasured one. Clearly, no non-local interaction is needed to account for this astounding fact. What sticks in peoples craw is the quantum nature of the observables not the correlation.

Now, in regard to the photon emission, how exactly is the time of emission to be known?

 

[ftr]

One could construct a classical version of EPR using two counter rotating gyroscopes. A small explosive device separates the two. Sometime later you measure one gyroscope and, as if by magic, in an instant, you predict the direction of the unmeasured one. Clearly, no non-local interaction is needed to account for this astounding fact. What sticks in peoples craw is the quantum nature of the observables not the correlation.

Now, in regard to the photon emission, how exactly is the time of emission to be known?

The explanation you allude to is not a universal one(a minority). As for emission, that is exactly the point, the formalism imply it is instantaneous.

 

[Paul Colby]

As for emission, that is exactly the point, the formalism imply it is instantaneous.

Which formalism is that? Last I checked the formalism provides one with an absorption or detection rate. How is this a statement about emission time?

 

[ftr]

Because there is no statement, that implies it is instantaneous.

 

[ftr]

Moreover, I don't know why you find it strange since with superposition an electron has undefined state before measurement. That is even more stronger than "instantaneous".

 

[Paul Colby]

Because there is no statement, that implies it is instantaneous.

Not following your logic here. My point, in the limit one finds this interesting, is how exactly is one to frame this question from an experimental or observational point of view? It's unclear to me one may even define the emission time for an individual decay.

 

[ftr]

The emission did happen, right?

 

[Paul Colby]

The emission did happen, right?

One may measure with a finite accuracy or time interval a time of detection. One may then infer again with some finite time interval the emission time based on the distance to the emitter. Neither of these may be confused with instantaneous. Nor do either of these numbers relate directly to the time it took to emit. For this one needs the time of excitation. How do you propose to determine that, and to what accuracy?

 

[ftr]

The first quantization of interaction of light and matter does not go into mechanism. I think we are going in a circle.

 

[Paul Colby]

I think we are going in a circle.

I was trained as an experimentalist. If you think this is a circle that's likely because you frame questions based on an incomplete view of the theory. What is or is not an observable is a non-trivial question. Not everything in field theory is observable. For example the "blue" component of the quark field can't be observed directly because this would violate color symmetry. So, are you asking a question about something that is in principle observable? Framing even an idealized experiment helps give one insight into this type of question.

The first quantization of interaction of light and matter does not go into mechanism.

Okay, so what does the second quantization treatment tell us? Why limit your discussion to a theory known to be incomplete.

 

[ftr]

Ok, since you are an experimentalist

http://www.nature.com/nature/journal/v458/n7235/full/458157a.html

indicating what the formalism implies.

 

[Paul Colby]

Well, "almost instantaneous" is not very quantitative. Can't say I'm interested enough to pay for the article. Things to remember, typical optical frequencies are many order of magnitudes above those encountered in electronics, though this gap is closing. The switching speeds of an optoelectronic device are likely larger than the absorption and emission times. These are still not zero.

 

[vanhees71]

The first quantization of interaction of light and matter does not go into mechanism. I think we are going in a circle.

You cannot describe the emission of photons in first quantization. It's a typical "creation process", which is described by QFT or "2nd quantization" although 1st and 2nd quantization are misnomers, because there's one and only one quantum theory (in this case of electromagnetically interacting particles and radiation, i.e., QED).

 

[vanhees71]

You seem to have made a sweeping statement. EPR is considered to be an instantaneous phenomenon(not interaction).

May be it is considered as such, but there's nothing instantaneous about it, at least not in the minimal statistical interpretation of QT (which is one of the strongest reason for me to consider this interpretation as the only consistent one known so far).

 

[Joyal Babu]

I think they might have used instantaneous because the time in which the phenomena happened was too small to measure experimentally.

 

[DrChinese]

I think people are not viewing this thread (especially the expert ones)

OK. I ask in a different way.

What does instantaneous mean?

1. No time lag.

2. A very-very-very small time lag.

This would be easy, hopefully...

Ha, fat chance. In the quantum and relativistic worlds, these can be interpreted in many ways. You really have to specify an exact context and then ask a question about that. Certainly experimental constraints, including the uncertainty principle, are issues. You have the notion of simultaneity as well. When someone says instantaneous, there is no expected time lag but there may be no way to really verify that. Many times the word "instantaneous" is used without necessarily imply that can be demonstrated rigorously.

 

[Traruh Synred]

" an electron emits a photon instantaneously" or consider other instantaneous reactions.

What is meant by instantaneous here?

Does it mean there is no time lag between the emission? Does it mean that the emission takes place at a speed greater than the speed of light?

It's not clear what 'instantaneous' means. The uncertainty principle limits how well we can know when a photon is emitted.

 

[xAxis]

Can you give the link of the video. Maybe instantaneous in the sense that the speed is immediately c?

 

[Android Neox]

" an electron emits a photon instantaneously" or consider other instantaneous reactions.

What is meant by instantaneous here?

Does it mean there is no time lag between the emission? Does it mean that the emission takes place at a speed greater than the speed of light?

Despite experimental verifications of Special Relativity, which requires non-simultaneity, contemporary quantum mechanical interpretations of entanglement depend upon instantaneous transfer of information. Contemporary physics has no self-consistent model for time.

 

[Paul Colby]

contemporary quantum mechanical interpretations of entanglement depend upon instantaneous transfer of information.

I know we go round and round on this, but, this statement isn't correct is it? Consider two Gyroscopes which are counter rotating which are separated using a torque free explosive device. These spins are correlated and require no information transfer and the situation is perfectly understandable. The problem arrises when the Gyroscopes are replaced with quantum ones. The very same experimental situation occurs in QM except the information is quantum mechanical, not classical. So, while we can think of each gyroscope carrying it's very own pointing direction we are forced by QM to consider each spin as carrying it's own state vector.

 

[DrChinese]

... we are forced by QM to consider each spin as carrying it's own state vector.

Clearly, an entangled system is one system (at least on the entangled basis) - so there is really no "separate state" vector in that sense.

Without a useful mechanical model to discuss, it is difficult to agree with Android's statement: "contemporary quantum mechanical interpretations of entanglement depend upon instantaneous transfer of information." If there is information being transferred, where is it going from/to? No contemporary interpretation seems to answer this in a satisfactory manner. (Even as the underlying formalism works.)

 

[Paul Colby]

If there is information being transferred, where is it going from/to?

I attempted to add a classical example in my previous post so I might isolate the horse and flog on it some more. The classical gyroscopes are separated carrying their "information" with them. When they are measured as isolated systems they are correlated not via some magic instantaneous information transfer but because they they were prepared as such in the past. In the quantum case it is membership in a quantum ensemble that is carried with each system pair. When the quantum systems become isolated they very much do have individual state vectors upon measurement of either component. The "myth" of non-local "interaction" is due to the belief that one is adding randomness through choice of measurement. Arguments I've read in this forum clearly makes this not the case.

 

[DrChinese]

The classical gyroscopes are separated carrying their "information" with them. When they are measured as isolated systems they are correlated not via some magic instantaneous information transfer but because they they were prepared as such in the past. In the quantum case it is membership in a quantum ensemble that is carried with each system pair. When the quantum systems become isolated they very much do have individual state vectors upon measurement of either component. The "myth" of non-local "interaction" is due to the belief that one is adding randomness through choice of measurement.

Ah, what you describe is a local realistic model. Those are of course ruled out by Bell.

There are models (interpretations) that are either non-local or non-realistic that feature elements to explain the quantum correlations. In most of the non-realistic group models, there are no FTL influences and c is respected - but there are other drawbacks. And of course there are non-local models, dBB (Bohmian) being the most well-known.

 

[Paul Colby]

Ah, what you describe is a local realistic model. Those are of course ruled out by Bell.

I clearly don't understand then. All I'm trying to convey is it is a quantum world, no more no less. This is classified (more like branded) as non-realistic world view. Then so be it. All evidence appears to support this non-realistic world.

 

[DrChinese]

I clearly don't understand then [why Bell rules this out, I assume].

The Bell proof makes it clear that the outcome correlations from Alice's and Bob's measurement choices are too strong to be independent (as you described). The problem is: no one understands the underlying mechanism. Again, in some interpretations there is FTL "communication" and in others c is respected.

 

[Paul Colby]

The Bell proof makes it clear that the outcome correlations from Alice's and Bob's measurement choices are too strong to be independent (as you described)

Correct me if I'm wrong (always a safe assumption). Bell's mathematical statement assumes some underlying classical variable(s) are present which accounts for the correlation. In my classical example this variable would be the gyroscope axis direction. No such variables are allowed by the data on QM systems. From a QM viewpoint there is no issue as far as I can tell other than this annoys people who demand a classical resolution which experiments show can't be forthcoming.

This leaves me wondering. There is a whole group of people who insist that there is an essentially mechanical length contraction "ether" explanation behind special relativity. It is accepted that they are wrong and space-time obeys a Lorentz symmetry within our current experimental accuracy. Observation indicates nature is simply that way, yet they refuse to get over it. This is all fine and good. What does trouble me somewhat is the steady rain of QM experiments and papers which are worded to make the naive reader think otherwise WRT the usual QM.

 

[DrChinese]

Correct me if I'm wrong (always a safe assumption). Bell's mathematical statement assumes some underlying classical variable(s) are present which accounts for the correlation. In my classical example this variable would be the gyroscope axis direction. No such variables are allowed by the data on QM systems. From a QM viewpoint there is no issue as far as I can tell other than this annoys people who demand a classical resolution which experiments show can't be forthcoming.

... What does trouble me somewhat is the steady rain of QM experiments and papers which are worded to make the naive reader think otherwise WRT the usual QM.

The Bell proof does not demand any particular type of hidden variable - it could be a function, for example, or a set of functions. Simply calling it "quantum" does not resolve the issue.

The reason there are so many papers worded the way they are is that quantum non-locality can be demonstrated in so many ways - and is so fascinating. Note that quantum non-locality, the phase, is simply referring to the interaction between Alice and Bob (whatever mechanism that accomplishes such).

It may help you to know that it is possible to entangle objects which have never existed in a common light cone. And that can be done *after* they are measured. Quantum non-locality can take many unusual forms.

 

[vanhees71]

I clearly don't understand then. All I'm trying to convey is it is a quantum world, no more no less. This is classified (more like branded) as non-realistic world view. Then so be it. All evidence appears to support this non-realistic world.

Yes, of course, with overwhelming evidence the Bell inequalities are violated with overwhelming significance, and quantum theory is right. So we live in a quantum world. That's it. Case closed. We can switch back to physics in our discussion!

 

[Android Neox]

I know we go round and round on this, but, this statement isn't correct is it? Consider two Gyroscopes which are counter rotating which are separated using a torque free explosive device. These spins are correlated and require no information transfer and the situation is perfectly understandable. The problem arrises when the Gyroscopes are replaced with quantum ones. The very same experimental situation occurs in QM except the information is quantum mechanical, not classical. So, while we can think of each gyroscope carrying it's very own pointing direction we are forced by QM to consider each spin as carrying it's own state vector.

The "Action-at-a-Distance" interpretation of quantum entanglement requires simultaneity... the wavefunction collapse takes place instantly, across all of space, from the perspective of the observer that sees him/her-self as the one to make the first observation of part of the entangled pair.

Since Special Relativity requires non-simultaneity and action-at-a-distance requires simultaneity, I set up a thought experiment to highlight the issue:

 

[Paul Colby]

I set up a thought experiment to highlight the issue

My position is QM requires no additional "interpretation" so I may be boring to talk to. The measurements Bob and Alice do are independent. If either measures their particle they gain knowledge of the other's particle QM state. At this point I say so what? The wave function is more akin to a probability distribution and is only one mathematical piece of QM. The wave function doesn't make a noise when it "collapses".

As to what is observed in your EPR setup is Bob will measure 50% in the up state and 50% in the down state. What Alice will see is also 50% up and 50% down with the added knowledge that she has a particle by particle readout of what Bob will measure suitably inverted in her data. So, since Alice and Bob's measurements are isolated by the finiteness of c, we can conclude that the randomness Bob sees is not caused by an additional randomness introduced by his measurement device (provided it's aligned with Alice's). And, of course, Alice and Bob's roles are entirely reciprocal.

 

[DrChinese]

My position is QM requires no additional "interpretation" so I may be boring to talk to. The measurements Bob and Alice do are independent. If either measures their particle they gain knowledge of the other's particle QM state. At this point I say so what? The wave function is more akin to a probability distribution and is only one mathematical piece of QM. The wave function doesn't make a noise when it "collapses".

As to what is observed in your EPR setup is Bob will measure 50% in the up state and 50% in the down state. What Alice will see is also 50% up and 50% down with the added knowledge that she has a particle by particle readout of what Bob will measure suitably inverted in her data. So, since Alice and Bob's measurements are isolated by the finiteness of c, we can conclude that the randomness Bob sees is not caused by an additional randomness introduced by his measurement device (provided it's aligned with Alice's). And, of course, Alice and Bob's roles are entirely reciprocal.

Some of what you say is agreeable, some not so much.

QM needs no interpretation, the formalism is fine as is (as far as anyone knows). The randomness Bob sees is not caused by an additional randomness introduced by his measurement device. And Alice and Bob's roles are entirely reciprocal. But...

Alice and Bob's choice of measurements may be independent, but their outcomes are NOT. The case of the aligned measurements does not show that, as EPR discovered: it actually implies the existence of hidden variables (and a common cause).

But at almost any other settings, the actual results do not display outcome independence. And the formalism itself demands outcome dependence without consideration of time or distance. For example, entangled photon polarization coincidence is a function of the difference in Alice and Bob's measurement settings (theta). And the correlation is too tight for independence to exist (that's from Bell).

 

[Paul Colby]

And the correlation is too tight for independence to exist (that's from Bell).

Bell's statement of independence doesn't accommodate a QM world. Game over.

 

[DrChinese]

Bell's statement of independence doesn't accommodate a QM world. Game over.

I think it is more that the quantum mechanical world we live in does not feature observer independence. An observer's choice of measurement here is somehow connected to an outcome there, where distance and time interval are not a factor.

 

[Paul Colby]

I think it is more that the quantum mechanical world we live in does not feature observer independence. An observer's choice of measurement here is somehow connected to an outcome there, where distance and time interval are not a factor.

In a QM world observations are always dependent on the observer. For an isolated spin 1/2 system the outcome of a measurement must depend on the measurement made. That this works even non-locally is amazing, yes but completely understandable if one is willing to concede QM as fundamental. Many here can't or won't make that jump. Bell's statement has a classical bias that is actually wrong given the data.

 

[DrChinese]

In a QM world observations are always dependent on the observer. For an isolated spin 1/2 system the outcome of a measurement must depend on the measurement made. That this works even non-locally is amazing, yes but completely understandable ...

Use of the word "isolated" is somewhat ambiguous to me in this context. I might re-phrase: For an isolated system of an entangled pair of spin 1/2 particles, outcome correlations are dependent on the the observers of each. "That this works even non-locally is amazing, yes but completely understandable..."

 

[newjerseyrunner]

Wouldn't it be different depending on if you're in the context of QM or GR? Would these be correct?

In QM instantaneous is any delta in time that is less than the precision allowed by the uncertainty principal.
In GR, it's completely non-sensical since there can be disagreement between observers in the flow of time.

 

[Paul Colby]

Use of the word "isolated" is somewhat ambiguous to me in this context.

isolated as in single non-entangled spin. Spin component depends on the measurement direction

 

[DrChinese]

isolated as in single non-entangled spin. Spin component depends on the measurement direction

Somehow that's what I thought you meant. So I am glad I clarified. The same applies in an entangled 2 particle system; the outcomes are consistent with the measurement choices of both observers. They are not independent as you keep trying to imply.

 

[Paul Colby]

They are not independent as you keep trying to imply.

They are correlated, how is it I implied otherwise? They are independent in that Bob and Alice's measurements may be made in either order in a way that doesn't interact with the other. Alice's measures doesn't effect or in anyway cause Bob's outcome. Alice's data merely determines the single particle QM state of Bob's particle. This is a fact that is true independent of Bob's chosen measurement. For arbitrarily aligned measurements It does so in a way that upsets Bell's concept of "dependence" which fails to hold in QM.

 

[DrChinese]

They are independent in that Bob and Alice's measurements may be made in either order in a way that doesn't interact with the other. Alice's measures doesn't effect or in anyway cause Bob's outcome. Alice's data merely determines the single particle QM state of Bob's particle. This is a fact that is true independent of Bob's chosen measurement.

Alice's measurement choice apparently affects Bob's outcome (by placing Bob into a eigenstate compatible to Alice's), or alternately Bob's measurement affects Alice's outcome - or some mixture of both. The QM prediction does not support any other variables, and requires both choices to explain the results.

That may be the same as what you say in your last 2 sentences, not sure.

 

[votingmachine]

I think people are not viewing this thread (especially the expert ones)

OK. I ask in a different way.

What does instantaneous mean?

1. No time lag.

2. A very-very-very small time lag.

This would be easy, hopefully...

I would say the first, no time lag.

That should not bother anyone. If I hold a ball in my hand and let go, it INSTANTANEOUSLY begins to accelerate downward (it was all along, but my hand stopped the process). There is no time lag between the removal of my restraining force, and the beginning of acceleration. There is no "road-runner-coyote" moment where the coyote pauses, and then falls.

I'm going to say that the electron emitting a photon instantaneously does not bother me. That does not mean the PROCESS is proven to take no time. But the entire process is invisible. If there is an energetic electron, and later a less energetic electron and a photon, there is no intermediate state (that we see) of an electron forming a photon from its decreasing energy. We see the kinetic energy being (continuously) formed, from gravitational potential energy, in the falling ball. But we don't see the (quantum) photon energy being formed from electron-state energy.

My understanding if the models is that the electron is sufficiently "wavy" that it does not behave like a particle (a "ball"), and move from place "A" to place "B". There is a moment it is at "A" and a subsequent moment it is at "B", with ambiguity about how, when, and if there is a transition. In general, the electron cannot be in between, and in general the mass-energy of the electron is conserved. Something that cannot have a path between "A" and "B", and yet goes from "A" to "B", probably goes instantaneously.