Quantum Jumps and Schrodinger's Cat are predictable

[Demystifier]

Well, this is often the case with Nature papers. I find this disturbing too! The only point is that if you read the text, it becomes clear that the abstract and introduction is just "popular-science gibberish", and in the rest of the paper the science usually gets correctly stated. That's the difference to many popular-science articles, where often you don't even understand the science, if you are an expert in the field ;-)).

I think Nature has policy that all their readers (biologists etc.) can understand abstracts and introductions of all their papers. That, I believe, is where the pop-science gibberish comes from.

 

[Demystifier]

To the contrary, may I ask you to tell me, where you need a collapse? You cannot use standard QED and propose the collapse argument, because that's contradicting each other.

Standard QED can be expressed as a claim that the state $|\psi(t)\rangle$ evolves as $e^{-iH_{QED}t}|\psi(0)\rangle$ where $H_{QED}$ is the local QED Hamiltonian, except when a measurement is performed in which case $|\psi(t)\rangle$ collapses. In this form there is no logical contradiction between collapse and locality of $H_{QED}$. The problem is that such a formulation is ambiguous because it is not clear what exactly is a measurement and what isn't, but you would probably agree that it is only a philosophical problem because in practice one always knows what is a measurement and what isn't.

Of course, it doesn't mean that collapse is necessary. But if you want quantum theory without the collapse, you need either many worlds or additional variables. In particular, the minimal statistical ensemble interpretation is a theory in which the additional variables are implicit but one refuses to talk about them explicitly. (One refuses to talk about the additional variables because one cannot say much about them with certainty without introducing some philosophy in the form of additional hypotheses that cannot be directly tested in experiments). Bohmian mechanics can be thought of as an extension of the minimal statistical ensemble interpretation, in which one risks his reputation of a serious scientist by deciding to talk about the additional variables explicitly.

 

[A. Neumaier]

the use of these outdated notions by physicists even in scientific papers and (even worse) introductory textbooks is indeed a bad habit, but what can you do...

The term ''quantum jump'' is used as a standard, well-defined in quantum optics in a very appropriate way, even in very highly cited technical work. The usage of the term has steadily grown a lot since Herzberg 1944: A google scholar search for ''quantum jump" in quotation marks give 173 papers up to 1960, growing in the following decades 1961-1970 to 547, 1971-1980 to 1880, then to 2320, 3240, 5950, and 2011-2019 to 6730.

But though not working in the field you feel qualified to decree what is outdated. When did the notion become outdated, and according to which criteria?

 

[vanhees71]

Standard QED can be expressed as a claim that the state $|\psi(t)\rangle$ evolves as $e^{-iH_{QED}t}|\psi(0)\rangle$ where $H_{QED}$ is the local QED Hamiltonian, except when a measurement is performed in which case $|\psi(t)\rangle$ collapses. In this form there is no logical contradiction between collapse and locality of $H_{QED}$. The problem is that such a formulation is ambiguous because it is not clear what exactly is a measurement and what isn't, but you would probably agree that it is only a philosophical problem because in practice one always knows what is a measurement and what isn't.

Of course, it doesn't mean that collapse is necessary. But if you want quantum theory without the collapse, you need either many worlds or additional variables. In particular, the minimal statistical ensemble interpretation is a theory in which the additional variables are implicit but one refuses to talk about them explicitly. (One refuses to talk about the additional variables because one cannot say much about them with certainty without introducing some philosophy in the form of additional hypotheses that cannot be directly tested in experiments). Bohmian mechanics can be thought of as an extension of the minimal statistical ensemble interpretation, in which one risks his reputation of a serious scientist by deciding to talk about the additional variables explicitly.

That's my very point! Collapse proponents claim that you need to envoke some mysterious event when a measurement is made which is outside the dynamics of the very theory you try to interpret. That makes no sense since after all measurement apparati are made of usual matter and thus function according to the general physical laws as any other piece of matter. It doesn't make sense to claim that only because something is measured the interaction between the measurement apparatus and the measured object must be described by some esoteric law outside the general physical laws.

In practice, however, measurement apparati are constructed by using the general physical laws, and the observational fact that they function as predicted is proof enough that the general physical laws are applicable to measuremente devices as to any other piece of matter.

 

[vanhees71]

The term ''quantum jump'' is used as a standard, well-defined in quantum optics in a very appropriate way, even in very highly cited technical work. The usage of the term has steadily grown a lot since Herzberg 1944: A google scholar search for ''quantum jump" in quotation marks give 173 papers up to 1960, growing in the following decades 1961-1970 to 547, 1971-1980 to 1880, then to 2320, 3240, 5950, and 2011-2019 to 6730.

But though not working in the field you feel qualified to decree what is outdated. When did the notion become outdated, and according to which criteria?

Sigh. As I said, it's very clear in which sense the notion of "quantum jump" is meant. It's NOT the outdated view a la Bohr within "old quantum mechanics". It's the transition between energy eigenstates of some Hamiltonian due to perturbation. E.g., the usual energy eigenstates of the hydrogen atom are calculated leaving out terms of the full QED Lagrangian. As soon as you take the corresponding radiative corrections into account, you get spontaneous emission and thus quantum jumps from excited states to lower states. This is, because the approximate energy eigenstates are not energy eigenstates of the full Hamiltonian, and the spontaneous emission of a photon in that case is not a quantum jump of "old quantum mechanics" but a dynamical process as any other in QED, and for sure it's not instantaneous.

Aside from this, only that some wording is frequently used is not necessarily a hint that this might be good practice didactics wise.

 

[A. Neumaier]

Sigh. As I said, it's very clear in which sense the notion of "quantum jump" is meant. It's NOT the outdated view a la Bohr within "old quantum mechanics". It's the transition between energy eigenstates of some Hamiltonian due to perturbation.

Yes, and everybody in this thread except you understood it in this way. You alone ranted against the name. You want to reserve the name quantum jump for Bohr's old understanding, but others find the term far too descriptive to put it permanently to rest.

the spontaneous emission of a photon in that case is not a quantum jump of "old quantum mechanics" but a dynamical process as any other in QED, and for sure it's not instantaneous.

This non-instantaneous dynamical process is called in modern quantum optics (and already long ago) a quantum jump. (As any jump in real life it takes time, but can often be idealized as being instantaneous.)

In quantum mechanics (which can be used without invoking QED), the quantum jump is represented by a collapse of the state (another very common term that you decree to be taboo) when a small quantum system passes a filter where it undergoes scattering, or when a single atom is manipulated in an ion trap.

If you would stop fighting for your ideosyncratic restriction of this common terminology in the scientific literature on quantum mechanics, some of the repetitive overhead in the foundational discussions would go away.

 

[vanhees71]

This socalled "collapse" is also a dynamical process, not a quantum jump and nothing that's outside of the dynamics of QT.

Of course, in non-relativistic QM you can enwoke instantaneous processes as an "explanation" without being in conflict with causality, but you cannot do so within relativistic local microcausal QT, because that would be a contraction.

I think it's very important to emphasize this point, and if it comes to debates on the foundations, the use of clear and unambiguous language is utmost important. That's why in my opinion one should not use some of the (in my opinion unfortunate) standard notions in the scientific literature (among them "quantum jumps", "collapse"). Even worse are philosophical notions like "realism"...

 

[A. Neumaier]

This socalled "collapse" is also a dynamical process, not a quantum jump and nothing that's outside of the dynamics of QT.

Of course. But it is a dynamical process of QED, and must therefore be postulated explicitly in simple quantum mechanics for nonexperts.

That the collapse cannot be instantaneous follows already from the fact that performing a measurement or passing a filter takes time, and was well-known very early in the discussion of foundations. For example, in his 1932 book, von Neumann writes:

we have repeatedly shown that a measurement [...] must be instantaneous, i.e., must be carried through in so short a time that the change [...] is not yet noticeable

He makes clear that instantaneous is just an idealization for ''very short time''.

Of course, in non-relativistic QM you can invoke instantaneous processes as an "explanation" without being in conflict with causality, but you cannot do so within relativistic local microcausal QT, because that would be a contradiction.

So what? In relativistic local microcausal QT you can not even invoke Born's rule - since it implies positive probabilities of a system prepared locally for being observed one second later light years away. See Hegerfeldt's paper
Instantaneous spreading and Einstein causality in quantum theory,
Annalen der Physik 7 (1998), 716--725.

 

[vanhees71]

Indeed, spontaneous emission is one of the (amazingly few) things you cannot make plausible in the semiclassical interpretation. Everything else you can, e.g., putting the hydrogen atom in a weak classical em. radiation field (e.g., a plane wave solution) and discuss the corresponding absorption and induced-emission "quantum jumps" via (first-order) time-dependent perturbation theory (usually in the dipole approximation, leading to the usual well-known selection rules for em. transitions). From this calculation you see very well that you get the literal quantum jump only in an idealizing approximation. Otherwise the occupation probabilities for the hydrogen eigenstates turn out to be smooth functions of time (oscillatory in this case).

 

[Demystifier]

That's my very point!

So do you agree, as I argued in the post, that the minimal statistical ensemble interpretation is a theory in which additional variables are implicit?