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Physical review. D. Particles, fields, gravitation, and cosmology/Physical review. D. Particles and fields

Comment on "Observer dependence of quantum states in relativistic quantum field theories"

Ingram Bloch

Published Apr 15, 1984• 1 citation
Publisher

Abstract

In response to Malin's recent paper it is suggested that the important aspect of timing in relativistic descriptions of position determinations is the timing with which a pure state is converted to a mixture, rather than the timing of the mixture's reduction to a new pure state; this suggestion removes some of the subjectivism that Malin finds in quantum states. It is suggested also that viewing quantum mechanics as a branch of psychology raises more questions than it answers.

DOI
10.1103/physrevd.29.1854

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In response to Malin's recent paper it is suggested that the important aspect of timing in relativistic descriptions of position determinations is the timing with which a pure state is converted to a mixture, rather than the timing of the mixture's reduction to a new pure state; this suggestion removes some of the subjectivism that Malin finds in quantum states. It is suggested also that viewing quantum mechanics as a branch of psychology raises more questions than it answers.
We study the instantaneity of the state-reduction process in relativistic quantum mechanics. The conclusion of various authors that this instantaneity will restrict the set of relativistic observables to purely local ones (i.e., that the measurement of any nonlocal property of a system at a well-defined time would give rise to violations of relativistic causality) is found to be erroneous, and experiments (of a kind not encountered before in measurement theory) are described whereby certain nonlocal properties of some simple physical systems can be measured at a well-defined time without violating causality. The attempts of certain authors to reconcile the reduction process with the covariance of the relativistic quantum state are considered and found wanting, and it is argued that the covariance of relativistic quantum theories resides exclusively in the experimental probabilities, and not in the underlying quantum states. The problem of nonlocal measurement is considered in general: distinctions (which are not to be met with in the nonrelativistic case) arise in relativistic quantum mechanics between what can be measured for fermions and what can be measured for bosons, between what can be measured for individual systems and what can be measured for ensembles, and between what kinds of states can be verified by measurement and what kinds of states can be prepared by measurement; and these pose difficult questions about the nature of measurement itself.
I suggest that measurement in quantum theory should be regarded as a sense of time (of things happening), which is as important as the conventional relativistic notion of time. A key question -- of basic physical interest whether one accepts the arguments here or not -- is, What physical criteria determine when a measurement takes place? I suggest a way in which the answer to this may be bound up with the resolution of some pathologies associated with the stress-energy operator, and may at the same time determine the cosmic flow of time. The problem of reconciling the quantum sense of time (measurement) and the conventional relativistic one gives some indication that the the correct "quantization" of gravity is essentially different from that of other fields.
The words: determinism, hidden variables, subjectivism, information, objectivism, informational-theoretic axioms,observers have some connection with physical reality? What we mean with "description" of physical reality? When we say that we understand this reality? Certain parameters:position, velocity are sufficient? We will focus only to conceptual considerations regarding the relation between the "questions" and the relative "answers" in general and specifically in quantum mechanics. It is usually believed that the answers are more important of the questions, for this reason we can read many answers everywhere and in different field of knowledge. We need to add and clarify some things: (i) usually an answers require a question, (ii) but, as we know, their relation is not so simple and immediate, (iii) For instance: a)an epistemic questions give us ontic answers? b)the answer has a connection with the question and vice versa? c)we could to infer a question starting from an answer? d)there are answers without questions? These answers could be in some framework considered as ontic answers? The relative scientific works are the same time ontic? Speaking of quantum mechanics we see around many answers in the meantime we do not see the correspondent questions, these answers seem completely independent, and this seem a right road, the road of the independent nature unlinked from human thoughts. We retain instead that questions can affect the possible answers. Exist "something" before the question?
Recent work has shown that relativistic time dilation results in correlations between a particle's internal and external degrees of freedom, leading to decoherence of the latter. In this note, we briefly summarize the results and address the comments and concerns that have been raised towards these findings. In addition to brief replies to the comments, we provide a pedagogical discussion of some of the underlying principles of the work. This note serves to clarify some of the counterintuitive aspects arising when the two theories are jointly considered.
The measurement of nonlocal properties of relativistic quantum-mechanical systems, the compatibility (or lack of it) of two or more such measurements, and various other of their characteristics are considered; and with these notions in hand an old problem (which is to produce a covariant description of the state reduction associated with the measuring process) is attacked, and succumbs. The solution requires us to depart (not as we did in part I of the present work, but in an entirely different direction) from the usual picture of the time evolution of quantum states.