Artículos con la etiqueta ‘History and Philosophy of Physics (physics.hist-ph)’

Heating the coffee by looking at it. Or why quantum measurements are physical processes

Por • 17 feb, 2014 • Category: Crítica

Using a very simple Gedankenexperiment, I remind the reader that (contrary to what happens in classical mechanics) the energy of a quantum system is inevitably increased just by performing (some) textbook measurements on it. As a direct conclusion, this means that some measurements require the expenditure of a finite amount of energy to be carried out. I also argue that this makes it very difficult to regard measurements as disembodied, immaterial, informational operations, and it forces us to look at them as physical processes just like any other one.

Can the clicks of the detectors provide a complete description of Nature?

Por • 16 feb, 2014 • Category: Educacion

No matter how counterintuitive they are, quantum phenomena are all simple consequences of the laws of Quantum Mechanics. It is not needed to extend the theory with hidden mechanisms or additional principles to explain what Quantum Mechanics already predicts. This indubitable fact is often taken as supporting the view that all we can know about the universe comes from the outcomes of the quantum observations. According to this view, we can even learn the physical laws, in particular the properties of the space, particles, fields, and interactions, solely from the outcomes of the quantum observations. In this article it is shown that the unitary symmetry of the laws of Quantum Mechanics imposes severe restrictions in learning the physical laws of the universe, if we know only the observables and their outcomes.

Toward a principle of quantumness

Por • 13 feb, 2014 • Category: Ciencia y tecnología

Quantum correlations and other phenomena characteristic to a quantum world can be understood as simply consequences of a principle derived from the projection postulate. This principle states that these specifically quantum phenomena are caused by the tension between the constraints imposed by incompatible observations. This tension is found to be at the root of Bohr’s complementarity, Heisenberg’s uncertainty, results concerning non-locality, contextuality, quantum correlations in time and space.

The Two Bell’s Theorems of John Bell

Por • 7 feb, 2014 • Category: Crítica

Many of the heated arguments about the meaning of «Bell’s theorem» arise because this phrase can refer to two different theorems that John Bell proved, the first in 1964 and the second in 1976. His 1964 theorem is the incompatibility of quantum phenomena with the dual assumptions of locality and determinism. His 1976 theorem is the incompatibility of quantum phenomena with the unitary property of local causality. This is contrary to Bell’s own later assertions, that his 1964 theorem began with that single, and indivisible, assumption of local causality (even if not by that name). While there are other forms of Bell’s theorems — which I present to explain the relation between Jarrett-completeness, «weak locality», and EPR-completeness — I maintain that Bell’s two versions are the essential ones. Although the two Bell’s theorems are logically equivalent, their assumptions are not, and the different versions of the theorem suggest quite different conclusions, which are embraced by different communities. For realists, the notion of local causality, ruled out by Bell’s 1976 theorem, is motivated implicitly by Reichenbach’s Principle of common cause and explicitly by Einstein’s relativity Principle, and it is the latter which must be forgone.

Two Kinds of Discovery: An Ontological Account

Por • 6 feb, 2014 • Category: Educacion

What can we discover? As the discussion in this paper is limited to ontological considerations, it does not deal with the discovery of new concepts. It raises the following question: What are the entities or existents that we can discover? There are two kinds of such entities: (1) actual entities, and (2) possible entities, which are pure possibilities. The paper explains why the first kind of discovery depends primarily on the second kind. The paper illustrates the discoveries of individual pure possibilities by presenting examples such as the Higgs particle, Dirac’s positron, and Pauli-Fermi’s neutrino.

How do life, economy and other complex systems escape the heat death?

Por • 6 feb, 2014 • Category: Opinion

The primordial confrontation underlying the existence of our universe can be conceived as the battle between entropy and complexity. The law of ever-increasing entropy (Boltzmann H-theorem) evokes an irreversible, one-directional evolution (or rather involution) going uniformly and monotonically from birth to death. Since the 19th century, this concept is one of the cornerstones and in the same time puzzles of statistical mechanics. On the other hand, there is the empirical experience where one witnesses the emergence, growth and diversification of new self-organized objects with ever-increasing complexity. When modeling them in terms of simple discrete elements one finds that the emergence of collective complex adaptive objects is a rather generic phenomenon governed by a new type of laws. These ‘emergence’ laws, not connected directly with the fundamental laws of the physical reality, nor acting ‘in addition’ to them but acting through them were called by Phil Anderson ‘More is Different’, ‘das Maass’ by Hegel etc

Gauge Symmetry, Spontaneous Breaking of Gauge Symmetry: Philosophical Approach

Por • 3 feb, 2014 • Category: Leyes

This paper deals with the ontology of the vector potential. When the state of the system has the full gauge symmetry of the Hamiltonian, the electromagnetic vector potential may be interpreted as a convenient tool of a mathematical formulation, with no ontological meaning. I argue that this interpretation is in difficulty because the vector potential becomes proportional to the supercurrent in the superfluid phases, which are spontaneously broken gauge symmetry phases, where particle number is not conserved. I suggest that when gauge symmetry is spontaneously broken, the vector potential becomes an emergent material object of nature.

Is ergodicity a reasonable hypothesis?

Por • 30 ene, 2014 • Category: Leyes

In the physics literature «ergodicity» is taken to mean that a system, including a macroscopic one, visits all microscopic states in a relatively short time. We show that this is an impossibility even if that time is billions of years. We also suggest that this feature does not contradict most physical considerations since those considerations deal with correlations of only a few particles.

The Nature of the Controversy over Time-Symmetric Quantum Counterfactuals

Por • 29 ene, 2014 • Category: Leyes

It is proposed that the recent controversy over «time-symmetric quantum counterfactuals» (TSQCs), based on the Aharonov-Bergmann-Lebowitz Rule for measurements of pre- and post-selected systems, can be clarified by taking TSQCs to be counterfactuals with a specific type of compound antecedent. In that case, inconsistency proofs such as that of Sharp and Shanks (1993) are not applicable, and the main issue becomes not whether such statements are true, but whether they are nontrivial. The latter question is addressed and answered in the negative. Thus it is concluded that TSQCs, understood as counterfactuals with a compound antecedent, are true but only trivially so, and provide no new contingent information about specific quantum systems (except in special cases already identified in the literature).

Einstein’s first gravitational field equation 101 years latter

Por • 27 ene, 2014 • Category: Educacion

We review and strengthen the arguments given by Einstein to derive his first gravitational field equation for static fields and show that, although it was ultimately rejected, it follows from General Relativity (GR) for negligible pressure. Using this equation and considerations folowing directly from the equivalence principle (EP), we show how Schwarzschild metric and other vacum metrics can be obtained immediately. With this results and some basic principles, we obtain the metric in the general spherically symmetric case and the corresponding hydrostatic equilibrium equation. For this metrics we obtain the motion equations in a simple and exact manner that clearly shows the three sources of difference (implied by various aspects of the EP) with respect to the Newtonian case and use them to study the classical tests of GR.