Recordações do Futuro. Por que não?

Por que não lembramos do futuro? Essa pergunta faz parte de discussões de física teórica sobre a flecha do tempo, viagens no espaço-tempo que poderiam violar causalidade, um universo imaginado pelo cérebro de Boltzmann etc.

A física não sabe porque o passado é diferente do futuro, uma vez que todas as leis conhecidas da física moderna podem funcionar do futuro para o passado de maneira similar ao do passado para o futuro.

O escritor tem uma resposta antrópica:

Somos a memória que temos, sem memória não saberíamos quem somos.

Saramago.

Ao final das contas, a discussão cai sobre o tempo. Qual é a sua natureza e significado?

A FQXi Community promoveu um concurso de ensaios sobre a natureza do tempo. Foram escolhidos e classificados 24 ensaios.  Além dos ensaios premiados, destaco o da nossa colega do blog Theorema Egregium

On the Nature of Time – Or Why Does Nature Abhor Deadlocks?
By Christine Córdula Dantas

This essay aims at introducing a novel point of view on the nature of time, inspired by a synthesis of three seemingly unrelated concepts: Bergson’s notion of duration, Dijkstra’s notion of concurrency, and Mach’s notion of inertia.

Vejam os primeiros classificados do concurso da FQXi Communit:

The Nature of Time
By Julian Barbour

A review of some basic facts of classical dynamics shows that time, or precisely duration, is redundant as a fundamental concept. Duration and the behaviour of clocks emerge from a timeless law that governs change.

Does Time Exist in Quantum Gravity?
By Claus Kiefer

Time is absolute in standard quantum theory and dynamical in general relativity. The combination of both theories into a theory of quantum gravity leads therefore to a ‘problem of time’. In my essay I shall investigate those consequences for the concept of time that may be drawn without a detailed knowledge of quantum gravity. The only assumptions are the experimentally supported universality of the linear structure of quantum theory and the recovery of general relativity in the classical limit. Among the consequences are the fundamental timelessness of quantum gravity, the approximate nature of a semiclassical time, and the correlation of entropy with the size of the Universe.

What if Time Really Exists?
By Sean Carroll

Despite the obvious utility of the concept, it has often been argued that time does not exist. I take the opposite perspective: let’s imagine that time does exist, and the universe is described by a quantum state obeying ordinary time-dependent quantum mechanics. Reconciling this simple picture with the known facts about our universe turns out to be a non-trivial task, but by taking it seriously we can infer deep facts about the fundamental nature of reality. The arrow of time finds a plausible explanation in a “Heraclitean universe,” described by a quantum state eternally evolving in an infinite-dimensional Hilbert space.

Forget Time*
By Carlo Rovelli

Following a line of research that I have developed for several years, I argue that the best strategy for understanding quantum gravity is to build a picture of the physical world where the notion of time plays no role at all. I summarize here this point of view, explaining why I think that in a fundamental description of nature we must “forget time”, and how this can be done in the classical and in the quantum theory. The idea is to develop a formalism that treats dependent and independent variables on the same footing. In short, I propose to interpret mechanics as a theory of relations between variables, rather than the theory of the evolution of variables in time.

The Flow of Time*
By George F. R. Ellis

Current theoretical physics suggests the flow of time is an illusion: the entire universe just is, with no special meaning attached to the present time. This paper points out that this view, in essence represented by usual space-time diagrams, is based on time-reversible microphysical laws, which fail to capture essential features of the time-irreversible nature of decoherence and the quantum measurement process, as well as macro-physical behaviour and the development of emergent complex systems, including life, which exist in the real universe. When these are taken into account, the unchanging block universe view of spacetime is best replaced by an evolving block universe which extends as time evolves, with the potential of the future continually becoming the certainty of the past; spacetime itself evolves, as do the entities within it. However this time evolution is not related to any preferred surfaces in spacetime; rather it is associated with the evolution of proper time along families of world lines. The default state of fundamental physics should not be taken to be a time irreversible evolution of physical states: it is an ongoing irreversible development of time itself.

Free will, undecidability, and the problem of time in quantum gravity*
By Rodolfo Gambini & Jorge Pullin

In quantum gravity there is no notion of absolute time. Like all other quantities in the theory, the notion of time has to be introduced “relationally”, by studying the behavior of some physical quantities in terms of others chosen as a “clock”. We have recently introduced a consistent way of defining time relationally in general relativity. When quantum mechanics is formulated in terms of this new notion of time the resolution of the em measurement problem can be implemented via decoherence without the usual pitfalls. The resulting theory has the same experimental results of ordinary quantum mechanics, but every time an event is produced or a measurement happens two alternatives are possible: a) the state collapses; b) the system evolves without changing the state. One therefore has two possible behaviors of the quantum mechanical system and physical observations cannot decide between them, not just as a matter of experimental limitations but as an issue of principle. This first-ever example of fundamental undecidability in physics suggests that nature may behave sometimes as described by one alternative and sometimes as described by another. This in particular may give new vistas on the issue of free will.

Electron time, mass and zitter

By David Hestenes

de Broglie’s original idea that the electron has an internal clock has recently received experimental confirmation by measuring the period of the clock in an electron channeling experiment. This result has been explained by a new model of the electron, called the zitter model because it incorporates Schroedinger’s qualitative zitterbewegung concept into a fully specified interacting particle model. The zitter electron is a lightlike charged particle with intrinsic spin that maintains it in a helical spacetime path, with curvature and frequency determined by the electron mass. Thus, electron mass is fully reduced to clock frequency in electron motion. This essay discusses details of the model and its implications.

Read more: Time reversal, by A. Zee, Boltzmann Antropic Brain, Arrow of Time FAQ, Feynman on Boltzmann Brain. Tudo é muito legal, mas a gente gasta um bom tempo lendo e pensando. Gasta tempo?

Vamos filosofar.