Abstract
The ’’problem of time” in present physics substantially consists in the fact that a straightforward quantization of the general relativistic evolution equation and constraints generates for the Universe wave function the Wheeler-De Witt equation, which describes a static Universe. Page and Wootters considered the fact that there exist states of a system composed by entangled subsystems that are stationary, but one can interpret the component subsystems as evolving: this leads them to suppose that the global state of the universe can be envisaged as one of this static entangled state, whereas the state of the subsystems can evolve. Here we synthetically present an experiment, based on PDC polarization entangled photons, that shows a practical example where this idea works, i.e. a subsystem of an entangled state works as a ’clock” of another subsystem.
Highlights
[1] As Augustinus Hipponensis present physicists are in the situation where time is an essential physical parameter whose meaning is intuitively clear, but several problems arise when they try to provide a clear definition of time
First of all the definition of time is different in different branches of physics as classical and non-relativistic quantum mechanics, special relativity or general relativity[2]
In the following we will only consider temporal degrees of freedom. This idea of a static universe surprisingly re-proposes, rephrased in modern language, ideas stemming from the work of Parmenides of Elea [”the phenomena of movement and change are appearances of a static, eternal reality”] and diffused in roman-hellenistic word [”Tempus item per se non est, sed rebus ab ipsis consequitur sensus, transactum quid sit in aevo, tum quae res instet, quid porro deinde sequatur” 1 Titus Lucretius Carus, De Rerum Natura]
Summary
“Quid est ergo tempus? si nemo ex me quaerat, scio; si quaerenti explicare velim, nescio.” [1] As Augustinus Hipponensis present physicists are in the situation where time is an essential physical parameter whose meaning is intuitively clear, but several problems arise when they try to provide a clear definition of time. We epitomize an experimental approach [37] to this problem by providing an emblematic example of Page and Wootters idea at work, visualizing how time could emerge from a static (with respect to an abstract external time) entangled state.
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