Newsgroup: it.scienza.fisica
Titolo: Tempo quantistico

Ciao, se andate su sci.physics era stato aperto da me un thread sul tempo
quantistico e non avendogli detto che non ero un fisico mi hanno PURE
rimproverato alcuni errori :-)))
Cmq tutto è nato dal seguente estratto dal Scientific American:
Is time quantized? In other words, is there a fundamental unit of time that
could not be divided into a briefer unit?
John Baez is a member of the mathematics faculty at the University of
California at Riverside and one of the moderators of the on-line
sci.physics.research newsgroup. He responds:
"The brief answer to this question is, 'Nobody knows.' Certainly there is no
experimental evidence in favor of such a minimal unit. On the other hand,
there is no evidence against it, except that we have not yet found it. There
are no well-worked-out physics theories incorporating a fundamental unit of
time, and there are substantial obstacles to doing so in a way that is
compatible with the principles of General Relativity. Recent work on a
theory of quantum gravity in which gravity is represented using loops in
space suggests that there might be a way to do something roughly along these
lines--not involving a minimum unit of time but rather a minimum amount of
area for any two-dimensional surface, a minimum volume for any
three-dimensional region in space and perhaps also a minimum 'hypervolume'
for any four-dimensional region of space-time."
William G. Unruh is a professor in the department of physics and astronomy
at the University of British Columbia. He offers this reply:
"There is certainly no experimental evidence that time--or space for that
matter--is quantized, so the question becomes one of whether there exists a
theory in which time is quantized. Although researchers have considered some
theories in which there is a strict quantization of time (meaning that all
times are an integer multiple of some smallest unit), none that I know of
has ever been seriously regarded as a viable theory of reality--at least,
not by more people that the original proponent of the theory.
"One could, however, ask the question in a slightly different way. By
putting together G (Newton's constant of gravity), h (Planck's constant) and
c (the velocity of light), one can derive a minimum meaningful amount of
time, about 10-44 second. At this temporal scale, one would expect quantum
effects to dominate gravity and hence, because Einstein's theory links
gravity and time, to dominate the ordinary notion of time. In other words,
for time intervals smaller than this one, the whole notion of 'time' would
be expected to lose its meaning.
"The biggest obstacle to answering the question definitively is that there
exists no really believable theory to describe this regime where quantum
mechanics and gravity come together. Over the past 10 years, a branch of
theoretical physics called string theory has held forth the greatest hope,
but it is as yet far from a state where one could use it to describe the
nature of time in such a brief interval."
Another, somewhat iconoclastic perspective on this question comes from
William G. Tifft, a professor of astronomy at the University of Arizona:
"There are several ways to answer this question. 1) There is no conclusive
evidence that time is quantized, but 2) certain theoretical studies suggest
that in order to unify general relativity (gravitation) with the theories of
quantum physics that describe fundamental particles and forces, it may be
necessary to quantize space and perhaps time as well. Time is always a
1-dimensional quantity in this case. 3) My own work, which combines new
theoretical ideas with observations of the properties of galaxies,
fundamental particles and forces, does suggest that in a certain sense time
may indeed be quantized. To see this we need some background information; in
this scenario, time is no longer 1-dimensional!
"My colleagues and I have observed that the 'redshifts' of galaxies seems to
be quantized. The redshift is the apparent shift in the frequency of light
from distant galaxies. This shift is toward the red end of the spectrum and
its magnitude increases with distance. If redshifts were due to a simple
stretching of light caused by the expansion of the universe, as is generally
assumed, then they should take on a smooth distribution of values. In fact,
I find that redshifts appear to take on discrete values, something that is
not possible if they are simply due to the cosmic expansion. This finding
suggests that there is something very fundamental about space and time which
we have not yet discovered.
"The redshifted light we observe is consists of photons, discrete
'particles' of light energy. The energy of a photon is the product of a
physical constant (Planck's constant) times the frequency of the light.
Frequency is defined as the reciprocal of time, so if only certain redshifts
are possible, then only certain energies are present, and hence only certain
frequencies (or, equivalently, time intervals) are allowed. To the extent
that redshifts of galaxies relate to the structure of time, then, it
suggests an underlying quantization.
"In our newest theoretical models we have learned to predict the energies
involved. We find that the times involved are always certain special
multiples of the 'Planck time,' the shortest time interval consistent with
modern physical theories. The model we are working with not only predicts
redshifts but also permits a calculation of the mass energies of the basic
fundamental particles and of the properties of the fundamental forces. The
model implies that time, like space seems to be three dimensional.
We now think that three-dimensional time may be the fundamental matrix of
the universe. In this view, fundamental particles and objects--up to the
scale of whole galaxies--can be represented as discrete quantized structures
of 3-d time embedded within a general matrix of 3-D time. The structures
seem to be spraying radially outward from an origin point (time = 0): a
big-bang in 3-D time. Any given chunk, say our galaxy, is flowing outward in
3-D time along its own 1-dimensional track, a 1-D timeline. Inside our
(quantized) chunk we sense only ordinary 3-D space, and the single
1-dimension time flow of our chunk of 3-D time.
"Now we can finally attempt to answer the original question, whether time is
quantized. The flow of time that you sense corresponds to the flow of our
chunk of 3-D time through the general matrix of 3-D time. This time is
probably not quantized. Both ordinary space and ordinary 'operational' time
can be continuous. On the other hand, the structure of the time intervals
(frequencies and energies) that make up the 3-D chunks of time which we call
galaxies (or fundamental particles) does appear to be quantized in units
connected to the Planck scale. In the 3-D time model, space is a local
entity. Galaxies are separated in 3-D time, which we have misinterpreted as
separation in space.
"What matters in 3-D time is the time intervals needed to send signals
between galaxies; separation of galaxies in time, not space, is fundamental.
The general matrix of 3-D time appears to contain discrete 'particles' of
3-D 'time.' These particles are the galaxies. When photons travel between
galaxies, the result is a quantized structure that we see as quantized
redshifts. When photons travel within a single 3-D temporal structure, we
see only ordinary 3-D spatial dynamics and continuous flowing time. Believe
it or not, it seems that we can have it both ways--the underlying structure
of time can be 3-D and quantized, but structures in time can flow
continuously."
Che cosa ne pensate? A me - da profano - la quantizzazione del tempo appare
ovvia visto che  'tutto' è quantizzato.
Ovvero, parlando con un ingegnere elettronico una volta sono finito a
discutere della lunghezza di Planck e un pò semplicisticamente, per farmi
capire le cose, questo ingegnere mi disse che sotto tale scala il nostro
universo 'cessava di esistere'. Ora prendiamo la cosa più piccola che possa
esistere e vediamo che tempo ci mette a reagire. Se questo è il tempo più
breve possibile, ovvero se nient'altro reagisce ad una velocità maggiore,
nenanche a livello teorico, allora possiamo dire che quello è il quanto del
tempo. Infatti noi misuriamo il tempo calcolando la durata di certi eventi.
Ma se esiste un evento la cui durata rappresenta il più breve intervallo di
tempo misurabile, ANCHE IN PRINCIPIO, allora quello è il quanto del tempo. O
no?
Ho pure coniato un possibile nome per tale quanto: il CRONONE, da kronos=
tempo.....come fotone da photos o gravitone da...gravità :-)))
Ciao ciao.

  Ma guarda, la questione della quantizzazione del  tempo
  e` difficile e anche molto di piu` di quello
  che Unruh e C. vogliono far credere.
  La meccanica quantistica NON e` in grado matematicamente di supportare
  un modello di tempo quantizzato per vari motivi (come ha provato  tra
  l`altro Pauli 50 anni fa, ma tanta gente non conosce certe cose...)
  Esisitono tentativi  seri recenti basati sulle misure a valori di
  operatori positivi.
  La roba con i loops secondo me lascia il tempo che trova per queste 
  questioni fini...
  Ciao, valter
 

Salve mi piacerebbe conoscere gli o l'articolo dove Pauli dimostro' cio'
che affermi o anche un testo sul quale se ne faccia riferimento
Grazie!
Saluti Corrado

Grazie ! :-)