On the physical basis of cosmic time

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Garth
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I have always stressed the need, when talking about cosmological concepts, such as length, mass, energy and time, to define how such entities are measured.

In particular, when talking about time we need to relate the unit 'second' to a physical clock by which that second is measured. Problems arise in the very early universe when such clocks might not yet exist.

A paper on today's arXiv makes the same point: On the physical basis of cosmic time.

In this manuscript we initiate a systematic examination of the physical basis for the time concept in cosmology. We discuss and defend the idea that the physical basis of the time concept is necessarily related to physical processes which could conceivably take place among the material constituents available in the universe. It is common practice to link the concept of cosmic time with a space-time metric set up to describe the universe at large scales, and then define a cosmic time t as what is measured by a comoving standard clock. We want to examine, however, the physical basis for setting up a comoving reference frame and, in particular, what could be meant by a standard clock. For this purpose we introduce the concept of a `core' of a clock (which, for a standard clock in cosmology, is a scale-setting physical process) and we ask if such a core can--in principle--be found in the available physics contemplated in the various `stages' of the early universe. We find that a first problem arises above the quark-gluon phase transition (which roughly occurs when the cosmological model is extrapolated back to 10-5 seconds) where there might be no bound systems left, and the concept of a physical length scale to a certain extent disappears. A more serious problem appears above the electroweak phase transition believed to occur at 10-11 seconds. At this point the property of mass (almost) disappears and it becomes difficult to identify a physical basis for concepts like length scale, energy scale and temperature -- which are all intimately linked to the concept of time in modern cosmology. This situation suggests that the concept of a time scale in `very early' universe cosmology lacks a physical basis or, at least, that the time scale will have to be based on speculative new physics.
My own work defines two gauges in which time is measured, one in which fundamental particle masses are constant and the other in which the energy of an individual photon in the CMB is defined to be constant.

We define the two time systems by sampling two photons, one emitted by a caesium atom the other sampled from the CMB radiation.

The first, an "atomic" second, is defined as the duration of exactly 9.19263177x109 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom.

The second, a "photonic" second, is defined as the duration of exactly 1.604x1011 periods of the radiation corresponding to the peak of the CMB black body spectrum.

Both systems of time measurement are physically significant and agree with each other in the present era, although they will diverge from each other at other times.

The “photonic” second retains a physical basis even at very early times when particles and particle masses do not.


Garth
 
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Garth, I don't want to de-emphasize the importance of measurement. But I find the subversion of the measurement urge (by reality itself) to be one of the most interesting things... about reality.

For photons especially, being uncertain in their state vector, non-local and acausal even when lounging at home.

Uncertainty:
"It is not possible, according to Heisenberg, to construct for the present physical state a description D(i) as precise as is needed if we wish to predict a future state with a probability close to 1" p212


Reversability:
Though I'm sure the precision of what you describe would be astounding for even non-everyday modelling, doesn't Reversibility (aside from uncertainty) make it impossible to establish a theoretically non-statistical (non-varying) model for time? i.e. There are no known physical processes that span the phenomenon of time (only statistical 2nd Law TD even points to it). All others are time-independent. If a metric is not causally conistent then it is surely variably precise.

"It appears that mixing processes, in the most general sense of the term, are the instruments which indicate a direction of time. They do so becauses they translate the... directional symmetry of the time ensemble into an asymmetry of the space ensemble. This leads to two distinct meanings of phrase "probability that a low-entropy state si preceded by a state of high entropy" and makes it possible to account for the inference from entropy to time" p122-123

References: "The direction of Time" Hans Reichenbach, Dover Press, NY 1956 ISBN 0-486-40926-0 (pbk)
 
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Wallace
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Garth, I think I'm missing the point here (not to critize, I'm just missing something)? What is the significance of the two gauges for measuring time that you are referring to, what might we learn from considering these different gauges?

I'm not so sure that the 'photonic' time would work in the very very earlier universe, since at that there are no particles as such, and that includes, at least in my understanding of it, photons. In any case, I'm also not seeing why measuring time in the very very early universe is a huge issue, since it's an uncertain epoch in a lot of ways and the precise length of that epoch is not a particularly important question.
 
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Garth
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Garth, I think I'm missing the point here (not to critize, I'm just missing something)? What is the significance of the two gauges for measuring time that you are referring to, what might we learn from considering these different gauges?

I'm not so sure that the 'photonic' time would work in the very very earlier universe, since at that there are no particles as such, and that includes, at least in my understanding of it, photons. In any case, I'm also not seeing why measuring time in the very very early universe is a huge issue, since it's an uncertain epoch in a lot of ways and the precise length of that epoch is not a particularly important question.
In building a theory we have to assume certain physical laws hold on which that theory is constructed.

In the earliest universe it is generally taken that Planck time and distance still hold and a temperature may be assigned to that particular epoch, which has been calculated back from the present epoch CMB temperature. So that at 10-43 seconds the temperature is given as nearly 1033 K. Given that temperature the characteristic photon energy may be assigned of ~ 1019 Gev and from Wien's law a frequency of ~ 1044 secs-1. It is this frequency that becomes the "'core' of the 'photonic' clock" at the Planck era.

A normal 'atomic' clock that is based on the mass of a fundamental particle cannot be so defined at this epoch. That was the point of the Rugh & Zinkernagel paper.

I was just pointing out that it is possible to define an alternative clock for these epochs.

Note that using the alternative 'photonic' clock the universe is measured to be static and eternal (photons 'expand' with the universe - [itex]\lambda \propto R[/itex]) - this insight may resolve some of the problems of origin in the standard theory, and may have relevance to arguments put forward in the Kinematic GR models of expansion thread.

Garth
 
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Chronos
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Your approach also suggests alternative approaches are viable, Garth. How do they compare? I think you could write a very good review paper on this subject.
 
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Garth
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Your approach also suggests alternative approaches are viable, Garth. How do they compare? I think you could write a very good review paper on this subject.
As you know Chronos my own SCC theory had such a 'photonic clock' yet in its 2002 manifestation did not pass the GP-B geodetic precession test.

It may be that a viable alternative theory, possibly a re-write of the 2002 theory or another completely different one, may yet use physically significant 'photonic' time.

In the SCC theories gravitational clocks, i.e. ephemeris time, follow this 'photonic' time.

One intriguing observation is that the use of such ephemeris time would explain the Pioneer Anomaly. See Peter Ostermann's eprint; Relativity theory and a Real Pioneer Effect

Garth
 

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