Richard Livingston <
richalivingston@gmail.com> wrote:
On Wednesday, July 20, 2022 at 1:39:22 AM UTC-5, Richard D. Saam wrote:
On 7/14/22 1:55 AM, Jonathan Thornburg [remove -color to reply] wrote:
...
(As Phillip Helbig noted
in another message in this thread, in our universe 1/CMBR_temperature
can serve as such a global time coordinate.)
Is there another?
...
I'm inclined to agree with JT about the 8 hour period in the JPL data.
There is too much human and earth related effects to be confident that
that is something universal. And there is absolutely no theoretical
basis for such an effect.
Would that be an 8 hour (siderial) or an 8 hour (solar) period?
The cosmic background temperature is one good possibility for a
universal time, although it would be very difficult to use it to
determine the time with any precision useful for humans. It should be recognized that any such "time" would be a convention that everyone
concerned would have to agree to, and as such there are many
possibilities.
The cosmic background temperature isn't really a clock.
It could be used to define an epoch though.
(like epoch 10 Kelvin)
In practice such an epoch would be completely useless,
since it is in practice nothing but a distance scale.
(distance away ~ distance in the past)
Or you may give the equivalent redshift.
It is nowadays feasable to actually measure
the cosmic background temperature long ago/far away
by high resolution spectroscopy.
The occupation of hyperfine levels of long-lived states
gives the cosmic background temperature that applied then/there.
A little more practical and accurate would be a time scale based on the distance between two galaxies as measured in some specified inertial
frame. Another would be based on some master clock in a single
particular location in the universe.
Which would be time tied to that particular location,
so by definition not a universal time.
With an understanding of special
relativity it is possible for everyone everywhere to calculate the time
on that clock for the observers location IN THE MASTER CLOCK INERTIAL
FRAME. (Sorry about yelling, but that last part is important!) This
would not be a simple calculation for any observer that is accelerating,
but in principle can be done. If the master clock is transmitting time signals by radio, the observer can measure the distance back to the
master clock and determine the current time, again having to take into account the observers motion wrt the master clock and the distance. Of course at great distances when massive objects are near the line of
sight, gravitational lensing would complicate this calculation.
But special relativity is irrelevant here. (Lorentz invariance applies) Gravitational effects determine the clock rates.
Typical examples: Here on Earth clock rates vary with altitude.
(more precisely, with the Newtonian potential)
This has been easily measurable for at least fifty years.
To cope with it TAI has been invented.
TAI is a weighted average, computed by the BIPM,
taking the different clock rates into account,
by reducing the rates to mean sea level.
(beware of technicalities!)
TAI is good enough for all timekeeping on Earth.
TAI is not good enough though for precision tracking of satellites,
when taking relativistic corrections into account.
So the IAU has defined TCG, which is the time of a clock
that is co-moving with the Earth, but located 'infinitely' far away.
(in practice out of the gravitational potental well of the Earth.
It sufices for 'sublunar' orbital calculations. (such as GPS)
(don't know about L2)
Again, TCG is not good enough for tracking motions in the solar system.
So, (get the pattern?) the IAU has defined TCB, which is the time
of a clock that co-moves with the solar system barycentre,
but which is out of the gravitational well of the solar system. [1]
(and next the galaxy, the local cluster, the ....,
if we lived long enough)
So 'universal time' is not possible
because gravity has an infinite range,
and you can never be free from it.
There cannot be an absolute zero of the gravitational potential,
where you could station a universal clock.
A universal clock would have to be outside the universe,
which cannot be.
So we must do with practical clocks, that are practically adequate,
for a given purpose,
Jan
[1] These corrections are huge, by modern standards.
For example, TCB runs about half a second/year fast wrt to TAI.
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