We can know the proportion of isotopes at the time the rock solidified by measuring it. The usual technique is the Isochron Method, see
Isochron Dating for an explanation. For a general introduction to Radiometric and other dating methods I would suggest:
Radiometric Dating - A Christian Perspective.
This is still presupposing a
constant half life, which is prescinding from possible relativistic considerations.
Creationist sources are not reliable when discussing dating methods, they have an agenda and they select what they say according to that agenda. Woodmorappe failing to point out the temperature implications of “plasma” is a case in point. You would not go to an atheist website to learn about Catholicism; neither should you go to a creationist website to learn about science.
Without going too in depth - on a tangent - this goes both ways. Evolutionist scientists could also be said to have an agenda. The only way to hope to attain to the truth is to take all arguments on their own merits.
There are some uncertainties about the early universe, but those uncertainties are confined within known limits by what we can observe here and now.
I would turn it around. The fact that we only know our velocity
relative to the celestial phenomenon we
can observe means that we do not even know our relativistic distortion -
for certain - in the here and now - much less 1000 or 1000000 years ago.
All we have are theories of relativity and quantum mechanics - which we admit we do not fully understand - and what we can perceive. We have no way of knowing our speed relative to phenomena we have yet to observe.
Those known limits completely rule out a 10,000 year old universe, as favoured by AiG. These early uncertainties have nothing to do with the formation of the earth, which happened around nine billion years after the start of the universe.
Again - you are throwing time figures around like they are absolute. Time is not absolute, only the speed of light is. That is my whole point.
Firstly, there is no such thing as “absolute speed” in a relativistic universe. All speeds are relative because there is no absolute frame of reference.
As far as we know - and excluding the speed of light itself of course. In principle, there is no other absolute speed. However, if there is an unobserved, unmoving substance/object (perhaps dark matter even) then as a matter of practice, all other speeds would be relative to that which is immovable.
Secondly, neither our galaxy nor the local cluster existed in the early universe.
Not relevant to my point. The point is even in the here in now, all we know is relative speed, to observable phenomena. We have no way of know our relative speed to other, unobserved phenomena. In other words, we do not even know our relative speed - only our speed relative to the objects we can observe. Nor do we know all the gravitational forces exerted on our whole galaxy - as witnessed by the need some scientists feel to hypothesize dark matter to explain the fact that not all motion is accounted for by the gravitational forces of the observed celestial phenomena.
It took around 700,000 years for the universe to cool down enough for solid matter to form, and it took even longer for galaxies and local clusters to form once the matter had condensed. By that time significant relativistic effects would have been confined to the immediate vicinity of black holes. Elsewhere relativistic effects are not very significant locally, which is why Newton’s non-relativistic theory of gravity worked so well for so long. Even now it is accurate enough to be used for travel between planets, except Mercury which is close enough to the Sun to require some corrections for the curvature of space.
Again - you are assuming the whole solar system is not under the influence of other, more all-encompassing relativistic effects. The point is we cannot even explain all the measured gravity in the known universe by recourse to observed phenomena. And that is in the present. It seems to me we have very little present surety on which to based rock-solid afffirmations about the distant (or not so distant) past.