Once again, this is where young earthism comes into conflict with the core, central issue at stake: the need for accurate reporting of what scientists actually do. “A uniformitarian presupposition is inserted into the equation to obtain millions of years of age” is simply not how radiometric dating — or anything else for that matter — is done in reality.
Contrary to young earth claims, scientists do not just blindly assume that rates of change were constant. Nor do they do so just to get millions of years out of the equation. They only assume that rates are constant if they have strong theoretical or experimental reasons to do so, and they rigorously test those assumptions every way that they can.
Take, for example, the fundamental constants of nature. Things such as the speed of light, the gravitational constant, Planck’s constant, the electron to proton mass ratio, and others. These values aren’t taken as constant just to support “evolutionism” or “secularism” but because so many other observations depend on them. Everything from how atoms hold together to how the Sun produces energy. Change them even slightly, and you could alter chemistry, collapse stars, or make atoms themselves unstable. Life as we know it would not be able to exist.
Even then, scientists still do not blindly assume that the fundamental constants are constant. They carefully measure how much they could have changed in the past. For example, ultra-precise atomic clocks are sensitive to changes in those constants of one part in 1018 per year. These show no detectable drift. Astronomical observations show that atomic spectra from distant stars billions of light years away are exactly the same as those on Earth today. That would simply not happen if the fundamental constants of nature had changed.
For the earth to be six thousand years old, some of these constants would have to have changed by a factor of a billion in order for us to see the results that we observe in nature today. That wouldn’t just be a small tweak; it would have reduced the Earth to a ball of hot plasma. Clearly, that didn’t happen, neither in reality nor in the Biblical record.
So no, a “uniformitarian presupposition” is not just “inserted into the equation to obtain millions of years of age.” Constant rates are only treated as constant because experimental conclusions from multiple independent lines of evidence, from lab experiments to the furthest reaches of the cosmos, force us to conclude that that was indeed the case.