Question

Why scientist generally do not use carbon-14 dating to go back further than seven half-lives?

Answer 1

Because the atmosphere hasn't reached equilibrium yet. This also proves that the earth is less than 30000 years old.

Answer 2

\[1/2^7 \approx 0.0078\] is very small

would be more accurate to use a isotope that has been through a fewer number of half-lives