The illustration below shows the three isotopes of carbon.
Some isotopes of certain elements are unstable; they can spontaneously change into another kind of atom in a process called “radioactive decay.” Since this process presently happens at a known measured rate, scientists attempt to use it like a “clock” to tell how long ago a rock or fossil formed.
Assumptions in the scientific community are extremely important.
If the starting assumption is false, all the calculations based on that assumption might be correct but still give a wrong conclusion. Libby’s original work, he noted that the atmosphere did not appear to be in equilibrium. Libby since he believed the world was billions of years old and enough time had passed to achieve equilibrium. Libby’s calculations showed that if the earth started with no If the cosmic radiation has remained at its present intensity for 20,000 or 30,000 years, and if the carbon reservoir has not changed appreciably in this time, then there exists at the present time a complete balance between the rate of disintegration of radiocarbon atoms and the rate of assimilation of new radiocarbon atoms for all material in the life-cycle.2C in the atmosphere.
The earth has a magnetic field around it which helps protect us from harmful radiation from outer space. The stronger the field is around the earth, the fewer the number of cosmic rays that are able to reach the atmosphere.
This would result in a smaller production of The cause for the long term variation of the C-14 level is not known.
Note that, contrary to a popular misconception, carbon dating is not used to date rocks at millions of years old.