 Applied to radioactive decay, the Born Rule of quantum theory is taken successfully to predict such things as the half-life of the first excited state of the hydrogen atom—that the probability that an atom of hydrogen in this state will be found to have decayed to the ground state after \(1.1 \times 10^{-9}\) seconds (i.e., just over a billionth of a second) is ½. This prediction has been experimentally confirmed by measuring how the frequency with which photons are emitted by a large number of hydrogen atoms in the decay of this excited state decreases over time. Most physicists regard this and other probabilities predicted by quantum theory as objective physical features of the world, typically identifying the probability of decay with the relative frequency of decay as measured in such an experiment.