A scientist initially measures 12,000 bacteria in a growth medium. 4 hours later, the scientist measures 24,000 bacteria. Assuming exponential growth, the formula \(P=C(2)^{rt}\) gives the number of bacteria in the growth medium, where \(r\) and \(C\) are constants and \(P\) is the number of bacteria \(t\) hours after the initial measurement. What is the value of \(r\)?