This is a problem from my textbook:
"A proton or neutron sometimes 'violates' conservation of energy by emitting and then reabsorbing a pi meson, which has a mass 135MeV/$c^2$. This is possible as long as the pi meson is reabsorbed within a short enough period of time $\Delta t$ consistent with the uncertainty principle. Consider $p \to p + \pi$. By what amount $\Delta E$ is energy conservation violated? (ignore any kinetic energies)"
For this, I thought it appropriate to take the "violation" of energy conservation's only constituent to be the rest energy of the pion, $E_0 = mc^2 = 135MeV$.
"For how long a time $\Delta t$ can the pi meson exist?"
Using the energy-time uncertainty relation, I do the following calculations:
$$\Delta E \Delta t \approx \hbar $$
$$\Delta t \approx \frac{\hbar}{\Delta E}$$
Taking $\Delta E $ to be the rest energy of the pion calculated above, the resulting $\Delta t$ comes out to be $4.88 \times 10^{-24}s$. But checking the Wikipedia article on pi mesons, the lifetime is said to be $2.6 \times 10^{-8}s$. Is there something that I'm missing in this process? Was my choice of the rest energy for $\Delta E$ perhaps incorrect? Why am I coming out with the wrong value for the lifetime of the pion?
Best Answer
Basically, DZ has given a good answer to this question. There are only a couple of points I would like to add from pure physics point of view, just for pedagodical purpose, if I may.
The time you are calculating is the time the $\pi^0$ is in 'flight' from the moment it is emitted by the proton until it is captured again. So it is ok to use the uncertainty principle to find an estimate. Since this is a strong interaction process it will be very short time, which you find. That does not mean the $\pi^0$ lives that short time. It is like you throw a fire cracker up in the air after it has been 'programmed' to explode in 1 min say. The fire cracker will fly upwards, reverse its motion and hit the ground in a fiew seconds, and then will explode - the $\pi^0$ does not do that when it is recaptured by the proton though. If the $\pi^0$ had the freedom to fly longer it would 'explode' in the programmed time of about $2.6\times10^{-8}s$. The free-$\pi^0$ life time is the time the particle actually lives in free flight so to speak. It is much longer than the other time because the mechanism that drives its decay is an electromagnetic interaction, as stated in DZ's answer. These two life times are calculated using different Feynman diagrams. I hope this clarifies the difference between the two 'lives' of the $\pi^0$.