There is also an uncertainty principle for energy and time: delta e delta t h show that both sides of this expression have the same units. the relationship introduced in problem 1 -48 has been interpreted to mean that a particle of mass m (e = mc^2) can materialize from nothing provided that it returns to nothing within a time delta t h / mc^2. particles that last for time delta t or more are called real particles; particles that last less than time delta t are called virtual particles. the mass of the charged pion, a subatomic particle, is 2.5 x 10^-28 kg. what is the minimum lifetime if the pion is to be considered a real particle