From: Gary Annat 3/02/99 0:54:58 Subject: Photons... post id: 90 This might be an easy question for some of you, but it is something that I have great trouble with. Question: Given that a Photon has no rest mass, how can it have a value other than zero for momentum? From: James Richmond 3/02/99 11:25:40 Subject: re: Photons... post id: 122 One way to think about this (not necessarily the best way) is that momentum is a property that energy aquires when it moves relative to something else. The relativistic formula for momentum, which works for photons as well as massive objects is p2 = (E/c)2 - (mc)2, where E is the total energy due to motion and (possibly) rest mass, and m is the rest mass. To find the momentum of a massive object, we need to subtract the rest mass energy from the total; hence the second term on the right hand side. Photons have no rest mass. In this case, the equation reduces to p=E/c. Notice that the values of energy and momentum assigned by an observer to an object depend on the observer's state of motion relative to the object. If you're sitting in a car moving along the road, then without looking outside you'll say it has no momentum and only rest mass energy. But if you stand on the road and watch the car drive past, you'll say it has momentum and some extra (kinetic) energy due to its motion. This forum is un-moderated. The views and opinions expressed are those of the individual poster and not the ABC. The ABC reserves the right to remove offensive or inappropriate messages.