Monday, October 12, 2009

Mass of a Photon

Elvis: hey
does photon has mass?
Sent at 10:38 PM on Monday

me: no rest mass, but they got energy, and they are affected by gravity, so their gravitational mass can be said as E=mc^2

Elvis: why e=mc^2??
u mean the m in this equation
hence, what is the momentum for photon then?

me: p=E/c
ya, the m is the effective mass for a photon
Sent at 10:44 PM on Monday

Elvis: so if a qn asks abt does photos have a mass?
what will be ur answer?

me: is that the full question?
it depends on the context
if it is just year one physics, or GEm module, then no
if it ask in terms of General relativity and light bending then perhaps

Elvis: why is that a diff?
Sent at 10:48 PM on Monday

me: normal people don't need to know too much that will confuse them
Sent at 10:51 PM on Monday

Elvis: but what theory that makes it diff?

me: it has no rest mass
but since it has energy, it has mass, since energy= mass
nothing much about theory
ya just the last answer, it's the most accurate one

Elvis: but what is rest mass?

me: mass that you can measure when you're in the rest frame of that particle
Sent at 10:55 PM on Monday

me: you can argue that since photon is always travelling at speed of light with respect to you, you can never measure the rest mass of a photon, but that's not the reason that it doesn't have a rest mass
Sent at 10:56 PM on Monday

me: basically it's the mass that we are familiar with in everyday life
and in everyday life, as far as we are concerned, light has no mass
bring in relativity, and it becomes confusing to non-physicist

Elvis: but i guess we see it that it might have mass because it always travel at the speed of light and hence because of the formula and the energy it has, it will have mass?

me: you're not making any sense in that sentence,
what module is this for?

Elvis: nature's thread

me: ok

Elvis: coz i dun understand abt speed of light

me: it is invariant under any transformation of frame
anything with rest mass cannot travel to that speed
anything massless must travel at that speed
so I think a suitable answer for your module is that light has no rest mass.

Elvis: oh
make sense

me: thanks
Elvis: thanks alot!


No comments: