Thread: photons, mass and energy

Hi,

below are three statements that I believe are true, but in my mind they lead to a contradiction. Obviously there is something here that I don't understand, could someone please nudge me in the right direction?

1) Mass equals energy
2) A photon has energy
3) A photon is massless

Perhaps a photon shouldn't be considered as a moving particle at all, but just an instantaneous transfer of energy between massive particles? (Instantaneous in that the space time interval is zero)

Or something else...

Thinking about this a bit more I believe my confusion came from a bad understanding of equality/identity and mass/rest mass.

I personally disagree on ur 3rd point although it has been assumed by physicists that photon is massless.Photoelectric effect proves the particle nature of photons and a particle widout mass is simply unimaginable.

Originally Posted by sushy_bhat

I personally disagree on ur 3rd point although it has been assumed by physicists that photon is massless.Photoelectric effect proves the particle nature of photons and a particle widout mass is simply unimaginable.

Maybe for you. Not for scientists. Nature doesn't care about what anyone can imagine or not. Photons ARE massless.

Originally Posted by Vemedu

Thinking about this a bit more I believe my confusion came from a bad understanding of equality/identity and mass/rest mass.

Energy and mass are not the same thing, but they are equivalent, i.e. mass can be transformed into energy and vice versa.

May i noe on wat basis it has been inferred tht a photon is massless

Many, many repeated experiments carried out over the last (approximately) 100 years. As with basically everything in science, things are claimed as true exactly because they have stood up to rigorous attempts to prove them false.

I'll add that we can never be 100% sure that anything is true, but when enough evidence stacks up, it becomes a waste of time and energy to assume otherwise until some better experiment or piece of evidence comes along. (They do, but so far have only confirmed our current view.)

How can photons be massless and have momentum...that should get you guys thinking! :-D

Originally Posted by Dishmaster

Originally Posted by sushy_bhat

I personally disagree on ur 3rd point although it has been assumed by physicists that photon is massless.Photoelectric effect proves the particle nature of photons and a particle widout mass is simply unimaginable.

Maybe for you. Not for scientists. Nature doesn't care about what anyone can imagine or not. Photons ARE massless.

Originally Posted by Vemedu

Thinking about this a bit more I believe my confusion came from a bad understanding of equality/identity and mass/rest mass.

Energy and mass are not the same thing, but they are equivalent, i.e. mass can be transformed into energy and vice versa.

No photons are not massless. They simply have zero rest mass.

They do have energy and momentum and thereby have mass. at least if you take mass to be related to inertia.

Mass and energy are the same thing. That is crucial to relativity and is evidenced in particle physics.

It is possible to take a somewhat contrary stance, but only by declaring "mass" to mean "rest mass", and if you do that you run into additional difficulties in terminology in handling mass and energy conversion.

This gets into the can of worms surrounding the issue of "relativistic" mass vs "rest mass". If you don't like relativistic mass, and there are many physicists who do not, then limit "mass" to "rest mass" and photons have none. But you also complicate the equation E=mc^2 and have to keep track of momentum separately.

It is ultimately a matter of convention and one can adopt either convention and from that develop a consistent theory of mechanics.