Energy = mass * c^2 where c=speed of light

[pmb_phy]

Photons is energy, electromagnetic, and therefore has no mass and travels in wave forms like light. So photons is a quantum of electromagnetic energy. So the equation should fit both ways... be it fusing 2 photons or something? Or putting photons into matter with mass.

Photons are not energy. They have energy. Then again all particles have energy.

 

[expscv]

Hard to say. I've never seen any firm rule on this apply. It seems to be a guessing game. For instance - Even for someone with a "medium level" understanding it is still uncertain what they know. It often depends on how they learned it and what question is being asked.

The quesion of the relativistic mass for light is a good example. If a student has a text which defines inertial (aka relativistic) mass, m, as

$$m = \frac{m}{\sqrt{1-v^2/c^2}}$$

then they might think that the relativistic mass for light is undefined. They might think that its zero too. However if the students text defines inertial mass in the correct way, i.e. as m = p/v, then its obvious that anything with a finite momentum and finite speed has a finite m. This fact is not a well known fact but this is the most rigorous and the most precise way to define inertial mass and its the definition upon which all derivations of m are based. Its an unfortunate fact that basic physics texts don't explain this. However texts which are devoted to relativity often elxplain this.

Hence my suggestion to define precisely what one means by "mass" when speaking about whether or not light has mass or not.

hoho the next time i post anything i will state my level hehe

 

[pmb_phy]

hoho the next time i post anything i will state my level hehe

I don't think that's neccesary. As I indicated I think its a guessing game. Recall that I mentioned that two people who learned at the same level but from different texts might not know the same ideas.

Besides. I wouldn't want to post at a discussion board where people thought that they had to post their background to get an answer.