Are you telling me that the equation E = mc^{2} is not workable?

Sorry but I'm not native english speaking, so I don't understand what exact meaning to associate to "workable" in this context, so I'll try to explain better what I've already written.

The equation, as any other equation, is valid only in specific cases. This one is valid *only if the system is stationary, that is it doesn't move*. In physics, even the phrase *it doesn't move* have to be precisely specified: if you have a system consisting of two equal iron balls moving fast in opposite directions, then the system is stationary, because its centre of mass has velocity = zero.

You can define the concept of "stationary" even if you have a system made up of massless objects, like photons. Instead of talking about centre of mass, you talk about *total momentum*: if two equal energy photons are flying in opposite directions, then the system of the two photons is stationary because its total momentum is zero. Then you can apply E = mc

^{2} even to such a kind of system and you discover that this system

*has mass*. I mean,

not *relativistic* mass, but real, invariant mass. A single photon is massless, but two photons travelling in opposite directions has mass! (and even if they are not travelling in opposite directions but simply in

*different* directions, because it turns out that in this case you can always find an inertial frame of reference where they travel in opposite directions).

In conclusion, you can apply E = mc

^{2} *but only if the system is stationary*.

I have found out that the constant for Mass is the Kilogram. I assume that matter must be converted to Kilograms in order to work the equation E = mc^{2}. What would prevent that? Thanks for comments. Joe L. Ogan

Can you explain better what you mean?