can we really say that mass and energy are equivalent if we are not really able to identify what is energy?

Answer 1

Simply give "energy" an arbitrary meaning that fits all observations - such as the ability to perform work. It isn't important that this be an absolute definition, only that it serves our criteria of determining if mass and energy are indeed equivalent.

With this in mind, it's an easy step to see that when we convert mass in a fission reactor to energy, we have confirmed the validity of their equivalence according to Einstein's famous equation, E=MC^2.

Answer 2

Yes, E=MC2, Put simply the faster mass goes the more it expands and they have to remain equal, so if one gram of salt is to move at the speed of light it's mass would grow expeditiously, even here on earth you can witness this if you where to measuree your mass standing still and then measur it again in a centrifuge moving 100mph you mass would be a fraction bigger. This is fact Mass and Energy are equal and must remain so.

Answer 3

Obviously nuclear weapons exist, so mass and energy are equivalent.

Answer 4

Yes