Oscar Benjamin wrote:
If we want to be precise then it's pointless to even refer to the "rest mass" of something that is never at rest.
Which just shows that the term "rest mass" is a bit silly. It came from some confused thinking very early in the development of relativity. The physicists soon sorted that out, but unfortunately the textbooks didn't catch up, and we've ended up with several generations of confused students as a result. :-( The masslessness of photons comes from an extrapolation
that leads to a divide by infinity: strictly speaking it's just undefined.
No, it's not. The total energy of a particle is given by E**2 == c**2 * p**2 + m**2 * c**4 where p is the particle's momentum and m is its mass. For a photon, m == 0. No division by zero involved. For a massive particle at rest, p == 0 and the above reduces to the well-known E == m * c**2
Something I don't know is if there's some theoretical reason why the binding energy could never exceed the sum of the energies of the constituent particles (resulting in an overall negative mass).
Conservation of energy would be one reason. If you put two particles together and got more energy out than went in, where did the extra energy come from? If you find a way to make that trick work, watch out -- the secret cartel of energy companies that's suppressing all the free-energy inventions will want to disappear you... -- Greg -- https://mail.python.org/mailman/listinfo/python-list
