[Still off-top] Physics [was Requests author discusses MentalHealthError exception]

[Gregory Ewing]

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