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

[Oscar Benjamin]

On 4 March 2016 at 10:38, Marko Rauhamaa <marko at pacujo.net> wrote:
> Oscar Benjamin <oscar.j.benjamin at gmail.com>:
>
>> The mass is carried by the new particles. The new particles may have a
>> total *rest mass* which differs from the total rest mass of the
>> previous particles. However the total mass is the rest mass plus the
>> mass associated with the "kinetic energy" of the particles.
>>
>> [...]
>>
>> Mass and energy are not interchangeable in the sense that you can
>> exchange one for the other with e=mc^2 giving the exchange rate.
>> Rather mass and energy are *the same thing*. Although they are
>> different concepts defined in different ways and having different
>> dimensions and units they are inseparable: e=mc^2 gives us the
>> proportion in which the two appear together.
>
> A physicist mentioned to me that the word "mass" has replaced the term
> "rest mass" in modern Physics lingo.

It depends on the context. Rest mass or similar can still be used
where you want to stress the difference (as I was doing).

> That's why you say a photon is
> "massless" even though every observable photon has a relativistic mass.
> It's all in the terminology.

That's just a casual use of terminology. If we want to be precise then
it's pointless to even refer to the "rest mass" of something that is
never at rest. The masslessness of photons comes from an extrapolation
that leads to a divide by infinity: strictly speaking it's just
undefined.

> As for the existence of a negative mass, it is interesting to note that
> the (rest) mass of an alpha particle is less than the sum of the (rest)
> masses of its constituents. About 1% of the mass is "missing."

Since the binding is associated with negative energy it has a negative
contribution to the energy/mass of the particle as a whole. This is
true of any bound state.

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).

--
Oscar