# [hist-analytic] Materialism and mass as a unit of measurement

Roger Bishop Jones rbj at rbjones.com
Sat Jan 29 08:10:04 EST 2011

```I don't know enough to offer more than the odd comment on
this topic, but here are a couple of points

On Friday 28 Jan 2011 21:19, Steve Bayne wrote:

> 1. A photon is massless

Should be something like "has no rest mass".
It always does have mass.

> This is a given of physics. Another given of physics is
> the momentum of a photon can be expressed this way,
> where 'P' is momentum:
>
> 2. P = E/C (where 'C' is the speed of light; and 'E' is
> (rest) energy)

E is not "rest" energy since that would be the same as rest
mass (units apart).
This simplification of the relationship only applies in the
case that the rest mass is zero.

> Now we, also, have it as a given that
>
> 3. E = MC^2 (Yippee!) And so, from (2) and (3) we get
>
> 4. P = MC^2/C
>
> Leaving us with
>
> 5. P = MC
>
> But wait! Doesn't (5) contradict (1), since 'M' means
> 'mass'?

No because the mass involved is not rest mass.

> 6. P = MV (for picky people assume we are dealing with
> vector properties where applicable).

This is a non relativistic equation, and therefore is not
correct if M is the rest mass.
I think it is still correct if M is the total mass.

> That is: does 'M' in both (5) and (6) get indicated by
> the same units of measure?

Yes I think so.  But you must bear in mind that 5 only
applies to objects with zero rest mass, and I think such
objects are always moving at the speed of light, so when the
two equations are both applicable, they both say the same
thing.

Roger Jones

```