Re: [Phys-L] mass, energy, and spacetime

[John Denker <jsd@av8n.com>]

On 03/16/2013 05:58 AM, Brian Blais wrote:
> The two sentences are not mutually exclusive, and mass is, in a
> sense, “equivalent” to energy.  I'd prefer to use the phrase that
> mass is a form of energy, that in this way there is an equivalence
> between mass and energy - not the total energy, of course.  just as a
> hamburger is a form of energy, but not the entire energy.
>
> I think this is word-play, but still important because so many of
> these concepts can be confusing.

To my ears, "equivalence" is a very strong word.  An equivalence relation
is reflexive, symmetric, and transitive;  for details see
  http://mathworld.wolfram.com/EquivalenceRelation.html

One would not say that /Lake Baikal/ is equivalent to /water/, because
some of the world's water is in Lake Baikal but some is not.  By the same 
token, one should never say that /mass/ is equivalent to /energy/, because 
some of the the world's energy is in the form of mass but some is not.

I just now reworded the paragraph in question at
  http://www.av8n.com/physics/spacetime-welcome.htm#eq-e-mc2