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From: rpw3@fortune.UUCP
Newsgroups: net.physics
Subject: Re: Re: eV Revisited - (nf)
Message-ID: <2513@fortune.UUCP>
Date: Sat, 11-Feb-84 11:04:02 EST
Article-I.D.: fortune.2513
Posted: Sat Feb 11 11:04:02 1984
Date-Received: Wed, 15-Feb-84 04:18:54 EST
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#R:rlgvax:-168600:fortune:8600013:000:2555
fortune!rpw3    Feb 11 05:04:00 1984

Guy is right. It's just that jargon is confusing to some. Physicists
have gotten so used to talking about mass in eV that they don't realize
that people don't understand that they don't mean the mass of an electron
IS 511KeV, they mean the mass of the electron is EQUIVALENT to 511KeV
when said mass is converted to energy (via the c^2).

See, in an accelerator near the speed of light it all gets blurred anyway,
since nobody really CARES what the rest "mass" is when the relativistic
"mass/energy" is 100-1000 times rest mass. So the language gets
sloppy. People naturally contract "rest mass equivalent energy" ==>
rest mass energy => rest "mass".

<<fun ON>>

Note that other units can be converted for amusement and amazement...

Anybody remember Cpt. Grace Hopper's standard guest lecture, with
the punch-line when she reaches into the bag and pulls out some
"nanoseconds"? Little pieces of telephone wire, 11.9 inches long!

Somebody once showed me the corresponding equivalence of mass and length,
if I can just remember it. Let's see (this is a bit weird)...

	1. F = m * a 			force due to acceleration
	2. F = G * (m1 * m2) / r^2	force due to gravity (r = distance)
1,2=>	3. a = G * m / r^2		acceleration (of a point mass)
					due to gravity.	
	4. a = S / t^2			Def'n of acceleration (S = distance)
3,4=>	5. S / t^2 = G * m / r^2
or,	   m = 1/G * S * (r/t)^2	(rearranging)
	6. c = c0 * (r/t)		(let c0 be just the number ~2.99e8
					 with no units)
5,6=>	7. m = 1/(G*c0^2) * c^2 * S

and voila, pulling the same trick of scaling by c^2, we get

	8. m = K * S			where constant K = c^2 / (G * c0^2)
or,	   S = K' * m			where K' = 1 / K

Anyway, the dimensional analysis holds, if you want to play the game.
The way I heard it, by the time you have all the scaling constants
worked out, the mass of the Sun is ~ 1.1 kilometers, which led a wag to
suggest that that's how far the Sun has ejected itself from our space-time
due to its mass. Rubber-sheet geometry lives! :-)

Rob Warnock

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p.s. You can also get length in terms of mass by starting with
the following two equations and playing the same games (hint:
expand everything out to mass, distance, and time, and start
cancelling terms 'til you're left with m = S), but the constant
of proportionality comes out different. Any explanations?

	1. E = m * c^2			Einstein
	2. E = F * S			force * distance
	...
	N. S = k" * m			(what's K"?)