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

Baynesr at comcast.net Baynesr at comcast.net
Sat Jan 29 09:58:58 EST 2011


Good point, although I don't think it affects the criticism of Block-Stalnaker. You remark: 

> 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)." 

You are right that 'E' in (2) can't be rest mass mass since a photon has no 
rest mass. I had intended it for (6) and added it hurriedly just in case readers were apprised of the difference, as they (you) obviously were. However, this doesn't materially affect the main point which was that the identities involving 'M' differ depending on whether you are talking about photons or, say, helium atoms (alpha particles). So I have no problem with your comment on (6) that it is not relativistic. That was why it was included. But there is a deeper philosophical question here. Whether relativistic or nonrelativistic equations apply depends on the ratio of mass to momentum. A photon has no mass, so the equations are highly relativistic. Still, this is a degree concept; there is no "absolute" determination of when rest energy is such as to use a nonrelativistic characterization of momentum. But set this aside, since it was not directly involved. 

Your comment: 

"No because the mass involved is not rest mass" 

is consistent with my main point, since it was precisely the difference between rest mass and nonrest mass that raised the question of whether we are using the same *units of measure* (rather than values of these units as between photons and "particles"). This is what creates the problem the problem with applying identities in the context of explanation the way Block-Stalnaker do it. No problem here, as far as I can tell. 

So, your points all involve the "ambiguity" of 'E=MC^2' and it was exactly this ambiguity that called into question the distinction between using 'M' as a unit of measurement in the case of a massless photon in contrast to 'M' applied to tables and chairs etc. And it was the question of this use of 'M' in both cases that led to the challenge to Block-Stalnaker, etc. When I use 'M' in 'E=MC' regardless of whether M is or is not rest mass is not the main issue; the main issue is whether in either case the units of measurement are the same, particularly where 'E' characterizes a massless photon. If 'M' did not apply, then 'E=MC' would be meaningless. It does apply, but only because it is being used as a unit of measurement which converts, just as feet and meters, and not as indicating some ontological feature of photons. If it did, then I would have no case against Block-Stalnaker, since I claim that matter, when understood as mass, cannot be all there is. 


Steve Bayne 

----- Original Message ----- 
From: "Roger Bishop Jones" <rbj at rbjones.com> 
To: hist-analytic at simplelists.co.uk 
Sent: Saturday, January 29, 2011 7:10:04 AM 
Subject: Re: Materialism and mass as a unit of measurement 

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 

Roger Jones 
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