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- From: dk@imager (Dave Knapp)
- Newsgroups: sci.physics
- Subject: Re: Converting the masses
- Message-ID: <131637@lll-winken.LLNL.GOV>
- Date: 28 Jul 92 06:34:35 GMT
- References: <1992Jul25.194550.1970@smsc.sony.com> <131516@lll-winken.LLNL.GOV> <1992Jul28.004259.9052@smsc.sony.com>
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- Organization: Laboratory for Experimental Astrophysics
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-
- In Article <1992Jul28.004259.9052@smsc.sony.com> markc@smsc.sony.com
- (Mark Corscadden) writes:
-
- > The general recommendation was that relativistic mass is an idea
- > who's time has come and gone and that you should use the word
- > "mass" to mean "rest mass" or the intrinsic mass of a particle or
- > a system.
-
- Here's where I have a problem with what you are saying, Mark: you
- are, in effect, saying that a system has some "intrisic mass" as if
- it is some property of a system that cannot be changed. That is not
- what my perspective is at all: for example, the N(1440)P11+ particle
- has the same quarks as a proton, but it is in an excited state. So
- a given collection of 3 quarks doesn't have an "intrisic mass."
-
- > Here's a brief description of part of what I thought people were
- > saying. I still think this is the position that's being held by
- > "rest mass only" people like David Knapp, but since it's *their*
- > position only they can say whether or not I am expressing it
- > correctly:
-
- [parts of example deleted...]
-
- > We say that the mass of the cannon ball is 1 kg, period. The mass
- > of a cannon ball is an intrinsic property which doesn't depend
- > upon your frame of reference.
-
- Partly yes, partly no. As was pointed out earlier, the mass of
- the cannon ball depends (for example) upon its temperature. On the
- other hand, the mass doesn't depend on the reference frame.
-
- > If it is moving at 0.99 lightspeed in your reference frame, then
- > we say that you have a 1 kg cannon ball which happens to possess a
- > very large amount of kinetic energy in your frame of reference.
- > This kinetic energy is not mass, its energy!
-
- Right. I'll buy off on this part, and the rest of your example.
-
- > I think that you *can* give a clear unambiguous meaning to the
- > concept of the one and only intrinsic rest mass of a given system,
- > based upon nothing more than identifying the objects which
- > comprise the system. However I'm still puzzled because (as far as
- > I can tell) it requires you, among other things, to treat the
- > kinetic energy of the parts of the system, measured relative to
- > the system as a whole, as mass. I'm puzzled because at first I
- > though that treating kinetic energy as mass was not kosher,
- > period.
-
- I can certainly understand your confusion here. However, I think
- that the problem is identical in the case of subatomic particles; in
- the example above of an excited proton, the quarks have more energy
- associated with them, which gives the particle more mass.
-
- I'll quote your next example in detail, because I have a point to
- make about it:
-
- > You have two cannon balls (again). Each has a mass (always read
- > "rest mass" or "intrinsic mass" here) of 1 kg. They are separated
- > by one million kilometers, so for all practical purposes they are
- > independent of each other.
-
- > They are moving directly away from each other at 0.995 lightspeed.
- > Then the mass of this system, consisting of the two cannon balls
- > together, is 20 kg. It is not 2 kg.
-
- > On the other hand, if the center of mass of this pair of cannon
- > balls is moving at 0.9999 lightspeed in your frame of reference,
- > then their mass is still 20 kg. Their mass is an intrinsic
- > property of the system and does not depend in any way upon how the
- > two-cannon ball system is moving with respect to you.
-
- > Is that right?
-
- Yes, that's right. I think you are getting confused because it is
- artificial to treat the "system" you have described as a single
- object: why would you want to consider this system as a single mass?
- What purpose does that serve? Because of the size of the system,
- it's not useful to think about interactions with the system as a
- whole, so the mass is useless in that context. If you want to
- calculate the gravity of the system, it is also not very useful, once
- again because of the size of the system. I suppose that from a large
- enough distance, the gravitational interaction of your system would
- look like a point mass of 20 kg; but closer to the system, one would
- have to make a more detailed calculation of the energy-momentum
- tensor. In other words, the essential quantities for your system are
- the energy and momentum, not the mass.
-
- My point here is that in order to arrive at an example that makes
- my definition of mass unclear, you have been forced to use an example
- in which: 1.) the definition of "system" is fuzzy, 2.) the usefulness
- of the concept of "mass" for the system is questionable, and 3.) the
- concept of "relativistic mass" would not evade the problems.
-
- -- Dave
-
- --
- *-------------------------------------------------------------*
- * David Knapp dk@imager.llnl.gov (510) 422-1023 *
- * 98.7% of all statistics are made up. *
- *-------------------------------------------------------------*
-