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- Path: sparky!uunet!usc!sdd.hp.com!elroy.jpl.nasa.gov!ames!agate!agate!matt
- From: matt@physics2.berkeley.edu (Matt Austern)
- Newsgroups: sci.physics
- Subject: Re: Converting the masses
- Date: 28 Jul 92 00:05:09
- Organization: Lawrence Berkeley Laboratory (Theoretical Physics Group)
- Lines: 64
- Message-ID: <MATT.92Jul28000509@physics2.berkeley.edu>
- References: <131163@lll-winken.LLNL.GOV> <1992Jul25.194550.1970@smsc.sony.com>
- <131516@lll-winken.LLNL.GOV> <1992Jul28.004259.9052@smsc.sony.com>
- Reply-To: matt@physics.berkeley.edu
- NNTP-Posting-Host: physics2.berkeley.edu
- In-reply-to: markc@smsc.sony.com's message of 28 Jul 1992 00:42:59 GMT
-
- In article <1992Jul28.004259.9052@smsc.sony.com> markc@smsc.sony.com (Mark Corscadden) writes:
-
- > question 2: Are physicists in general who use "mass" to mean "rest mass"
- > or "intrinsic mass" really all in agreement as to how the concept works?
-
- Yes. Assuming that it is possible to tell what the energy of a system
- is, and that it is possible to tell what its momentum is, then the
- concept is unambiguous: you go to the rest frame of that system (i.e.,
- the frame in which its spatial momentum is zero). The energy of that
- system, in its rest frame, is what we call its mass. By the way, I
- have never heard anyone use the phrase "intrinsic mass"; this is not a
- customary term.
-
- Oh, and note that if a system interacts with the outside world (i.e.,
- if it exchanges energy with the outside world), then its mass can
- change; there's nothing terribly surprising about that. Of course, if
- it interacts terribly strongly with the outside world, you might want
- to rethink whether or not you really want to call it a "system" at
- all.
-
- > 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. Or rather, I haven't checked the math; I'll take it on faith
- that the 20 kg figure is correct. Certainly, the invariant mass of
- this system is larger than 2 kg.
-
- And, in fact, it isn't quite as unnatural as it might seem to talk
- about these two cannonballs as a "system". There is one very similar
- case in which I would do that: you have an electron and a positron,
- which are separated by a few meters, which aren't interacting with
- each other, and which are flying away from each other. If you think
- that the electron and positron might come from the decay of some other
- particle, it is very common to define, and work with, the invariant
- mass of that electron-positron system.
-
- A less abstract example, though: consider the deuteron, i.e., a bound
- state of a proton and a neutron. The mass of the deuteron (according
- to the latest Particle Data Book) is 1875.613 MeV. The mass of the
- proton is 938.272 MeV, and the mass of the neutron is 939.566 MeV.
- Again: the mass of the proton-neutron system (the deuteron) is not
- equal to the mass of the proton plus the mass of the neutron.
- --
- Matthew Austern I dreamt I was being followed by a roving band of
- (510) 644-2618 of young Republicans, all wearing the same suit,
- matt@physics.berkeley.edu taunting me and shouting, "Politically correct
- austern@theorm.lbl.gov multiculturist scum!"... They were going to make
- austern@lbl.bitnet me kiss Jesse Helms's picture when I woke up.
-