jfh (08/09/82)
I wish to respond to Bob Morris' comments (floyd.428). First,
however I want to say that the first sentence of my reply ("Surely you jest!!")
was unwarranted, and I apologize to him for it.
I have no quarrel with either the assertion that the center of mass
does not change during a supernova explosion (within limits of asymmetries),
or with relativistic mechanics. However, I believe that Morris has made some
erroneous assumptions.
"Again, during a supernova explosion, the change in
mass is zero, both instantaneously and long term."
Consider the fusion of four hydrogen atoms to form helium. (I know the
overall reaction is much more complicated than this makes it sound, but
please bear with me. This is an approximation for illustration only.)
The mass of a hydrogen atom is 1.00797 atomic mass units (amu), so four
of them comprise a total mass of 4.03188 amu. However, a helium atom
has a mass of only 4.0036 amu. (data from Handbook of Chemistry and Physics,
46th edition) This leaves a difference of 0.02828 amu, which is converted
to energy in accordance with Einstein's equation. Surely no one will
argue that the mass of four hydrogen atoms is equal to the mass of one
helium atom.
"Photons do just as good a job of causing gravitation
as anything else of the same mass."
This is the statement to which my original reply was directed. Photons have
no REST mass; hence the are able to move at the speed of light. I believe
that this is a requirement for any entity which can attain this velocity, since
it can be demonstrated that any massive body would require infinite energy
input to reach light speed. I certainly do not deny that photons possess
energy, and that this energy is equivalent to a certain mass.
However, while I acknowledge that energy and mass are equivalent, it
appears to me that Morris is claiming that energy and mass are the SAME.
It is not at all clear to me that this is a defensible position. Does a
body composed of a certain number and type of particles become more massive
if it is strongly heated (vibrational energy is presumably also equivalent
to mass)? The crux of the matter seems to reside in the (paraphrased)
question asked by Mike Knudson (ihnss.126), "Does energy exert gravity?"
Specifically, is it possible for photons to emit gravitons, or is this
property reserved to those particles with non-zero rest mass?
It is equally unclear that I have illuminated the situation at all.
One could presumably argue in the fusion example that as long as you were able
to contain all the energy in a closed system things haven't really changed.
Does the ability to emit gravitons "define" mass? Comments on these questions
or on my interpretation of the arguments are welcome. Although I saw the
announcement of net.physics a few minutes ago, I am also posting this to
net.space also, just in case.
Fran Heidlage
duke!phs!jfhrhm (08/09/82)
O.K. I will reply specifically to duke!phs!jfh
1. If four hydrogen atoms conspire somehow to form a helium atom, then
the mass of the helium is exactly that of the hydrogen atoms.
2. Mass and energy are the same.
3. Yes, the mass of a collection of particles increases if they are heated.
4. Energy exerts gravity.
Altogether, rest mass doesn't have much do with anything.
These positions may or may not be defensible, but I will let any physics
text do the defending for me. They are hardly disputed questions.jqw (08/09/82)
It has recently been shown (experimentally) that two beams of light have gravitational attraction for each other. All forms of energy have mass and some additionally have rest mass. The sum of the masses of all the particles in the fusion reaction are equal before and after (if you include photons and neutrinos.) The center of mass (and of gravity) of a supernova explosion remains in the same place whether or NOT there are asymmetries. If a greater mass goes in one direction, it will have a lower velocity and hence the center of gravity will not move. This is a rather general principle: if a rocket explodes in flight, the center of gravity of all the pieces will follow a parabolic path. Centrifugal acceleration IS the same thing as gravitational acceleration. The force is created by the mass of the entire universe that you are rotating with respect to. If you are the only object in the universe (*ego*), then when you rotate, there is no force. You also aren't rotating, since there's nothing to rotate with respect to, but that's the whole point. If there are only a few, relatively low mass objects in the universe, the centrifugal force will be small. Linear acceleration is also the same as gravitational acceleration. You are accelerating with respect to the entire universe (as represented by your local space-time), and hence the force. The same effect would be accomplished if every other object in the universe accelerated in the opposite direction. (Actually, because of the aforementioned center-of-mass-in-same-place effect, this couldn't really happen.) If, again, you were in a relatively empty universe, this time positioned between 2 hundred-foot diameter boulders, you could easily cause a large acceleration between these rocks with little force, because they would mass very little.