🚀 go-pugleaf

Fred McGalliard wrote:
> 
> Chris Muffett
> You just said that the neutrino has a well defined mass, then took it
> back by suggesting it might not have a rest mass. There is certainly a
> class of physics where rest mass is important. Chemistry for example.
> But why does it impact issues such as the mass found in space in
> general? As I recall, there is no particular reason to seperate rest
> mass from any other kind of mass for purpose of calculating
> gravitational effects?? I think I am still having this argument, so this
> just establishes what I currently think.

Fred,

    I'm not sure I fully understand your point, but I will try an
clarify what I said.

    The neutrino has an extremely small rest mass, but whether it has
zero rest mass is still unknown. Certainly it's rest mass is less than
1/20,000th of an electron, and so it is sometimes assumed to be
identically zero.

    There is a cosmological theory that suggests that neutrinos do have
a very small mass, and because there are so many of them in the
Universe, this could account for the discrepancy in the mass of the
Universe. This discrepancy is known as "hidden mass" or "dark matter"
and is responsible for about 90% of the Universe's mass.

    The difference between "rest mass" and "real" (inertial or
gravitational) mass becomes important when considering particles that
travel at relativistic speeds (speeds approaching 1.0c). For example,
the rest mass of a photon is zero, but actual mass of a moving photon
will not be zero. Gravitational effects must always consider the real
mass, not the rest mass. Rest mass is simply a theoretical value used by
particle physicists.

    I'd like to know whether this has cleared up any of the questions
you have.


	Regards,
	 Chris.