I got an interesting email relating to an older topic on this blog, and with the permission of the author, Topher Cooper, I’ve decided to post it here.
Many thanks to Topher for this informative overview.
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I’ve been reading the archives of your blog and I just thought I
would write you a clarification of some stuff (at least my
understanding of it) from some closed discussions.
First off, the neutrino is really, truly, intrinsically invisible
(which is not true of the photon, obviously). This is because it
only interacts with other matter (including other neutrinos) via the
weak force (and the gravitational force — but it’s so weak on the
level of individual neutrinos and photons it really doesn’t
count). In particular, it doesn’t interact via the electromagnetic
force. Since "vision" has to do with electromagnetic radiation,
neutrinos are invisible. Another way of saying this is that they
cannot directly affect photons (which can only interact via the
electromagnetic force) in any way, nor vice versa. Neutrinos cannot
emit light, they cannot absorb light, they cannot reflect light, they
cannot deflect light. It’s as if light and neutrinos existed in
different universes.
Now for mass:
There really isn’t any controversy here, just some disagreement about
the best terminology.
The first thing to remember is the famous E=mC^2 formula, which says
that mass and energy are really two ways of looking at the same
thing. The mass of something is the same thing as the total energy
in it. In older writing about relativity, this included the kinetic
energy of the thing from its movement relative to the observer. This
mass was called "relativistic mass" when someone wanted to be clear
what mass they were talking about. But the relativistic mass of a
thing depends on the velocity of the observer relative to the
thing. It isn’t really a property of the thing itself. The
"intrinsic mass" (a.k.a., invariant mass, or proper mass) is that
part of the mass that does not depend on the rest frame of the
observer. When the intrinsic mass of something is not zero, then we
can also speak of the relativistic mass of that thing to an observer
who is not moving relative to it — that is called its "rest mass"
and is equal to the intrinsic mass. When the intrinsic mass is zero,
however, the speed of the object must be C for all observers, so it
can never be "at rest" and cannot, therefore be said to have a "rest mass."
Since the middle of the last century, however, there has been a
growing tendency among scientists to just use the term "mass" to
refer to the "intrinsic mass". The relativistic mass is hardly ever
mentioned — you can always just refer to the total energy or
momentum relative to a particular observer and be talking about the
same thing — and when it is, the whole phrase is now always said by
scientists. Note, however, that for complex objects, part of the
mass (intrinsic mass) is that which is due to the kinetic energy
internal to the object — such as heat energy (yes, things get more
massive as they heat up — but not enough under earthly conditions so
you could possible measure it). So the idea that kinetic energy
provides mass is not really banished, its just considered "cleaner"
to arrange things so that it’s kept off-stage.
So, photons have a zero intrinsic mass. It was assumed until
recently that neutrinos did also. But they do have a non-zero
relativistic mass, because they have energy and momentum. A box with
perfect, parallel mirrors inside will have a greater mass if there is
a photon bouncing back and forth between them than if it does
not. Because photons have mass of a kind (which can be thought of as
the "charge" of gravity) they can be deflected by a gravitational
field — and are, as the famous demonstration of General Relativity showed.
I’m pretty sure that according to current theory, anything that is
observable from a frame has relativistic mass in that frame. As we
look at frames closer and closer to one traveling at C in the same
direction as a photon, the photon’s energy decreases, its
relativistic mass decreases towards zero and it becomes more and more
red-shifted, until at the limit, it disappears.