Re: Questions (Space)

Ric Locke <warlocke@xxxxxxxxxxxx> wrote:
Tina Hall wrote:
Tim S <Tim@xxxxxxxxxxxxxxxxxxxxxxxx> wrote:
Jonathan L Cunningham wrote:

No, the thing I'm talking about is usually called "mass"

That depends who's doing the calling. Among physicists who work on
these things, "mass" means what you're calling "rest mass". Hence
"photons are massless". People really do say that. When people talk
about the masses of elementary particles, such as the photon, they
are talking about rest masses, since that is the sort of thing that
can be the property of a particle.

Thanks for pointing that out.

Mind explaining why some people talk about 'rest' mass? The 'rest'
bit I mean. I always read it as 'left over <from something>', but
now it occurs to me that it may mean 'while resting; inert,
unmoving'.

Yes, that's the sense being used here. "At rest" = "not moving".

Ah, thanks.

Why would that change at all? (If you've got simple terms to explain
that.)

According to Einstein, when you accelerate something some of the
energy goes into additional mass instead of additional speed. At low
speeds, anything you or I would ever personally experience, the
proportion is vanishingly small and need not be taken into account,
but as the speed gets close to the speed of light most of the
additional energy from acceleration goes into more mass rather than
more velocity.

The problem there is that I don't know why Einstein thought that. I'm
lacking the basis for that assumption, so I stay at the 'why'?

(And this, being just a conclusion on unknown data, isn't something that
'just is'.)

Satellites move fast enough for the effect to be detectable, and
planets move fairly slowly but are large enough that it has to be
taken into account when planning space-probe missions, so we know by
experiment that Einstein was right.

Don't know. Could be coincidence and something else is at work.

So the mass when the object is at rest is different from the mass
when it's moving, and it makes sense to talk about "rest mass".

I'm still wondering about the 'why would it be different'. (Saying
Einstein said so is still no 'why'.)

A photon is a special case. It always moves at the speed of light,
and is never "at rest". It has energy and therefore mass (because the
two are interchangeable, see below).

So far, I see energy and mass as in vaguely the same relation as the
electric and magnetic thing, only very very much more constant.
(Something is massive or energetic, it doesn't flip back and forth. But
if you _could_ flip it back and forth, you would get the same waves as
with the photon. Only one direction would be solid - 3D - matter, and
the other direction - perpendicular to it - would be energy, something
you can't touch.)

And that, to me, means that if it is constant energy at full 100%, it
has no mass.

Electrons might be somewhere on the line between both. (Would make sense
to me, anyway.)

(Now I wonder about flinging peas around, wandering thoughts and all.
But there you only add energy, you don't half turn the pea into energy.)

Jonathan objects to assigning a "rest mass" to a photon, because if
it's at rest it isn't a photon :-)

Makes perfect sense. :)

(But that doesn't strike me as what Jonathan objects to. I think it's
terminology, and claiming it's matter when moving. Which doesn't agree
with my idea, see above.)

What you are talking about, they usually call "energy".

That finally is something on the subject of a photon's 'mass' that
makes sense. And that gets more 'massive' with --since speed
(constant) isn't an option-- more frequency/wavelength? Higher
frequency = more beats = more energy?

(Please answer, even a 'no, but I don't feel like explaining' is
better than leaving it open.)

That's correct. Higher frequency = shorter wavelength = more
energy/mass. Sorry it took so much beating around the bush to get to
something that made sense to you.

It isn't easy to find that which makes sense to me. :)

kg being, of course, a unit of energy

I thought gram was mass. That's what I learned, anyway.

Energy and mass are the same thing -- E = Mc^2.

That's just a formula, means nothing to me.

Since the constant of proportionality, the speed of light squared, is
so large,

You've lost me there.

quite small masses mean quite large energies, and small energies mean
vanishingly small masses.

Why not small energies mean large masses? A pea on a plate is 100%
solid, no energy.

In the latter case, few call it "mass" although it's correct to do so.
1 kg =~ 10^17 joules, which is quite a lot of energy. (Somebody check
my arithmetic on that, please)

I'm lost already. I know joules only in relation to calories, what makes
one fat. :) (And that is 'I have seen the term'. I don't know what is at
the core of it.)

--
Tina
WIP: Space: 2420 words
WISuspension: Seasons & Elements trilogy | Magic Earth series
Posted to Usenet newsgroup rec.arts.sf.composition.

.

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