Re: Questions (Space)

Jonathan L Cunningham wrote:
Tim S <Tim@xxxxxxxxxxxxxxxxxxxxxxxx> wrote:



Jonathan, people are talking about two different things called mass here.

Not "people". Me and Tina.

The "zero mass" is rest mass. It really is zero for photons. And

You probably missed where I said already that

I saw the one that I read after I posted this. I probably shouldn't have leapt in.

(in at least two other
posts). The problem is that Tina has read in many places that photons
have "zero mass" when what is meant is "zero rest mass" (as I've said in
at least two other posts).

It will help that you are agreeing with me. (That the zero mass referred
to in what she has read is rest mass.) Tina is more likely to believe
two of us.

it's Lorentz-invariant, so it's zero no matter how you look at it. The
thing you're talking about is usually called "energy". (It's not

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.

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

I calculated
it above using the world's most famous equation E=mc^2, or in this case,
m (mass) = E/c^2 (energy divided by speed of light squared).

The quantity 2.8*10^-36 kg really is a mass (the "kg" is a clue).

kg being, of course, a unit of energy ... though not the SI unit of energy which goes by the name "the SI unit of energy" ...

I derived it from: the energy of a red photon is about 1.6 ev[*], multiply
that by 1.6*10^-19 to get Joules, divide by c^2.
^
Yes, I know. |
--------------------------------------------------------------|
eV



Why do you say it's usually called "energy"? Tina said (based on
incorrect information) that photons have no mass:
"No really, they don't have mass," she said.

And that's possibly because she read something written by a physicist, who used "mass" to mean what you're calling "rest mass". I know your usage is popular, but it's not universal, and there are people who complain that it's misleading.


This is not true. Talking about energy would (I believe) be regarded as
a digression by Tina. (I could be wrong: I'm still learning what kind of
explanations she understands.) She already knows they have energy. What
she doubts is that they have mass.

As far as explanations go, you may be right. As I said, you're a braver man than I am.


Lorentz-invariant, so you change it just by going at a different velocity.)

I'm not sure that thinking of Lorentz-invariance, when talking about a
zero quantity, makes the explanation simpler.

A _constant_ zero quantity, like mass (sorry ... rest mass), or a zero component of a vector, as expressed in a particular basis, like energy?

Tim
.

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