Re: Questions (Space)

Tim S <Tim@xxxxxxxxxxxxxxxxxxxxxxxx> wrote:

Tina Hall wrote:
Jonathan L Cunningham <spam@xxxxxxxxxxxxxxxxxxxx> wrote:

Photons do have mass. A red photon weighs about
0.0000000000000000000000000000000028 grammes (2.8*10^-36 kg), and a
blue photon nearly double that.

They're said to have no mass. So where do they get the weight without
mass?

Just noticed this. The following is really a note to Jonathan, so he can
try and explain it to you if he wants. (I'm afraid I have no idea how
I'd go about explaining photons to you. The people who are trying are a
lot braver than I am ... )

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 (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". 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). 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.

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.

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.

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.

Jonathan
[*] And *precisely* for the same reason, the voltage drop across a red
LED is about 1.6v. Measure the volate drop across a yellow LED, and the
reading will tell you how many ev of energy a yellow photon has (about
2.2, IIRC).

--
"I think too much - therefore I am mad!"
Agatha Clay playing Lucrezia Mongfish.
.

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