[quote=plandon;174448]The mispronunctiation I can't stand is is the singular "Math".[/quote]
Quaint, archaic, pretentious and American.
What's the problem?

David

[QUOTE=xilman;174511]Two things:

a) a photon undoubtedly has inertial mass (or, more precisely, momentum) and you do not need quantum mechanics to show it theoretically. Measuring it experimentally, by photon scattering experiments for instance, shows that QM is required. Experiment also shows that if something emits a photon, the (inertial) mass of the initial something and the final something differ by precisely the predicted mass of the photon.

b) I concede that the gravitational mass of a photon, or collection of photons, has not yet been measured by experiment (as far as I know). Given that every experiment yet conducted suggests very strongly that inertial masses and gravitational masses are strictly proportional, I doubt that any serious physicist believes that a photon (or collection of photons) does not have a gravitational mass too.

(The famous 1917 eclipse observation indicated that photons are influenced by a "gravitational field" but that in itself was no great surprise. Newtonian gravity also predicts the same effect. The difference between the two theories is that GR predicts twice the deflection as Newtonian gravity. The observations were much better in accord with GR and, to that extent, they suggested that Einstein came closer to "the truth" than Newton.)

Paul[/QUOTE]
Yes, the way I have understood/thought-about GR is that it makes no distinction between mass and energy when it comes to gravitation - the gravitational field only cares about total mass/energy.

I seem to recall a story about Einstein inadvertently dropping the key factor of 2 in his initial calculations of bending of starlight by a gravitational field, and being dismayed after all those years of work to find his result was no different than Newton's. Luckily he quickly found the algebra error and the rest is history. There seem to also be some lingering questions as to whether the original eclipse expeditions measured the bending with sufficient accuracy to really distinguish between the 2 competing theories, or whether the triumphant "Einstein is right!" was as much a result of Eddington's being convinced of that in advance of the data. (There were of course other compelling reasons for Eddington to believe the "truth" of GR, e.g. the amazingly accurate prediction of the anomalous precession of Mercury in its orbit around the sun. But letting the "correct" result of one kind of test influence the interpretation of another is of course a no-no, in terms of objective science.) Not that there is any serious doubt anymore, now that we can make such measurements with much greater precision.

[quote=xilman;169207]I've not read the post in question, but have to take issue with "no-one tries to say .. E=mc^2". I've seen a number of statements which explicitly make that point that [B]all[/B] forms of energy have inertial mass given precisely by Einstein's equation.

I fail to see why chemical and nuclear reactions should be qualitatively different. Perhaps I need to read the post in question. Perhaps, also, I'm unusual in having studied molecular, atomic and nuclear spectroscopy and can see that they are all fundamentally similar processes despite taking place at different (though overlapping) energy scales


Paul

P.S. If need be, I'll try to dig up such statements but can't be bothered right now.[/quote]

You obviously missed my meaning:
[quote=davieddy;169177]No one tries to say that the energy released in a chemical reaction
exemplifies E=mc^2, so how is a nuclear reaction qualitatively different?

David[/quote]


Here is a post which may fit the bill although you responded to it
at some length, starting off with "To muddy the waters further..."

[quote=davieddy;168595]I don't consider the comparison between nuclear and chemical
binding energy as "specious" (woody word?) as the one I think is being
made here, namely comparing the energy from burning a mass m of
petrol with E=mc^2.

I know the energy released in a nuclear explosion is popularly attributed
to E = mc^2, but I prefer to look at it through the other end of the telescope, in view of nucleon number remaining constant:

The enormity of nuclear binding energy (compared to chemical)
still results in only a piddling "mass defect" given by E/c^2.[/quote]

The point I was making was that the distinction between the mass/energy
involved in chemical and nuclear reactions is indeed quantitative rather than
qualitative.

Call me paranoid, but I took your "Some folks badly misunderstand Albert"
(or words to that effect) personally.

David

[quote=davieddy;174521]Yawn[/quote]

Deleted that "thread hijacked" post; mods, feel free to delete this one too.

[quote=ewmayer;174506]rearranging Herr Einstein's famous formula as m[sub]r[/sub] := E/c[sup]2[/sup].[/quote]Au contraire! m = E/c[sup]2[/sup] [I]is[/I] Herr Einstein's formula, with substitution of the modern "E" for 1905-convention "L". See [URL]http://www.fourmilab.ch/etexts/einstein/E_mc2/www/[/URL] (just after "From this equation it directly follows that:-- ").

The famous E = mc[sup]2[/sup] is the rearrangement.

[quote=cheesehead;174524]Au contraire! m = E/c[sup]2[/sup] [I]is[/I] Herr Einstein's formula (see [URL]http://www.fourmilab.ch/etexts/einstein/E_mc2/www/[/URL] , just after "From this equation it directly follows that:-- ") with substitution of the modern "E" for 1905-convention "L".

The famous E = mc[sup]2[/sup] is a rearrangement.[/quote]
Many thanks

[QUOTE=xilman;174511]b) I concede that the gravitational mass of a photon, or collection of photons, has not yet been measured by experiment (as far as I know). Given that every experiment yet conducted suggests very strongly that inertial masses and gravitational masses are strictly proportional, I doubt that any serious physicist believes that a photon (or collection of photons) does not have a gravitational mass too.[/QUOTE]Actually, the gravitational redshift of photons, as measured by the Mössbauer effect for instance, comes pretty close to measuring the gravitational mass of a photon.

Close, but no cigar, because it measures the effect on a collection of photons in a gravitational field, not the gravitational field arising from a collection of photons.


Paul

[QUOTE=davieddy;174519]Call me paranoid, but I took your "Some folks badly misunderstand Albert" (or words to that effect) personally.[/QUOTE]Very well, if you insist.

You are paranoid.

I was emphatically [[b]not[/b] including you in the class "some folks".


Paul

"Maths", everyone knows that math is the proper abbreviation for mathematics.

[quote=Uncwilly;174532]"Maths", everyone knows that math is the proper abbreviation for mathematics.[/quote]
Not until you invade us to teach us the virtues of the death penalty,
freedom, democracy, torture, imprisonment without trial....

Oversexed, overpaid and over here etc

[quote=xilman;174528]Very well, if you insist.

You are paranoid.

[/quote]

Remind me to call you a taxi.
Just took time out to watch "Have I Got News For You" and
"Reggie Perrin".
Unsurprisingly, the former mentioned the Speaker of the
House of Commons:

He used to be a sheet metal worker.
Wasn't much better as a Speaker.

David

PS this is veering towards getting back on the thread topic.