Re: New version of a relativity FAQ

On Jun 25, 7:01 am, "Pmb" <peter.m.br...@xxxxxxxxxxxxx> wrote:
"Daryl McCullough" <stevendaryl3...@xxxxxxxxx> wrote in message

news:g3tdh70176h@xxxxxxxxxxxxxxxxxx

Pmb says...

Tom Roberts wrote on Wed, 25 Jun 2008 08:18:52 +0200:

Because that theory is, in essence,
geometrical, regardless of whether or not there are other
interpretations of it.

Nope. Even Einstein himself disagreed with that kind of thinking!

What Tom said is correct. General Relativity can be interpreted as
a geometrical theory of gravity. He didn't say that that was the
*only* interpretation, or that it was Einstein's interpretation.

That is not what Tom said. He said *is* in essence geometrical. His later
assertion "regardless of whether or not there are other interpretations of
it." implies that he is saying that there can be no other interpretations.
He is of course wrong. There are more than one interpretations. I believe
that what Einstein said is correct.

Anyway, I wish people would stop invoking Einstein, or Feynman,
or Newton or Maxwell as support for whatever it is that they
are claiming. If Einstein made a good argument in favor of
some claim, then you can quote the argument. But claiming
that Einstein was on your side is dirty pool, in my opinion.
He's not around to confirm that he agrees with you.

You're mistaking fact from view. Theories are constructions of the human
mind and are therefore subject to opinion. When Einstein's theories are
being discussed it is of great importance what a genius like Einstein said.
At least to me. If you don't like quotes then simply don't read them. People
here love to quote Tom and I love to quote Einstein. Big whoop!

But Einstein *did* put it into writing in a letter to Max Abraham. Would you
like me to quote him in that letter? I'll post that later today if so.

Pete



Ok, I had a look at your solution : http://www.geocities.com/physics_world/sr/cyclotron.htm

No wonder you were so cagey in sharing it, there are quite a few
mistakes. I hope that you will take the following as constructive
criticism and that you'll correct the page. Here it goes:

1.It is quite clear that you can't solve the problem by using
relativistic mass, the "m" in your equations is rest mass. The reason
is quite simple, starting from eq.1 you should be writing:

d(\gamma*m*v)/dt=q(E+vxB)

2. You keep avoiding writing the ODE above, instead you go through all
kinds of girations and you arrive to...

3. ....the following commedy of errors:

"From the diagram it is easy to see that a positive charge q moves in
a clockwise motion around the circle. w is known as the cyclotron
frequency. To find an explicit expression for w we start with the
expression for the magnitude of the Lorentz force which is F = qvB.
Since the motion is transverse to the acceleration we also have F = ma
= mRw2 = qvB = qRwB or w = mv/qB = p/qB. "

4. You are now mixing relativity with Newtonian mechanics (F=ma),
exactly like Juanshito did earlier in this thread. This is a "no-no".
You simply cannot have any "F=ma" in a relativistic page, if you want
to be taken seriously.

5. This is the first time that mass, "m" appears in your solution (it
should have appeared from the very beginning, in eq.1) but you were
too focused on avoiding it. If you had it early on, in the ODE
d(\gamma*m*v)/dt=q(E+vxB), you would have avoided this embarassing
error.

6. Now, as a result of the error at point 4, you have a completely
wrong formula for the "cyclotron frequency". Contrary to what you
derived, w is NOT equal to mv/qB = p/qB.
Nowhere close.

7. Now, you have spent quite a lot of effort writing a lot of formulas
but you are missing the most basic thing (aside from botching the
formula for w). The particle moves in a circle, so what is the value
for its radius? You come close to calculating the radius from eq.13
but not quite.

Pete,

You are not a bad guy but you have a bee under your bonnet when it
comes to relativistic mass. If you were doing the computations
honestly you would have arrived to the correct results. In order to do
that , you need to use rest mass, ok? I know that this pains you but
this is the truth.

Now, I will also transfer to Dave the solution for the case when you
have BOTH a non-null B AND a non-null E. Don't even dream of trying to
solve this problem by using relativistic mass, you can't. On the other
hand, if you start with d(\gamma*m*v)/dt=q(E+vxB) and you pay
attention to the math, you might get the correct result. This is a
tougher problem, so I suggest that you get cracking. Oh, and you will
need to use the rest mass m, ok :-)
.

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