Re: New comer
From: Bill Hobba (bhobba_at_rubbish.net.au)
Date: 06/16/04
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Date: Wed, 16 Jun 2004 08:02:19 GMT
"Bilge" <dubious@radioactivex.lebesque-al.net> wrote in message
news:slrnccvsr4.m6q.dubious@radioactivex.lebesque-al.net...
> Mike:
> >
> >"Bill Hobba" <bhobba@rubbish.net.au> wrote in message
> >news:d_Kzc.29402$sj4.25753@news-server.bigpond.net.au...
> >>
> >> "Kuki" <kukiyky@hotmail.com> wrote in message
> >> news:can559$g84$1@justice.itsc.cuhk.edu.hk...
> >> > Hi all, I am a new beginner in Relativity.
> >> > I have got some enquiries about this topic, and hope you can give me
> >some
> >> > guidances on those problems, thanks!
> >> >
> >> > 1. We all know that light travels more slowly in glass than in air,
so
> >can
> >> I
> >> > say this contradict the thoery of special relativity?
> >>
> >> No - because it says that the speed of light in a vacuum is invariant.
> >> Actually SR has little to do per se with light - see
> >> http://arxiv.org/abs/physics/0110076.
> >>
> >> >
> >[snip]
> >
> >There can be no SR without the postulate about the constancy of the
> >speed of light.
>
> That isn't true. Special relativity is a theory of spacetime, not
> electromagnetism. The second postulate is more of an historical artifact
> than something which is necessary for special relativity. From the first
> postulate, one obtains the lorentz transforms simply as the most general
> linear transformation for a spacetime displacement and galilean invariance
> as a well-defined limiting case.
>
> Einstein chose the speed of light to represent the quantity `c', for
> the simple reason his goal in 1905 was to provide a ``natural''
> explanation for maxwell's equations in terms of spacetime. Fixing the
> constant `c' to be the speed of light in maxwell's equations did that.
>
> Physics has changed since that time. We now know of two interactions
> which do not have 1/r potentials, so it's really more natural to place
> things which are related to E&M in a theory about E&M. Relativity then
> places constraints on E&M because charge conservation then depends
> upon the speed of light being equal to `c'. On the other hand, it's
> fairly straight forward to create a relativistic theory of E&M which
> has a massive photon and doesn't conserve charge. The most natural
> thing to do is choose c == 1, so that space and time really are on
> equal footing and let experiment decide which phenomena (if any) are
> null vectors.
>
> >The article you site is a piece of crap as far as the assertions it
> >makes. Any experiments to determine mu in that paper must involve the
> >auxiliary hypothesis of the constancy of the OWSL otherwise they are
> >impossible to perform. The only way to support that view is to make a
direct
> >OWSL measurement. This is impossible obviously in the context of SR.
Thus,
> >the postulate must be retained and the derivation in that paper is plain
> >posteriori.Furthermore, it entails many more possibilities than SR
entails
> >as mu can take any value. Seems like a compensatory theory rather than
SR
> >without c = c.
>
> The derivation in that paper seems overly complicated, but it's
> aimed at lower level students who don't have the mathematics or
> physics background necessary to derive relativity in the simplest
> way that best suits the spirit of the theory.
That is true - and is obviously aimed at exactly the audience you describe -
which is the reason I like quoting it. In fact is it is an expansion of the
derivaiuton found in Rindler Introduction to Special Relativity. The
derivation of linearity, for example, is much more complicated than it
needs to be. I think Tom Roberts derivation based on group theory is better
again.
>
> If you want a very simple derivation all you need to do is to
> assume that time and space are on equal footing without assuming
> the metric is lorentzian, initially. The lorentzian signature
> is easy to justify, based on the fact the neither of the two other
> signatures would be consistent with any observation. Then take an
> arbitrary vector X = (t, x, y, z), perform an infinitessimal spacetime
> displacement and construct the finote transforms. You get 3 rotations
> and 3 boosts from that. You get the galilean transforms as a well
> definined limiting case.
>
I rather like the derivation along those lines from the paper by Victor
Stenger that I have given the link to many times. He shows the signature
from QM considerations. Trouble is he withdrew it for some reason which I
think is a pity.
Thanks
Bill
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