Re: [Phys-l] Neutrinos going faster than speed of light?

[David Bowman <David_Bowman@georgetowncollege.edu>]

Regarding Alex Brown's response to Brian W's question:

> phase velocity can, but group velocity can't...
>
> --- On Sat, 24/9/11, brian whatcott <betwys1@sbcglobal.net>
> wrote:
>
> From: brian whatcott <betwys1@sbcglobal.net>
> Subject: Re: [Phys-l] Neutrinos going faster than speed of
> light?
> To: "Forum for Physics Educators"
> <phys-l@carnot.physics.buffalo.edu>
> Date: Saturday, 24 September, 2011, 17:38
>
> > On 9/24/2011 10:58 AM, chuck britton wrote:
> > > I SAID it was naive and the BRIEF reading had me still
> > > thinking of a vacuum chamber that had withstood the
> > > earthquakes/drifting etc.
> > >
> > > yikes ;-)
> >
> > More in the same naive vein: I seem to recall that there is a
> > measurable entity in EM waves that has super-luminal velocity:
> > is it phase velocity vs group velocity?
> > Gamma rays have wave properties too of course!   :-)
> >
> > Brian W

Often, but not always, the group velocity tends to coincide with the signal/information velocity (the speed limited by c).  But in certain absorptive media it is possible for an EM wave to have a superluminal group velocity.  See: http://www.eleceng.adelaide.edu.au/personal/dabbott/publications/PIE_withayachumnankul2010.pdf  for more details.  But the speed of energy transfer is always luminal or subluminal.  The local velocity of energy transfer is given by the local quotient of the Poynting flux divided by the EM energy density, i.e. 2*(E x H)/(E·D + H·B).

David Bowman