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From: ronon on 3 Feb 2010 15:38

I was just wondering if the relativistic mass of a photon could curve
space time around it and form a tunnel ?




--
ronon
From: Tom Roberts on 6 Feb 2010 17:54
Paul Cardinale wrote:
> If I'm not mistaken, a system of photons (which can have a non-zero
> rest* mass) can consist of a single photon.

A collection of photons [#] can certainly have a nonzero norm of their total
4-momentum (i.e. sum the individual 4-momenta and then take the norm). For a
single massive object, the norm of its 4-momentum is its mass, and that is the
value of its rest mass. But that collection of photons has no "rest", and if the
set consists of a single photon then the norm of its 4-momentum is zero.

Note that if the set of photons [#] all have the same 4-momentum, then the norm
of their total 4-momentum remains zero. This is also true of a classical light ray.

[#] I'm ignoring the quantum aspects of photons here.


> * When refering to a system of particles, is the word 'invariant'
> applicable?

Certainly. As used around here, "invariant" means that a given quantity does not
vary under a change of coordinates; it says nothing about what the quantity
might model. For example, the norm of any 4-vector is invariant, regardless of
what it represents.


Tom Roberts
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