Re: Photon bend another photon


Randy M. Dumse wrote:
<rambus2005@xxxxxxxxx> wrote in message
news:1156001002.217451.177670@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Nice post. A small correction, though, the "box full of photons" is a
bad example. What you are measuring is the effect of the increased
"pressure radiation" which manifests itself as a measurable force,
not the increased mass of the system.


Thanks. I hear, and appreciate, your objection. I even worried a bit
about putting it as I did. Notice I had choosen mass-energy, as opposed
to the more correct stress-energy, to more easily be understood by the
current audience. I also included a pressure comment to keep the door to
larger concepts open. So you are right that in this example pressure
plays the major role.

But I don't know if I am comfortable saying the mass of the system does
not increase. I feel if I left out the box completely, and the "system"
was just the two (antiparallel) photons it would still have mass.

But given the box is closed, you don't know what's inside. You can
measure the mass of the system. Weight it, or try to accelerate it. In
either case, it exhibits mass. Calling it mass, or calling it "pressure
radiation manifesting itself as a measurable force" seem just two ways
of looking at the same thing due to equivalence.

Would you call the "mass without mass" of Wheeler's geon due to pressure
radiation? I wouldn't. Would radiation thrown down a black hole increase
it's radiation pressure component? or does it just become "mass".

So I don't see the mirrored box as a terribly bad example, just one
complicated by the presence of the mirrored box, to make it is easier to
conceptualize two photons as a system. The mirrors help us imagine them
remaining in close proximity.

--
Randy M. Dumse

Caution: Objects in mirror are more confused than they appear.

Very good points. There are several reasons for steering clear of
"mass" in the unfortunate "photon in the box" example.

1. As you well point out, the box is closed, so we can only observe the
increase in inertia. If one does the computations, one can show that
the increased inertia is due to the forces tangential to the vertical
box walls resulting from pressure radiation.

2. The forces at point 1 are dependent on the direction of the photons
, parallel or antiparallel . In the case of a parallel pair photons the
resulting "invariant pass" is sqrt(Sigma_E^2-(Sigma_pc)^2)=0 while in
the case of antiparallel case the "invariant mass" is Sigma_E (because
Sigma_p=0) . This fact produces a "paradox", the "mass added to the
photon box" is direction dependent which does not bode well with the
notion of mass being a scalar. This is why I call the "photon in a box"
a very unfortunate example. I emaile Baez on this, he corrected the
website a little , but not enough.

.

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