On Fri, 18 Aug 2006 02:20:49 +0000, "Danny Dot"
wrote in :


"Cosmik Debris" wrote in message
news
On Fri, 18 Aug 2006 12:39:03 +1200, Bill Hobba wrote:


"Danny Dot" wrote in message
...
I am a lowly engineer in aerospace, but have taken a few advanced
physics
questions.

My question is, if two photon's pass really, really, really close to
each
other -- will their paths be changed by one interaction with the gravity
of the other.

Well very small distances in QM are problematical due to the Heisenberg
uncertainty principle. But yes it is predicted that EM radiation will
interact gravitationally. Strangely though it depends on the direction

Actually the prediction is that if gravity is already
generated from something that has mass, then photons
will be subject to it from the curvature of space-time.
However, photons never generate gravity since they have
no mass.

the
beams are traveling - I forget which is which but if they pass in one
direction no interaction - if they pass in the opposite direction then
they
interact. But the effect is so small there is little chance it will
ever be
experimentally testable.


Beams parallel no attraction, anti-parallel attraction.


Was the interaction gravity or some other force??
[...]

I'm not sure I believe these assertions. It would be nice
to get an exact reference for the claim. The interaction,
if any, would definitely not be gravitational, even though
there I think I could not rule out some nonlinear thing
from EM itself. However, I kind of doubt the claim.

--
// The TimeLord says:
// Pogo 2.0 = We have met the aliens, and they are us!

The TimeLord wrote:
On Thu, 17 Aug 2006 20:34:12 +0000, "Danny Dot"
wrote in :

I am a lowly engineer in aerospace, but have taken a few advanced physics
questions.

My question is, if two photon's pass really, really, really close to each
other -- will their paths be changed by one interaction with the gravity of
the other.

No. Photons do not have mass. Thus they generate no
gravity. Thus they do not interact gravitationally.

--
// The TimeLord says:
// Pogo 2.0 = We have met the aliens, and they are us!

It is not mass which causes gravitation, but energy.

The TimeLord wrote:
On Thu, 17 Aug 2006 20:34:12 +0000, "Danny Dot"
wrote in :

I am a lowly engineer in aerospace, but have taken a few advanced physics
questions.

My question is, if two photon's pass really, really, really close to each
other -- will their paths be changed by one interaction with the gravity of
the other.

No. Photons do not have mass.

That statement is correct.

Thus they generate no
gravity. Thus they do not interact gravitationally.


That statement is not correct. GR predicts, that despite their lack of
mass, photons are not only deflected by gravitational fields but,
because of their energy and momentum, they can be the source of
gravitational fields. Remember that gravity is an inertial force.
Hence the acceleration of a test particle is independent of its mass.
Ironically, although it was unknown to Newton, his theory of
gravitation predicts deflection of light rays in a gravitational field,
but provides only one half the deflection seen in nature. GR provides
the correct value.

"Danny Dot" wrote in message
...
I am a lowly engineer in aerospace, but have taken a few advanced
physics questions.

You are asking in a dangerous place. Many answers are uniformed, casual,
some incorrect. Always check references, if offered, to know which are
correct.

My question is, if two photon's pass really, really, really close to
each other -- will their paths be changed by one interaction with the
gravity of the other.

GR predicts light rays traveling in parallel do not interact. This is
easily confirmed experimentally. Look through a telescope. Are distant
images blurred by gravitational interaction smearing out of parallel
traveling rays? or can distant rays be focused with telescoping images?
Q.E.D.

GR predicts antiparallel rays to attract. The gravitational interaction
of antiparallel rays is so small, and the event of crossing so short,
this is almost impossible to imagine an experiment with current
technology that could prove this prediction. (I very probably would have
heard if such an experimental result was published or even attempted.)

However, the math of GR clearly makes the predition antiparallel light
to light attraction occurs. You can review this math if you wish in the
original R.C. Tolman's 1934 text, "Relativity, Thermodynamics and
Cosmology" if you wish, see Pg 274. Or you can refer to my 1999
coauthored paper http://xxx.lanl.gov/abs/gr-qc/9811052. See page 12 for
an estimate (with rather dated construction parameter estimates) of the
gravitational interaction in the beams of LIGO.

The acceleration of a test ray relative to a pencil of light can be
thought of as gravitational deflection, or it can be envisioned as the
interaction of the gravitoelectric and gravitomagnetic effects. When the
beams are parallel these later effects cancel so there is no effect.
When the beams are antiparallel these effects reinforce so there is
twice the effect.

Notice I avoided the use of "photon" as this is a quantum term, and GR
is a classical theory. To the best of all experimental knowlege to date,
photons have no detectable mass.

In GR, it is mass-energy that gravitates, and not just mass. Even
photons (and even pressure!) participate. If you put two photons in a
mirror box, its mass would increase (a tiny, but non-zero, amount). Fill
the mirrored box with many photons, and externally, you can't tell the
difference between the box with photons in it and the box with an
equivalent mass.

--
Randy M. Dumse

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

"The TimeLord" wrote in message
news
On Fri, 18 Aug 2006 02:20:49 +0000, "Danny Dot"
wrote in :


"Cosmik Debris" wrote in message
news
On Fri, 18 Aug 2006 12:39:03 +1200, Bill Hobba wrote:


"Danny Dot" wrote in message
...
I am a lowly engineer in aerospace, but have taken a few advanced
physics
questions.

My question is, if two photon's pass really, really, really close to
each
other -- will their paths be changed by one interaction with the
gravity
of the other.

Well very small distances in QM are problematical due to the Heisenberg
uncertainty principle. But yes it is predicted that EM radiation will
interact gravitationally. Strangely though it depends on the direction

Actually the prediction is that if gravity is already
generated from something that has mass, then photons
will be subject to it from the curvature of space-time.
However, photons never generate gravity since they have
no mass.

What makes you think that? The source of gravity is the stress-energy
tensor so the source of gravity need not have a rest mass - all one needs to
do is write down a lagrangian and you have a stress energy tensor - indeed
that is the modern definition of stress-energy tensor. It is then trivial
to see that light must interact gravitationally - ie EM fields have a
lagrangian. What is not trivial and quite interesting is the interaction
depends on the direction of the beams.

Thanks
Bill


the
beams are traveling - I forget which is which but if they pass in one
direction no interaction - if they pass in the opposite direction then
they
interact. But the effect is so small there is little chance it will
ever be
experimentally testable.


Beams parallel no attraction, anti-parallel attraction.


Was the interaction gravity or some other force??
[...]

I'm not sure I believe these assertions. It would be nice
to get an exact reference for the claim. The interaction,
if any, would definitely not be gravitational, even though
there I think I could not rule out some nonlinear thing
from EM itself. However, I kind of doubt the claim.

--
// The TimeLord says:
// Pogo 2.0 = We have met the aliens, and they are us!

"Randy M. Dumse" wrote in message
...
"Danny Dot" wrote in message
...
I am a lowly engineer in aerospace, but have taken a few advanced physics
questions.

You are asking in a dangerous place. Many answers are uniformed, casual,
some incorrect. Always check references, if offered, to know which are
correct.

My question is, if two photon's pass really, really, really close to each
other -- will their paths be changed by one interaction with the gravity
of the other.

GR predicts light rays traveling in parallel do not interact. This is
easily confirmed experimentally. Look through a telescope. Are distant
images blurred by gravitational interaction smearing out of parallel
traveling rays? or can distant rays be focused with telescoping images?
Q.E.D.

GR predicts antiparallel rays to attract. The gravitational interaction of
antiparallel rays is so small, and the event of crossing so short, this is
almost impossible to imagine an experiment with current technology that
could prove this prediction. (I very probably would have heard if such an
experimental result was published or even attempted.)

However, the math of GR clearly makes the predition antiparallel light to
light attraction occurs. You can review this math if you wish in the
original R.C. Tolman's 1934 text, "Relativity, Thermodynamics and
Cosmology" if you wish, see Pg 274. Or you can refer to my 1999 coauthored
paper http://xxx.lanl.gov/abs/gr-qc/9811052. See page 12 for an estimate
(with rather dated construction parameter estimates) of the gravitational
interaction in the beams of LIGO.

The acceleration of a test ray relative to a pencil of light can be
thought of as gravitational deflection, or it can be envisioned as the
interaction of the gravitoelectric and gravitomagnetic effects. When the
beams are parallel these later effects cancel so there is no effect. When
the beams are antiparallel these effects reinforce so there is twice the
effect.

Notice I avoided the use of "photon" as this is a quantum term, and GR is
a classical theory. To the best of all experimental knowlege to date,
photons have no detectable mass.

In GR, it is mass-energy that gravitates, and not just mass. Even photons
(and even pressure!) participate. If you put two photons in a mirror box,
its mass would increase (a tiny, but non-zero, amount). Fill the mirrored
box with many photons, and externally, you can't tell the difference
between the box with photons in it and the box with an equivalent mass.

--
Randy M. Dumse

Nice response Randy - a pleasure to read. Good to see you posting.

Thanks
Bill


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

"Bill Hobba" wrote in message
...

"The TimeLord" wrote in message
news
On Fri, 18 Aug 2006 02:20:49 +0000, "Danny Dot"
wrote in :


"Cosmik Debris" wrote in message
news On Fri, 18 Aug 2006 12:39:03 +1200, Bill Hobba
wrote:


"Danny Dot" wrote in message
...
I am a lowly engineer in aerospace, but have taken a few advanced
physics
questions.

My question is, if two photon's pass really, really, really close to
each
other -- will their paths be changed by one interaction with the
gravity
of the other.

Well very small distances in QM are problematical due to the
Heisenberg
uncertainty principle. But yes it is predicted that EM radiation will
interact gravitationally. Strangely though it depends on the
direction

Actually the prediction is that if gravity is already
generated from something that has mass, then photons
will be subject to it from the curvature of space-time.
However, photons never generate gravity since they have
no mass.

What makes you think that? The source of gravity is the stress-energy
tensor so the source of gravity need not have a rest mass - all one needs
to do is write down a lagrangian and you have a stress energy tensor -
indeed that is the modern definition of stress-energy tensor. It is then
trivial to see that light must interact gravitationally - ie EM fields
have a lagrangian. What is not trivial and quite interesting is the
interaction depends on the direction of the beams.

BTW if you are interested in the technical detail of this I checked my
reference which is Wald - it is found in appendix E.

Thanks
Bill


Thanks
Bill


the
beams are traveling - I forget which is which but if they pass in one
direction no interaction - if they pass in the opposite direction then
they
interact. But the effect is so small there is little chance it will
ever be
experimentally testable.


Beams parallel no attraction, anti-parallel attraction.


Was the interaction gravity or some other force??
[...]

I'm not sure I believe these assertions. It would be nice
to get an exact reference for the claim. The interaction,
if any, would definitely not be gravitational, even though
there I think I could not rule out some nonlinear thing
from EM itself. However, I kind of doubt the claim.

--
// The TimeLord says:
// Pogo 2.0 = We have met the aliens, and they are us!

Sorcerer wrote:
"Danny Dot" wrote in message
...
|I am a lowly engineer in aerospace, but have taken a few advanced physics
| questions.
|
| My question is, if two photon's pass really, really, really close to each
| other -- will their paths be changed by one interaction with the gravity
of
| the other.

No.
Photons are not billiard balls, although they share some things in common,
such as momentum. What makes a photon different is that they can pass
right through each other. That isn't so strange, though, they can pass
right through glass too, a billiard ball cannot.
Androcles

Actually billiard balls can pass thru glass and photons cannot!

The first case should be obvious, unless you have bullet-proof glass.
(I make a joke!) And then you go get more glass.

In the second case, photons are thought to be absorbed by the
atomsin the medium, and the the excited atoms radiate new
photons. John Fox, Am. J. Phys., 33,1 (1965), calls this an
extinction process. The new photons are not, strictly speaking,
the originals, but replicas. I'd guess that a little energy is lost
in each absorption/re-emission event.

In my humble opinion there is a lot of wrong think about
so-called photons. Here is an alternate description of how
atoms disturb one another at a distance electromagnetically.

Ritz's (1908) ballistic emission theory held that charges emit,
more-or-less continuously, what he called fictitious particles.
It would be fairly easy to picture vibrational disturbances
being impressed on the outgoing fluxes of fictitous particles.
(Real particles with gaps between them might be permissible.)

If you have a gazillion atomic sized Ritzian* e/m radiators all
doing their thing, their emission fluxes, with vibrational
disturbances impressed, spread out in more or less all
directions, in buck-shot (or water spray droplet) fashion,
rather than bulltet-like fashion, and pass amongst one another
mostly unscathed.

Every now and then (quite frequently, actually) some of these
"wave-like" disturbances arrive at a given chunk of space in
phase with each other, and constructive interference produces
a strong enough wave action (at that place and time) that
can induce a "measurable" atomic disturbance.

*Ritz theorized that atoms produced e/m radiation as a result
of their electron orbits vibrating with respect to their atomic
magnetic fields. (No spin-flips or shell hopping required!) This
would be similar to the well known e/m radiation produced by
molecular vibrations.

For more on Ritz's emission theory, please see:
http://www.shadetreephysics.com/crit/1908a.htm
http://www.shadetreephysics.com/ritz.htm

Bob Fritzius
Electrical Engineer gone bad

Randy M. Dumse wrote:
..

In GR, it is mass-energy that gravitates, and not just mass. Even
photons (and even pressure!) participate. If you put two photons in a
mirror box, its mass would increase (a tiny, but non-zero, amount). Fill
the mirrored box with many photons, and externally, you can't tell the
difference between the box with photons in it and the box with an
equivalent mass.


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.

Dear fritzius:

wrote in message
ups.com...
....
In the second case, photons are thought to be
absorbed by the atomsin the medium, and the
the excited atoms radiate new photons. John
Fox, Am. J. Phys., 33,1 (1965), calls this an
extinction process. The new photons are not,
strictly speaking, the originals, but replicas.
I'd guess that a little energy is lost in each
absorption/re-emission event.

You describe "transmission", in which c_medium c. Gamma and
radio photons pass through glass without slowing.

In my humble opinion there is a lot of wrong
think about so-called photons. Here is an
alternate description of how atoms disturb
one another at a distance electromagnetically.

We are receiving photons generated just before the CMBR quenched.
13-15 billion years separate the emitting and receiving charges.
Wrap your head around that.

David A. Smith