Re: recoiling photons evidence?

"ralph sansbury" <[EMAIL PROTECTED]> wrote in message
news:[EMAIL PROTECTED]
>
> "George Dishman" <[EMAIL PROTECTED]> wrote in message
> news:[EMAIL PROTECTED]
> >
> > "ralph sansbury" <[EMAIL PROTECTED]> wrote in message
> > news:[EMAIL PROTECTED]
> > > When a bullet and the exploding gas shoots out of the barrel
> of a
> > > gun, the gun moves in the opposite direction at a lesser
> velocity
> > > because of its greater mass according to conservation of
> > > momentum..
> > >    But the emission of a photon which has no mass except the
> > > ascribed E/c^2 is not quite the same.
> >
> > That is the wrong equation.The correct form is:
> >
> >  E^2 = m^2 * c^4  +  p^2 * c^2
>
> No this is the wrong equation. The correct form is
> E^2 = (m_0)^2 * c^4  +  p^2 * c^2

Physics these days has generally dropped the old 'relativistic
mass' presentation as anachronistic and confusing. "Mass" means
invariant mass.

>   The correct implicit equation I was using was
> m=E/c^2 not m_0=E/c^2.
>    And since p=mv=Ev/c^2
> and mc^2=(m_0)c^2/(1-v^2/c^2)^1/2 you get
> the corrected equation above.

The equation you used implies non-zero mass while the
(invariant) mass of a photon is zero, hence the possible
confusion. If you want to use "relativistic mass", it is
best to use that phrase so everyone understands you.

> > hence
> >
> >  m = sqrt(E^2 - p^2 * c^2) / c^2
> >
>
> > For a photon that is zero as far as anyone has measured.
> >
>      The rest mass may be zero but it is never at rest

And that comment is exactly the reason the these terms have
been dropped. The "rest mass" is actually invariant so is
a fundamental property that applies regardless of motion.
The "rest mass" of a photon is zero at all speeds.

> and  the
> difference between the
> rest mass and the mass in motion is not zero.

The difference is the kinetic energy, it is not mass of any
form. Do you see why these terms are considered confusing?

> A gamma ray photon
> impinging upon a nucleus can be converted into the rest mass of
> an electron
> and a positron and the kinetic energy of these particles.

Yes, total energy and total momentum are both conserved.

>    In the Compton effect you mention the xray photon impinging on
> an electron produces the energy of its oscillation at a lower
> frequency
> plus the kinetic energy of its linear motion.

It is not clear what "its" means in that sentence. The frequency
(hence energy) of the photon is reduced while the kinetic energy
of the electron is increased. The momentum is similarly
redistributed but still conserved.

>    But lets get back to my question as to the evidence of photon recoil
> in the emission of a photon? When the distance between the source
> and the receiver is small as in the Compton effect and various
> other cases eg
> http://www.aip.org/enews/physnews/1999/split/pnu450-1.htm,

You need a subscription to read the article but from the abstract
is seems clear they are using radiation pressure to cancel the
motion of the mirror. Since the momentum of the mirror is changed
by the reflection of the photons, there is your evidence.

>  The action between the source magnetic force of the oscillating electrons
>  on the receiver magnetic property of the oscillating electrons produces
> a repulsion and part of the energy and momentum is this and part
> is the electrical field producing the oscillation.

The mechanism is not relevant to your question, you only asked
for evidence that photons carry momentum when they recoil.

>    If however you start an oscillation of charge in the source
> by thermally exciting bound electrons in tungsten for example
> the oscillating electrons may recoil from one another but the
> whole source would not be recoiling against anything.

In thermal agitation in tungsten, there are many particles
being accelerated in random directions so the mean cancels.

>    To say that some equation that works in the above cases
> implies that it would work in this case may not be correct.

There is no reason to suspect it would differ, and since
the same model is successful at the particle level and for
macroscopic measures such as the mirror you cite, it is a
usable model.

> http://math.ucr.edu/home/baez/physics/ParticleAndNuclear/photon_m
> ass.html
> >
> > >    Instances of light pressure on reflective surfaces and Compton
> > > like scattering of xrays at lower frequencies with the missing
> > > energy transformed into the linear movement of the electron
> > > suggests a recoil of the electron but not of the photon.
> > >    What is the evidence for recoiling photons?
> >
> > If the photon did not carry momentum, there would be
> > no effect at all on the electron in the Compton effect.
> >
>    It is not the momentum of the photon but the magnetic effect
> which causes the linear movement at some angle is between
> oscillating charges in the xray source and the oscillation of the
> receiver electrons and the electric effect is the oscillation at
> a slightly lower frequency.

However you describe the mechanism, the effect is the same,
momentum is transferred and the amount is quantised.

>  >If the incoming photon carried momentum but the outgoing
> > photon did not, the angles would be different to what is
> > observed.
> >
>    In the case I am talking about there is no incoming photon.

You only asked:
> > > When a bullet and the exploding gas shoots out of the barrel of a
> > > gun, the gun moves in the opposite direction at a lesser velocity
> > > because of its greater mass according to conservation of
> > > momentum..
> > >    What is the evidence for recoiling photons?

The Compton effect demonstrates that both the incoming and
outgoing photons must carry momentum proportional to their
frequency.

George