On Dec 30 2011, 7:19 am,
richard.desan...@comcast.net wrote:
It appears that you are trying to analyze this situation by
classical physics, by which I mean the physics before Einstein.
However, you are making mistakes with regards to the classical
physics. You have made some statements that aren't true in either
classical physics or relativistic physics.
You need some lessons in classical physics, wmeaning the
mechanics of Newton and the optics of classical Doppler. I will ignore
SR, GR and QM for now. I will answer your questions in terms of
classical physics.
> In other words, the first bounce gives the wall twice the momentum of
> the ball, w/o the wall gaining practically no KE. I think you are
> wrong about the zero KE. The wall cannot gain momentum w/o gaining
> KE.
This doesn't make sense even in Newtonian physics. Momentum is a
vector, not a scalar. Therefore, there is an issue with the direction
of the momentum. Momentum can change direction without changing the
kinetic energy.
>A brick that gains of 2x momentum of the ball, must also gain
> four times the KE of the ball.
The kinetic energy in any direction depends on the speed of the
ball. Therefore, if the ball merely reverses direction it has not
changed its kinetic energy. This is basically what happens when a ball
bounces against the wall.
There is a tiny change in kinetic energy of the ball. You can
hear some of the kinetic energy that leaves the ball. However, it is
very small.
>Momentum=mv. 2x momentum = 2mv. KE =
> m(2v)^2 = 4mv^2.
Here, v is a speed, not a direction. Direction in this case is
characterized by the sign. The square of a number is equal the square
of the negative of that number. So,
(-v)^2=v^2
m(-2v)^2 = 4m(v)^2.
Reversal of sign changes the component of momentum in the
direction of the wall. It does not change the kinetic energy.
>
> Throughout this thread, you have been saying that redshift caused by
> relativistic emitter speed does not violate the conservation of energy
> law.
Kinetic energy is observer dependent in classical physics. One
doesn't need relativity to see that the kinetic energy changes with
the observer. The way that the kinetic energy changes in different in
classical and relativistic physics.
Conservation of energy is not violated just because an observer
sees a different kinetic energy than another observer. The kinetic
energy seen by two observers, one moving toward a source and one not
moving relative to the source, will be different even in classical
physics.
Conservation of energy is violated if the same observer traveling
at a constant speed sees a change in energy. However, that is not the
Doppler shift. It is not the relativistic Doppler shift and it is not
the classical Doppler shift. I will stick with classical Doppler for
now.
>I still think you are wrong.
The energy density of a light wave depends on the amplitude of
the electric field. The amplitude of the light wave is not changed by
the classical Doppler shift. Therefore, the energy density of the wave
is not changed by the Doppler shift.
The apparent speed of light changes for the observer headed toward
the light source. The flux of energy is the density of energy times
the speed of the light wave. The flux of energy passing a certain
point increases for the observer heading toward the source.
Therefore, observed energy absorbed by the observer increases for an
observer headed toward the source relative to an observer standing
still with respect to the source.
>
> Regardless of frame, SR or GR, E=hv defines a photon's energy. If you
> think otherwise, provide reference.
That formula is quantum mechanical. I said I will ignore quantum
mechanics for now.
The formula defines the photons energy, not the energy of of the
light wave absorbed by the observer. The energy density of the light
wave depends on the amplitude of the wave, meaning the magnitudes of
the electric field and magnetic field. Doppler predict that the
apparent frequency of the wave is changed by the observers motion. The
number of peaks hitting the observer changes. Peaks are not photons,
but they move at the same speed as the photons.
The total amount of energy headed toward the observer per unit
time is given by the flux of energy. It doesn't matter if the energy
is in discrete chunks or continuously distributed.
> that if a galaxy (moving at relativistic speed) emits a yellow photon,
> that photon will be yellow within the galaxy and will be yellow on
> Earth (assuming propagation through a vacuum).
Not if the instrument on earth is moving at high speed relative to
the center of the earth, or the other galaxy.
> If redshift causes the
> energy used to create the yellow photon to be higher than the energy
> of the photon, a violation of the conservation of energy law occurred.
Two observers on earth, moving at different speeds toward or away
from the other galaxy, would measure different frequencies and
different photon energies.
This is with or without relativity. Kinetic energy is always
observer dependent. Relativity doesn't qualitatively matter in this
regard. Kinetic energy measured by two observers moving at high
velocity relative to each other will always be different.