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Photon Motion
Name: Robert G. C.
Status: educator
Age: 60s
Location: N/A
Country: N/A
Date: 12/7/2004
Question:
All mass, from elementary particles to superclusters of
galaxies, moves in response to one or more of the four basic forces.
However, the photon seems unique in its ability to move, not in response
to a force, but in the absence of any force.
Replies:
Does science have any idea *why* photons move? Never mind (for now) why
they move at velocity c. Just why do they move at all?
science does better with "how" not so well with "why"; nonetheless, the
Compton effect establishes experimentally that photons and free electrons
collide exactly in such a way that the angle of the scattered electron
(experimentally measurable) and the shift in the wavelength of light
(experimentally measurable) obey the conservation laws of momentum and
energy. The experimental result is: All photons travel at exactly the
same speed (in a vacuum), the "speed of light", and this speed is
independent of the wavelength of the incident photon. A detailed analysis
of the Compton effect is too long to give in detail here, but it has been
verified innumerable times since Compton received the Nobel prize in
1927.
Three points need to be clarified: 1. Light travels at slower speeds
than "c" when traveling through matter. It is the speed of electromagnetic
radiation in a vacuum that is constant. 2. Photons DO interact with other
forces in nature. A famous such interaction is the deflection of light
from distant sources as the light passes through a strong gravitational
field. This was predicted by Einstein's general theory of relativity, and
subsequently has been observed innumerable times. 3. It is possible to
arrange an experiment to try to "trick" a photon to travel through a
transparent medium at speeds exceeding the speed of light in that medium.
But the photons aren't fooled. Radiation is emitted that brings the
photon's speed to its "proper" value. This is called Cherenkov radiation.
Again, the details are too long to go into here but a starting point for
info on that topic is: http://www.infoplease.com/ce6/sci/A0811680.html
However, this does not stop physicists from conjecturing. Physicist J.
Magueijo has a recent popular book entitled "Faster than the Speed of Light"
that makes just that conjecture -- what happens if one postulates the
possibility? Where does it lead? His is a serious physicist who discusses
the ramifications.
Finally, there are elegant, albeit arcane experiments in which the speed
of light is brought to a virtual stop in a state of matter known as Bose
Einstein condensates. This occurs only at temperatures a a few nano kelvins
above absolute zero. See, for example:
http://www.sciencenews.org/articles/20010127/fob1.asp
However, this is not what one usually means by light moving in the absence
of a force.
Vince Calder
Robert:
Photons always move (at the speed of light) since they have zero rest mass
and so have no energy (and so do not exist at all) if they are at rest (or
are moving at less than the speed of light). Maxwell's equations describe
the motion of light waves and shows that they proceed at the speed of light.
You are wrong, however, in stating that photons are unique in their ability
to move in the absence of any force. Newton's third law (F = ma) states
that the acceleration (a) of ANY object is zero if the force acting on it
(F) is zero.
Zero acceleration means that if the object is at rest it will stay at rest,
but that if it is moving, it will continue moving at constant velocity
(constant speed in the same direction).
In fact, he considered this special case (F=0) so important (and surprising)
that he dignified it as his First Law of mechanics.
Best, Dick Plano
Science does better with "how" not so well with "why"; nonetheless, the
Compton effect establishes experimentally that photons and free electrons
collide exactly in such a way that the angle of the scattered electron
(experimentally measurable) and the shift in the wavelength of light
(experimentally measurable) obey the conservation laws of momentum and
energy. The experimental result is: All photons travel at exactly the same
speed (in a vacuum), the "speed of light", and this speed is independent
of the wavelength of the incident photon. A detailed analysis of the
Compton effect is too long to give in detail here, but it has been
verified innumerable times since Compton received the Nobel prize in 1927.
Three points need to be clarified: 1. Light travels at slower speeds
than "c" when traveling through matter. It is the speed of electromagnetic
radiation in a vacuum that is constant. 2. Photons DO interact with other
forces in nature. A famous such interaction is the deflection of light from
distant sources as the light passes through a strong gravitational field.
This was predicted by Einstein's general theory of relativity, and
subsequently has been observed innumerable times. 3. It is possible to
arrange an experiment to try to "trick" a photon to travel through a
transparent medium at speeds exceeding the speed of light in that medium.
But the photons aren't fooled. Radiation is emitted that brings the photon's
speed to its "proper" value. This is called Cherenkov radiation. Again, the
details are too long to go into here but a starting point for info on that
topic is: http://www.infoplease.com/ce6/sci/A0811680.html
Vince Calder
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