Department of Energy Argonne National Laboratory Office of Science NEWTON's Homepage NEWTON's Homepage
NEWTON, Ask A Scientist!
NEWTON Home Page NEWTON Teachers Visit Our Archives Ask A Question How To Ask A Question Question of the Week Our Expert Scientists Volunteer at NEWTON! Frequently Asked Questions Referencing NEWTON About NEWTON About Ask A Scientist Education At Argonne 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



Click here to return to the Physics Archives

NEWTON is an electronic community for Science, Math, and Computer Science K-12 Educators, sponsored and operated by Argonne National Laboratory's Educational Programs, Andrew Skipor, Ph.D., Head of Educational Programs.

For assistance with NEWTON contact a System Operator (help@newton.dep.anl.gov), or at Argonne's Educational Programs

NEWTON AND ASK A SCIENTIST
Educational Programs
Building 360
9700 S. Cass Ave.
Argonne, Illinois
60439-4845, USA
Update: June 2012
Weclome To Newton

Argonne National Laboratory