Friday, October 6, 2017

photons - Does anything give light an initial velocity?


I am curious as to how Photons began moving to begin with. Were they moving when they were first created or did some force have to act on it to start its movement?



Answer



An alternate point of view is to look at photons as a perturbation in the electromagnetic field. Consider a positive charge at rest. From it emerges an electric field whose lines extend in the universe, at the speed of light. This speed of light is maybe to be seen as the maximal speed at which any perturbation can be transmitted rather than the speed of light itself. Now if you start to move the charge back and forth, ripple will appear on those lines, propagating at this maximal speed (like a wave on a string), the speed of light. But photons are just quanta of this excitation of the field, hence they propagate at the speed of light.


You can see this with this in this animation :http://www.tapir.caltech.edu/~teviet/Waves/empulse.html Or with this applet: https://phet.colorado.edu/sims/radiating-charge/radiating-charge_en.html


In a sense your question is also why waves propagate a the speed they do. The answer lies in the medium properties, which in the case of light is vacuum, and its properties are the the vacuum permittivity and permeability.


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