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Question:
I have noted a question posed by Mike T and answered by Alcir Grohmann, Vince Calder, and Ken Mellendorf.

(http://www.newton.dep.anl.gov/webpages/askasci/phy00/phy00391.htm)

Mike T's question is, "How do emitted photons instantaneously travel at the speed of light since they were not accelerated? At one instant there is no photon, and at the next instant, it miraculously is already travelling at the speed of light."

I have noted sections of Alcir Grohmann's answer "...When the photon appears it behaves like all EM waves. It does not need to be accelerated. ...", and of Ken Mellendorf's answer, "...We do not know whether it requires any time at all." ('It' refers to a change to or from a photon.)

I have recently read Einstein's Theories of Relativity, and it seems to me that the primary impetus of the Special Theory is "The Apparent Incompatibility of the Law of Propagation of Light with the Principle of Relativity", specifically the incompatibility in regard to "The Theorem of the Addition of Velocities Employed in Classical Mechanics."

Velocity is distance per unit of time. If the unit of time measured is zero, the only rational value for velocity is also zero.

Here is my problem. If a) EM waves do not require acceleration or if b) 'it' does not require any time at all, and instantaneous velocity of any object is zero, emitted light cannot inherit the velocity of the object transmitting the light.

OK, here is my question. Does the obvious effectiveness of the Lorentz transformation mean that light is NOT instantaneously propagated and thus can inherit the velocity of the object emitting the light - in other words, 'the change' DOES require time?



Replies:
You have put your finger on a "technical issue" that is usually neglected in introductions to the interaction of light and matter -- and the reason for the neglect is that the "answer" is buried in the "details" which become both conceptionally and mathematically challenging. The first issue is that quantum mechanics -- the mechanics that apply to atoms, molecules, and smaller -- does not lend itself to our macroscopic intuition. Consequently, we have to follow "the math" and see where it leads. The downside of that is a description that is not intuitively "satisfying". But that is the tradeoff. Given the major "audience" of NEWTON sometimes you have to trade rigor for comprehension. In fact "transitions" are not instantaneous, but to dig into the details requires a degree of mathematical sophistication that the 'audience' usually does not have. For those who care to get into the 'real' reasons, you can start with:
http://jchemed.chem.wisc.edu/JCEWWW/Articles/DynaPub/DynaPub.html The issue is: How many of us have the sophistication to get into the "explanation"?

Vince Calder



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