The discussion revolves around the nature of photons, specifically addressing whether they accelerate to the speed of light upon emission, their rest mass, and the implications of these properties in contexts such as refraction and light propagation through different media.
Participants generally disagree on whether photons can have zero velocity at any point in their existence. While some maintain that photons must always travel at c, others propose that there may be a transitional state during emission. The discussion on refraction also reveals differing interpretations of how light behaves in various media.
Participants reference concepts such as conservation laws and the electromagnetic force, but there are unresolved assumptions regarding the timing of photon emission and the nature of light propagation in different media.
This discussion may be of interest to those exploring the fundamental properties of light, the behavior of photons in various contexts, and the implications of special relativity in understanding massless particles.
G01 said:Photons always travel at c in a vacuum. So, no. They don't accelerate. They are traveling at the speed of light from the moment they are emitted.
A moment indicates some passage of time. So doesn't it take some amount of time to emitt the photon? There must be a transition between being emitted and actually emitted and if there is such a transition then is the photon moving at C during the transition? Does the Photon go from zero velocity to C absolutely instantaneously with zero passage of time? the photon must at some point in its life be at zero velocity. Right?
No. Why "must" it have zero velocity? There is no physical reason I can think of, your preconception is not a physical reason.sinebar said:the photon must at some point in its life be at zero velocity. Right?
DaleSpam said:No. Why "must" it have zero velocity? There is no physical reason I can think of, your preconception is not a physical reason.
First, photons mediate the electromagnetic force, as far as I know there is no force that acts on photons, therefore there is nothing that could accelerate them.
Second, photons are massless, so if they have any v<c they don't exist.
Third, if they don't begin at c then energy and momentum (i.e. four-momentum) are not conserved.
Those are all physical reasons that they don't accelerate. I urge you to put aside your preconceptions.
Photons always travel at C irrespective of the medium through which light is travelling. What is seen as refraction is actually the decrease or increase in the group velocity.Mr Noblet said:In refraction, what causes the change of direction and speed of the photon? This speed change is also discontinuous, right?
Special relativity requires that any massless particle always travels at C. So yes, for a photon to exist it must be traveling at C.sinebar said:Ok so at no point in the life of a photon does it ever have zero velocity? Even at the absolute instance of creation it is traveling at C? In other words a photon has never been at rest? If it has never been at rest then there must be absolutely no passage of time from creation to C?
Correct. Otherwise it would violate all sorts of conservation laws.sinebar said:Ok so at no point in the life of a photon does it ever have zero velocity? Even at the absolute instance of creation it is traveling at C? In other words a photon has never been at rest? If it has never been at rest then there must be absolutely no passage of time from creation to C?
Correct.Mr Noblet said:So the photons themselves travel at c in all mediums, but the light propagates at different speeds?
Whilst this analogy is commonly used to explain this phenomenon it isn't an accurate description of the process. Perhaps the FAQ https://www.physicsforums.com/showpost.php?p=899393&postcount=4" may help.Mr Noblet said:Is it because some photons are absorbed or deflected, so it slows the total rate at which the light moves through the medium? I'm not too familiar with a lot of optics, so maybe I'm completely wrong.