photons

[littlebaskets]

does a photon pick up speed when it begins ?

[insane_alien]

no, the instant it is formed it is travelling at c. it is incapable of travelling at any other speed.

[KennyC]

That's my understanding as well. It is immediately traveling at C. Interesting huh?

 

Another sort of related question, what determines the direction it moves when it is "born?"

[timo]

Another sort of related question, what determines the direction it moves when it is "born?"

The direction is determined by the momentum of the photon which is to some extent restricted (read: determined) by the conservation of total momentum (details depend on the particular process).

[Bob_for_short]

...Another sort of related question, what determines the direction it moves when it is "born?"

 

A photon, as a wave, is created with its source (like with an antenna). It is a particular process in the source that determines the photon direction, in my opinion.

[littlebaskets]

if a photon is immediate on speed C does it contradict that law about immediate transformations ?

[timo]

When the photon does it, that means it is not illegal. More seriously: I don't know what law you are referring to so there probably is no such law.

[swansont]

The Heisenberg Uncertainty Principle? No, it is not violated. Being created moving at c does not imply the interaction took no time.

[Amr Morsi]

Energy, direction and phase can all be determined from the Quantum Electrodynamics Theory. It is to be noted that the direction is a probablistic one; i.e. under a great number of emissions the probability gets determined.

......... as far as I think.

[Coneys]

no, the instant it is formed it is travelling at c. it is incapable of travelling at any other speed.

 

It is true that it travels at c from the moment it is created, however it is capable of traveling at different speeds. Photons only travel at c in a vacuum. They can be slowed down momentarily.

[insane_alien]

no, the photons themselves travel at c withing a medium always. they just get absorbed a lot by the material so the average velocity appears lower. in truth, the photons are travelling at c or they exist as potential energy in an atom/bond. but when they are photons they travel at c

[Coneys]

no, the photons themselves travel at c withing a medium always. they just get absorbed a lot by the material so the average velocity appears lower. in truth, the photons are travelling at c or they exist as potential energy in an atom/bond. but when they are photons they travel at c

 

You are correct. I was mistaken. I now understand that when a photon travels through a dense medium, it is absorbed by atoms momentarily and then a new photon is emitted again at c... this makes it take longer for the photon to travel over the total distance, but each photon is still traveling at c.

[toastywombel]

You are correct. I was mistaken. I now understand that when a photon travels through a dense medium, it is absorbed by atoms momentarily and then a new photon is emitted again at c... this makes it take longer for the photon to travel over the total distance, but each photon is still traveling at c.

 

To expand upon what insane alien said, it really depends on how you look at the photon, from a classical point of view or a particle point of view. This is from wikipedia,

 

This is what insane alien was talking about.

 

"The factor by which the speed of light is decreased in a material is called the refractive index of the material. In a classical wave picture, the slowing can be explained by the light inducing electric polarization in the matter, the polarized matter radiating new light, and the new light interfering with the original light wave to form a delayed wave."

 

But in particle physics the photon gains mass as it moves through a medium, this is because it blends with the surrounding matter. This causes the effective mass of the photon to increase, thus making it impossible for the photon to travel at c, it must travel less than c.