## Why does light move?

 Quote by RichyB Or how does light move? There's a source of light, for example a torch but what is it that propelled the photons into my eye from the torch? I'm new to this so I don't know, I mean I know light is a wave but it's also a photon init, so why does that photon move?
As you already indicate, the simplest description/interpretation is that light moves as a wave. The photon concept can be confusing; however that description basically implies that light isn't created as continuous waves but as "wave packets". That should not be confounded with the idea of massive particles (I'm just guessing what may be bugging you).

In relativity theory the speed of light waves is a property of "space" and according to general relativity this property is affected by the local presence of mass. Consequently it was predicted (and verified) that light rays ("photons") should bend around the Sun even though light does not consist of particles with rest mass.

 Quote by RichyB [..] But light isn't gaining momentum from anywhere there's no force that initially accelerated it is it?
In fact conservation of momentum also applies to light; light has both energy and momentum. In the very high vacuum of space this momentum could in theory be used for "sailing" space ships away from the sun.
 I like the idea that 'light does not experience time' because it sort of reflects Einsteins great thought experiment about what a clock woul look like if you were moving away from it at the speed of light....time would stand still. From this came the relativity that we all love and find hard to get to grips wit

 Quote by Dickfore It may be shown that the parallel and perpendicular accelerations of a massless particle subject to a force F is: $$\mathbf{a}_{\|} = \frac{c^2}{E} \, \left(\frac{m c^2}{E} \right)^{2} \, \mathbf{F}_{\|}$$ $$\mathbf{a}_{\bot} = \frac{c^2}{E} \, \mathbf{F}_{\bot}$$
Where did you get the first equation from? The second equation is simply F=ma
 I, also, am not certain where first equation comes from. I cannot find it in my text books...(which do not go beyond degree level)

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 Quote by RichyB Or how does light move? There's a source of light, for example a torch but what is it that propelled the photons into my eye from the torch? I'm new to this so I don't know, I mean I know light is a wave but it's also a photon init, so why does that photon move?
I am not positive, but I think this is what “propels the photons into your eye from the torch”:

“In the physics of electromagnetism, the Abraham–Lorentz force is the recoil force on an accelerating charged particle caused by the particle emitting electromagnetic radiation. It is also called the radiation reaction force.
The force is proportional to the square of the object's charge, times the so-called "jerk" (rate of change of acceleration) that it is experiencing. The force points in the direction of the jerk. For example, in a cyclotron, where the jerk points opposite to the velocity, the radiation reaction is directed opposite to the velocity of the particle, providing a braking action.”

 Quote by RichyB Or how does light move? There's a source of light, for example a torch but what is it that propelled the photons into my eye from the torch? I'm new to this so I don't know, I mean I know light is a wave but it's also a photon init, so why does that photon move?
I don't know why or how light moves, and I'm not convinced anyone else knows. It is observed to do so, that's all.

The wave/particle duality went out of style in 1950, but is as persistent as crabgrass. All it will do is confuse you. Think quantum field theory, in which photons and particles are both "excitations of a field."

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 Quote by ImaLooser I don't know why or how light moves, and I'm not convinced anyone else knows. It is observed to do so, that's all. The wave/particle duality went out of style in 1950, but is as persistent as crabgrass. All it will do is confuse you. Think quantum field theory, in which photons and particles are both "excitations of a field."
Problem is how to get that across to elementary students (younger than University age). The historical approach is something that I think we have to stick with. (Unless you can think of a way of NOT telling schoolkids about light and electromagnetism, yet giving them some sort of introductory Science education)

 Quote by truesearch The source of 'typical' (!!) light is NOT the transition of electrons in a nucleus from a higher to a lower energy level.
I'm a chemist and am a bit confused by how you seem to imply that electrons originate from the nucleus.
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Quote by elemis
 Quote by truesearch The source of 'typical' (!!) light is NOT the transition of electrons in a nucleus from a higher to a lower energy level.
I'm a chemist and am a bit confused by how you seem to imply that electrons originate from the nucleus.
I guess you missed the word NOT in his post.

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 Quote by Doc Al I guess you missed the word NOT in his post.
With or without the "NOT", the statement is faulty. It's wrong without the "NOT" and irrelevant with it. We all know there are no electrons in the nucleus. What could be said, however, is that the source of light 'is' the movement of electrons relative to the nucleus.

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 Quote by sophiecentaur With or without the "NOT", the statement is faulty. It's wrong without the "NOT" and irrelevant with it. We all know there are no electrons in the nucleus. What could be said, however, is that the source of light 'is' the movement of electrons relative to the nucleus.
Yes, and I believe it was Naty1 who made the initial misstatement (presumably a typo) in post #4. Truesearch was only pointing it out, as I tried to do in post #12.

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Poor wording by me....I just meant orbital electrons in an ATOM [not nucleus] changing energy levels as an example.

 When an electron absorbs energy either from light (photons) or heat (phonons), it receives that incident quanta of energy. But transitions are only allowed in between discrete energy levels such as the two shown above. This leads to emission lines and absorption lines. When an electron is excited from a lower to a higher energy level, it will not stay that way forever. An electron in an excited state may decay to a lower energy state which is not occupied, according to a particular time constant characterizing that transition. When such an electron decays without external influence, emitting a photon, that is called "spontaneous emission".
http://en.wikipedia.org/wiki/Laser

or
 If a light source ('the atom') is in the excited state with energy , it may spontaneously decay to a lower lying level (e.g., the ground state) with energy , releasing the difference in energy between the two states as a photon.
http://en.wikipedia.org/wiki/Spontaneous_emission

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 I agree with you jnorman and I love the summary that I once read (can't remember where !!!) Light takes no time to get from one point to the next and for light there is no distance between one point and the next. Wish I understood it !! I believe you are saying that light does not experience time, as it travels at c and according to the math time should stop at that velocity. I'd say that the math simply doesn't work when you input a velocity of c into the equations, so you cannot depend on it.
Such stuff, which is ok intuitively to some extent, leads to comments like this, which I like:

 Eternity is no time at all for a photon.
Richard Feynman may have been the wise guy [genius] who coined that, I'm not sure. But it sounds like something he would have said!!!!

I think the math does 'work' but just for light;it's not a realistic result. The issue is that 'nothing' [no mass, no test instrument] can ever reach the speed of light so nothing can be in the frame of light. One way to understand this is that no matter how fast you think you are going, locally light will always pass you at 'c'...so 'nothing' can attain such speed 'c'. The math tells you that IF you are moving at speed c ,as a photon does, time appears to stop and [Lorentz-Fitzgerald] length is zero. [You can't use photons to measure any time delay because they don't age!!]

Hence the other common adage:
 Photons don't age.
 It was a mistake to state that 'typical' light came from electrons in a nucleus. This mistaken statement appears to have been backed up in post *10 my a 'science advisor' no less. I hope that we would all agree that mistakes of this sort must be corrected.
 'We all know there are no electrons in the nucleus' yet we all know (!!!) that beta radiation (electrons) come from the nucleus. Physics is fascinating.

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 Quote by truesearch 'We all know there are no electrons in the nucleus' yet we all know (!!!) that beta radiation (electrons) come from the nucleus. Physics is fascinating.

“An unstable atomic nucleus with an excess of neutrons may undergo β− decay, where a neutron is converted into a proton, an electron and an electron-type antineutrino.”
http://en.wikipedia.org/wiki/Beta_particle