# Why does light move?

1. ### RichyB

15
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?

2. ### Dickfore

Although this is far from a good analogy, but what's propelling the Earth to revolve around the Sun?

3. ### RichyB

15
Ummm, as far as I understand it something moves through space at the same speed forever unless its path is altered by another force? I'm new to this I'm sorry if it's a stupid question.

But as I think about it, the Earth, fundamentally moving through space because of energy is received from some other place, like...maybe it gained momentum from the gravity of something and swung off into space?

But light isn't gaining momentum from anywhere there's no force that initially accelerated it is it?

I'm not sure, as I say I'm a total n00b compared to all of you I was never smart enough at maths to do physics but I can generally get my head around concepts if I'm given the opportunity to ask questions.

4. ### Naty1

The source of typical light is the transition of electrons in a nucleus from a higher energy to a lower energy orbit. When they give off energy some of it is visible light, more is electromagnetic radiation we cannot see by eye. All electromagnetic radiation moves at a constant speed, 'c', and so do the quanta of electromagnetic radiation, photons. When a photon of the right energy interacts with an electron say in silicon, a different form of eenergy is produced: an electric current.

Ultimately nobody really knows 'why' things operate this way, but we have a lot of math that describes how things operate; if they did not operate very nearly as they do we do know we would not be here to observe such activity. Without electrons orbiting nuclei to form the elements and light being absorbed and emitted, life could not exist....in fact neither would stars, plants,planets nor much of anything else.

5. ### Naty1

Thats another basic of our universe: gravity. It also progagates at the speed of light. When earth formed it did so from revovling gases...and that anguklar momentum remained as the earth solidified....nearby gases formed elsewhere and became other planets, sun, asteroid, ice moons, and so forth. Even entire galaxies have such rotations, like our own Milky Way.

6. ### Dickfore

Correct, although this is true in non-relativistic mechanics. In relativistic mechanics, there is the possibility of the existence of massless particles, such as the photons. 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 $E = c \, \sqrt{p^2 + (m c)^2}$ is the relativistic energy of the particle, and m is the rest mass. It is easily seen that massless particles do not feel any parallel acceleration (since their speed is always c), but the radius of curvature of the trajectory is given by:
$$\frac{c^2}{R} = \frac{c^2}{E} \, F_{\bot} \Rightarrow \frac{1}{R} = \frac{F_{\bot}}{E}$$

But, for photons, I don't know what force may act on them, since they are uncharged. You cannot use the above formulae for a gravitational field, since they are derived in special relativity.
The photon gains momentum and energy in the process of its emission, and keeps it until absorbed.

7. ### RichyB

15
So the movement of one electron from one orbit in the nucleus to another gives off one photon?

How many different orbit levels are there for an electron to have? Cause I was once told that every electron in the universe has a different one, and once one changes energy levels, every other one in the universe has to shift instantly to another one because they can't occupy the same one?

Brian Cox said that.

8. ### Dickfore

Please give a reference to this claim. "I was once told..." by no matter whom does not count as a reputable source.

9. ### truesearch

349
The source of 'typical' (!!) light is NOT the transition of electrons in a nucleus from a higher to a lower energy level.

10. ### davenn

3,389
ohhh really ? and where do you think the photon comes from ?

Dave

11. ### K^2

2,470
What somebody was probably trying to tell you, and I'm just guessing based on your description, is that every electron must be in different quantum state. That is true, because electrons are fermions, and no two fermions can have exactly the same state. This is called Pauli Exclusion Principle. Feel free to look it up for more details. However, two electrons in two different atoms are already inherently in different states, as location is part of the state. So transition of electron in one atom doesn't depend on state of electron in another atom. (This is simplifying things quite a bit, but it should at least clear up some confusion.)

### Staff: Mentor

Is that what you meant to say?

13. ### RichyB

15
Yeah, yeah this is what he said. It was in a talk he gave, thanks for that, it's on youtube in a video called 'A night with the stars'.

I wish I could spend a week with Brian Cox I'd have so many questions.

14. ### truesearch

349
Depends what you mean by 'typical' light. If you mean em radiation detected by the eye (known as 'visible' radiation) then this comes from changes in electron energy levels of orbiting electrons in atoms.

X-rays (not visible to the human eye) come from changes in electron energy levels closer to the nucleus of an atom.
Gamma rays come from the nucleus.
Any basic physics text book will explain this

Last edited by a moderator: Jun 2, 2012
15. ### jnorman

308
light does not move. in our particular reference frame, it appears to travel at the fixed velocity of C. however, at that speed, there is no where for it to travel from or to - time has stopped and all distance references have been reduced to zero. the photon essentially occupies all of spacetime in between the time it is emitted and the time it is absorbed.

16. ### truesearch

349
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 !!

### Staff: Mentor

Light most definitely moves. How else would it get from a light bulb to your eye? The issue is that you cannot assign a reference frame to light because there is no reference frame where light is at rest. But this does not mean it does not move, as it moves relative to our frames. I've read a explanation of everyone and everything in the universe moving at c through the time dimension in their rest frames, while light simply moves at c in the space dimension, as it doesn't have a rest frame.

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.

18. ### harrylin

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.

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.

Last edited: May 29, 2012
19. ### truesearch

349
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

20. ### vin300

518
Where did you get the first equation from? The second equation is simply F=ma