How Does Light Move and What Propels Photons?

[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?

 

[Dickfore]

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

 

[RichyB]

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.

 

[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.

 

[Naty1]

but what's propelling the Earth to revolve around the Sun?

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.

 

[Dickfore]

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.

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:
<br /> \mathbf{a}_{\|} = \frac{c^2}{E} \, \left(\frac{m c^2}{E} \right)^{2} \, \mathbf{F}_{\|}<br />
<br /> \mathbf{a}_{\bot} = \frac{c^2}{E} \, \mathbf{F}_{\bot}<br />
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:
<br /> \frac{c^2}{R} = \frac{c^2}{E} \, F_{\bot} \Rightarrow \frac{1}{R} = \frac{F_{\bot}}{E}<br />

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.

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

The photon gains momentum and energy in the process of its emission, and keeps it until absorbed.

 

[RichyB]

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.

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.

 

[Dickfore]

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.

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

 

[truesearch]

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

 

[davenn]

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

ohhh really ? and where do you think the photon comes from ?
peer reviewed paper references please

Dave

 

[K^2]

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?

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.)

 

[Doc Al]

The source of typical light is the transition of electrons in a nucleus from a higher energy to a lower energy orbit.

Is that what you meant to say?

 

[RichyB]

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.)

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.

 

[truesearch]

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 textbook will explain this

 

[jnorman]

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.

 

[truesearch]

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 !

 

[Drakkith]

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.

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 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.

 

[harrylin]

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.

[..] 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.

 

[truesearch]

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

 

[vin300]

It may be shown that the parallel and perpendicular accelerations of a massless particle subject to a force F is:
<br /> \mathbf{a}_{\|} = \frac{c^2}{E} \, \left(\frac{m c^2}{E} \right)^{2} \, \mathbf{F}_{\|}<br />
<br /> \mathbf{a}_{\bot} = \frac{c^2}{E} \, \mathbf{F}_{\bot}<br />

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

 

[truesearch]

I, also, am not certain where first equation comes from. I cannot find it in my textbooks...(which do not go beyond degree level)

 

[Bobbywhy]

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.”
http://en.wikipedia.org/wiki/Radiation_reaction

 

[ImaLooser]

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."

 

[sophiecentaur]

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)

 

[elemis]

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.

 

[sophiecentaur]

I wondered about that too.

 

[Doc Al]

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.

 

[sophiecentaur]

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.

 

[Doc Al]

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.

 

[Naty1]

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

 

[Naty1]

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.

 

[truesearch]

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.

 

[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.

 

[Bobbywhy]

'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

 

[Leland]

Photon has kinetic energy, and therefore it moves.

 

[sophiecentaur]

Momentum, yes. But KE? How would you detect it?

 

[Subductionzon]

Is there a way to measure the kinetic energy of a photon using relativity? I know you could try to measure it with Newtonian mechanics but I am pretty sure that the answer would be wrong.

 

[Drakkith]

Is there a way to measure the kinetic energy of a photon using relativity? I know you could try to measure it with Newtonian mechanics but I am pretty sure that the answer would be wrong.

Not really. Per the definition from wiki:

The kinetic energy of an object is the energy which it possesses due to its motion.[1] It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its acceleration, the body maintains this kinetic energy unless its speed changes. The same amount of work is done by the body in decelerating from its current speed to a state of rest.

A photon requires no energy to accelerate up to c, it simply moves at c always. Furthermore, a photon could never be decelerated either. In addition to that, a photon always moves at c regardless of who is observing it, which means that it's kinetic energy could not be based upon it's motion, as it moves at the same velocity in all frames yet it's energy differs.

 

[truesearch]

I have received an 'infraction' as a result of my posts following post *10.
Therefore, as much as I would like to contribute to this discussion I fel that I must keep quiet.

 

[Drakkith]

I have received an 'infraction' as a result of my posts following post *10.
Therefore, as much as I would like to contribute to this discussion I fel that I must keep quiet.

Well, as long as you don't do whatever the infraction was telling you not to do you should be fine!
And don't worry too much. An infraction is like a "notification" that just tells you that you've done something you maybe shouldn't have. It's not a warning and it doesn't count against your points on the forum. It's just there to let people know they've strayed a little too far from the main road before they get too far off and start to get warnings and such.

 

[truesearch]

what I did was to repeat what a 'science advisor' did... I quoted his/her quote.
I am interested in physics not other peoples quotes.
Say no more about it... let's concentrate on physics

 

[sophiecentaur]

Is there a way to measure the kinetic energy of a photon using relativity? I know you could try to measure it with Newtonian mechanics but I am pretty sure that the answer would be wrong.

You could 'infer' a value for KE, perhaps, but not measure it. In any case, the energy of a photon is just its energy and of an type that is specific to photons. Why should it be related to half m vsquared? (If it doesn't have a mass) People make the same mistake about the momentum of a photon and try to relate it to an mv definition.

 

[Leland]

My bad, momentum is a much better word.