# Photon Emission

1. Dec 5, 2008

### jmltinc

Can someone tell me about photon emission?

If an electron bound to a nucleus emits a photon and drops to a lower energy level, in what direction does the photon go?

- Does it leave the electron perpendicular to the tangent line of the electon's 'orbit' at the instant of emission - that is, at the time of emission, does it leave the electron in a direction away from the nucleus along the radius vector whose origin is the nucleus and passes through the electron?

- Are photons emitted toward a point in space of lower energy?

- Or, is the direction of emission of a photon not known, and only a probability of a direction known?

Thank you,
-John

2. Dec 6, 2008

### clem

Your first three questions are too classical to be relevant.
The probability of the photon's direction depends on the orbital angular momentum of the two states. For decay from a P (L-1) state to an S (L=0) state, the probability is proportional to cos^2\theta, where \theta is the angle from the quantization axis.
For other cases, it will be the square of a different Legendre polynomial.

3. Dec 6, 2008

### malawi_glenn

This is atomic physics.

Don't think of the atom as the Bohr model, with small electrons going around the nuclei as the planets orbit around the sun... that is not the correct view.

The direction of a single photon is probabilistic, but the anngular distribution function depends on what levels are involved due to quantum angular momentum conservation.

Edit: Clem was faster! :-D

4. Dec 6, 2008

### edguy99

I dont think so as it would have to leave in such a way as to preserve angular momentum. What makes you think "perpendicular or along the radius vector"?

5. Dec 6, 2008

### jmltinc

Boy, I hope the last line is not an invitation to leave.

I am a simple man who had formal training in electronics, but never was fortunate to pursue University for Physics – my real love. Since my early twenties, I have accumulated quite a library of QED, QCD, Relativity, Classical, Cosmology, and ‘some way out there’ textbooks; both conceptual in nature as for the layman; from Feynman’s QED to Born’s Einstein’s Theory of Relativity, and mathematically structured books from Dirac’s Lectures on Quantum Mechanics to Shelter Island. You can guess from which I get the most. Probably half my life, I have fallen asleep pondering Nature.

I understand the basic concepts of QED and QM, however lack the mathematical training, which are the cornerstone of these theories. So, in my library (which is reasonable in content) there are certainly equations which answer my questions, but since they are strictly mathematical (and, of the highest order), I have no idea what they are saying. While the Standard Model relies on equations of probability, matrices, series, and techniques such as Perturbation Theory and Renormalization to describe Nature that is far removed from our usual concepts of the world, it does not necessarily preclude a mind from imagining the process. But it does require someone to translate the equations into concepts for someone like me.

I understand the Bohr model of the atom was thrown out nearly 100 years ago, and I understand the reasons why. I understand that QED is immensely successful in predicting an outcome. I am also aware of its shortcomings.

Having said that, I will probably continue to ask questions which I hope may not irritate the Group. My questions will probably be all over the board (hell, after 30 years of pondering, my mind is loaded with questions). After some time I may even become so presumptuous to challenge the Standard Model, and look forward to the arguments which will defend it.

I take it from the answer to my original question, that photons can be emitted in any direction. I also assume that photons emitted by a proton in the nucleus are not necessarily absorbed by an electron within the atom (and vice-versa). Am I correct?

Thank you for the replies.
-J

6. Dec 6, 2008

### malawi_glenn

tell me about the shortcomings of QED.

Why should a proton in a nucleus emit a photon?

7. Dec 6, 2008

Staff Emeritus
That would be presumptuous. It's equivalent to saying "I don't understand the theory [because one cannot understand a theory without understanding it quantitatively -- with all the mathematics that entails] but I am sure it is wrong."

A far more fruitful path would be to study it for what it is, ideally including the mathematical underpinnings, rather than to presuppose that it's incorrect.

8. Dec 7, 2008

### edguy99

I think you are correct that they can be emitted in any direction.

Generally the photon emission is related more to electrons then to protons.

I am not sure about this, but I will throw it out. If an electron is in a magnetic field, it will orientate its spin in an perpendicular direction to the magnetic field. Any photons shooting off from this state come out perpendicular.

9. Dec 7, 2008

### malawi_glenn

Nope, wrong.

10. Dec 7, 2008

### jmltinc

Newton’s Third Law and the consequence of the Conservation of Momentum. Actually, I should have written, ‘emitted within an angle, somewhere between the radius vector and the momentum vector'. It seemed to me any emission outside this 90 deg area would could vectorally subtract from the vector that represents the drop in energy level (which, I assume is along the radius vector and toward the nucleus).

Have I frustrated you too much yet?

A short list would include:
- It can only describe (admittedly, with great accuracy) processes which are not complex (those that require few electrons and couplings).
- While it enjoys great success, it is poorly understood (conceptually). It offers no explanation why Nature works the way She does. If all you are looking for is an answer to a simplistic interaction, QED is the finest theory ever conceived, but it does not explain Electrodynamics in full.
- The value of the theoretical and experimental values of the magnetic moment of an electon do not agree (admittedly by a small amount, but different never the less. Perhaps experiment will correct this).
- Renormalization: Even Feynman called it a “dippy process” and a “shell game”. 1/137 still is a mystery.

I take it from the question it does not. From my point of view, a proton in a nucleus (let’s talk of an atom with a higher atomic number), is under great coulomb forces and moves about the nucleus. Even a hydrogen atom is not stationary in space. As the charged particle is accelerated, it will radiate. No?

Agreed. I have already admitted such sin. And I am not proposing I have an answer derived from my ignorance. However, I do not see challenging accepted theories taboo. I will learn something; you will have to think about it. It is a win-win.

If I had the privilege to have taken college, Under-grad, and Graduate courses, I would not bother the Group with such seemingly foolhardy questions. At age 53; work, and family prohibits studies. I am sorry that I seemingly am beneath you. But, I am not a dolt, and any replies will be researched, analyzed, and digested, and appreciated.

It is too bad that the state of physics must rely so heavily on abstract mathematics to obtain agreement with experiment. Physicists have an idea, and go to the Mathematician to create an equation to produce a desired outcome. The Physicist does not understand the mathematics, and the Mathematician does not understand the physics.

Thanks to all,
-J

11. Dec 7, 2008

Staff Emeritus
I said no such thing.

Do you have any idea how insulting this sounds? That physicists don't understand the mathematics that they use every day?

Do you have any evidence for this remarkable claim?

12. Dec 7, 2008

### malawi_glenn

i) Newtonian physics is not relevant in Quantum physics, hence the NEED of quantum physics...

ii) Basically what you are arguing against is the holistic description of nature. QED has its limits since it is a QUANTUM theory, hence applicable in quantum world. BTW herd of QED in condensed matter physics? Works well even there..

iii) The question of WHY will ALWAYS be followed by another question WHY, for all eternity. And no, QED is not the final theory, since it is only describing electromagnetism in quantum physics. So it is in general a mean field theory of a deeper lying one (maybe string theory is correct) BUT since QED is so powerful and accurate, every model and theory which will give QED as its mean field theory, have to live up to the results of QED, which are fantastic accurate. So QED really puts constraints on any deeper lying theory.

iv) Renormilazation is beutiful and today a rigourous mathematical discipline. Back in Feynmans days, it was just a "magic trick", but science had make progress and I invite you to do an update.

v) All theories will deviate from experiment, the thing is that QED is the best theory of them all..http://en.wikipedia.org/wiki/Precision_tests_of_QED

vi) Sorry but then you are back to the bohr model, that a particle acceperating will emit photons, loose energy and collapse to the ground state. You must give up classical description of physics when dealing with quantum objects.

vii) Then you are the one who understands everything right? You have 3nobel prizes? And what is WRONG with physics beeing described by, for you, advanved math? Is there an A PRIORI reason for why it should not be so? The thing is that the math is SIMPLE and SYMMETRIC, and "BEAUTIFUL" if you know its languange. Take our good ol QED as an example, the U(1) gauge symmetry which forces you to let your deriviate transform in the same way as the field does which will give you a description of phyics which is accurate to the order of 10^-11 is REALLY what physics is about -> Finding Symmetries!!!

viii) Well if you are 53, dont have time with physics, and have not studied or anything, WHY bother? What is the point then? Must we physicsits adjust ourself to YOU?

13. Dec 7, 2008

### jmltinc

Boy, did I get off on the wrong foot. I apologize to the Group for any transgressions I may have committed. I assure all, they were unintended. My only purpose is to glean any knowledge as I can from you folks. My bedside manner is that of (apparently poor) humor, which I assume has been taken wrongly.

for which you replied:
No sir or madam, you did not. I did, and meant it. I do not; for one instant believe I am in your league. I am definitely beneath you and humbly acknowledge it.

I said:
To which you replied:
I am sorry if you feel insulted. Insult was not the intent. The intent was to mark how today’s physics is deeply steeped in very advanced and abstract mathematics.

It is not a remarkable claim, just a paraphrase from Richard Feynman’s, The Character of Physical Law, chapter 2, 'The Relation of Mathematics to Physics'.

I am sorry if it offended you.

malawi-glenn wrote:
Agreed and understood.
I do not believe I have argued against anything. I asked a question, and answered several posed to me. I understand the difference between Classical and Quantum theories and the whys. We touched on Solid-State Physics in Electronics. I have no doubt that QED applies as it is of electronic structure. Is it not also called Condensed-Matter Physics?
I never argued that QED is a final theory. I know it only describes the EM interaction. You asked me what QED lacked, and I simply gave you my opinion. Right or wrong.
What updates am I missing? QED did not "work", and Feynman had to resort to (a briliant) Renormalization. If it has been updated, I am unaware. Another reason to join the Group. Do you not consider Renormalization may be a patch for a fundamentally flawed theory?
Agreed.
I understand the core difficulty with the Bohr model of the atom - that is an electron in its ground state must not emit 'real photons' else it collapse into the nucleus due to its ever increasing acceleration and consequent EM emission. But I never suggested that.
I don't even know how to respond. I understand very little. that is why I am here. I don't believe I deserved the Nobel crack. I never suggested an alternative, nor did I claim one. I never said there was anything wrong with physics being described by 'advanced math'; just that I was incapable of understanding it. I can think of no A Priori reason why Nature should only be described by such mathematics, except, since you brought it up, there could be a much more simple, elegant explaination that has yet to be found. I believe that math is SIMPLE and SYMMETRIC, and BEAUTIFUL to those who know her language. I believe Nature is symmetric and finding symmetries are invaluable in the understanding of particle physics (Lie groups have untangled the mess of the plethora of particles and given us order). Why is it that symmetry is what physics is all about? With all due respect, I can arrange checkers symetrically in several ways, but may deduce several (or no) ways of playing the game.
Why bother? Do you not look at someting as beautiful as Nature and ask "what makes it work?" Because I am an old man, am I expected to give up on my love? Should my curiosity cease? The point is to learn and understand. I do not understand the mathematics. Do you understand human nature? And no, you physicists do not have to adjust yourself to me. Do you think I am beneath you?

To those that will give me a chance, I will appreciate it. I was unaware of this group. I have a family member who is an Associate Dean of the Physics Department at the University of Chicago. I am fortunate to have ties to my favorite interest. Rather than have an Undergrad or Graduate student spend his or her valuable time with me, it was suggested I try this forum first. It does not sound like I made a good first impression.

-John

14. Dec 7, 2008

### malawi_glenn

Neither have we said that QED is the final theory, by putting it "It has ot shortcommings", in the same breath as that you want to question the standard model, one might get the illusion that you are a crackpot. The correct wird to use is "limitations" since it only describes the electromagnetiv force, BUT the entire standard model is built up on the same structure as QED, just different gauge groups.

So QED does not "lack" anything I would say, since it does what it is supposed to do. And the things about renormilization: ...

For renormlization, pick up any quite new textbook on it. I am sure that weinbergs QFT books cover it. The sucess of renormilzation as a stringent mathematical theory wasn't developed until after feymans death. So it is quite popular for crack pots to cite feynman on just that point.

Regarding symmetries: We expect that nature should NOT depend on us, the observers, that is what gauge symmetry is. Each observer may choose his/her own convetntion regarding unobservable things, like phases of wavefunctions etc. Other symmetries are mirror symmetry, time reversal symmetry, rotation symmetry. The "strange" thing is that symmetry works -> We impose SU(3) local gauge symmetry in QCD and viola, one has a theory that describes the strong interaction at a almost unreal presicion.

Another thing is that electron only emits photons when going down in discrete energy states, it does not emit photons due to an acceleration process... You suggested that the proton should emit photons due to its motion inside the nucleus.

15. Dec 8, 2008

Staff Emeritus
Actually, what you said was that the physicists themselves didn't understand it.

16. Dec 8, 2008

### Tac-Tics

17. Dec 8, 2008

### malawi_glenn

Which question?

18. Dec 8, 2008

### jmltinc

Vandium_50 posted:
Dear sir or madam, I was trying to make the point I did not understand the math. As I pointed out in my last post defending this statement, they are not my words, but that of Richard Feynman. Allow me to quote his statement:
“When the problems in physics become difficult we may often look to the mathemeticians…” “In fact the total amount that a physicist knows is very little. He only has to remember the rules to get him from one place to another and he is alright, because all the various statements about equal times, the force being in the direction of the radius, and so on, are all interconnected by reasoning” – Richard Feynman, The Character of Physical Law, Chapter 2. To read the chapter is to understand the meaning.

So you see sir or madam, I was only making a point regarding my ignorance of the math using the words of a Genius in your field.

I also ticked you off with this statement: "After some time I may even become so presumptuous to challenge the Standard Model, and look forward to the arguments which will defend it."

I do not possess the arrogance to believe that I have a better theory than QED. I meant that I was excited at the prospect of challenging the Standard Model and learn from your defense. Instead I found myself defending my statements.

I hope I have explained myself sufficiently to you to put this to bed, and hope you will have patience with my ignorance and lack of decorum, and help me find the answers I pursue.

Thanks,
-John

19. Dec 8, 2008

### cdotter

Why is everyone being so rude to jmltinc? He asked a question and admitted he didn't know the mathematics behind it. What does he get in a response? An answer followed by a page of smug remarks and implied superiority over him because he doesn't understand the mathematics. Why not stop being a bunch of jerks, and help him and suggest some reading materials to help him understand the mathematics behind it.

20. Dec 9, 2008

### ZapperZ

Staff Emeritus
OK, that's enough.

This is exactly the very reason why, if you have a question, to ask it directly and clearly, without any "editorializing". If not, your comments can easily be taken the wrong way and you can end up offending the sensibilities of others.

I don't know if anyone else has anything extra to contribute to this thread with on-topic response. If there is, please PM me and I'll reopen it. But in the mean time, this is going nowhere and it is done.

Zz.