# What's the source of EM radiation

1. Mar 25, 2013

### Naveen3456

An electron and a positron annihilate and energy is released in the form of quantized EM (gamma etc.) rays:

Now some childish yet inquisitive questions;

1. Where were there quanta prior to annihilation, inside the electron or positron (or both)?

2. Were these quanta really present before or were created at the instant of annihilation?

3. If they were present already, what 'mechanism- I don't know any other word' led them out?

4. If they were created at the instant, did the matter wave associated with the electron and positron turn into the electromagnetic wave (quanta)?

2. Mar 25, 2013

### Staff: Mentor

The photons did not exist before, they are created in the annihilation process.
Sort of.

3. Mar 25, 2013

### Naveen3456

How come matter wave which has some intrinsic mass (however small) turn into something that has no intrinsic mass?

Did this matter wave gave its speed (or velocity) to the photon (which then continues to maintain that speed as if by magic, without any perpetual source of energy behind it for the maintenance of this speed.)

Plz be a little bit elaborate..........

4. Mar 25, 2013

### Staff: Mentor

Quantum field theory. There is no (known) deeper reason "why" things happen, it is just an observation.

No.
Without any force acting on them, all things keep their velocity, momentum and energy.

5. Mar 25, 2013

### ftr

I don’t know your level of education, but at least you need to be at 2nd year college level (science or engineering to have any hope of understanding with good deal of effort.

Even in classical physics all we do is model. Like express properties of electron such as mass and charge by matching experimental results to formulas. When you get to QM similar activity takes place, however, the objects and their mathematical representation become more abstract. The functions which are used to model then are converted using Fourier transforms and manipulated to compute interactions. Spontaneous emission is modeled using what is called First quantization.

http://en.wikipedia.org/wiki/Quantization_of_the_electromagnetic_field

6. Mar 25, 2013

### Naveen3456

Is there any time lag in the emission of a photon?

What if some super-super high photography (just suppose) is done and we see an emerging photon part by part.

If it’s instantaneous, how come time is not involved in it?

Do you mean to say that at its 'birth' only the photon is traveling at the speed of light? It must have taken some time to reach this speed. How can this be explained?

What's the reason (mechanism) for this 'keeping' of velocity, momentum and energy?

Suppose a photon enters an area of space where there is absolute zero ( or near absolute zero) temperature, would it still keep its speed?

7. Mar 25, 2013

### bhobba

As one guy said you really need to be at least at second year college level to even begin to understand it - if not all you will get is hand-waving type stuff.

But in that vein here is what I think is the best way of looking at it - in Quantum Field Theory electric and magnetic fields involve the exchange of what are called virtual photons - changing fields create real photons by, in a rough way, shaking some of those virtual particles loose so it becomes real - in Quantum processes in general you have a probability of something happening rather than an actual elapsed time.

Thanks
Bill

8. Mar 25, 2013

### xAxis

No

You made here two false assumptions.
1. We cannot see photon
2. Photons are point particles, they don't have parts.

If it's instantaneous, no time between anihilation and radiation, time is not involved in it. It would be strange if time was involved
How could you explain accelerating something that has no mass (photon in this case)?
It's been known for about 350 years now that no force or mechanism is required to keep velocity of any object (first Newton's Law)

Yes. The speed of light is constant in vacuum, regardless of the temperature, or anything else.

9. Mar 26, 2013

### Naveen3456

We surely see light, don't we? Even single photons are detectable?

I think Einstein said that nothing can happen at a speed greater than the speed of light.

10. Mar 26, 2013

### Staff: Mentor

Sure, but they do not have any parts to observe.
It also happens at a single place, there is no speed involved.
Note that this is just an attempt to describe a process in words, to avoid details of quantum field theory.

11. Mar 27, 2013

### Naveen3456

Even if 'it' happens at a single place, it is for sure that one process (state) ends and another process (state) begins.

This ought to involve time as far as i think.

12. Mar 27, 2013

### Staff: Mentor

Not if both things happen at the same time.
But this discussion won't lead to anything. In quantum field theory, nothing propagates faster than light, it is possible to show that.

13. Mar 27, 2013

### LayMuon

The important thing is the energy conservation. Mass is a form of energy: E = mc^2, where m is the dynamical mass. It is the rest mass of photon that is zero.

14. Mar 27, 2013

### Born2bwire

Sure, by absorbing the photon. We can't track a photon, we can only state that it was created here (most likely) and then detected (thereby annihilated) over here at this point in time.

15. Mar 27, 2013

### LayMuon

i think it does. think of em radiation by an excited atom. it takes some time for an electron cloud to change it's shape to the final state, and during that shape-shifting EM is being emitted.

16. Mar 27, 2013

### Bill_K

Nope, that's false too. Your classical intuition misleads you. Photon emission by an excited atom is also instantaneous. There is no elapsed time required for the electron to readjust. It is either in the initial state or the final state - there is no intermediate stage.

17. Mar 27, 2013

### LayMuon

But if you right down and solve schroedinger equation the process is continuous, the the instantaneity question reduces to how to interpret wave function.

18. Mar 27, 2013

### Bill_K

The process that's "continuous" is just the growing of the probability that the system is in the final state. Regardless of what your favorite interpretation is, the system will always be found to be in one or the other of the discrete states. There is no period during which "the electron cloud is changing shape."

19. Mar 27, 2013

### bhobba

Not so. The solutions of the Schrodinger equation gives the possible energy states and in general they are discrete - most certainly so in an atom. If it emits or absorbs a photon it changes to another energy state and that change is not continuous - nor can it be since the solution to the Schrodinger equation are the only allowable states. This is part of the weirdness of QM - hard to wrap your mind around but is, as far as we can tell today, how the world works.

As an aside a massive amount of work has been done over the years on why QM is like that and I recently came across a paper that for me really got to the heart of the matter about whats going on - check it out - very interesting:
http://arxiv.org/pdf/0911.0695v1.pdf

Basically it would seem if you have entanglement then you must have QM - it more or less follows from some very reasonable assumptions that leads to only two choices - QM and bog standard probability theory - but what distinguishes QM is it allows entanglement.

Thanks
Bill

20. Mar 28, 2013

### Jano L.

Bill, what is the reason for you assertions? I concur with Laymuon, if we consider the basic equations of the theory, they are differential equations with no indication that changes in the atom are instantaneous jumps. Such instantaneous jump would violate wave equation for the EM field.

The changes in atoms can possibly be very quick due to interaction with the other atoms and radiation, but they are unlikely to be instantaneous if described by differential equations. The emission of radiation is often quite slow - the spontaneous emission from hydrogen atom takes times of order of a nanosecond, a very long time in which the electronic density oscillates back and forth cca millions of times (period in order of femtoseconds). Or take the Rabi oscillations - if the laser is tuned to resonance with some pair of levels and the strength of the electric field is weak, we can change the state of the atoms and make the $\psi$ function to oscillate between the ground state and the excited state very slowly (with frequency $\boldsymbol \mu_{12}\cdot \mathbf E_0 / \hbar$).