# Real photon - virtual particle collisons

1. Mar 9, 2004

### outandbeyond2004

Seems to me that in deep space real photons collide often with virtual particles. Is that a reasonable theory? If so, what are the (approximate or ball-park) statistics? What does happen? Does the putative virtual particle involved in a collision with a real photon become real? What are the consequences that we can observe, such as "tired light"?

I see that heisenberg posted questions like the above already. My apologies to him. They never got replies, though.

Last edited: Mar 10, 2004
2. Mar 10, 2004

### arivero

Well, a virtual particle is a particle whose momenta is not relate to its energy. This is a violation of energy preservation, thus the virtual particle can only exist a very small time h/E; during this time the uncertainty principle "covers" the crime. If during this time the virtual particle is able to react with other, "stealing" energy, then it can become real.

Other possible reaction for the virtual entity is to dissapear during the reaction, but granting its momentum to the real particle it has reacted with. In this way virtual particles transfer momentum between two real particles, say two electrons interacting via a photon.

3. Mar 10, 2004

### outandbeyond2004

I need a clarification of the statement that a virtual particle's momemtum is not related to its energy. I think you were too terse.

In relativity theory (not including QM), the energy-momentum vector of a particle in its rest frame is given by

[ mc^2 0 0 0 ]

where m is the particle's rest mass and c is the vsol

In a frame of reference where the particle has velocity v, the vector is
[E px py pz]
where E is the relativistic energy, and px is the momentum in the x direction, etc.

Looks like even a real particle's momentum is not related to its energy at least in special relativity.

4. Mar 10, 2004

### arivero

$$E=\sqrt{p^2 c^2+ m_0^2 c^4}$$ is true for real particles and false for virtual ones.

5. Mar 10, 2004

### outandbeyond2004

What should we take to be the observer's frame of reference? One in which the CMRBackground dipole is zero, is that OK?

I think I understand now: The HUP says that if you know the energy well, then you do not know the momentum well, is that correct? However, can't we *assume* a energy-momentum vector whose magnitude is the rest mass of the virtual particle, for the purpose of generating statistical predictions?

6. Mar 10, 2004

### Nereid

Staff Emeritus
Equally interesting must surely be 'in deep space real photons collide with real particles'.

From example, the Sunyaev-Zel'dovich effect, and inverse Compton scattering of high energy particles by the CMB.

7. Mar 10, 2004

### outandbeyond2004

Nereid, I tried to think of why you would say that collisions by real photons with real particles are equally interesting, or what you are hinting at. I'm too dense, I guess. Perhaps you are assuming that if a collision between a real particle and a virtual particle DID occur, then it would be as if the virtual particle IS real.

8. Mar 10, 2004

### Nereid

Staff Emeritus
Hi outandbeyond2004, and welcome to Physics Forums (I didn't notice that you're new here before)!

It's interesting - to me anyway - because it shows that our understanding of physics seems to apply 'in deep space', in regimes that we cannot hope to attain in our Earth-based labs, at least in my (and maybe your) lifetime, and also in deep time.

The relevance to virtual particles I'll gladly leave to arivero, lethe, Tom, GRQC, and others whose grasp of the underlying theory is far stronger than my own. In an astrophysical setting, the only place I can think of where tests may be possible is black holes, and we are at least a century away from having good enough observations of those (IMHO)

9. Mar 10, 2004

### outandbeyond2004

Nereid, thanks for the kind welcome! Re tests near black holes, are you thinking of Hawking radiation and black hole "evaporation"?

10. Mar 10, 2004

### Nereid

Staff Emeritus
Yes.

Also, since you're talking about photon interactions 'in deep space' (whatever that means), a black hole with a patchy accretion disk; lots and lots of photons, across the whole EM spectrum, passing as close as you like to the event horizon.

I guess if you insist on 'deep space', the black hole would need to be isolated (no accretion disk), and we could observe what happens to the photons from a GRB which occurs 'behind' it, on our line of sight.

11. Mar 10, 2004

### outandbeyond2004

'Deep space' means far, far, far from Earth, like a certain satellite of Neptune.

12. Mar 10, 2004

### Nereid

Staff Emeritus
... where some of the daughters of Nereus and Doris went when global warming became too much (or was it to get away from the bright city lights, all the better to do astronomy? )

13. Mar 11, 2004

### outandbeyond2004

Two massive stars orbit each other. One goes supernova, causing the other to go supernova, too. They thus recoil from each other. After a while one enters intergalactic space, so becomes somewhat of a bare hole = accretion disk pretty thin. We could see gravitation lensing and maybe some Hawking radiation from near the event horizon? Such observations would yield some limits on the real photon - virtual particle collision stats that could be collected. Nah, too unlikely. Even if such a hole exists, we would have to be very lucky to detect it if at all possible.

I still do not know why arivero thinks that the energy of a virtual particle is not related to the momentum in the usual way. Frankly, it does not make sense. Is there a post somewhere that makes sense out of it?

14. Mar 11, 2004

### arivero

Well, it is the definition of virtual particle. Did you see the formula for the energy in my previous reply? In a pedantic way, it is said that virtual particles can be "off-shell".

15. Mar 11, 2004

### arivero

Consider the whole process of two real particles interacting via a virtual particle. This means that the two real particles exchange a momentum Dp and an energy DE. The possible values of Dp and DE are given by relativistic colision theory, but they are also the actual momentum and energy of the virtual particle. IF one checks the equations, then it can be seen that the on-shell equation does not work generally for these values (DE, dp).

16. Mar 11, 2004

### TeV

There's no danger in using term virtual particle as long as one consider it helpful in describing real QM phenomena and processes*.
Therefore, introducing of it is purely theoretical and is part of convenient math notatation.Postulated as such it cannot be observed or detected .Not even in a principle (by theory).If used incorrectly
it can violate almost every physical law including some laws of QM itself (not just relativity and causality).
______
* We don't know why observable reality obey rules of quantum theory.
The nature most likely "disguists" of mathematical infinities
etc,etc..

17. Mar 11, 2004

### outandbeyond2004

Let's consider the hypothetical situation: A photon is going through space that is empty except for that photon. (I don't know if such an situation is ever possible, but I am guessing anyway.) There is then no virtual particle for the photon to interact with. Well, no, there is the self-gravitation predicted in General Relativity. On the other hand, the symmetry of the situation (which implies conservation via Noether's laws) does seem to say that the photon simply continues (like inertia) without any loss of energy or momentum. My mind is conking out here, so -- help!!

18. Mar 12, 2004

### arivero

What is the problem? The photon simply continues, describing a nice geodesic across spacetime.

19. Mar 12, 2004

### outandbeyond2004

Arivero, because there is nothing wrong with your mind, and you agree with me, therefore my mind is OKay. Yah!!

In that situation, there is no collision with any virtual particle! Right? Maybe virtual particles do not even exist in that situation.

20. Mar 12, 2004

### TeV

Ah,just listen to yourself..
Exist?What to exist?We are not even sure what means "existance", and what extent of reality we should attribute to the free propagating real photon if it doesn't collide with another particle in postemitting epoch.(please,don't confuse nonabsorbed photon with a virtual one).
Remember ,virtual particles are just a figure of speach.They cannot exist in a real spacetime (in HUP restrictions) ,but are byproduct of equation formalism describtion of QM fields.
It is easier to follow what is going on with QM vacuum energy fluctuations , if we stick to the conservations of momentum , symetry of equations,virtual particle language becomes a usual terminology.
QM vacuum has an energy.It is quite mystery why and still uncertain how much of it.This potential energy can be relized under certain ,so to speak "provoking" circumstances set in our observable real universe,and manifestations are quite a real:Casimir effect,antimatter,or some dare say inflation of the Universe is the most dramatic one.

best regards