# Do particle pairs avoid each other? Please end my musings

Can you explain what happens when a particle and its antiparticle are created. Do they whiz away from each other at the speed of light or what? I suppose that they don't because otherwise they would never meet and annihilate each other, but then, if I had just been created with an antiparticle I would do all I could to stay away from him/her. On the other hand, for the sadistic/suicidal type, they might actually be attracted to one another.

[the question is serious, even though it's written light-heartedly. Please explain.]

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So there are two different things you might be talking about which often get conflated in popular level discussions of the topic:

• Real particle/anti-particle pair production in reactions. An example reaction (one among many) would be $p + \gamma \rightarrow p + e^{-} + e^{+}$ (a proton and a gamma ray collide to produce a proton and electron positron pair). In this case the electron/positron pair will typically fly away from each other at near the speed of light. They are attracted to each other due to the electromagnetic interaction, but have a velocity far greater than the escape velocity, so the attraction will never be enough to bring them back together. This is typical in these sorts of reactions, though you might find some exceptions.

• "Virtual" pair production. I think this may be what you're talking about. But if you haven't heard of this don't worry too much about it - typical presentations of the topic are often more confusing than they are worth. There are certain ways of organising calculations in quantum field theory that are made simple by talking about particle/anti-particle pairs spontaneously forming out of the vacuum. These particles interact with other things before annihilating in a very short time. They only leave a visible trace through their brief interactions with other things. These "particles" don't obey the usual relationship between energy and momentum or the laws of motion obeyed by "real" particles - hence the name "virtual." What you end up doing in the calculation is adding up all the possible ways the particles could interact with other things and move around before coming back together and annihilating. So it's probably best to think about them as a short cut for a calculation rather than get hung up about whether they "really exist" - whatever that means - or what their trajectories are. You certainly do need to include them in the calculations to get the right answer, but don't take them too literally.

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thanks. Your explanation of virtual particles makes things quite clear, but I think I was referring to what happens to real particles. So, what you are saying is that they are not attracted to or deterred from each other in any way, just that they are released from the original particle at such a velocity that they would never meet? –  layman238 Dec 31 '12 at 14:36
Which type of particles, real or virtual are those on the event horizon of a black hole, when one gets swallowed up in the BH and one stays on the outside and glows? –  layman238 Dec 31 '12 at 14:37
@layman238 the idea that Hawking radiation is due to one member of a virtual pair crossing the event horizon is just a metaphor. See physics.stackexchange.com/questions/30597/… for more details. –  John Rennie Dec 31 '12 at 14:47
I've actually seen Hawking radiation described this way quite clearly in a reputable physics text book. If as you say it is just a metaphor, then why DO black holes emit radiation? –  layman238 Dec 31 '12 at 20:16

This is a clarifying answer.

Here are particle antiparticle pairs in a bubble chamber photo from Fermi lab, created by the reaction

gamma + proton -->electron positron pair + proton

The gamma is unseen before it hits the proton ,because it does not ionize and when it hits the proton, most of its energy is taken up by the new pair production.

As one can see the products do not have the velocity of light but maybe a few hundreds to a few MeV each. The momentum can be calculated from the curvature given the value of the magnetic field, which is perpendicular to the page. The tracks end in spirals because the electrons ( positrons) ionize the fluid, and so we can see the tracks, but there is loss of momentum and energy due to this ionization, creating the spiral.

In the center of mass of the initial state "proton gamma" the three particles acquire their momentum from the conservation of momentum and energy laws. The matrix element kinematics are such that the target proton is a spectator, very seldom seen with a mm track in the chamber and the electron positron pair take most of the energy of the gamma..

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I think you should mention that there isa strong magnetic field present in these images lest viewers get the idea that the curves and spirals are a usual feature of the path of ionizing particles. –  dmckee Dec 31 '12 at 17:04
@dmckee thanks, will do –  anna v Dec 31 '12 at 17:20

Do they whiz away from each other at the speed of light or what? I suppose that they don't because otherwise they would never meet and annihilate each other,

Do you mean that you can have particles moving at speed of light? The difficulty of deaccelerating those particles what makes you thinking that renders the annihilation impossible?

It looks like you think that the only way to annihilate is to meet the original antiparticle. But, all elementary particles of the same type are identical. So, there is no need to wait for the recombination of the pair to annihilate. Why don't you consider that your particle may dispart and marry another different one?

Take some force that pulls the particles apart initially. Once they reach some distance, they do not annihilate and engage in interactions with other particles, where they also can annihilate. There is no need for speed (of light) nor need to beware the original counterparticle, once particles are sufficiently apart.

Might be physicists have another opinion

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all elementary particles of the same type are identical. So, there is no need to wait for the recombination of the pair to annihilate. o.k. but what does happen to the original pair? are they attracted or deterred from each other? –  layman238 Dec 31 '12 at 13:26
The same applies to the Big Bang: why Universe expand if we know that gravity pulls everything together. Big Hole had to result when it was tiny and not let anything to escape. Yet, we see that expansion takes place. So, obviously, there is some force that pulls them apart. There are some quantum vacuum fluctuations that create the paris. These are the forces. But, I am not familiar with them. When you have force pulling things apart, you do not need the speed to do this. Also, I do not understand why you think it need to be the speed of light. –  Val Dec 31 '12 at 13:33
I have also seen that virtual particles normally annihilate but some forces can pull them apart so that a real pair is formed. But no need for the speed of light. The particles may attract each other but other forces dominate at large distances so that it does not matter anymore. –  Val Dec 31 '12 at 13:52