
#1
Sep3004, 09:50 AM

P: 3

Hello, I'm new here and I have a question about antimatter. Does annihilation occur when a particle and it's antiparticle touch, or does any combination of particles and antiparticles annihilate? Can someone help explain this to me?




#2
Sep3004, 10:43 AM

P: 2,828

Welcome Slacker !
You may like to view antimatter as Feynman understood it : antimatter is ordinary matter traveling backwards in time. Hard to swallow. Well, it is very efficient to picture it this way, and besides, nobody could tell you "you are wrong" with this conception. Imagine a one + one dimensional world : there is one time dimension, and only one space dimension. It is very convenient, because spacetime is reduced to a plane, and we can actually visualize it. Now, there is only one particle around. What do we see : a single curve. The particle moving in spacetime draws a curve. (This is the same in actual 3+1 diemsional spacetime, we just cannot visualize spacetime, that would require to go in a 5 dimensional space and contemplate the 4dim hypersurface with a curve in it. See, let us stick to the 1+1 world for now.) OK. In order to make the discussion easier, the time axis is vertically oriented, future goes up. The unique space dimension is horizontal. Now our fuzzy particle decides to change direction in time. Let us say, it should emit a photon in order to conserve energy and momentum, but let us not care about this outgoing photon for now. It goes backwards in time for a while (down), then meets another photon, and reverses once again to go back forwards in time (up). Let us also not care for now about the ingoing photon required to make the second flow reversal. We can consider 3 regions : in the far past, there is only our single and sad particle going towards the future. Likewise, in the far future, there is the same sad single curve. But in between somewhere, we see two curves ! As it appears to us, they look like different particles. Besides, the curve corresponding to our buddy travelling backwards in time has all the properties of an antiparticle : if for instance our particle carries electrical charge, this charge will seem to us to be opposite when the flow is towards the past ! Now let us go back to our photons : there are two photons in the plane. As it seems to us, one photon in the lower part is creating a particleantiparticle pair, the particle escapes to infinity, and the antiparticle eventually meets an identical particle and this meeting produces a decay in the outgoing photon. The photon carry no quantum number such as electrical charge, or other kinds of charges (except for angular momentum, but this is also linked to spatial movement, so it does not need to be conserved at the level of internal charges). You see that you need exactly the opposite quantum numbers to produce annihilation. And from the Feynmanbackwardsintimeflow view point, this is obvious. 



#3
Sep3004, 11:28 AM

P: 3

OK, I think I understand. So, let's say there was an isotope of hydrogen with one neutron. If an atom composed of one antiproton, one neutron, and one positron were to touch it, they would annihilate. Does the number of neutrons affect whether or not they annihilate?




#4
Sep3004, 11:44 AM

P: 2,828

Antimatter questionBesides, is there really a bound state antiproton/neutron ? The annihilation would have already occured at the quark level before one can even produce the second "atom". 



#5
Oct204, 06:20 AM

P: 2,057





#6
Oct204, 06:23 PM

P: 260

When they annihilate, their energy, momentum and information content is carried by the decay products: usually photons for charged particles, but other end products are also possible.
Personally, I find that thinking of antiparticles as particles going back in time is just a useful mathematical trick  while some physics about the particle/antiparticle symmetry emerges, we always observe the antiparticle as moving forward through time and we can't observe stuff moving backwards through time anyway. So I try not to read too much into this  someone here can correct me if I'm missing something. 



#7
Oct204, 07:44 PM

P: 2,057

Thank's zefram_c.




#8
Oct2304, 11:13 AM

P: 161

When it is remembered that the spin axis of the antineutron is merely the opposite end of the spin axis of the neutron, it merely flips 180degrees. The neutron binds equally well to either real or anti protons. Cheers, Jim 



#9
Oct2704, 08:26 AM

P: 2,828

I am not saying [tex]p^+/p^[/tex] annihilation can not occur. I was questionning whether in the case of hydrogen/antihydrogen the energy released by [tex]e^+/e^[/tex] (which occurs first) could prevent the protons from meeting at all. It might depend on the experimental conditions really.




#10
Nov204, 11:14 PM

P: 161

Hi Humanino,
In actual experiments, anti protons are fired into a hydride target with the result that 4 plus pions and 4 minus pions were pictured in a bubble chamber. If the exposure had been longer all mesons would have decayed to like charged muons. One positive pion that moved more slowly than the others did show a kink in its trajectory when the muon appeared. Cheers, Jim PS. Lest your forget; opposite charges attract each other so that nothing can happen before annihilation. 



#11
Nov304, 04:17 AM

P: 2,828





#12
Nov604, 09:13 AM

P: 26




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