Properties of anti-matter

  • #1

Main Question or Discussion Point

As the positron has a positive charge and is the mirror of the electron and the antiproton is negative and a mirror of the proton, will then anti-matter produce anti-gravity. If not then why not? Is this linked to the photon pairing of electron and positron?
 

Answers and Replies

  • #2
mathman
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As the positron has a positive charge and is the mirror of the electron and the antiproton is negative and a mirror of the proton, will then anti-matter produce anti-gravity. If not then why not? Is this linked to the photon pairing of electron and positron?
I'll turn the question around. Why should it produce antigravity? Gravity depends on mass and energy - whether the stuff is matter or antimatter doesn't matter.
 
  • #3
I am assuming that you mean because the charges balance out there will be the same net effect? if so have anti-matter molecules been created yet?
 
  • #4
Bill_K
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:confused: No balancing is involved. The mass of an antiparticle is positive. The energy of an antiparticle is positive. The effect of gravity on an antiparticle is in the same sense (attractive) as the corresponding particle. Particles attract particles. Particles attract antiparticles. Antiparticles attract antiparticles.
 
  • #5
I notice that anti-helium-4 nuclei have been produced now.
 
  • #6
BruceW
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in response to the original question, anti-particles have opposite 'charge' of the normal particles. There are several different kinds of 'charge', including electrical charge and lepton number, etc. But mass is not one of these types of 'charge'. So the mass of anti-particles is the same as mass of normal particles.
 
  • #7
in response to the original question, anti-particles have opposite 'charge' of the normal particles. There are several different kinds of 'charge', including electrical charge and lepton number, etc. But mass is not one of these types of 'charge'. So the mass of anti-particles is the same as mass of normal particles.
Thanks. I believe some of the proposals at CERN are to detect if there is even a negligibly small repulsive force. They don't seem to favour this though. I wonder what they will find to explain the discrepancy between the amounts of matter and antimatter in the universe.
 
  • #8
Bill_K
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The CERN experiment is called AEgIS. See here and here. They will use a beam of antihydrogen atoms.
 
  • #9
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Would someone please explain? Based on BruceW, Bill_K comments... it appears our universe does not have "conservation of mass" as a basic law of particle interaction - two particles can anihilate with mass disappearing. Since mass is in many ways equivalent to energy, is there a discrepancy here and how is this normally explained (or is it an unsolved problem)? Thanks
 
  • #10
BruceW
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When two particles annihilate, some other form of energy must be given off. For example, photons, which carry the energy away.
 
  • #11
When two particles annihilate, some other form of energy must be given off. For example, photons, which carry the energy away.
What I am trying to grasp is the electron/positron pairing in photons when matter and anti matter annihilate. When photons are released from matter only what are they then composed of? Electrons only?
 
  • #12
BruceW
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eh?! Let's say that an electron and positron collide and annihilate. This means that the electron and positron disappear. But then two photons will appear instead. (And in higher-energy collisions, other massive particles can be created).

So in the simple case, we start off with an electron and a positron, and we end up with two photons. These photons are not composed of anything.
 
  • #15
What exactly don't you understand?
Well high energy photons can produce an electron and a positron pair. From where?
 
  • #16
Drakkith
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Well high energy photons can produce an electron and a positron pair. From where?
From the energy they possess. That's all I know. Particle creation can happen according to some very complex rules, but it all comes down to having enough energy/mass to create the new particles.
 
  • #17
Sorry I should have said earlier composed from electron energy only.
 
  • #18
Generating a positron from a collision with matter seems only feasible if matter and antimatter have a connection beyond simple annihilation. Do quarks and anti quarks co-exist in some sense?
 
  • #19
Drakkith
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Generating a positron from a collision with matter seems only feasible if matter and antimatter have a connection beyond simple annihilation. Do quarks and anti quarks co-exist in some sense?
I'm unsure as to what you are asking. I know of no "connection" between matter and antimatter other than what the standard model of particle physics tells me.
 
  • #20
I'm unsure as to what you are asking. I know of no "connection" between matter and antimatter other than what the standard model of particle physics tells me.
Well if you think about it a high energy photon hitting matter should produce two electrons as it has not hit anti matter. It makes sense that a photon hitting anti matter would generate the electron's anti particle, the positron. Yet here we have a positron being produced by a photon hitting matter. If an electron will annihilate a positron how can they both be produced without an immediate annihilation taking place? They are after all produced by the same photon. This can be explained I suppose by them leaving in opposite directions. However, we now are in the position where this positron will quickly annihilate with another electron. This will produce two photons, not one, so how did one photon gain the ability to produce the electron and positron if annihilation does not produce a single photon?
 
  • #21
Drakkith
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Well if you think about it a high energy photon hitting matter should produce two electrons as it has not hit anti matter. It makes sense that a photon hitting anti matter would generate the electron's anti particle, the positron. Yet here we have a positron being produced by a photon hitting matter. If an electron will annihilate a positron how can they both be produced without an immediate annihilation taking place? They are after all produced by the same photon. This can be explained I suppose by them leaving in opposite directions. However, we now are in the position where this positron will quickly annihilate with another electron. This will produce two photons, not one, so how did one photon gain the ability to produce the electron and positron if annihilation does not produce a single photon?
Due to conservation of momentum a single photon cannot create matter. It requires either another photon or something like an atomic nucleus to interact with.
 
  • #22
BruceW
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Well if you think about it a high energy photon hitting matter should produce two electrons as it has not hit anti matter. It makes sense that a photon hitting anti matter would generate the electron's anti particle, the positron. Yet here we have a positron being produced by a photon hitting matter. If an electron will annihilate a positron how can they both be produced without an immediate annihilation taking place? They are after all produced by the same photon. This can be explained I suppose by them leaving in opposite directions. However, we now are in the position where this positron will quickly annihilate with another electron. This will produce two photons, not one, so how did one photon gain the ability to produce the electron and positron if annihilation does not produce a single photon?
In pair production, a single photon does decay into an electron and positron. BUT there must be an atom nearby, to help in conservation of momentum. So really a single photon on its own, without any atoms nearby, cannot create an electron-positron pair.

P.S. to everyone, does the line through his name mean that he is no longer a member of PF?

P.P.S. I just realised I pretty much just repeated what Drakkith said. Sorry about that!
 
  • #23
Drakkith
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P.S. to everyone, does the line through his name mean that he is no longer a member of PF?
I think so.
 
  • #24
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<<Would someone please explain? Based on BruceW, Bill_K comments... it appears our universe does not have "conservation of mass" as a basic law of particle interaction - two particles can anihilate with mass disappearing. Since mass is in many ways equivalent to energy, is there a discrepancy here and how is this normally explained (or is it an unsolved problem)? >>

Rest mass is not conserved, but mass is. It is the mass of an object, not its rest mass, that is proportional to energy. So energy is conserved.

If a particle and an antiparticle annhiliate to form two photons, then the energy of that particle antiparticle system becomes exactly the energy of the two photons. Rest mass has gone to zero, but mass/energy unchanged in the reaction.
 
  • #25
BruceW
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Rest mass is not conserved, but mass is. It is the mass of an object, not its rest mass, that is proportional to energy. So energy is conserved.

If a particle and an antiparticle annhiliate to form two photons, then the energy of that particle antiparticle system becomes exactly the energy of the two photons. Rest mass has gone to zero, but mass/energy unchanged in the reaction.
I agree. To add to that, the invariant mass of the system is also conserved. So the sum of the rest masses of the individual particles is not conserved, but the invariant mass is conserved.

In the case of a single particle, the invariant mass and rest mass are the same. So if there is 1 particle, its rest mass is conserved. I think this is where people sometimes fall into the mistake of thinking that the sum of the rest masses for a multi-particle system would be conserved. But this is simply not true. (Because the invariant mass is no longer equal to the rest mass, for a multi-particle system).
 

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