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Does anti-matter exist?

  1. Jan 16, 2009 #1
    I was watching a documentary about Paul Dirac and his work and I have a pretty basic question.

    Matter is made of sub atomic particles that comprise atoms. The proton is the opposite of an electron, but an electron is not an anti-proton.

    Why do Dirac's equations predict anti-matter? Doesn't it just predict an opposing force that isn't "anti" anything such as positrons?
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  3. Jan 16, 2009 #2


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    You need to think carefully on what you consider as "opposite". A proton is the "opposite" of an electron if you only consider charge as the sole criteria. In other respects, a proton is not opposite of an electron. They each have different masses, a proton is a hardron while an electron is a lepton, etc... etc. In other words, there's a series of properties in which they both are not "opposite" to each other.

    A positron and an electron are practically identical to each other in many ways. There are several symmetry operations one can do that allows one to get one from the other. So the differences between the two are actually quite "minor", and also via symmetry operations. It's like having an identical twin, but really each one is, say, a mirror image of the other, i.e. the left gets transposed as a right on the other one.

    That is what we then call the "antiparticle" of that particle.

  4. Jan 16, 2009 #3
    Why can't they be particles that have no interaction with eachother whatsoever?

    Why is one "anti"? Can't they simply be unique?
  5. Jan 16, 2009 #4


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    Er.. they both have charge, and therefore, can interact!

    Aren't these just "name game"? Don't get hung up on the name. Pay attention, instead, to the physics. If you don't like to have the word "anti" in front of it, call it something else. It's not going to change what it is.

  6. Jan 16, 2009 #5
    I guess the reason I am hung up on the term "anti" is this whole notion that if they come into contact with each other than they are destroyed.
  7. Jan 17, 2009 #6


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    The mutual destruction is a physical fact, so just accept it. It doesn't matter what its called.
  8. Jan 17, 2009 #7
    Well, positron not only exist, but we use it regularly, e.g. in PET scan in hospital!
  9. Jan 17, 2009 #8


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    Energetic annihilation between particles and anti-particles is routine. If you will explore CERN's website, you will see that they have a program in place to produce antiparticles, trap them and cool them to produce neutral anti-hydrogen. A critical test that can be done with such materials is possible falsification of the Weak Equivalence Principle. Is the gravitational infall rate of neutral antimatter the same as that exhibited by neutral matter? This is a really big question, because charge effects are so large that they swamp gravitational effects in any lab experiments, and the role of mass in gravitational attraction is critical to our understanding of the intersection between the quantum world and the classical.
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