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Q & A: Matter - antimatter differences

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Most recent answer: 10/22/2007
Q:
Is the mass of antimatter similar to the mass of matter, with charge only exchanged?
- Stuart Doblin (age 45)
Fallbrook, CA, USA
A:
The fundamental particles that make up our world have a number of characteristic properties that can be measured in the laboratory. Examples of the particles would be electrons, positrons, protons, neutrons, a variety of mesons, etc.  For some of the properties, like electric charge, so-called 'color', 'charge conjugation', etc, the algebraic sign of the anti-particle changes.   For other properties, for example  mass, angular momentum, and lifetime, the value is the same for particle and antiparticle.    

The most spectacular thing  is that if a particle meets up with its antiparticle partner...  Kablooie.  They mutually annhilate and give off radiation of various sorts.   Energy is conserved, of course.

LeeH


(published on 10/22/2007)

Follow-Up #1: Energy is conserved, as usual

Q:
If they annhalate each other how is energy conserved? How does that work?
- charlie (age 42)
kalkaska, mi , usa
A:
Energy conservation is one of the touchstones of physics.  The existence of the neutrino, for example, was predicted by the 'apparent' non-conservation of energy in certain nuclear beta decay reactions.   There are countless other examples.  One of the conceptual problems is that energy can manifest itself in many ways:  kinetic energy, mass-energy, thermal energy, potential energy, ...  so you have to be careful to add it all up in reactions.

 In matter anti-matter annihilations the energy goes into the decay products in the form of other, smaller, masses  and  kinetic energy.   For example when an electron and a positron (anti-electron)  annihilate two photons are emitted.  Each photon carries the same amount of energy as the mass of the electron times velocity of light squared.   In a neutron anti-neutron annihilation (by the way neither carries any electric charge) the decay products are two or more pi-mesons.  The mesons have mass (smaller than the neutron) in addition to kinetic energy. But add all the decay energy up and you will find, yet again, total energy is conserved.

LeeH

(published on 10/22/2007)

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