β+ decay: voiding conservation of mass or creating something with negative mass

treehouse

From Wikipedia: "Positron emission or beta plus decay (β+ decay) is a type of beta decay in which a proton is converted, via the weak force, to a neutron, releasing a positron (the antimatter counterpart of an electron) and a neutrino.
Isotopes which undergo this decay and thereby emit positrons include carbon-11, potassium-40, nitrogen-13, oxygen-15, fluorine-18, and iodine-121."
Carbon-11 is used in positron emission topography.

Positrons are massive; therefore, β+ decay either disproves that mass is conserved or generates something with negative mass. How is the concept of negative mass anything other than non-sense?

If the universe is not massive enough to stop cosmological expansion and cosmological expansion could make parts or all of the universe impractical to occupy, could β+ decay be used to control cosmological expansion?

jtbell

In nuclear and particle-physics interactions, the sum of the rest-masses of the particles before the interaction does not generally equal the sum of the rest-masses of the particles after the interaction. ß+ decay is far from unique in this.

What counts is conservation of energy: The total energy of the particles before equals the total energy of the particles afterward, provided that you include all the forms of energy: rest-mass energy mc^2, kinetic energy, and potential energy when appropriate.

treehouse

What are some others?


But can't we make energy without destroying mass? Aren't there nuclear reactions in which rest mass is conserved which generate thermal energy?

treehouse

One of my other threads got further into conservation than this one: "Some stars get less massive; but some stars which emit photons that can push solar sails turn into black holes - and when stars get less massive it is because as part of a nuclear reaction generating an immense amount of energy they eject matter which was previously relatively stationary inside the star." - http://www.physicsforums.com/showthread.php?t=510083

treehouse

If you take energy and use it to emit photons into a star isn't all the energy in those photons lost if the photons are not intense enough to knock electrons off the atoms in the star? Wouldn't the photons just get the electrons in the atoms more excited just to be wasted as the electrons leave the atoms with whatever energy they do in the particular nuclear reaction they are involved in in the star?