Special & General Relativity Questions and Answers
Is anti-matter the same as negative mass?
Not quite. Anti-matter is simply matter which has the opposite electric charge from normal matter. Example, the anti-electron has exactly the same mass as a regular electron, but has a positive charge rather than a negative charge. The anti-proton has the same mass as a regular proton, but carries a negative electric charge. There are also quarks and anti-quarks which carry opposite color-charge, but otherwise are identical to regular quarks. Anti-neutrinos carry opposite 'neutrino-charge' from ordinary neutrinos.
Negative mass behaves very differently. There is no experimental evidence for this oddity of mathematics. Gravitationally, if one particle had ordinary 'positive' mass, and one had 'negative' mass, because the force of gravity is the product of their masses, the sign of the force would be (+m) x (-m) = - m^2 which means that they would repel each other rather than attract as for ordinary gravity.
If a particle traveled faster than the speed of light, it would appear to have 'imaginary' mass as in the square-root of (-1) times its ordinary mass. We have no idea how regular matter and 'imaginary' matter interact, but all forces between real and imaginary matter involving their mass would be 'imaginary' rather than 'real' forces which means we might not actually detect any 'real' effects from them, gravitationally. Two imaginary masses would affect each other via an anti-gravity effect because the product of their imaginary masses would yield a negative sign or 'repulsive' gravitational force.
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All answers are provided by Dr. Sten Odenwald (Raytheon STX) for the NASA Astronomy Cafe, part of the NASA Education and Public Outreach program.