Answers provided by
Dr. Christopher S. Baird

# Does the negative mass of antimatter mean that it's gravitationally repelled from stars?

Category: Physics      Published: June 19, 2013

Antimatter does not have negative mass. In our universe, there is no such thing as negative mass. Mass only comes in positive form. In contrast, electric charge can be positive or negative. The nonexistence of negative mass has profound consequences. First, gravity is always attractive and never repulsive. Presumably, if negative mass existed, it would be repelled by objects with positive mass such as the earth or sun. A cup made out of negative mass would fall up and not down when you let it go. As interesting as this sounds, negative mass does not exist, so heavier objects always fall down. Second, the lack of negative mass means that gravitational fields can never be shielded, blocked, or canceled. In contrast, electric charge comes in both positive and negative varieties. An electric field created by a positively charged object can therefore be blocked by a wall of negative charges. This is the principle used in metal shielding, which keeps the microwaves in a microwave oven from cooking everything in the kitchen. But there is no negative mass to cancel out gravity. If negative mass did exist, you could use it as a form of anti-gravity. If you built a floor on earth out of negative mass of sufficient size, then people above that floor would feel no gravity and would float around. Again, negative mass does not exist, so genuine anti-gravity is not possible. Gravity permeates every cell of every creature, and into the deepest dungeon with the thickest walls.

Antimatter is a physical entity with positive mass that is identical to regular matter in every way except that the charge and some other properties are flipped. Every bit of matter in the universe has a potential antimatter counterpart. Every fundamental particle of regular matter has an antimatter version. For instance, the antimatter version of the electron is the positron. Electrons and positron have the exact same mass, the same spin, and the same charge magnitude. The only difference is that electrons are negatively charged and positrons are positively charged (positrons should not be confused with protons which are completely different particles) and a few other properties are flipped. When an electron meets a positron, they annihilate each other and their combined mass is converted completely into energy in the form of gamma rays. This effect is used routinely in medical PET scans. In general, antimatter annihilates its regular matter version when they meet. Antimatter can be thought of as regular matter traveling backwards in time. In this picture, a particle-antiparticle annihilation event can be thought of as a forward-time-traveling particle being knocked by gamma rays so that it becomes the same particle, but just traveling backwards in time. Note that this time travel concept applies only to specific antimatter events that obey the conservation of energy and does not open up the possibility for humans to travel back in time, which would violate the conservation of energy. Although antimatter comes in very small quantities in our universe, it is not as exotic or unnatural as once thought. Every minute of every day, high energy cosmic rays from distant supernovas are slamming into earth's atmosphere and creating a very small amount of antimatter. Also, the natural radioactive decay that takes place constantly in earth's rocks creates small amounts of antimatter. But this antimatter does not stick around for long because it quickly bumps up against regular matter and destroys itself in the process.