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Hypothetical forms of matter that would respond to forces in reverse—push them, and they accelerate toward you; pull them, and they accelerate away. [Negative mass] would violate everything we know about physics while enabling reactionless drives, warp bubbles, and [perpetual motion] machines. Anti-mass is even stranger, potentially canceling out normal mass and creating all sorts of paradoxical effects. Neither has ever been observed, and most physicists suspect they're impossible. But the math allows them, and where math leads, dreamers follow. [Negative mass] and anti-mass are the ultimate engineering fantasy: materials that would let you build starships, time machines, and devices that make your commute actually enjoyable. They're also the ultimate scientific cautionary tale: just because you can write an equation doesn't mean you can build a thing.
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