(A Twitter thread)

5. Negative Mass
Negative mass is a hypothetical type of exotic matter whose mass is of opposite sign to the mass of normal matter. For example, if a normal object has a mass of 5 kg, a negative-mass object would have a mass of -5 kg. Unlike normal matter, negative mass would exhibit strange properties such as the oppositely oriented acceleration for an applied force orientation. This means that if you push an object with negative mass, it accelerates towards you instead of away from you.

In 2017, physicists at Washington State University created a fluid with negative mass in laboratory conditions. They cooled rubidium atoms to just above absolute zero, creating a Bose-Einstein condensate. By using lasers to change the way the atoms spin, they created conditions where the rubidium behaved as if it had negative mass.

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