Supplee's paradox

Proper noun

(physics) A physical paradox that arises when considering the buoyant force exerted on a relativistic bullet (or in a submarine) immersed in a fluid subject to an ambient gravitational field. If a bullet has neutral buoyancy at rest in a perfect fluid and is then launched at relativistic speed, observers at rest within the fluid would conclude that the bullet should sink, since its density will increase due to the length contraction effect. On the other hand, in the bullet's proper frame it is the moving fluid that becomes denser and hence the bullet would float.

Japanese Meaning

(物理学) 相対論的状況下での浮力に関する逆説。具体的には、完全な流体中で中性浮力を保つ弾丸（または潜水艦）が、相対論的速度で運動させられた場合、静止している流体の観測者からは弾丸の密度が長さ収縮により増加して沈むと判断される一方、弾丸の固有系では移動する流体の密度が増すため、弾丸が浮くと見なされるという矛盾が生じる現象