我們考慮在五維時空的超光速粒子，其度量張量分別為正則和共型平坦。計算四維等效的宇宙常數。修正的牛頓萬有引力公式不同於低於光速的粒子。並討論真空不穩定性。

We consider tachyons in five-dimensional spacetime with the canonical met-
ric and the conformally-at metric respectively. The four-dimensional effective cosmological constant Λ is calculated. The modified Newton's gravitational force law is different from that of slower-than-light particles. The vacuum instability is also discussed.