Negative-energy gravitational collision
= Negative-energy gravitational collision
{title2=$t_{\mathrm{coll}}=\pi\sqrt{a^3/[8G(m_1+m_2)]}$}
Two point <masses> released from rest at separation $a$ have negative <mechanical energy> $-Gm_1m_2/a$ but collide in time $\pi\sqrt{a^3/[8G(m_1+m_2)]}$. Their separation approaches zero along a regular trajectory before collision. The <energy> sign alone therefore does not supply a positive lower separation bound.