pressure - Is negative 20 psi / 1.5 bar possible?

If I understand correctly, negative pressure usually means relative pressure: the difference between inside and outside.

If outside is normal (1 bar, 15 psi, 100 kPa etc), how low can the (relative) pressure inside be? Can it be -20 psi / -1.5 bar / -150 kPa?

Absolute pressure can't be negative, right?