Ghosts don't have hearts. They have Spectral Health (or SP). One SP point gets taken away every second the Proton Stream comes in contact with the ghost. Once all they're SP is drained, they become vulnerable.
- The Para-Goggels show the ghost's SP along with other information.
- Unlike hearts, SP points don't get divided into halves, so if you're trying to weaken a ghost with 10 SP points, it'll take 10 seconds.