A particle falling into a black hole is doomed. Whether or not any information about it is ultimately preserved (an area of active research), that particle can never return to the outside Universe, nor can it send any signals about its fate. For the particle, the black hole is the end of time, its entire future. The boundary of the black hole — its event horizon — is the undiscovered country from whose bourn no traveler returns.
However, the event horizon is in our future, too. Not that we ourselves will cross it: just that any particle we observe falling into a black hole will enter it at some point in the far future from our perspective. That’s because from our perspective, the particle’s time appears to slow down, so that we never see it cross the event horizon. Even though light takes time to reach us, in a sense we’re observing an event that is yet to come. Forget Doctor Who: a black hole is the true end of time. There’s a whole other blog post in this topic, which (if I’m feeling procrastinatory this week in the face of heavy deadlines) I might write soon.
Analogously, white holes are a kind of beginning of time: particles originating inside the white hole event horizon must exit and never return to the region inside. If that feels weird to you, it should: white holes don’t act like normal gravitational objects. In fact, there’s good reason to believe white holes don’t exist at all, as I explain in my latest for Nautilus:
Black holes are common in the cosmos—nearly every large galaxy harbors a supermassive one in its nucleus, not to mention smaller specimens. However, astronomers have yet to identify a single white hole. That doesn’t rule out their existence entirely, since it might be hard to see one: If they effectively repel particles, there’s a small possibility they could be lurking out there somewhere, invisible. Nevertheless, none of all the diverse objects astronomers have observed seem to resemble what we’d expect from white holes. [read more...]