Common knowledge tells us that a journey into a black hole ends, well, badly.
That’s because its center — the singularity — is an infinitely dense region of space-time, from which nothing can escape.
But, as our twisted universe would have it, there’s actually a certain type of theoretical black hole that may lack a singularity — one that you could safely enter, and one which could, theoretically, allow travel though time.
The Kerr Black Hole
In 1963, mathemetician Roy Kerr proposed the idea of a rotating black hole through which something could travel without being crushed into oblivion. It’s called a Kerr black hole, or a Kerr ring.
It would form after the collapse of a neutron star; instead of collapsing into the common, infinitely-dense singularity, it would create a ring of rotating neutrons, their centrifugal force preventing the singularity from forming at all.
This would, theoretically, allow safe passage through a wormhole within the black hole, avoiding the aforementioned spaghettification and opening the way for a very interesting possibility:
On the other side, somewhere out there, would be a “white hole,” ostensibly the opposite of a black hole, spewing out the contents that had been sucked in. It would act as a bridge that would allow travel through both time and space or, perhaps, into another universe.
There is, of course, no evidence of the existence of either rotating Kerr black holes or white holes, but let’s assume for a moment that they do exist.
Using Kerr Black Holes For Time Travel
Could a rotating black hole be used to travel through time?
The problem with using a Kerr black hole as a means of travel, through time or otherwise, is that you never know where you’re going to end up. Unless you developed some way to calibrate or position the “white hole” on the other side, there’s no telling where, when, or in what universe you’d find yourself.
If you could control the other end of the wormhole, however, you could potentially create a closed time-like curve.
You’d have to somehow keep the rotating black hole stationary, while sending its other end, the “white hole,” on a near-lightspeed voyage into the future. You could then pass through the black hole and end up on the other side, and vice versa, knowing (more or less) where and when you’d be going.
That wouldn’t allow you to travel to the past from your point-of-view, but it would allow those in the future to visit your present, or their past. Time travel for everybody, then, one way or the other.
But it all boils down to the ability to control a theoretical type of black hole. In other words, it’s nothing but extraordinary speculation. Perhaps, then, we should just throw caution to the wind and ask a certain alleged time traveler his thoughts about this madness.
John Titor & The Kerr Black Hole
According to John Titor, his time machine used two microsingularities to distort the gravitational field around the machine enough to “replicate the affects (sic) of a Kerr black hole.” This ostensibly achieved the fantastical time-travelling abilities of a Tipler cylinder, proposed by Frank Tipler in 1974.
The microsingularities, said Titor, were “manipulated by ‘injecting’ electrons onto [their] surface.”
Now, while I’m fuzzy on the science involved in manipulating black holes (haha), it was theorized by Kip Thorne in the 1980s that you could control (i.e., “hold open”) a black hole’s wormhole by using some kind of “negative energy.”
So, in the grand scheme of theoretical gobbledygook, it sounds okay.
In the end, John Titor’s microsingularities not only took him through time, they took him into another universe entirely, what he called a “worldline.” As the story goes, because it was another universe altogether, any and all temporal paradoxes were cleanly avoided. That’s one of the possibilities of venturing through a Kerr black hole.
So, if the universe did allow for time travel, I actually do think it would end up much like Titor described it: using black holes, and avoiding paradox through the existence of many worlds.
A Kerr black hole (or something similar), then, seems a likely candidate for the world’s first time machine.