Black holes are arguably the single densest things in existence, so much so that their gravity causes an inescapable pull on everything around it that even light can't escape.
But with how dense they are, could they function as essentially time machines?
This might sound crazy, like the plot of pure science fiction. After all, black holes have been associated with time travel before, thanks to popular sci-fi media like the movie Interstellar or the Netflix series Dark, but it isn't as odd as you might think.
Here's what you need to know about black holes and whether they could allow someone to travel through time.
What is a black hole?
A black hole is essentially the remains of a dying star, with gravity having caused the star to implode on itself, forming a gravitational singularity that pulls everything in and lets nothing out.
Black holes in general are incredibly mysterious. They are, in essence, just massive concentrations of gravity so strong that nothing, not even light, is able to escape. Scientists weren't even sure they existed 20-30 years ago, and the only way we know the black hole exists is because it has an enormous gravitational pull that influences matter around it.
Note that the term "massive" here doesn't mean big in size. Massive, when talking about black holes and other astronomical objects, refers to how much physical mass is in it. This does not equate to size, since something like gas can cover a huge area but not have as much mass in it.
And broadly speaking, more mass means stronger gravity.
For example, compare Earth to Jupiter. The gravity on Jupiter is much stronger than on Earth, and things will seem heavier there as a result. Then you have something like the Moon or Mars, where the gravity is much weaker.
There are also a lot of black holes out there, with one study even suggesting that there may be as many as 40 quadrillions, also known in scientific notation as 40 x 10^18 in the observable universe alone.
This was determined by evaluating a number of factors like the rate of star formation, the amount of stellar mass and metallicity - the abundance of metals, which, in astronomy, refers to any element heavier than hydrogen and helium.
And some of these are especially huge. These are known as supermassive black holes, and they reside in the center of many galaxies, including our own Milky Way, which is centered on the supermassive black hole Saggitarius A*.
But how does this relate to time travel?
This is where some good old-fashioned Albert Einstein physics comes into play.
Einstein's most famous accomplishment, aside from the atom bomb, was his general theory of relativity, which is still accepted as the description of gravity to this day, well over 100 years later.
To go into detail about this theory would be difficult, considering the complicated math involved. But to put it in simple terms, it describes the relationship between matter, energy and gravity. Black holes were actually predicted by the general theory of relativity and later observations confirmed their existence.
Now, to briefly summarize how this is relevant, consider: Space is bent and stretched by matter and energy, and space will be bent more and stretched further the more massive a given object is.
What happens, in this case, is that space becomes distorted, bent in ways that things will fall into, courtesy of gravity's pull. It's why gravity pulls things to it and why the closer you get to something, the stronger the gravity is.
But this is where time comes in, because if space is stretched, so is time.
Say, hypothetically, one was to have an old clock near an extremely massive object. If the object is massive enough, the clock will tick slower.
In other words: Intense gravity slows down time. A month near a black hole might be equal to several years back on Earth.
This is step one for black hole time travel: Just fly near one for a bit and then come back. You'll find that a lot of time has passed, far more than will seem for you.
Of course, just be careful not to go too close, crossing what scientists have dubbed the Event Horizon. If you do that, there's no going back.
But that's just time travel into the future, what about going back?
So this is going to get very complicated but bear with me.
Remember how black holes stretch and bend space and time because of how massive they are? Well, regarding time, it doesn't just bend it so it slows everything down. It can basically make a time loop on itself like folding a blanket or rolling up a newspaper.
Scientists all this a closed timelike curve (CTC) and was first discovered by Willem Jacob van Stockum over 20 years after Einstein posited the general theory of relativity. Basically, you would go forward in time and end up before you left, since you were going backward by going forwards.
Still with me? Good, because this gets even more complicated.
After all, all of this isn't even taking into account the exact complicated mechanics of it all. There are all the issues of cause and effect, the speed of light, worldlines and various extremely complicated mathematical equations and concepts that have been the focus of nearly a century of research and theorizing.
But suffice to say, this is where we get concepts like wormholes from.
Now, theoretical debates aside, does this mean that time travel is possible? Well, technically, it would count as time travel.
But by no means should you do it.
Now, putting aside the fact that it will be impossible to do it unless you happen to have access to a spaceship that can fly to a black hole fast enough that you'd be able to live long enough to make it, and then live long enough to come back, there are good reasons why time travel via black hole is a terrible idea.
The biggest problem, though, is the aforementioned event horizon. Is that time loop caused by the black hole gravity? It's likely going through the event horizon. And if you go inside, it will end very badly.
The fact that this process is called "spaghettification" should raise alarm bells about bad it is.
So yes, time travel is almost definitely possible. And you'd die doing it.