Can humans use black holes to travel through time?
Black holes are natural time machines that allow travel to both the past and the future. But don’t expect to go back anytime soon to visit the dinosaurs.
At the moment we don’t have a spacecraft that will take us close to one black hole. But even disregarding that small detail, attempting to travel to the past using a black hole might be the last thing you ever do.
What are Black Holes?
A black hole is an extremely massive object that is typically formed when a dying star collapses on itself.
Like planets and stars, black holes have gravitational fields around them. A gravitational field holds us to the Earth and causes the Earth to revolve around the Sun.
As a rule of thumb, the more massive an object, the stronger its gravitational field.
Earth’s gravitational field makes it extremely difficult to get into space. That’s why we build rockets: we have to travel very fast to escape from the Earth gravity.
The gravitational field of a black hole is so strong that not even light can escape it. That’s impressive, because light is the fastest known to science!
That’s why black holes are black, by the way: we can’t bounce light off a black hole like we can bounce a flashlight’s beam off a tree in the dark.
Albert Einstein’s general theory of relativity tells us that matter and energy have a curious effect on the universe. Matter and energy bend and stretch space. The more massive an object is, the more space around it stretches and bends.
A massive object creates a kind of valley in space. If objects come close, they fall into the valley.
That’s why if you get close enough to any massive object, including a black hole, you fall towards it. It’s also why light can’t escape a black hole: the sides of the valley are so steep that light doesn’t travel fast enough to climb out.
The valley created by a black hole gets steeper and steeper as you approach it from a distance. The point at which it becomes so steep that light cannot escape is called the event horizon.
Event horizons are not only of interest to would-be time travelers: they are also of interest to philosophers, as they have implications for how we understand the nature of time.
When space is stretched, so is time. A clock near a massive object ticks more slowly than a clock near a much less massive object.
A clock near a black hole will tick very slowly compared to a clock on Earth. One year near a black hole can mean 80 years on Earth, as you may have seen in the movie Interstellar.
For example, black holes can be used to travel to the future. If you want to jump off Earth in the future, just fly near a black hole and then return to Earth.
If you get close enough to the center of the black hole, your clock ticks slower, but you should still be able to escape as long as you don’t cross the event horizon.
Loops in time
What about the past? This is where things get really interesting. A black hole bends time so much that it can wrap itself around.
Imagine taking a sheet of paper and joining the two ends together to form a loop. That’s what a black hole seems to do with time.
This creates a natural time machine. If you could somehow get into the loop, which physicists call a closed time-like curve, you would find yourself on a trajectory through space that starts in the future and ends in the past.
Within the loop, you would also find that cause and effect are difficult to disentangle. Things that are in the past cause things to happen in the future, which in turn cause things to happen in the past!
So you’ve found a black hole and you want to use your trusty starship to go back and visit the dinosaurs. Good luck.
There are three problems. First, you can only travel to the past of the black hole. That means if the black hole formed after the dinosaurs went extinct, you can’t go back far enough.
Second, you would probably have to cross the event horizon to get into the loop. This means that to get out of the loop at some point in the past, you have to leave the event horizon. That means traveling faster than light, which we’re pretty sure is impossible.
Third, and probably worst of all, you and your ship would”spaghettificationSounds delicious, right?
Unfortunately that is not the case. As you passed the event horizon, you would be stretched flat, like a noodle. In fact, you would probably be stretched so thin that you would just be a series of atoms spiraling into the void in a spiral.
So while it’s fun to contemplate the time-warping properties of black holes, that visit to the dinosaurs will have to stay in the realm of fantasy for the foreseeable future.
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