Are black holes time machines? Yes, but there is a catch.
Black holes form natural time machines that allow travel to both the past and the future. But don’t expect to return anytime soon to visit the dinosaurs.
At present, we don’t have a spacecraft that can get us close to a black hole. But, even leaving that little detail aside, trying to travel back in time 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 usually forms when a dying star collapses in on itself.
Like planets and stars, black holes have gravitational fields around them. A gravitational field is what keeps us glued to the Earth and what makes the Earth revolve around the Sun.
As a general rule, the more massive an object is, the stronger its gravitational field.
Earth’s gravitational field makes it extremely difficult to reach space. That’s why we build rockets: we have to travel very fast to get out of Earth’s gravity.
The gravitational field of a black hole is so strong that not even light can escape. That’s impressive, since light is the fastest thing known to science!
By the way, that’s why black holes are black: we can’t bounce light off a black hole the way we might bounce a flashlight off a tree in the dark.
albert einstein 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 is stretched and bent around it.
A massive object creates a kind of valley in space. When objects get close, they fall into the valley.
This is why when 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 go fast enough to get out.
The valley created by a black hole becomes 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 interesting to would-be time travelers: they are also interesting to philosophers, because they have implications for how we understand the nature of time.
When space stretches, so does time. A watch that is close to a massive object will tick more slowly than one that is close to a much less massive object.
A clock near a black hole will tick very slowly compared to one on Earth. One year near a black hole could mean 80 years on Earth, as you may have seen illustrated in the movie. Interstellar.
In this way, black holes can be used to travel to the future. If you want to jump into the future of Earth, 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 will slow down, 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 back around itself.
Imagine taking a piece of paper and joining the two ends together to form a bow. That’s what a black hole seems to do over time.
This creates a natural time machine. If you could somehow get into the loop, which physicists call a closed time loop, you’d find yourself on a trajectory through space that starts in the future and ends in the past.
Within the cycle, you will also find that cause and effect become difficult to disentangle. Things in the past make things happen in the future, which in turn make things happen in the past!
So, you have found a black hole and you want to use your trusty spaceship 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 was created after the dinosaurs went extinct, then you won’t be able to go back far enough.
Second, you would probably have to cross the event horizon to enter the loop. This means that to exit the loop at a certain time in the past, you must exit 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 suffer.”spaghettification“. Sounds delicious, right?
Unfortunately, it is not. Crossing the event horizon, he would be stretched out, like a noodle. In fact, you’d probably be so stretched out that you’d just be a chain of atoms spiraling into the void.
So while it’s fun to think about the time-warping properties of black holes, for the foreseeable future that visit to the dinosaurs will have to remain in the realm of fantasy.
Sam Baron is Associate Professor of Philosophy of Science at the Australian Catholic University. This piece first appeared in The conversation.