Black holes create natural time machines that allow you to travel to both the past and the future. But don’t expect to return to dinosaurs anytime soon. Currently, we do not have a spacecraft that can bring us closer to a black hole. But even if that’s a bit of an aside, trying to go back in time with a black hole might be the last thing you do.

What are black holes?

A black hole is an extremely large object that is usually formed when a dying star collapses on itself. Like planets and stars, black holes have gravitational fields around them. The gravitational field is what keeps us on Earth and makes the Earth revolve around the Sun. As a rule, the larger the object, the stronger the gravitational field.

Earth’s gravitational field makes it extremely difficult to get into space. That’s why we rocket: We have to travel very fast to escape Earth’s gravity. The gravitational field of a black hole is so strong that even light cannot escape it. That’s great because light is the fastest thing science has ever known! By the way, that’s why black holes are black: we can’t reflect light from a black hole the way we reflect a flashlight from a tree in the dark.

stretch of space

Albert Einstein’s general theory of relativity tells us that matter and energy have strange effects on the universe. Matter and energy bend and stretch space. The larger the object, the more stretched and curled the area around it. A large object forms a kind of valley in space. As objects get closer, they fall into the valley. Therefore, when you get close enough to any large object, including a black hole, you will fall towards it. It also prevents light from escaping the black hole: The valley’s slopes are so steep that the light does not travel fast enough to escape.

The valley formed by the black hole becomes steeper as it approaches. The point at which no light can escape is called the event horizon. Event horizons are not just of interest to time travelers: they are interesting to philosophers, as they affect how we understand the nature of time.

time stretching

As space expands, so does time. A clock near a large object will run slower than a clock near a much smaller object. A clock near a black hole runs very slowly compared to a clock on Earth. As you may have seen in the movie Interstellar, one year near a black hole can mean 80 years on Earth. In this way, black holes can be used to travel into the future. If you want to jump into the future of Earth, fly past the 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 can still 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 enough to wrap itself around itself. Imagine taking a piece of paper and joining the two ends to form a loop. This is what a black hole does over time. This creates a natural time machine. If you could somehow enter the loop that physicists call a closed time-like curve, you would find yourself on a trajectory of space that begins in the future and ends in the past. In the loop, you will also find that it is difficult to work out cause and effect. What happened in the past causes what will happen in the future, which in turn causes what will happen in the past!

hitch

So you’ve found a black hole and you want to use your trusty spaceship to come back and visit the dinosaurs. Good luck There are three problems.

First, you can only pass by a black hole. This means that if a black hole was created after the dinosaurs died, you couldn’t go back far enough.

Second, you probably need to cross the event horizon to get into the loop. This means that to get out of the loop at a certain time in the past, you have to get out of the event horizon. That means traveling faster than light, which we’re pretty sure is impossible.

Third, and probably worst, you and your ship will “get spaghetti.” Sounds delicious, right?

Unfortunately, this is not the case. You’ll stretch like noodles as you cross the event horizon. In fact, you’d probably be so subtle that you’d be a chain of atoms spiraling into space. While it’s fun to think about the time-bending properties of black holes, this visit to the dinosaurs will be a dream come true for the foreseeable future.