Could humans use black holes to time travel?

Black holes form natural time machines that allow travel into the past and future. But don’t expect to be back to visit the dinosaurs anytime soon.

Currently, we have no spacecraft that can take us anywhere near a black hole. But even with 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 stuck to Earth and what keeps Earth orbiting 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. This is why we build rockets: we have to travel very fast to get out of Earth’s gravity.

Read more: How gravitational waves can ‘see inside’ black holes

The gravitational field of a black hole is so strong that even light cannot escape it. This is impressive, since light is the fastest thing known to science!

Incidentally, this is why black holes are black: we can’t bounce light off a black hole any more than we can bounce a flashlight beam off a tree in the dark.

Stretching space

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 stretches and bends around it.

A massive object creates a kind of valley in space. When the objects get close, they fall into the valley.

3D illustration of Earth and Sun in warped space-time. (Image credit: vchal via Shutterstock)

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 the light isn’t going fast enough to escape.

The valley created by a black hole gets 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 aren’t just interesting to would-be time travelers: they’re also interesting to philosophers because they have implications for how we understand the nature of time.

Stretching time

When space is expansive, so is time. A clock that is close to a massive object will tick more slowly than a clock that is close to a much less massive object.

A clock close to a black hole will tick very slowly compared to that on Earth. One year near a black hole can 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 into the future. If you want to jump into Earth’s future, just fly past a black hole and then return to Earth.

If you get close enough to the center of the black hole, your clock will tick more slowly, but you should still be able to escape as long as you don’t cross the event horizon.

Spray on time

What about the past? This is where things get really interesting. A black hole bends time so much that it can coil back on itself.

Imagine taking a sheet of paper and bringing the two ends together to form a loop. This is what a black hole seems to do to time.

This creates a natural time machine. If you could somehow get into the loop, which physicists call a closed time curve, you would find yourself on a trajectory through space that starts in the future and ends in the past.

Inside the loop, you’ll also find that cause and effect are hard to sort out. Things in the past cause things to happen in the future, which in turn cause things to happen in the past!

Catching

So you’ve found a black hole and want to use your trusty spaceship to go back and visit the dinosaurs. Good luck.

There are three problems. First, you can only travel past the black hole. This means that if the black hole was created after the dinosaurs died, then you won’t be able to go back far enough.

Second, you’ll probably need to cross the event horizon to enter the loop. This means that to exit the loop at a certain time in the past, you will need to exit the event horizon. That means traveling faster than light, which we’re pretty sure is impossible.

Third, and perhaps worst of all, you and your ship will undergo “spaghettitification”. Sounds delicious, right?

Alas, it is not. As you crossed the event horizon, you would stretch out, like a noodle. In fact, you’d probably stretch yourself so thin that you’d just be a string of atoms spinning in 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 stay in the realm of fantasy.

This article is republished from The Conversation (opens in new tab) under a Creative Commons license. Read the original article (opens in new tab).