Black holes are natural time machines that allow travel to the past and the future. But don’t expect to be back to visit dinosaurs anytime soon.
Right now, we don’t have a spacecraft that can bring us anywhere near a black hole. But even leaving those small details aside, trying to travel into the past with 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. The gravitational field is what keeps us stuck to the Earth, and what keeps the Earth revolving around the sun.
As a general rule, the greater the mass of an object, the greater its gravitational field.
Earth’s gravitational field makes it very difficult to get into space. That’s why we make rockets: We have to travel really fast to get out of Earth’s gravity.
The gravitational field of a black hole is so strong that not even light can escape from it. That’s pretty impressive, because light is the fastest thing known to science!
By the way, this is why black holes are black: We can’t bounce light off a black hole the way we might bounce light from a torch off a tree in the dark.
space span
Albert Einstein The general theory of relativity It tells us that matter and energy have a strange effect on the universe. Matter and energy bend and stretch space. The larger the body, the more space it stretches and curves around.
A massive body that creates a kind of valley in space. When things get close, you fall into the valley.
This is why, when you get close enough to any massive object, including a black hole, you fall toward it. This is also why light cannot escape a black hole: the sides of the valley are too steep for light to travel fast enough to get out.
The valley created by the black hole becomes steeper and steeper the closer you get to it from the distance. The point at which it becomes too steep for light to escape is called the event horizon.
Event prospects are not only interesting to would-be time travelers: they are also interesting to philosophers, because they have implications for How do we understand the nature of time?.
dilation time
When space expands, so does time. A clock close to a massive object will tick slower than one close to a much less massive object.
A clock close to a black hole will tick much more slowly than a clock on Earth. One year near a black hole could mean 80 years on Earth, as you may have seen 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 near a black hole and then come back to Earth.
If you get close enough to the center of the black hole, your clock will set slower, but you can still escape as long as you don’t cross the event horizon.
episodes in time
What about the past? This is where things get really interesting. A black hole bends time so much that it can wrap around itself.
Imagine that you take a sheet of paper and connect its two ends to form a loop. This is what a black hole appears to do with time.
This creates a natural time machine. If you can somehow get into the loop, which physicists call a closed time curve, you will find yourself on a path through space that begins in the future and ends in the past.
Within the loop, you’ll also find the difficulty of disentangling cause and effect. Things that were in the past cause things to happen in the future, which in turn cause things to happen in the past!
catch
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 a black hole. This means that if the black hole was created after the dinosaurs became extinct, you wouldn’t be able to go back far enough.
Second, you probably have to cross the event horizon to get into the loop. This means that to exit the loop at a certain time in the past, you would need to exit the event horizon. This means traveling faster than light, which we are sure is impossible.
Third, and perhaps worst of all, you and your ship will be subdued.spaghettieffLooks delicious, right?
Unfortunately, this is not the case. When it crosses the event horizon, it will stretch flat, like noodles. In fact, you would probably be so screwed up that you would just be a string of atoms spiraling through the void.
So while it’s fun to think about the time-warping properties of black holes, visiting dinosaurs in the foreseeable future should remain in the realm of fantasy.
Sam Barron is Associate Professor of Philosophy of Science at the Australian Catholic University. This piece appeared for the first time Conversation.