Black holes can work like a time machine, but there is an unpleasant nuance

Black holes are actually natural time machines that can allow humans to travel both to the future and to the past. This is if we talk about the theoretical side of the issue of time travel using black holes. However, from a practical point of view, such manoeuvres will have, to put it mildly, unpleasant consequences for the time traveller.

Sam Baron, Associate Professor of Philosophy of Science at the Australian Catholic University, explained the details of theoretical time travel in The Conversation. The scientist explains that the whole story of time travel using black holes is purely theoretical, as it contains too many "but".

In particular, humanity simply does not have a spaceship that could take us to a black hole. The closest black hole to the Earth is about 1560 light years away (one light year = 9.46 trillion kilometres). This is very, very, very far away.

Not to mention that even if you were near a black hole, you would need an engine capable of reaching speeds above the speed of light to pull off the time trick (and humanity doesn't even know if this is possible!).

But, first things first...

What are black holes?

A black hole is a supermassive object that is formed as a result of the collapse of a dying star. Black holes are characterised by an extremely strong gravitational field, which depends entirely on how massive the black hole is.

Gravity is known to exist on planet Earth and all other planets. This is the very factor that prevents us from exploring space freely, as we are forced to build super-powerful rockets to get up into the sky.

But the gravitational field of the Earth is just a toy compared to the similar force of a black hole. The gravity of black holes is so powerful that even light, whose speed reaches almost 300,000 kilometres per second, cannot escape from it. Meanwhile, humanity does not know anything that moves faster than light.

Deformation of space

According to Albert Einstein's theory of relativity, matter and energy have the ability to bend and stretch space. The more massive an object is, the more deformation the space around it undergoes.

Therefore, supermassive black holes create a kind of valley in space into which all objects that dare to get too close fall, and the gravitational force is so powerful that it is simply impossible to get back out.

But to become a prisoner of a black hole, you need to get close enough to the so-called event horizon - the point beyond which even light cannot escape.

Unstable time

When a black hole stretches the space around it, the same thing happens to time. Scientists know that the more massive the object next to which a clock is located, the slower it will go. This phenomenon is known as gravitational time dilation. It has even been experimentally confirmed using an atomic clock on Earth. The difference between the clock on the surface and the clock high above the Earth was only nanoseconds, but the further we fly away from the planet into space, the faster time will pass.

The gravitational time dilation trick was clearly demonstrated in Christopher Nolan's science fiction film Interstellar, when the crew is split into two parts, one of which goes to a planet that is too close to a black hole, while one of the crew members remains on the Endurance mothership in orbit. As a result, the astronauts, who have spent several hours on the planet, return to find the crew member on Endurance 23 years older.

That is why, to get to the future, it is enough to fly up to a black hole, get into "slow time", and then return back to Earth. The main thing is not to cross the event horizon.

Time loops

Oddly enough, it is also possible to go back in time thanks to a black hole, but it will be a bit more difficult. The fact is that a black hole distorts time so much that it starts to twist it around itself (like a scarf). Physicists call this phenomenon a closed time curve. And from a theoretical point of view, we can call it a natural time machine.

Now that you are close to the black hole, your task is to get into this time loop. There you would suddenly find yourself moving not just through space, but also through time.

Perhaps the task would be somewhat more complicated, since there are no cause-and-effect relationships in such a loop, and past events affect future events, which in turn change the past. Just keep your head on straight and you will succeed.

But there is a nuance. You can go back in time in this way only to the point where the black hole was formed, so choose responsibly, as you risk not reaching your destination.

Consequences

In theory, time travel with the help of a black hole sounds very simple, of course. However, there are two nuances that should not be forgotten.

To get to the past through a time loop, you will most likely have to cross the event horizon. To break out of the loop, you'll have to fly faster than light (which is not even theoretically possible yet). Think carefully about whether you have a powerful enough engine and enough fuel.

But these are trifles, since it is the approach to a black hole that will most likely kill you. You didn't think that the Universe would allow an ordinary person to just play with time, did you?

What will happen is that the closer you are to the event horizon, the more you will be stretched like a noodle. Eventually, you, along with the spaceship, will be stretched to a long string of atoms. And in such a state, you must admit, you will not be very comfortable enjoying the views of the future or the past.

Earlier, OBOZREVATEL also talked about what will happen when the Milky Way galaxy crashes into Andromeda.

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