The idea that we could go back in time to change the past has become one of the favorite techniques in films, literature and television series. Harry Potter, Back to the Future, Groundhog Day and many other films promised us the opportunity to re-select in our past. For most people, this opportunity will remain fantastic, because all the laws of physics indicate that moving forward in time is inevitable and necessary. There has even been a paradox in philosophy highlighting the absurdity of this possibility: if time travel were possible, you could go back in time and kill your grandfather before your parents even met, thereby eliminating the possibility of your own existence. For a long time it was believed that there was no way back. But thanks to the intriguing properties of space and time in Einstein's general theory of relativity, travel back in time may be possible, says physicist Ethan Siegel.
An illustration of the early universe of quantum foam, in which quantum fluctuations manifest themselves at the smallest scales. Positive and negative energy fluctuations can create tiny quantum wormholes.
Let's start with the physical idea of a wormhole. In the known universe on the smallest scales, tiny quantum fluctuations appear on the fabric of space-time. This includes energy fluctuations in positive and negative directions, which often occur very close to each other. A strong, dense, positive energy fluctuation can create curved space in a certain way, and a strong, dense, negative energy fluctuation will bend space in the opposite way. If you connect these two regions of curvature, you get - briefly - a quantum wormhole. If the wormhole survives long enough, you can try passing a particle through it, so that it instantly disappears in one place in spacetime and appears in another.
Accurate mathematical graph of the Lorentzian wormhole. If one end of a wormhole is constructed from positive mass / energy and the other from negative mass / energy, the wormhole will become traversable.
To scale all of this up, for example, and allow a human to walk through a wormhole, requires some work. Although all known particles in our Universe have positive energy and either positive or zero mass, it is possible for particles with negative mass and energy to exist within the framework of general relativity. Of course, we have not found them yet, but if you believe the theoretical physicists, there is nothing that would exclude the possibility of their existence.
If a substance with negative mass and energy exists, creating a supermassive black hole and its counterpart with negative mass and energy, and then combining them, will create a traversable wormhole. No matter how far you separate these two objects in alignment, as long as they have enough mass and energy - both positive and negative - the instant connection will remain. All of this is great for instant travel through space. But what about the timing? And this is where the laws of special relativity come into play.
According to the law of special relativity, stationary and moving parts age at different rates.
If you travel close to the speed of light, you experience a phenomenon known as time dilation. Your movement in space and movement in time are linked by the speed of light: the faster you move through space, the slower through time. Imagine that you have a destination 40 light years away, and you can travel at an incredible speed: over 99.9% of the speed of light. If you board a ship, travel to a star at almost the speed of light, then stop, turn around and return to Earth, something strange will be found.
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Due to the slowing down of time and shortening of length, you can reach your destination in as little as a year and then return in another year. But 82 years will pass on Earth. Everyone you know will get very old. This is how time travel is physically possible: you travel to the future, and time travel will only depend on your movement in space.
Is time travel possible? With a wormhole large enough, for example created by two supermassive black holes (positive and negative masses and energies), we could try.
If you build a wormhole like the one we described above, the story will change. Imagine that one end of a wormhole will be motionless, for example, somewhere near the Earth, while the other will travel at a speed close to light. After a year of rapid movement of one of the ends of the wormhole, you pass through it. What happens next?
Well, the year will be different for everyone, especially if everyone moves in time and space differently. If we are talking about the same speeds as before, the "moving" end of the wormhole will age 40 years, but the "calm" end - only 1 year. Step into the relativistic end of the wormhole and get to Earth only a year after the creation of the wormhole, and you yourself will age 40 years.
If 40 years ago, someone created such a pair of tangled wormholes and sent them on a similar journey, they could step into one of them today, in 2017, and travel to 1978. The only problem is that you yourself could not have been in this place in 1978; you had to be at one end of the wormhole or travel through space to catch up with it.
Warp travel as seen by NASA. If you create a wormhole between two points in space, so that one hole moves relativistically relative to the other, observers passing through it would age differently.
And by the way, this form of time travel also forbids the grandfather paradox! Even if the wormhole had been created before your parents were conceived, there was no way you could have appeared at the other end of the wormhole early enough to travel back in time and find your grandfather before that crucial moment. At best, you could take your newborn father and mother aboard a ship, catch up with the other end of the wormhole, let them mature, age, conceive you, and then travel back down the wormhole on their own. Then you will meet your grandfather in his prime, but technically it will be around the time your parents were born.
The universe gives free rein to the most unusual things. Especially if negative mass and energy really exists in the Universe and can be controlled. But travel back in time is something completely out of the ordinary. Due to the oddities of both special and general relativity, time travel to the past may not only be possible in fiction.