You don't need a lot of mind to travel in time. Each of us moves about 24 hours ahead every day. It is another matter that this movement remains as unintentional as inevitable.
The idea of the flow of time as something unchanging, constant, eternal and uniform sits somewhere very deep in our psyche. We measure it in seconds, hours, years, but the duration of these intervals can vary. As a river flow, which in fact is often compared to the flow of time, can now accelerate at sharp drops, then slow down, spreading widely, time itself is subject to changes. This discovery became, perhaps, the key in the scientific revolution, which in 1905-1915. performed the work of Albert Einstein.
The impermanence of time originates in its complex relationship with space. Three spatial dimensions and one temporal dimension form a single, inseparable continuum - the stage on which everything that happens in our world unfolds. The intricate interweaving and interactions of these four dimensions with each other give us hope that travel to the past and the future is still possible. To gain power over time, you just need to tame space. How is this possible?
For simplicity, let's imagine that the continuum of our universe includes not four, but only two dimensions: one spatial and one temporal. Every object, from a photon to Donald Trump, moves along this continuum at a constant speed. Whatever it does, whether it crosses the Galaxy or answers journalists' questions while sitting on a chair, the overall speed of its movement remains unchanged - simplifying, we can say that the sum of the speeds at which an object moves is always equal to the speed of light. If the president does not move in space, then all the energy of his movement goes into movement along the time axis. If a photon moves through space with the speed of light, then for a while it has no energy left, and for these particles time does not move at all.
We can say that movement in space "steals" movement from time. If Donald Trump accelerates - gets on a plane and crosses the Atlantic at a speed of about 900 km / h - he will slow down his movement in time and will be somewhere in the "future" by 10 nanoseconds, in the time that is for his "internal clock" has not yet arrived. The current record holder of being in space, Gennady Padalka, for 820 days on the ISS, during which he moved at a speed of about 27.6 thousand km / h, moved into the future by several tens of milliseconds. Upon reaching a speed of 99, 999% of light per year, you can move into the future for 223 "normal" Earth years.
This flow of motion from space to time and back should be extended to gravity. In the description of the General Theory of Relativity, gravity is a deformation of the space-time continuum, and in the vicinity of a black hole (and any other gravitating object) all four dimensions are "bent", and the stronger, the stronger the attraction. Time at the surface of the Earth flows more slowly than in orbit, and the super-precise clocks of the satellites run away by about 1/3 of a billionth of a second per day. This movement into the future is much more noticeable for bodies located near more massive objects.
The supermassive black hole in the center of our Galaxy weighs about 4 million suns, and if we start to cut circles near it, then after a while - when only a few days pass on our spacecraft - we may find ourselves in the Universe several years older than us.Again, in the future. As we understood, Einstein's formulas easily allow such movements, although in practice they are as complex as it is difficult to gain speed close to light, or survive in the vicinity of a supermassive black hole. But what about the past?
Back and up
Generally speaking, backward time travel is even easier than forward: just look at the starry sky. The diameter of the Milky Way is about 100 thousand light years, and the light of more distant stars and galaxies can travel to us for millions and billions of years. Looking around the night sky, we see flashes of the past - the Moon, as it was about a second ago, Mars - about 20 minutes ago, Alpha Centauri almost four years ago, the neighboring galaxy Andromeda Nebula - 2.5 million years ago.
The farthest limit available to this kind of "movement" in time is more than 10 billion years: a picture of that incredibly distant epoch can be seen in the microwave range, as traces of the relict radiation of the Universe. But, of course, such travels will not satisfy us; they seem to be something "fake" in comparison with how such movements look in fiction. You select the desired epoch on the screen, press the button - and …
Interestingly, Einstein's equations do not impose restrictions on such purposeful travel into the past. Therefore, some theorists, reasoning about this, assume that when moving at a speed greater than the speed of light, time in this frame of reference will flow in the direction opposite to the rest of the Universe. On the other hand, Einstein's theories still prohibit such a movement: the mass will become infinite upon reaching the speed of light, and to disperse an infinite mass even a little faster, infinite energy will be needed. But, most importantly, the introduction of such time machines is capable of violating an equally fundamental principle of cause and effect.
Imagine that you are a fierce supporter of Hillary Clinton and decide to go back in time to beat petty Donald Trump and drive him away from politics forever. If it worked, and Donald, after such "teaching" back in the 1950s, decided to focus entirely on business or on playing chess, then how would you even know about its existence, let alone inflame a dislike for this politician?.. These paradoxes are well revealed by the cult series of films "Back to the Future", and many scientists believe that they make travel to the past fundamentally impossible. On the other hand, we can always reason and fantasize. Let's try?
Through the ring
Approaching a sufficiently large black hole leads to time dilation. Falling inside is hardly an option: this activity is too dangerous and will not allow you and your car to travel in time to remain intact. However, there is an option in which a black hole may be a perfectly suitable "portal" to the past. It was indicated by calculations carried out back in the 1960s by the famous (and then still very young) New Zealand physicist Roy Kerr, who studied the gravitational field of rotating black holes.
Indeed, if an ordinary spherical body is compressed to a critical radius and forms a black hole singularity, then the mass of the rotating body is influenced by centrifugal forces. This angular momentum does not allow the formation of the usual "point" singularity, and instead a very unusual singularity appears - in the form of a ring of zero thickness, but nonzero diameter. And if the singularity of an ordinary black hole is not avoided by anyone who dares to approach it too close, then an observer approaching a ring-shaped singularity may well "slip through" it and end up on the other side.
Some scientists suggest that these properties can make "Kerr" black holes a kind of antipodes of ordinary ones - somewhere, in another space-time, they do not absorb, but, on the contrary, throw out everything that has fallen into them in ours. The lucky one who has escaped complete disintegration in the ring-shaped singularity will find himself somewhere in a completely different place and time. Where? Alas, even here no management is envisaged yet: how lucky. So far, we are not even sure of the existence of a singularity of such a suitable shape, not to mention how to control their occurrence and which parts of the space-time continuum they connect. Does this remind you of something?
Burrows and strings
If we remember our simplified two-dimensional continuum, which contains only one temporal dimension and one spatial dimension, then it will be easy for us to imagine how its tissue not only deforms and bends, but also breaks - as in the vicinity of massive bodies and in the singularity of a black hole … But where do such breaks lead? Apparently, again, - to another part of the continuum - as if we took a flat two-dimensional sheet and folded it in half, punching "holes" from one surface to another. No theory prohibits the existence of such holes in our four-dimensional space-time - objects commonly known as wormholes.
Physicists have practically never observed them anywhere, but there are a number of models describing such wormholes, and their authors include very authoritative figures, including the American Kip Thorne and the British Stephen Hawking. The latter believes that wormholes exist only on the Planck scale, in the "quantum foam" of virtual particles that are continuously born and annihilated in the vacuum of space-time. Together with them, countless wormhole tunnels are born and scattered, which for a tiny fraction of a second - randomly - connect completely different regions of space-time, and disappear again.
To use such holes for any benefit, they will have to learn how to stabilize and increase in size. Alas, calculations show that this will require colossal amounts of energy, unimaginable neither for the American president, nor for all of humanity in any more or less foreseeable future. Therefore, a slightly greater hope for free movement in time is given by another semi-fantastic concept, developed in the second half of the twentieth century. Thomas Kibble, Yakov Zeldovich and Richard Gott - we are talking about cosmic strings.
Do not confuse them with superstrings from another well-known theory: cosmic strings in Gott's view are very dense one-dimensional folds of space-time that arose at the dawn of the Universe. To put it simply, the “fabric” of space-time in that epoch had not yet “smoothed out”, and some of the folds of that time have survived to this day. They stretched out to tens of parsecs, but they are still unusually thin (about 10–31 m) and carry enormous energy (density about 1022 g per cm of length).
Thinner than an atom, cosmic strings pierce the space-time continuum, exhibiting the most powerful, albeit locally limited, gravity. But if we learn to manipulate them, bring them closer together, twist and weave, we can “tune” the space-time around as we like. Such superpowers promise already quite full-fledged travels to the past and the future at will, according to need or mood. Unless there are fundamental prohibitions on this. Remember Back to the Future?
Paradoxes and their resolution
Violation of cause-and-effect relationships when traveling to the past can confuse not only philosophers, but also any reasonable physical and mathematical calculations. The most famous example of this is the "murdered grandfather paradox", first described in science fiction back in the 1940s.The book by the French writer Rene Barzhavel tells how a careless time traveler killed his own grandfather, so that later he could not be born, fly into the past and kill grandfather … neither science nor our everyday experience accepts.
One of the solutions to this paradox may be "postselection" of events in the Universe itself. In other words, once in the past, the traveler will not be able to do anything that would disrupt the correct course of cause and effect. The pistol will not work, or he will not find his grandfather, or a thousand other accidents, oddities, embarrassments will happen, but the course of things will not allow to knock the Universe off its measured course. But in general it is difficult to imagine any action in the past that would not have far-reaching consequences. Let us recall another term that came from science fiction - the "butterfly effect", which indicates the property of some systems to amplify an insignificant influence to large and unpredictable consequences. Perhaps a post-selective solution to the paradoxes of time will still prevent us from traveling through it.
However, there is another approach that is much more promising. According to the hypothesis of the Multiverse, popular today, any possible (and impossible) option can be realized in the universe, they just all "diverge" in different parallel universes. You can travel back in time and shoot your grandfather, and he really will not give birth to your father, but he will not give birth to you, but in another, parallel world. Just like somewhere out there, Donald Trump may lose the election, or not be born at all, or turn out to be a famous cyclist. Just as somewhere there are worlds inhabited by green thinking jellyfish or generally obeying other laws of physics.
Thus, time travel paradoxically brings us to the problems of the fundamental structure of the space-time continuum. Problems that can be finally solved only by the first experience of real moving into the past - it is a pity that in our world this incredible event will have to wait for an indefinite time.