Never dreamed of going somewhere else? No, not with the usual speed with which we are “boring” going forward – second by second. Or:

– Faster, so that you can get far into the future, staying at the same age;

– Slower, so that you can do much more than others, for the same period of time;

– In the opposite direction, so that you can return to the era of the past and change it, possibly changing the future or even the present?

This may seem completely sci-fi, but not everything on this list will be purely “fantastic”: traveling through time is a scientifically possible process that is always with you. The only question is how you can manipulate it for your own purposes and control the movement in time.

When in 1905 Einstein put forward a special theory of relativity, the understanding that every massive object in the universe must travel in time was only one of its striking consequences. We also learned that photons – or other massless particles – can not experience time in their reference frame at all: from the moment when one of them is emitted, until the moment it is absorbed, only massive observers (like us) can see the flow of time. From the position of the photon, the whole Universe is compressed into one point, and absorption and radiation occur simultaneously in time, instantaneously.

But we have a mass. And everything that has mass is limited to always traveling at a speed less than the speed of light in a vacuum. And not only that, but also no matter how fast you move about anything – whether you are accelerating or not, it does not matter – for you the light will always move with one constant speed: c, the speed of light in a vacuum. This powerful observation and awareness comes with an amazing consequence: if you watch a person moving about you, his watch will go slower for you.

Imagine a “light clock”, or a clock that works by reflecting light back and forth in the up and down direction between two mirrors. The faster the person moves relative to you, the greater the speed of light movement in the transverse direction, and not in the up and down direction, which means that the slower the clock will go.

Similarly, your watch will move slower relative to them; They will see the time that flows more slowly for you. When you come back together, one of you will be older and the other younger.

But who?

This is the nature of the “twin paradox” of Einstein. The short answer: if you assume that you started in one frame of reference (for example, at rest on the Earth), and you will get to the same frame of reference later, the traveler will grow old, because for him time will go “slower”, and he who Stayed at home, will face a “normal” passage of time.

Therefore, if you want to accelerate over time, you will have to accelerate to near-light speed, move at such a rate for a while, and then return to its original position. Have to turn around a little. Do this and be able to move to days, months, decades, ages or billions of years into the future (depending on the equipment, of course).

You could witness the evolution and destruction of mankind; The end of the Earth and the Sun; Dissociation of our galaxy; Thermal death of the universe itself. As long as you have enough energy on the spacecraft, you can look as far into the future as you want.

But to come back is another story. Simple special relativity, or the relationship between space and time at a basic level, was enough to take us to the future. But if we want to return to the past, back in time, we need the general theory of relativity, or the relationship between space-time and matter and energy. In this case, we regard space and time as an inseparable tissue, and matter and energy – like what distorts this tissue, causes changes in the tissue itself.

For our Universe, as we know it, space-time is rather boring: it is almost perfectly flat, almost not curved, and in no form is obsessed with itself.

But in some simulated universes – in some solutions of Einstein’s general theory of relativity – you can create a closed loop. If space is fixated on itself, you can move in one direction for a long, long time to return to where you started.

Well, there are solutions not only with closed space-like curves, but also closed timelike curves. A closed timelike curve implies that you can literally travel in time, live in certain conditions and return to the same point from which you left.

But this is a mathematical solution. Does this mathematician describe our physical universe? It seems, not quite. The curvatures and / or discontinuities that we need for such a universe are wildly incompatible with what we observe even near neutron stars and black holes: the most extreme examples of curvature in our universe.

Our universe can rotate on a global scale, but the observed limits of rotation are 100,000,000 times stiffer than those that allow closed timelike curves that we need. If you want to go forward in time, you will need a relativistic DeLorean.

But back? Perhaps it will be better if you can not go back in time so as not to prevent your father from marrying your mother.

In general, summing up, we can conclude that traveling back in time will always fascinate people at the level of the idea, but, most likely, will remain in the unattainable future (paradoxical though it may seem). It is not impossible mathematically, but the universe is built on physics, which is a special subset of mathematical solutions. Based on what we observed, our dreams to correct our mistakes, going to the past, probably will remain only in our fantasies.