Ever dreamed of going somewhere else? No, not at the usual speed with which we "bored" go forward - second by second. Or:
- faster, so that you can climb far into the future, remaining at the same age;
- slower, so that much more can be done than others in the same period of time;
- in the opposite direction, so that you can return to the era of the past and change it, perhaps by changing the future or even the present?
It may sound completely sci-fi, but not everything on this list will be purely “fantastic”: travel 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 Einstein put forward special relativity in 1905, the realization that every massive object in the universe must travel in time was just one of its startling consequences. We also learned that photons - or other massless particles - cannot experience time in their frame of reference at all: from the moment one of them is emitted to the moment it is absorbed, only massive observers (like us) can see the passage of time. From the point of view of a photon, the entire Universe is compressed into one point, and absorption and emission occur simultaneously in time, instantly.
But we have mass. And anything that has mass is limited to always travel less than the speed of light in a vacuum. And not only that, but no matter how fast you move relative to something - whether you are accelerating or not, it doesn't matter - for you, light will always move at one constant speed: c, the speed of light in a vacuum. This powerful observation and awareness comes with an amazing consequence: if you observe a person moving relative to you, their clock will go slower for you.
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Imagine a "light clock" or clock that works by reflecting light back and forth in an up and down direction between two mirrors. The faster a person moves relative to you, the greater the speed of light movement in the transverse (along) direction, and not in the up and down direction, which means the slower the clock will go.
Likewise, your watch will move more slowly relative to it; they will see the time that flows slower for you. When you get back together, one of you will be older and the other younger.
But who?
This is the nature of Einstein's “twin paradox”. Short answer: if we assume that you started in the same frame of reference (for example, at rest on Earth), and get into the same frame of reference later, the traveler will age less, because time will go "slower" for him, and the one who stayed at home, will face the "normal" passage of time.
Therefore, if you want to accelerate in time, you will have to accelerate to near-light speed, move at this pace for a while, and then return to the original position. We'll have to turn around a little. Do this and you can travel days, months, decades, eras, or billions of years into the future (depending on the equipment, of course).
You could witness the evolution and destruction of humanity; the end of the Earth and the Sun; dissociation of our galaxy; heat death of the universe itself. As long as you have enough energy in the spaceship, you can look as far into the future as you like.
But coming back is another story. Simple special relativity, or the relationship between space and time at a basic level, was enough to get us into the future. But if we want to go back in time, back in time, we need general relativity, or the relationship between space-time and matter and energy. In this case, we regard space and time as an inseparable fabric, and matter and energy as what distorts this fabric, causes changes in the fabric itself.
For our Universe, as we know it, space-time is rather boring: it is almost perfectly flat, practically not curved, and in no way loops on itself.
But in some simulated universes - in some of the solutions of Einstein's general theory of relativity - a closed loop can be created. If space loops on itself, you can move in one direction for a long, long time to get back to where you started.
Well, there are solutions not only with closed space-like curves, but also closed time-like curves. A closed timelike curve implies that you can literally travel in time, live in a certain environment, and return to the same point from which you left.
But this is a mathematical solution. Does this mathematics describe our physical universe? It seems not quite. The curvatures and / or discontinuities 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 rotation limits are 100,000,000 times stiffer than the closed timelike curves we need. If you want to travel forward in time, a relativistic DeLorean is required.
But back? It might be best if you can't travel back in time to prevent your father from marrying your mother.
In general, summing up, we can conclude that travel back in time will always fascinate people at the level of ideas, but most likely will remain in an unattainable future (paradoxically). It's not mathematically impossible, but the universe is built on physics, which is a special subset of mathematical solutions. Based on what we have observed, our dreams of correcting our mistakes by going back in time will probably only remain in our fantasies.
ILYA KHEL