Traveling to the past is much more difficult than traveling to the future, but there are several hypothetical ways to travel to the past:
1. Through the so-called wormholes (English term wormhole - travel to the past)
General relativity admits the possibility of the existence of "wormholes". They are like tunnels (perhaps very short ones) connecting distant regions in space. Developing the theory of wormholes, K. Thorne and M. Morris noticed that if you move one end (A) of a short hole at high speed, and then bring it closer to the other end (B), then - due to the paradox of twins - an object that fell at the moment T at the entrance A, can leave B at the moment preceding T (however, in this way it is impossible to get into the time before the creation of the time machine).
It follows from Einstein's equations that the wormhole will close before the traveler can pass through it (as, for example, in the case of the "Einstein-Rosen bridge" - the first described wormhole), if it is not kept from this by the so-called "exotic matter" - matter with negative energy density. The existence of exotic matter has been confirmed both theoretically and experimentally (the Casimir effect).
2. When rotating around cosmic strings
In 1936, Van Stockum discovered that a body revolving around a massive and infinitely long cylinder would go back in time (later F. Tipler suggested that this is also possible in the case of a cylinder of finite length). The so-called cosmic string could be such a cylinder, but there is no reliable evidence that cosmic strings exist, and there is hardly a way to create new ones.
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The existence of cosmic strings was first predicted by British physicist Thomas Kibble in 1976, and their theory was developed by Soviet astrophysicist Yakov Zeldovich by 1981.
The diameter of cosmic strings is much smaller than the size of atomic nuclei (about 10-29 centimeters), the length is at least tens of parsecs, and the specific gravity is about 1022 grams per centimeter, that is, a thousand kilometers of a string has the mass of the Earth, which means that the strings have a high density.
According to the theory, cosmic strings emerged shortly after the Big Bang and were either closed or infinite. The strings bend, overlap and break. The dangling ends of the strings immediately join together to form closed pieces. Both the strings themselves and their individual fragments fly through the Universe at a speed close to the speed of light.
Of course, it is impossible to see a cosmic string, but it, like any very massive object, creates a gravitational lens: the light from sources behind it must bend around it.
3. Do nothing and wait for the time machine to appear
Finally, you can do nothing at all, but just wait until the time machine forms itself in the future. There is no reason to expect that this will happen, but it is important that if it does not contradict the laws of nature, then the time machine will still be invented and made. The simplest model of such a situation is the Deutsch-Politzer time machine.
This method is dubious, since nothing is formed by itself and you can not wait.