According to our current understanding of physics, there is no known speed that would allow an object or person to travel back in time. The concept of time travel is primarily explored in science fiction and remains speculative in terms of its feasibility in reality.
In the realm of theoretical physics, certain solutions to Einstein's equations of general relativity, such as wormholes or the Alcubierre drive, have been proposed as potential avenues for achieving time travel. However, these ideas involve complex and hypothetical concepts that require vast amounts of energy and involve exotic matter with properties not yet observed or understood.
It's important to note that these theoretical solutions are highly speculative and far from being realized or proven possible. As of now, time travel remains purely a concept of science fiction, and there is no scientific evidence or consensus on a specific speed or mechanism that would enable backward time travel.