It may surprise you that physicists don't understand time. Travel into the future appears plausible now.
Traveling back in time to meet our great-great-great grandparents seems implausible.
Albert Einstein helped us grasp time travel. His relativity theory described space, time, mass, and gravity.
For Einstein, time was relative. Relativity causes time to flow differently. Situations can hasten or slow time.
Its ability to speed up or slow down suggests time travel with real-world consequences.
Time passes faster with speed, but for the effect to be evident, you must travel at the speed of light.
Similarly, time moves slower in an extreme gravitational environment, such as a black hole.
Sky clocks tick faster than Earth clocks. Thus, the ones in the sky must be regularly adjusted for precision. If not, your map app may be six miles (10 km) off per day!
Traveling into the future is conceivable with relativity. Whether moving at the speed of light or in a strong gravitational environment, subjective time is brief.
Relativity does allow for backwards time travel, however this is still in the theoretical stage.
One 1949 study by Kurt Gödel suggested a closed time-like curve, where space and time loop back on itself, could allow past time travel.
No closed-circle loop exists in the universe, which undermines this notion. Pure theory.