One of the greatest enigmas of modern science is the possibility of time travel. Einstein proved through the general theory of relativity time travel into the future. However, time travel into the past is much more complex to understand in modern physics.
Michio Kaku explains in this video a theoritical model where the fabric of space time can be manipulated to travel into the past. This model impossible to build today with our current technology might be a reality in the distant future, making possible time travel possible in a commercial scale.