Possible Ways Time Travel Can Be Achieved (Continued)

1.) Infinite Cylinder 

A mechanism proposed by Astronomer Frank Tipler where 10 times the sun's mass would be rolled into a long and dense cylinder. "After spinning this up a few billion revolutions per minute, a spaceship nearby – following a very precise spiral around this cylinder – could get itself on a 'closed, time-like curve', according to the Anderson Institute." 

2.) Black Holes 

According to physicist Stephen Hawking, a ship moving rapidly around a black hole would experience just half the time everyone on Earth was experiencing. However, the travelers would need to travel around the speed of light to achieve this type of time travel. 

3.) Cosmic Strings 

Cosmic strings are "narrow tubes of energy stretched across the entire length of the ever-expanding universe." Left over from early cosmos, these strings are said to contain massive amounts of mass and could potentially wrap the space-time around them. In theory, "The approach of two such string parallel to each other would bend space-time so vigorously and in such a particular configuration that might make time travel possible." 

4.) Time Machines 

Research of time machines involves creating something known as a closed time-like curve, in which space-time is bent so far that time lines turn back on themselves to form a loop. To achieve this, an exotic form of matter with "negative energy density" would be needed, which is said to have numerous bizarre properties like moving in the opposite direction of normal matter when pushed. If this matter did exist, the quantity available would be far too small for time machine construction. Time machines may also be possible without the use of this exotic matter. "The work begins with a doughnut-shaped hole enveloped within a sphere of normal matter. Inside this doughnut-shaped vacuum, space-time could get bent upon itself using focused gravitational fields to form a closed time-like curve. To go back in time, a traveler would race around inside the doughnut, going further back into the past with each lap."  


https://www.space.com/21675-time-travel.html