The issue of travel to the past has been a topic of great interest for the scientific community as well as the fans of science fiction, yet its feasibility has equally been a topic of intensively heated debates.

Before the introduction of relativity theory (RT) and quantum mechanics (QM), our common-sense view of the universe was compatible with scientific theories. However, following the introduction of RT and QM that constitute the pillars of our understanding of the universe, our intuitive outlook of the world now seems to be at odds with such scientific theories. Indeed, due to its controversial nature, the topic of travel to the past has been a disputed territory for scientists, philosophers as well as ordinary people.

First theoretical base

The first scientific approach to this idea came in 1949, when the famous American logician and mathematician Kurt Gödel (1906-78), managed to prove that within the framework of Einstein’s general theory of relativity the idea of travel to the past could indeed turn into reality.

Solving Einstein’s gravitational field equations for a particular model of the universe, he proved that for a rotating world, it is possible for one to travel into regions of the universe that are in a past time according to every definition of time. Gödel went even further by calculating the amount of energy required by one to take a trip to its own past.

The calculated energy was so high that it made the feasibility of such travel almost impossible. Although later experiments proved that the model of a rotating world was not compatible with our universe, the fact insisted that in Gödel’s model, travel to the past was possible while no laws of physics were violated.

Further developments

This idea was further developed in the 60’s and 70’s. In 1974, the American physicist Frank Tippler proved that around a massive and highly dense rotating cylinder of finite size, space-time could be twisted to the extent that allows closed time loops. Based on his model, he suggested that a rotating cylinder just 10 kilometers wide and 100 kilometers long could indeed be used as a time machine.

According to him, a traveler leaving earth for such a spinning cylinder would feel a warped space close to the surface of the cylinder and a few times orbiting around the cylinder and moving back to the earth would see itself in the past!

In 1988, the American astronomer Kip Thorne suggested another idea by proposing that wormholes could be made to function as time machines (wormholes are hypothetical twists in the fabric of space-time that create a tunnel in the universe acting as shortcuts through time and space).

In 1991, Richard Gott suggested a space-time with two cosmic strings passing by each other at a high speed. Cosmic strings are hypothetical one-dimensional cracks in the universe. Their origin of creation goes back to billions of years ago. According to Gott, under such a condition, they would create a highly curved space-time topology making opening up of a closed time-like curve a possibility.

This idea was further substantiated and developed through Stephen Hawking’s work. He too suggested other criteria for travelling to the past, including the interior of a rotating black hole. A rotating black hole could create a wormhole taking one to a different point in time and space in a different universe.

Feasibility and further implications

Since the idea has been the focus of intense research, there have been numerous new ideas and suggestions about other circumstances under which time travel to the past can be attained. Indeed, there have been many suggestions for making time machines. Unfortunately, from just a purely technological view, none of these theories seem to be feasible at the present time or foreseeable future, owing to the fact that they all need an unimaginable gigantic amount of mass.

For example, for the Thorne’s wormhole time machine to operate, there needs to be a form of “exotic matter” for the wormholes to survive. In addition, while, no wormhole has ever been discovered, even if it is discovered, with its cross-section of about 10 -33 cm, it would be far too small to be used for practical time travel.

Also, apart from the issue of its feasibility, there are other problems and paradoxes arising from the idea of travel to the past. For example, while according to the theoretical point of view, the laws of physics as we understand them do not rule out this possibility, our common sense and daily experience imply the reverse, since the time manifests itself as being unidirectional.

There are also some famous paradoxes regarding the travel to the past like “The Grandfather Paradox” which poses such a question on what will happen if one travels back in time and kill his grandfather while he’s still a kid? Or even the worse, what would happen if one travels back in time and kills himself?!

Such examples indicate the violation of casualty principle thus rejecting any possibility of travel into the past. Of course, there have been some arguments quarreling these paradoxes, such as the “many worlds” hypothesis of Hugh Everett. He suggested that when one travels to the past, a new universe is created branching off from the original universe, thus making it untouched.

Since theories in physics are normally hundreds of years ahead of the technological progress, the only way to resolve such paradoxes and test these theories is a further advancement in technology, and until then there is no definite answer to this issue.

References:

Stephen Hawking A Brief History of Time

Stephen Hawking on Time Travel to the Future

www.space.com

www.thoughtco.com