Futurists, physicists and even ordinary Joes have already been obsessed with the idea of travelling back in time even before we enjoyed watching the misadventures of Marty McFly in โ€œBack to the Future.โ€ In fact, we always have this fantasy of going back in time to do things differently in our lives or perhaps considering the idea of killing โ€œbaby Hitlerโ€ to prevent all the atrocities of the Second World War from even happening! Well some experts believe that it is really possible to travel back in time just like in the movies and science fiction novels.

Although time travel is theoretically possible, our technology is not yet at par when it comes to the requirements of doing so. All the time machines that we saw on movies or read in novels have their own sophisticated mechanisms and laws of physics to make it possible. We donโ€™t have a Doc Brownโ€™s flux capacitor or Dr. Whoโ€™s time travelling phone booth. So let us consider the possible physics theories that somehow future inventors would probably think about. Interestingly, we are all progressively travelling in time but there is seems to be no going back and even going fast forward.

Special Theory of Relativity

According to Albert Einsteinโ€™s Special Theory of Relativity:

  • The laws of physics are invariant in all inertial systems.
  • The speed of light in a vacuum is the same for all observers, regardless of the motion of the light source.

In other words, space and time are on the same plane called space-time. There is a speed limit of 186,000 miles per second for any time machine that travels through space-time just like how fast light travels through an empty space.

When you move through space-time, especially when your speed relative to other objects is close to the speed of light, time goes slower for you than for the people you left behind. You wonโ€™t notice this effect until you return to those stationary people. By the time you come back, the people you have left behind have considerably aged and they will look older than you.

Time Dilation

Another aspect of time travel that not all of us know is time dilation. This phenomena happens when you travel close to the speed of light. To put it in simple terms, your motion through space and time is related to the speed of light so that the greater your motion through space then you have less motion through time.

Once we have the technology to build a vehicle than can travel as fast as the speed of light then it will be possible to travel back in time. In essence, not as fast as the speed of light in one go but consistently for a long period of time. If you want to travel 40 years into the future then you need to have a space vehicle than can travel that length at 186,000 miles per second! When you turn back and go back to Earth, you will find it odd that 80 years has already passed. Unfortunately, travelling through a spacecraft is only one way and you canโ€™t travel back in time that way.

Time Travel Options

Letโ€™s just have a thought experiment. If time travel is feasible right now, there are possible options that will help you travel back in time just like choosing the type of transportation in your daily commute from home to work.

Option #1 โ€” Wormhole

Going through a wormhole (also known as the Einstein-Rosen bridge) is like this: you are on the ground floor and you took the escalator to go to the upper floor and when you reach the end, you suddenly fell down a hole and now youโ€™re back in the ground floor. This what happens when you enter a wormhole, you travel back in time.

Once we manage to generate a wormhole or happened to find one created in space then we will see that one end of the wormhole will be almost motionless especially those remaining close to Earth while the other one goes off on a relativistic journey close to the speed of light. What happens when you then enter the rapidly-moving end of the wormhole after itโ€™s been in motion for perhaps a year?

As previously mentioned, one year is relative so when you go through the wormhole, the โ€œone yearโ€ for people left behind may be 50 or 100 years depending on the length of travel time you had. If you step into the relativistic end of the wormhole, you will probably arrive back on Earth only one year after the wormhole was created, while you yourself may have had 50 or 100 years of time to pass.

Interestingly, going through a wormhole would mean that it also forbids the grandfather paradox as there are major caveats for travelling back in time with this method. One of it is the fact that we need the technology to create wormholes and even if we manage to do one then it will only allow us to travel as far back as the point in time it was created. So in other words, it is like the elevator we ride on as it only reaches as far as it was constructed to be.

Option #2 โ€” Cosmic Strings

Travelling back in time could be done theoretically through cosmic strings โ€” the narrow tubes of energy stretched across the entire length of the ever-expanding universe that we know. Left over during the early cosmos, these thin regions are believed to contain huge amounts of mass that can warp the space-time around them.

Many scientists say that cosmic strings are either loops or infinite. Since strings are attached as parallel to each other then it can bend space-time that would make time travel a possibility as long as you have a spacecraft good enough to find these cosmic strings and fast enough to reach it.

Option #3 โ€” Black Holes

If its possible to have a spacecraft to enter a black hole or create it artificially by having a huge, rotating structure then time travel can be a possibility. According to physicist Stephen Hawking, โ€œAround and around theyโ€™d go, experiencing just half the time of everyone far away from the black hole. The ship and its crew would be traveling through time. Imagine they circled the black hole for five of their years. Ten years would pass elsewhere. When they got home, everyone on Earth would have aged five years more than they had.โ€

However, the crew would need to travel around the speed of light for this to work. It is also possible that your spacecraft might fall apart before being able to rotate that quickly.

Option #4 โ€” Infinite Cylinders

Proposed by astronomer Frank Tipler, the Tipler Cylinder is a mechanism that would take matter that is 10 times the sunโ€™s mass and roll it into a very long but very dense cylinder. The spinning of up to billions of revolutions per minute would allow a spacecraft to follow a precise spiral around this cylinder and get itself into a โ€œclosed, time-like curve.โ€ However, the cylinder needs to run infinitely long enough for time travel to work.

Option #5 โ€” Purpose-Built โ€˜Time Machinesโ€™

A time machine is a device that allows you to travel forward and backward in time. Aside from that, it also has to bend space-time so far that time lines turn back to become a loop known as โ€œclosed time-like curve.โ€

Unfortunately, a purpose-built โ€˜time machineโ€™ is not a simple device and even more complicated than a space shuttle because it needs an exotic form of matter with so-called โ€œnegative energy density.โ€ These types of matter has unique properties like moving in the opposite direction of normal matter when pushed. Even if you have this required exotic matter then it would probably be in quite rare quantity to be able to build that time machine.

Some scientists believe that it is possible to build a time machine without exotic matter. Anyone who wants to build a time machine would create a doughnut-shaped hole that is enveloped within a sphere of normal matter. Inside it is a vacuum space where space-time can be bent upon itself with focused gravitational fields to form a closed time-like curve.

If someone wants to go back in time, he has to race around inside the doughnut by trying to go back further into the past with every lap. However, there are untested scenarios where such time travel attempt can possibly go wrong. Gravitational fields have to be very strong and stable in order to make the closed time-like curve. Not to mention, manipulating them has to be very precise as well.

Final Thoughts

Time travel remains an elusive dream and physicistsโ€™ dream. It remains a novel thought experiment and a favorite science fiction staple. Whatever developments have been made towards achieving it remains far away from reaching reality. Only time will tell if we can do it.

It is said that history will become a science when time travel is now possible.

This article was published in the 1 November 2018 issue of FutureLapse.