What is time travel? In simple terms, time travel is the concept of movement between certain points in time, mostly by a person or a hypothetical device known as a time machine. A time machine can be of any shape and size, either in the form of a car or a portal connecting certain distant points in spacetime.
The concept of time travel has been more of a staple for popular science fictions for many years. Whether it is ‘Star Trek’ series, ‘Doctor Who’ or 1985’s ‘Back to the Future’, where we witness how scientist find a way to travel in time but in reality, it’s much more complex, even the concept is controversial. The scientific community is divided on whether time travel is possible or not, but who wouldn’t want to doodle around in their past or unravel the future. Now the question is it really feasible, is time travel possible? Let’s find out.
What is Time?
Before Albert Einstein, we thought that time is a constant entity as normal life would suggest, but he proved that time is actually relative, i.e. time passes at varying rates for people who are moving relative to each other. According to Einstein, time is the “fourth dimension.”
Suppose you and your cousin decided to synchronize time in conventional watches before leaving the Earth in a space shuttle. Travelling at a speed of 30,000 km/h relative to the Earth, you decided to take few orbits around the planet Earth before returning to the ground. Upon landing, if you compare your watch time with that of your cousin, you will discover a slight variation in time. This variation is called time dilatation.
One practical example of time dilation is experienced by astronauts when they returned to the Earth after spending 6 months on the International Space Station. Orbiting around the Earth at a distance of approximately 400 km, astronauts were aged 0.007 seconds less than those of us here on the Earth.
Increased Time Dilatation
This might be negligible, but one could experience an extreme amount of time dilation effect if he/she gets close to the speed of light. Theoretically, time dilation is almost negligible up to 50% of the speed of light and pose a little effect at 75% but it rises exponentially after that.
The theory of special relativity teaches us that how time speeds up or slows down based on your relative speed to someone or something else. Near the speed of light inside a spaceship, you would age much slower than your cousin here on the Earth.
You might be thinking, is it really a time travel? Well! Yes. According to the very nature of space-time, returning here on the Earth after traveling at 90% of the speed of light you would be 3 times younger than people on the Earth, which means you traveled years into the future relative to the time and space. Consider one example from Christopher Nolan’s Interstellar. In the quest to find a suitable planet for humans, Cooper’s team reached a massive watery planet which is locked extremely close to a super-massive black hole.
With a mass equivalent to that of 100,000,000 suns and a rotation rate of almost 99.8% of the speed of light, the supermassive black hole or Gargantua caused the time to slow down to the extreme limits so that one hour on the planet is equal to 7 years on the ship (spaceship Endurance). After returning to their mother ship (just 3 hours later), they found that their scientist friend aged 23 Earth years.
No, this is not just fiction, this is actually how it would happen if it was real life. So technically they traveled 23 years ahead of those people on the Earth. Until now we have explored the very possibility of traveling into the future, but what about our past?
Travelling Faster Than Light
Technically, Einstein’s theory of General relativity allows us to travel back in time. But achieving those mathematical equations physically might not be possible in the near future. This might become possible if we achieve speeds faster than the speed of light, which is approx. 300,000 km/s (299,792 km/s to be exact) in vacuum.
But again, Einstein’s equations tell us that an object can only achieve the speed of light if it has an infinite mass and zero length, which is by the way impossible in the real world. Though many researchers believe that Einstein equations can be modified further.
Creating “wormholes” is another possible way to time travel. The concept of a traversable wormhole is validated by the theory of general relativity. Although scientist hasn’t found any wormhole, the creation of a wormhole is only possible in the presence of “exotic matter” with negative mass/energy.
Image Courtesy: BrightR/Wikimedia Commons
The concept of time traveling into the past can lead us to various paradoxes and inconsistencies. One such is the famous grandfather paradox. Imagine that you have found a way to travel back into the past and decided to kill one of your parents or your grandfather, don’t ask me why. So if you killed your grandfather in the past, your father would never be born so neither would you, hence your life is forever altered.
Several scientists believe that, even if you wouldn’t be born in the current universe, you still take birth in the other. While others say that the probability of occurring such events (that change the past or cause a paradox) is zero (Novikov’s principle).
Other Popular Time Travel Theories
Image Courtesy: NASA
A short visit near a black hole is another way we can achieve time travel, the only catch is we have to move at the speed of light as if it is so easy. Imagine that you have a spacecraft powerful enough to propel you with a continuous acceleration of 1g, then you would be able to reach the center of the Milky Way galaxy (26,000 light years away) in several decades of your time.
Just to give you an idea of how far the galactic core really is, consider the fact that the nearest star to the Earth is situated at 4.3 light years away, and the Voyagers (farthest man-made thing) have traveled only half the distance so far.
After reaching near the black hole, take a short breather and then scoot over here on the Earth. When you get back on Earth, forget about meeting your family or friends, there is a greater possibility that the Earth you knew decades ago, now would just be a pile of rubble, as more than 40,000 years would have passed but you would age only 40 years or so. Some scientists even believe that this situation can be created artificially with a massive rotating structure.
Back in 2010, world renown physicists Stephen Hawking while speaking to the dailymail said
“Around and around they’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.”
Although this theory was initially discovered by Jacob van Stockum and Kornel Lanczos in the 1930s, it was unrecognized in the scientific world until the 1970s, when astronomer Frank Tipler analyzed this theory. Tipler proposed that in spacetime there is a massive and endless long cylinder with a mass 10 times that of the sun, spinning on its axis.
Spinning furiously at a speed of several billion turns per minute can lead to a frame-dragging effect. If a nearby spaceship makes precise revolutions around the cylinder, it could possibly get it into a “closed time-like curve”.
It’s a basic understanding that time traveling requires a special device, which we call time machine. According to physics, a time machine needs to bend space-time in order to shorten the distance between two points in space. Currently, physicists believe that one phenomenon essential for us to travel back in time, in which timelines move in loops, is “closed time-like curve.”
For time machines, to be able to achieve this state, it is believed that they need an “exotic matter” with negative energy density. Theoretically, these exotic matters exist and most probably they have some strange and unique properties which are not seen anywhere yet. But various simulations convey that the quantity at which these negative matters exists is very small and are not adequate for building a time machine.
So is time travel possible?
Two-dimensional space depicted in three-dimensional spacetime /Image Courtesy: K. Aainsqats
Based on the above arguments, the time travel doesn’t appear to be possible. But with constantly evolving field of physics who knows what might happen in the near future. Especially, the major advances in quantum fields could perhaps provide some understanding of how to overcome time travel paradoxes.