It is uncertain if time travel to the past is physically possible, and such travel, if at all feasible, may give rise to questions of causality. Forward time travel, outside the usual sense of the perception of time, is an extensively observed phenomenon and well-understood within the framework of special relativity and general relativity.
Time seems to follow a universal, ticktock rhythm. But it doesn't.
In the Special Theory of Relativity, Einstein determined that time is relative—in other words, the rate at which time passes depends on your frame of reference. Just as observers in two different frames of reference don't always agree on how to describe the motion of a bouncing ball, they also don't always agree on when an event happened or how long it took. A second in one reference frame may be longer compared to a second in another reference frame.
Albert Einstein's idea was that, theoretically, the closer we come to traveling at the speed of light (186,000 miles per second), the more time would appear to slow down for us from the perspective of someone who, in relation to us, was not moving. He called the slowing of time due to motion time dilation.
Imagine you're standing on Earth holding a clock. Your friend is in a rocket zooming past you at nearly 186,000 miles per second. Your friend is also holding a clock. If you could see your friend's clock, you'd notice that it seems to be moving a lot more slowly than yours. Your friend, on the other hand, thinks the clock in the rocket is moving just fine, while your clock on the ground seems to be moving very fast. Sound confusing? Well, remember, it took Einstein years to figure this out, and he was pretty smart.
Now let's talk about paradox. What is this paradox and what's its use in time travel?
Paradox is a situation or statement with two or more parts that seem strange or impossible together.
A paradox is a statement that contradicts itself. For example, the statement “I am not lying” is a paradox because it means one cannot be telling the truth while saying they are not lying.
The Grandfather Paradox is a paradox that has been pondered for decades. The story goes like this: what if you went back in time and killed your grandfather, before he had children with his wife.If the person who killed their grandfather was not around when he had children, then how can they exist now? But yet the person exists because they are going back in time and killing themselves!
This paradox is impossible to solve but it does lead us to wonder about free will.
THE TWIN PARADOX
Al and Bert, both of whom are 10 years old in their highly futuristic universe.
Al's parents decide to send him to summer camp in the Alpha-3 star system, which is 25 light-years away (a light-year is the distance light travels in a year). Bert doesn't want to go and stays home on Earth. So Al sets out on his own. Wanting him to get there as quickly as possible, his parents pay extra and send him at 99.99 percent the speed of light.
The trip to the star and back takes 50 years. What happens when Al returns? His twin brother is now 60 years old, but Al is only 10 and a half. How can this be? Al was away for 50 years but only aged by half a year. Has Al just discovered the fountain of youth?
Not at all. Al's trip into space lasted only a half year for him, but on Earth 50 years passed. Does this mean that Al can live forever? Nope. He may have aged by only half a year in the time it took 50 years to pass on Earth, but he also only lived half a year. And since time can slow down but never goes backwards, there's no way he could grow younger.
Is time travel possible? Short answer: Yes.
There are some ways that explain this:
1. Time travel via speed
This is the easiest and most practical way to time travel into the far future – go really fast.
According to Einstein’s theory of special relativity, when you travel at speeds approaching the speed of light, time slows down for you relative to the outside world.
2. Time travel via gravity
The next method of time travel is also inspired by Einstein. According to his theory of general relativity, the stronger the gravity you feel, the slower time moves.
As you get closer to the centre of the Earth, for example, the strength of gravity increases. Time runs slower for your feet than your head.
3. Time travel via suspended animation
Another way to time travel to the future may be to slow your perception of time by slowing down, or stopping, your bodily processes and then restarting them later.
Bacterial spores can live for millions of years in a state of suspended animation, until the right conditions of temperature, moisture, food kick start their metabolisms again. Some mammals, such as bears and squirrels, can slow down their metabolism during hibernation, dramatically reducing their cells’ requirement for food and oxygen.
4. Time travel via wormholes
General relativity also allows for the possibility for shortcuts through spacetime, known as wormholes, which might be able to bridge distances of a billion light years or more, or different points in time.
Many physicists, including Stephen Hawking, believe wormholes are constantly popping in and out of existence at the quantum scale, far smaller than atoms. The trick would be to capture one, and inflate it to human scales – a feat that would require a huge amount of energy, but which might just be possible, in theory.
5. Time travel using light
Another time travel idea, put forward by the American physicist Ron Mallet, is to use a rotating cylinder of light to twist spacetime. Anything dropped inside the swirling cylinder could theoretically be dragged around in space and in time, in a similar way to how a bubble runs around on top your coffee after you swirl it with a spoon.
According to Mallet, the right geometry could lead to time travel into either the past and the future.
Whether humans achieve the power to travel through time or not, but one thing is for sure, the research on time will surely take us into an unexpected future.
