Before discussing time travel to the past, it is essential to understand some key concepts. The nature of time is mysterious and not easy to grasp. We cannot directly sense time, but we experience is as it progresses. A stationary event is considered timeless, as time is the succession events. Wile travelling into the future is relatively less complicated, travelling back in time presents more challenges. Einstein’s theory of relativity provides a theoretical basis for moving forwards or backward in time, but it requires a time machine to make is possible. Currently, its considered impossible to travel back in time because we do not have the technology or machine capable of doing so, even though the theory suggests it could be feasible.
A Simple Explanation of Time
Everyone is acquainted to time. But it is not easy to define and understand time. As time has different definitions according to science, philosophy, religion, and arts, but most reliable is system of measuring it. Seconds, minutes, and hours provide base for clock. While the basis for the units of time evolved over time, from ancient Sumerian to present SI unit. The SI unit or modern unit of time is second. The electronic transition of the caesium atom defines the second. But still, need to understand exactly that what is time?
Definition of Time
According to Physics the succession of events from the past to the present and also into future. A system is timeless, if it is unchanging. Time is used to describe events in three-dimensional space, and fourth dimension of reality is time. We cannot see, touch, or taste the time, but it just happens. So, we are only able to measure the passage of time.
The Arrow of Time
There are two possibilities for time travel, positive time (time is moving forward into the future) and negative time (time is moving backward into past). However, there is only one direction is allowed for time in natural world, is known as the arrow of time. The biggest unresolved question in science is that why time is irreversible? The one of the best explanations is that the laws of thermodynamics are followed by our natural world. The statement of second law of thermodynamics says that entropy of an isolated system is always increasing or constant sometimes. If we consider our universe as an isolated system then its entropy (degree of disorder) is always in increasing direction and it can never be decreasing. It means that our universe can never reverse to its initial state exactly. So, its impossible to move backward into time frame.
Time Dilation
Time is same everywhere, in classical mechanics synchronized clocks preserve harmony. But Einstein’s theory of relativity says that time is a relative quantity because it dependent of an observer’s frame of reference. Due to relativity time is dilated, when observer moves with a high speed, closer to speed or light then time interval between events becomes longer. The clocks slowed down than the stationary clocks e.g., when a clock is placed in a jet or in an orbit, the time is recorded more slowly than the clocks on earth. When is falling, it decays more slowly as length contraction and time dilation is confirmed by Michelson-Morley experiment?
Time Travel
Time travel is defined as forward or backward motion into different points in time. In nature, jumping forward in time is occurring. When an Astronaut comes back on earth, jump forward in time because of its slower movement relative to the space. But travelling back in time, is not an easy task to achieve. It possesses many problems with it. Causality is the first important issue. A temporal paradox is caused by moving back in time and a classic example of problems with time travel back, is grandfather paradox. This paradox says that if a person travel back in time, can prevent his own birth by killing his grandfather before his parents were born. Its not possible to travel back in time, as believed by many physicists. But temporal paradox can solve also, like travelling between parallel universes or branch points.
Is Time Travel Really Possible? How Physics Explores?
Is Time Travel Possible?
The ability of time travelling into future and past, has been fascinating the physicists and science fiction writers for centuries. That’s why, is it possible to move backward in past and forward into future practically? To answer this fascinating question, we take the example of Dr Who, a fictional story’s character. Doctor had an advanced craft or time machine and could travel in past and also into future. By his travelling in time, had explained the temptations and paradoxes. But this provides no resemblance of tradis with real- world Physics.
But how can we relate it with real world. Are we able to build a time machine by which we could travel in time? We can answer this question after understanding the method of working of time. After the keen study of how actually time works? we can say that travelling in future is not as much problematic as traveling into past. So, till now there is no possibility in real world, to travel in past.
Einstein’s Theory
According to Einstein’s theory of relativity, flow of time is not steady but changes with motion of frame of reference. The theory of relativity provides evidence of time travelling into future or past. As this theory says that moving clocks runs slowly. One of the examples of time travelling is twin paradox, in which ageing of a twin in air craft moving (moving with very large speed, closer to speed of light) is slower than a twin on earth. When he come back on earth then he will be younger than his twin brother on earth. And gravity also effect the time flow, as time is slowed down near intense gravitational field e.g., near a black hole and vice versa. That’s why ageing of head is faster than feet because gravity is greater for feet. Because time can be compressed due to gravity or when moving relative the speed of light. It means that no time machine is needed to travel into future but very high speed as closer to speed of light or intense gravitational field. In both conditions, time will be compressed. In contrast, travelling back in past is very harder. So, for travelling back in time, we need a time machine always.
Time Machine
It is a hypothetical machine which works as a vehicle to put forward a cautionary vision of future of society. Travelling in time is same as in other three space dimensions except that along time, our consciousness moves also.
Tradis
We can move in time or in any dimension with a time machine called tardis. This time appears as smaller at out side but it’s bigger on the inside. Anyone can travel in time with tardis. It is appeared as influential. Because of Inadequate concealment, it was bogged down in the shade of an old British police box. The other name of camouflage function was chameleon circuit, to signal each other chameleon changes colour. But Einstein’s theory of relativity says that this craft can function only if it has ability to move with a peed, larger than speed of light. Due to this very larger speed requirement for functioning of tardis, we are not able to travel back in time. Because we have no such system to move a body with speed greater than speed of light, which starts with a speed less than speed of light. According to relativity not only time is dilated but mass also increases at speeds relative to speed of light.  So, it is impossible to travel back in dimensions of time.
Wormholes
The travelling back in time is theoretical possible, we will able to fold space-time as a piece of paper to make tunnel to be punched through and a shortcut is created between two widely separated points. These wormholes are possible in general relative theory. Most important problem is that wormhole has no evidence of its presence but they can exist mathematically. But these mathematical wormholes have very short lives. Mostly two black holes are joined together to produce a wormhole. That’s why, a incredibly intense gravitational field is associated with wormholes and would collapse under their own gravity. This size of wormhole is also too tiny to carry a person, or even a bacterium in it. These problems can be solved by negative energy, which not possible till now.
Problem Associated with Travelling Back in Time
If in future, we will succeed in making such time machines, as predicted by theories to move back in time. Then it arises many problems like:
- Collision with past thing
- Grandfather paradox
- Variation in absolute position
- Spatial extension of personal time
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Conclusion
Mystery of time is not an easy task to understand. The progression of events between past and future, is time. Time is considered fourth dimension and expressed in seconds. In our natural world time flow has only forward direction but we can to move back in past also because Einstein’s theory of relativity states that moving clocks run slower. The time travelling in future is less problematic than travelling back in past. It is impossible to move backward in dimensions of time till now. But theories provide us a ray of hope to travel into future or back to past. We can travel back in dimensions of time through a time machine like tardis or wormholes. But there are also many problems associated to travelling back in time. We can travel back in time only if we device a craft with a speed larger than speed of light. We not able to speed up a body, even a grain of sand, to achieve the time travelling. It is hoped that in future, we will succeed in making such time machines to travel in time without any problem and restriction.
By
Zeenat Mushtaque, Master of philosophy in Solid State Physics
Dr. Abid Hussain Nawaz, Ph.D. & Post Doc
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