Time travel ,possible or not?
Time travel
Time travel’s been one of mankind’s fantasy for centuries. You might have watched time travel and time machines in sci-fi movies like “Doctor Who” ,“Star Trek” ,“Back to future” , “Planet of Apes” and so on. Recently released interstellar also screened time travel to the future through a wormhole. The reality, however ,is more muddled. Not all scientists believe that time travel is possible.
Time travel is the concept of movement of a body between different points in time either to the past or to the future. Let us check possibility of different time travel theories. According to Einstein’s theories travelling to future is simple and travelling to past is nearly impossible. But there are some other theories which support backward time travel .Wormhole theory is the most prominent among them. Einstein’s relativity theories are the base of all time travel theories. So let us see what relativity theory is .
Einstein’s theory of relativity
The theory of relativity usually encompasses two theories by Albert Einstein: special relativity and general relativity.
Special Relativity theory
Special relativity theory was introduced in Einstein's 1905 paper "On the Electrodynamics of Moving Bodies". The special theory of relativity was based on two main postulates:
- The laws of physics are invariant (i.e. identical) in all inertial systems (non-accelerating frames of reference).
- That the speed of light in a vacuum is the same for all observers, regardless of the motion of the light source.
According to Einstein’s special relativity theory time must change according to the speed of a moving object relative to the frame of reference of an observer. However, the special theory of relativity only held true in the absence of a gravitational field. In this theory, Einstein showed that time and length are not as absolute as everyday experience would suggest: Moving clocks run slower, and moving objects are shorter. In other words ,as the speed increases time slows down and moving object becomes shorter. This is hard to digest ,because you can’t experience this effects in our surroundings. Here everything moves much slower when compared to light.
Just imagine you are in free space. No earth , no sun , no solar system , no galaxy nothing, only empty space. Put yourself in a spaceship and your friend in another one.Let us name your ship as S1 and your friend’s ship as S2. Imagine S1 is stationary and S2 is moving at half the speed of light(c/2) .In everyday life we add or subtract velocities to get relative speed. For light, it would be natural to expect that one could similarly add and subtract velocities. Suppose you (in S1)measure a particular light signal's speed, and find the usual value of 299,792.458 kilometers (186,000 miles) per second. If you see S2 moving at half the speed of light (c/2), you would expect that your friend on that spaceship would measure the speed of your light signal at merely c - c/2 = c/2, half the value that you measured. Not so, according to special relativity! Simply subtracting speeds would only give the correct answer if the observer on that space-ship measured space and time, distance and duration in the same way that you do. As we have seen above in special relativity section, that's not the case. From your point of view, for instance, the measuring rods on the speeding spaceship are shorter than your own, and its clocks run more slowly than your’s. Taken together, all of these relativistic effects combine in precisely the right way to result in a surprising phenomenon: Even from the point of view of an observer on the speeding spaceship, your light signal moves with exactly the same speed, c=299,792.458 kilometers per second.In simple words when both observers calculate speed of light using the formula “Speed=distance/time” the answers will be same.
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Two main consequences of Special relativity are:
- Time dilation : Clocks run slower for moving objects.
- Length contraction: Moving objects are shorter for a stationary observer.
General Relativity Theory
Since the special theory of relativity only held true in the absence of a gravitational field, Einstein strove for 11 more years to work gravity into his equations and discover how relativity might work generally as well. The result was General Relativity Theory .Einstein published his general relativity theory in the year 1915. The development of general relativity began with the equivalence principle, under which the states of accelerated motion and being at rest in a gravitational field (for example, when standing on the surface of the Earth) are physically identical.
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| deformed space due to mass of earth |
General relativity generalizes special relativity and Newton's law of universal gravitation, providing a unified description of gravity as a geometric property of space and time, or spacetime.Einstein considered space and time as a single entity called spacetime instead of considering them seperately. In general theory of relativity, space and time become even more flexible.According to Newton’s law of universal gravitation bodies having mass attract towards each other.This attraction is called gravity.However according to the theory of general relativity, matter causes space to curve. It is posited that gravitation is not a force, as understood by Newtonian physics, but a curved field (an area of space under the influence of a force) in the space-time continuum that is actually created by the presence of mass.
A popular explanation how general relativity works is the rubber sheet analogy. This involves a bowling ball making a depression on a trampoline and a marble either rolling around inside the depression or rolling directly into it. Here the bowling ball represents a planet, the marble a satellite, and the rubber sheet is space being bent through an additional dimension. This is just an analogy used for explaining general relativity in a simple way. In reality general relativity is much more complicated than this.
Three main consequences of general relativity are:
- States of accelerated motion and being at rest in a gravitational field (for example, when standing on the surface of the Earth) are physically identical.
- Clocks run slower in deeper gravitational wells. This is called gravitational time dilation.
- Rays of light bend in the presence of a gravitational field.
Proofs of relativity theory
Countless experiments have been carried out and Einstein’s theories hold true. Modern GPS uses Einstein’s theory of relativity to locate our position accurately. Clocks on GPS satellites are adjusted based on the predictions of special and general relativity theories.
In 1971 an experiment was conducted with four super accurate atomic clocks which were set to exact same time as reference clock. The clocks were placed aboard commercial flights and planes flew twice around the world. First eastward and the westward. When the clocks were returned and when compared with reference clock, clocks were no longer synchronized and differences were consistent with the predictions of special and general theory of relativity.
Special Relativity theory and time travel
According to Special Relativity theory time slows down for object travelling close to the speed of light. In other words if you are traveling at a high speed , you are moving forwarding in time.And time stops altogether for objects travelling at speed of light.Then follows ‘time must go backward for objects traveling faster than speed of light’. So far it’s been seen that no object can travel at the speed of light. On approaching speed of light object start to increase mass rather than speed. Look at the Einstein’s equation E=MC^2 .In this equation C=Speed of light remains constant. As the speed of object approaches light speed mass M approaches infinity. So it would require infinite amount of energy to accelerate object from there. In short it would take infinite energy for an object to achieve light speed. This is why we cannot travel back in time. We simply have not been able to reach greater speed than the speed of light. So, according to Einstein’s special relativity time travel to the past is impossible. Are you disappointed? No. There is still another possibility for time travel. The Wormholes.
Wormholes
A wormhole is a hypothetical topological feature that would fundamentally be a shortcut connecting two separate points in spacetime. A wormhole, in theory, might be able to connect extremely far distances such as a billion light years or more, short distances such as a few feet, different universes, and different points in time. A wormhole is much like a tunnel with two ends, each at separate points in spacetime. Spacetime can be viewed as a 2D surface (to simplify understanding) that, when 'folded' over, allows the formation of a wormhole bridge. A wormhole has at least two mouths that are connected to a single throat or tube.In three dimensional space a wormhole will appear spherical to our eyes and you can see through it the other end of wormhole.
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| 2D representation of a wormhole |
According to Einstein and his colleague Nathan Rosen, a wormhole is actually deformed space that has warped in such a way to connect two different points in space-time. The result is a tunnel-like structure that could be straight or curved, linking two areas of the Universe that are incredibly far apart. Einsteinian mathematical models predict that wormholes exist, but none have ever been found. So far, physicists haven’t determined a way in which wormholes would form naturally in the Universe.In 1962 two physicist John A. Wheeler and Robert W. Fuller published a paper showing that this type of wormhole is unstable, and that it will pinch off instantly as soon as it forms, preventing even light from making it through.
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| 3D illustration of a wormhole |
Even if wormholes exist they are too tiny for a human or spaceship to fly through. In 1988 noted theoretical physicist Kip Thorne and his graduate student Mike Morris demonstrated the possibility of traversable wormholes in general relativity. The type of traversable wormhole they proposed, held open by exotic matter. Exotic matter is a matter that has negative energy density and/or negative pressure. Negative properties of exotic matter might push the sides of a wormhole outward, making it large enough—and stable enough—for a person or a spaceship to fly through it. But exotic matter exists only in theory, we don't know what it looks like, and we have yet to know where to find it.
Wormholes and time travel
The theory of general relativity predicts that if traversable wormholes exist, they can also alter the speed of time. They could allow time travel.This would be accomplished by accelerating one end of the wormhole to a high velocity relative to the other, and then sometime later bringing it back. relativistic time dilation would result in the accelerated wormhole mouth aging less than the stationary one as seen by an external observer. However, time connects differently through the wormhole than outside it, so that synchronized clocks at each mouth will remain synchronized to someone traveling through the wormhole itself, no matter how the mouths move around. This means that anything which entered the accelerated wormhole mouth would exit the stationary one at a point in time prior to its entry.
For example, consider two clocks at both mouths both showing the date as 2010. After being taken on a trip at relativistic velocities, the accelerated mouth is brought back to the same region as the stationary mouth with the accelerated mouth's clock reading 2012 while the stationary mouth's clock read 2020. A traveler who entered the accelerated mouth at this moment would exit the stationary mouth when its clock also read 2012, in the same region but now eight years in the past. Theoretically you can convert a wormhole into a time machine. But you need to solve three problems 1. Existence of wormholes 2.Stabilization of worm holes using exotic matter and 3.Existence of exotic matter. Many more traversable worm holes have been proposed. But they also exist theoretically.
Conclusion
According to Einstein’s special relativity theory nothing can travel faster than light. To go backward in time ,faster than light travel is the only way. Faster than light travel can be achieved if we find out some shortcuts through space-time. Wormhole is one such shortcut which exist theoretically . So, for coming hundreds or thousands of years time travel will remain fantasy only. Let us wish after thousands of years our descendants will solve this mystery and come and visit us from future .
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