Possibility of Time Travel

                                               Is Time Travel Possible ?

Concept of Entropy 

Entropy is a scientific concept , as well as  a measurable physical property that is most commonly associated with a state of disorder , randomness , or uncertainty. The term and the concept are used in diverse field , from classical thermodynamics , where it was first recognized, to the microscopic description of nature in statistical physics , and to the principals of information theory . Entropy was defined in the mid-nineteenth century by German physicist Rudolph Clausius , one of the founders of the field of thermodynamics . He formulated it as the quotient of an amount of heat to the instantaneous temperature, in the dissipative use of energy during a transformation . Entropy predicts that certain processes are irreversible or impossible , aside from the requirement of not violating the conservation of energy , the latter being expressed in the first law of thermodynamics . Entropy is central to the second law of thermodynamics , which states that the entropy of isolated system left to spontaneous evolution cannot decrease with time , as they always arrive at a state of thermodynamics equilibrium , where the entropy is highest . 

   
                                      

Second law of Thermodynamics - The second law of thermodynamics requires that , in general , the total entropy of any system does not decrease other than by increasing the entropy of some other system. Hence , in a system isolated from  its environment , the entropy of that system tends not to decrease . It follows that heat cannot flow from a colder body to a hotter body without the application of work to the colder body. It follows from the second law of thermodynamics that the entropy of a system that is isolated may decrease .

Time Travel and the violation caused to the Second law of Thermodynamics 

Time travel is the concept of movement between certain points in time , analogous to movement between different points in space by an object or a person , typically with the use of a hypothetical device known as a time machine. It is uncertain if time travel to the past is physically possible. It is uncertain if time travel to the past is physically possible . The concept of forward time travel , outside the sense of the perception of time , is an extensively observed phenomenon and well understood within the framework of special relativity and general relativity. 

  

 

Wormholes

 

These are hypothetical warped spacetime permitted by the Einstein field equations of  general relativity . It works as to take , one entrance of the wormhole and move it within the gravitational field of an object that has higher gravity than the other entrance , and than return it to the position of the other entrance. The dilation causes the end of the wormhole that has been moved to have aged less , or become younger than the stationary end as seen by an external observer , however there is a difference in time connection inside the wormhole than outside it , so that synchronized clocks at either end of the wormhole will always remain synchronized as seen by an observer passing through the wormhole , no matter how the two ends move around. The current theories on  nature of wormholes suggest that  , construction  of a traversable wormhole would require the existence of a substance with negative energy , often referred to as exotic matter. 

Violation of Second Law of thermodynamics 

Entropy is a measure of the disorder of a system . It also describes how much energy is not available to do work . The more disordered a system  the higher the entropy, the less of a system's energy is available to do work.

In context of physics and astronomy after the Big Bang the universe is continuously  expanding and not contracting so physicist and astronomer define time as change in space, according to fundamental physics one second means that universe has expanded in space in comparison to the space it occupied one second earlier and since the energy in universe is quantized and as well as the number of elementary particles in the universe is same irrespective of its expansion.

 The second law of thermodynamics states that every change increases overall entropy of our universe. The more entropy defines as a  state with more possible ways of creating it. For traveling back in time we need to recreate a  condition of less entropy. Its not possible to create a state with less  possible ways of existing, which means entropy cannot be decreased . 

Theory of relativity 

It usually encompasses two interrelated theories by Albert Einstein which are special relativity and general relativity.

Special relativity 

It is a scientific theory regarding the relationship between space and time. This theory is based on two postulates. 

1.  The laws of physics are identical in all inertial frames of reference with no acceleration .
2. The speed of light is same in vacuum for all observers , regardless of the motion of the light source or observer. 

     This theory is proven to be the most accurate model of motion at any speed when gravitational and quantum effects are negligible. 

General relativity 

 

This theory describes the relation between the geometry of a four dimensional pseudo - Riemannian manifold representing spacetime. IF this theory is considered to be one of the two pillars of the modern physics , then quantum theory the basis of understanding matter from elementary particles to solid state physics , would be the other. 

Time travel to the past is theoretically possible in certain general relativity spacetime  geometries that permit traveling faster than the speed of light , such as cosmic strings , traversable wormholes, and Aluccbierre drives. The general relativity theory describes universe under a system of field equation that determine the metric , or distance function , of spacetime . 

Time Dilation 

It is the difference in the elapsed time as measured by two clocks . This is either due to a relative velocity between them or to a difference in gravitational potential between their  locations. Time dilation usually refers to the effect due to velocity. The observer will  measure the moving clock as ticking slower than a clock that is at rest in the observers own reference frame . In addition , a clock closer to a massive body ( which is at lower gravitational potential ) will record less elapsed time than a clock situated further from the massive body ( which is at the higher gravitational potential ). 

Gravitational Time Dilation 

It is a form of time dilation where there is an actual difference of elapsed time between two events as measured by observers situated at varying distances from a gravitating mass. At lower gravitational potential the clock is closer to the source of gravitation , the slower the time passes speeding up as the gravitational potential increases ( the clock getting away from the source of gravitation).  It was first described by Albert Einstein in 1907 . General relativity describes that inertial mass and gravitational mass are the same , and all accelerated reference frames ( such as uniformly rotated reference frame with its proper time dilation are physically equivalent to a gravitational field of same strength . 

 Black Hole's ability to bend light 

Black Hole is a region of spacetime where gravity is so strong that nothing, no particle or even electromagnetic radiation such as light can escape from it. General relativity theory predicts that a sufficiently compact mass can deform spacetime to form a black hole. Light rays passing close to the black hole get caught and cannot escape. The region around the black hole is a dark disk known as event horizon . Light rays that pass a little further away don't get caught but do get bent by the black holes gravity. Therefore you would see two duplicate of the same star on opposite sides of the black hole, because the light passing the either side of black hole get bend towards you.

Time dilation near black hole

For a distant observer , clocks near a black hole would appear to tick more slowly than those further away from the black hole. Which is due to effect known as gravitational time dilation
, an object falling into the black hole appears to slow as it approaches the event horizon . 

Concept of space time in general relativity 

According to general relativity space time is not flat but is curved by the presence of massive bodies . The spacetime curvature influences the motion of massive bodies within it , in turn, as massive bodies move in spacetime  the curvature changes and the geometry of the spacetime is in constant change in form . Then gravity provides a description of the dynamic interaction between matter and spacetime. 

 Time being directional 

Arrow of time , also known as time's arrow is the concept of positing the one way direction or asymmetry of time . The microscopic physical processes are believed to be entirely time symmetric. If direction of time were to reverse , the theoretical statement that describes them would remain true. Yet at the microscopic level it often appears that this is not the case there is an obvious direction or flow of time. The second law of thermodynamics states that in an isolated system entropy tends to increase with time . Also , the entropy can decrease with time in an open system. 

Gravity Being Fabric of Spacetime 

Spacetime is a mathematical model that fuses the three dimensional of space and one dimension of time into a single four dimensional manifold . Matter tells space how to curve and curved space tells matter how to move . Its the basic principle behind Einstein  General Relativity , which linked ,for the first time , the phenomenon of gravity with that of spacetime and relativity. 

Stephen Hawking's opinion on Time Traveler 

   

 Our subjective sense of time seems to flow in one direction , which is remember the past not future . Hawking states that our brain measures time in a way where disorder increases in the direction of time , we never observe it working in the opposite direction .  

    Hawking famously held the party for the time travelers but didn't send out the invitations until the party was over. No one showed up in the party . But Hawking's states the their is still some hope that traveling back in time could be possible according to the laws of universe . He suggest this notion on behalf of the something called  "M theory" that suggests the universe may contain seven hidden dimensions in addition to the four dimensions of space- time. 

                   Concept of Quantum Alternate Realities 

The many-worlds interpretation is (MWI) is an interpretation of quantum mechanics that asserts that the universal wavefunction is objectively real , and that there is no wavefunction collapse . This implies that all possible outcomes of quantum measurements are physically realized in some world or universe. 

   The American theoretical physicist Brian Greene says that our  universe might be really , really big but finite . Or it might be infinitely big . Brain says that there are possibilities , that there are so many ways that matter can arrange itself within that infinite universe. And if the universe is infinitely large , it is also home to infinite parallel universes. 

     

Time Travel paradoxes

•  Predestination paradox 
• Grandfather paradox 

   

Predestination paradox 

  It is also known as casual loop is a paradox of time travel . It exists when a time traveler is caught in a loop of events that " predestines" them traveling back in time . As consist  possibility of influencing the past while time traveling , one way of explaining why history does not change is by saying that whatever has happened must happen. The time traveler trying to alter the course of history in this model would only be playing their part in shaping the history as we already know it , rather than changing any aspect of the past. 

Grandfather paradox 

It is a time travel paradox in which inconsistencies emerge through changing the past. This paradox describes that a person travels to the past and kills their own grandfather before the conception of their father or mother , which prevents the time traveller's existence. Rather than this the paradox regards any action that alters the past , since there is a contraction whenever the past becomes different from the ways it was.