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(contracted; show full)ite=curious.astro.cornell.edu|access-date=2016-03-14}}</ref>. This is because the stars closest to earth are many light-years away, therefore meaning that the light emitted by them does not reach earth for years. This would mean that we are seeing what the stars looked like in the past – sometimes several hundred years ago. It is therefore possible that the star we are seeing has already died, but we are still receiving its light emission, thus looking back in time<ref name=":1" />.


== Einstein’s Postulates on Time ==
Einstein’s Theories of Relativity are at the root of time travel and the perception of time.  

The first theory, Special Relativity, postulates that:  

1.     The laws of physics are the same for all non-accelerating observers

(contracted; show full)ause motion effects the passage of time, someone who is moving will have a different conception of what is happening right “now”<ref name=":3" />. In this sense, the past is not gone, and the future isn’t non-existent. Einstein claims that the distinction between the past, present, and future is only an illusion, however persistent<ref name=":3" />. Yet if every moment in time already exists, how do we explain the naturally unfolding of time from the present to the future?


== Directionality of time ==
Interestingly, physics equations that we utilize show no preference for a directionality of time. In other words, the equations are equally as valid if computed using a backwards sequence of time.<ref name=":4">{{Cite web|url=http://www.pbs.org/wgbh/nova/physics/fabric-of-cosmos.html|title=The Fabric of the Cosmos|website=www.pbs.org|access-date=2016-03-14}}</ref>However, this is contrary to our everyday experience in which we only see time unfold in one direction, which is in a forwa(contracted; show full).<ref name=":4" /> Following this, these black holes this will decay due to a process known as hawking radiation and leaving behind random particles drifting through space. Once this occurs, there will be no change in entropy or events happening in the universe and thus no passage of time will occur.<ref name=":4" /> As increasing entropy and events are needed for time to occur. At this point, time within our universe will cease to exist.<ref name=":4" />

==
= Difference in now ===
Contrary to our everyday experience the concept about what is happening now can differ depending on the motion of two objects. If one object A is stationary and another object B is in motion away from the stationary object the object in motion experiences time more slowly when compared to the stationary object.<ref name=":4" />

As a result, the coordinate system expressing the current time in the frame of reference of B would be in the past of the coordinate system expressing the current time time in A.<ref name=":4" /> Conversely, if object A was moving towards object B the coordinate system expressing the current time in A would depict the future in B frame of reference.<ref name=":4" /> These changes are minute in our everyday experience, however over vast distances such as those present in the universe immense changes within time frames occur.

== Wormholes ==
A Wormhole is theoretical ‘bridge’ between two separate points in space-time<ref name=":5">{{Cite web|url=http://www.andersoninstitute.com/wormholes.html|title=Wormhole Time Travel|website=www.andersoninstitute.com|access-date=2016-03-14}}</ref>. A wormhole may be able to connect two separate points in space that are extremely far away, or connect two different points in time, theoretically allowing one to travel backwards or forwards in time depending on where the wormhole leads<r(contracted; show full)point in time prior to its entry. These ‘tunnels’ that connect space-time imply that future events and past events do not come and go but rather all exist in the fabric of space-time and the wormholes just simply allow us to travel between them, changing our current forward path of time to something that travels in a different direction, potentially allowing us to experience past, present and future depending on where the wormhole leads<ref name=":5" /><ref name=":6" />.

==
= Cosmic Strings ===
Cosmic strings are spatially one dimensional topological defects remnant from the big bang <ref name=":7">{{Cite web|url=http://www.andersoninstitute.com/cosmic-strings.html|title=Cosmic String Time Travel|website=www.andersoninstitute.com|access-date=2016-03-14}}</ref>. If they exist, the diameter of these strings would be comparable to a proton but they would be highly dense and would exert incredible amount gravitation force <ref name=":7" />.
(contracted; show full)

However, there are a few problems with this theory. When two cosmic strings are pushed together as high speeds, they may become unstable and create a black hole <ref name=":6" />. Since these strings are highly dense, the weight of the loop would be cosmological <ref name=":6" />. It is predicted that in order to go back a year the loop might weight half the mass of our galaxy<ref name=":6" />.


== Experimental Confirmation of Time Dilation ==

=== Hafele-Keating Experiment ===
The Hafele-Keating experiment was one of the most predominant experiments in demonstrating how time dilates as higher velocities. Four atomic clocks were placed on commercial planes and they flew twice around, once east and once west<ref name=":8">{{Cite journal|last=Schlegel|first=Richard|date=1974-03-01|title=Comments on the Hafele-Keating Experiment|url=http://scitation.aip.org/content/aapt/journal/ajp/42/3/10.1119/1.1987645|journal=American Journal of Physics|volume=42|issue=3|pages=183–187|doi=10.1119/1.1987645|issn=0002-9505}}</ref>.The clocks were then compared to ones that had remained stationary on the ground and it was found that the clocks disagreed with one another as the clocks in the planes were slightly behind the ones that remained stationary<ref name=":8" />. This showed that traveling at higher velocities slows down time in that reference frame and is consistent with predictions made by special and general relativity.

=== Muon Particles ===
[[File:Nonsymmetric velocity time dilation.gif|thumb|Visual representation of how a clock moves slower travelling at a great velocity. The clock moving faster in a circle is moving slower through time with respect to the stationary clock.]]
Muons are subatomic particles that have a half life of about 2.2 µs. An experiment was done where muons were allowed to travel close to the speed of light from a height of 1.9km to the ground. This took a time of 6.4 µs which means that due to the half life of the muons, only 13% of the original amount should have remained<ref name=":9">{{Cite web|url=http://www.alternativephysics.org/book/TimeDilationExperiments.htm#%5B8%5D|title=Experimental Evidence for Time Dilation|website=www.alternativephysics.org|access-date=2016-03-14}}</ref>. However, 82% of the concentration remained which means the high velocity of the particles caused the half life to extend showing that the time slows down for the particles at the high speed<ref name=":9" />.

=== Gravitational Effects on Time ===
In an experiment done where atomic clocks were used, raising a clock by just 12 inches showed that the clock ran faster at the slightly higher altitude. This effect is so small that it would add 90 billionths of a second in a 79-year lifespan<ref name=":10">{{Cite web|url=http://www.dailymail.co.uk/sciencetech/article-1314656/Scientists-prove-time-really-does-pass-quicker-higher-altitude.html|title=How to add 90billionths of a second to your life . . .  live in the basement: Scientists prove time really does pass quicker at a higher altitude|website=Mail Online|access-date=2016-03-14}}</ref>, but the effect exists none the less. This shows that the closer you are to the earth’s core, and thus the stronger the gravitational forces, the slower your clock will run<ref name=":10" />. This phenomenon that was proven is known as gravitational time dilation.

== Time Paradox ==
A time paradox is an apparent contradiction that is associated with the notion of time and time travel<ref name=":11">{{Cite web|url=http://www.astronomytrek.com/5-bizarre-paradoxes-of-time-travel-explained/|title=5 Bizarre Paradoxes Of Time Travel Explained|website=Astronomy Trek|language=en-US|access-date=2016-03-14}}</ref>. Time paradoxes fall into two main categories: causal loops and consistency paradoxes<ref name=":11" />.

=== Causal Loop ===
This category of time paradoxes occurs when an event in the future is the cause of an event in the past, which in turn is the cause of the future event<ref name=":12">{{Cite web|url=http://cns-alumni.bu.edu/~slehar/timeparadox/TimeParadox.html|title=The Paradox of Time|website=cns-alumni.bu.edu|access-date=2016-03-14}}</ref>. Both of these events exist in space-time, but their origin cannot be determined. A causal loop can involve information, object, signal, person and an event<ref name=":12" />. Here is an example of a causal loop.

Harry was told a story about how his mother had her necklace stolen by a stranger and as a result ending up meeting Harry’s father. Later in life, Harry builds a time machine and decides he wants to stop the man from stealing his mother’s necklace. Harry travels to the day his mother had her necklace stolen and rides on the subway with her. He ends up on the subway but doesn’t see anyone around, he continues to wait but no one comes. Eventually he begins to think if no one steals the necklace then he may not be born. So, Harry decides to steal the necklace, and he quickly goes back into the time machine and returns to the present<ref name=":12" />.

Causal loops are also referred to as an ontological paradox, bootstrap paradox or the predestination paradox<ref name=":12" />.

=== Consistency Paradox ===
Consistency paradoxes occur when a future event prevents the occurrence of an event in the past what was partly or entirely the cause of the future event, thus preventing the future event from occurring, resulting in a contradiction<ref name=":13">{{Cite web|url=http://www.unshieldedcolliders.net/2011/12/two-kinds-of-time-travel-paradox.html|title=Unshielded Colliders: Two Kinds of Time-Travel Paradox|website=www.unshieldedcolliders.net|access-date=2016-03-14}}</ref>. These kind of paradoxes occur whenever changing the past is possible. 

This category of paradoxes can be illustrated through what is called the grandfather paradox. This paradox was described in the early 1900s and revolves around the idea of preventing your birth by killing your grandparents<ref name=":13" />. The paradox is stated as follows: a time traveller goes into the past and kills his grandfather before his grandfather meets his grandmother. This results in the time traveller never being born<ref name=":13" />. This raises the question: if he was never born then how did he kill his grandfather? The grandfather paradox does not only discuss the impossibility of one’s birth, but it encompasses any action that eliminates the cause of an individual to travel back in time<ref name=":13" />. 

== References ==