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= Evolution of time =
Until about 1840, time was kept simply through use of sundials, or simply by determining the position of the sun in the sky<ref name=":0">{{Cite web|url=http://www.bbc.com/news/magazine-29476893http://www.bbc.com/news/magazine-29476893|title=Caesium: A Brief History of Timekeeping|last=Rowlatt|first=Justin|date=2014-10-04|website=BBC News|publisher=|access-date=}}</ref>. It was not until the use of railroads that time become more important in terms of synchronization across great distances. The faster the trains travelled, the more important accuracy of time became. It wasn’t until 1930 that the first quartz ring clock was developed, which uses the resonant frequency of quartz to keep time. The first caesium atomic clock was developed in 1955, which is far more accurate that using quartz to keep time as caesium has a much higher resonant frequency. Without caesium clocks, for example, GPS would be impossible as it involves use of synchronized caesium clocks to triangulate positions<ref name=":0" />.

In 1967, the second became defined as the duration of 9,192,631,770 periods of the radiation resulting from the transition between the levels of the ground state of the caesium-133 atom, meaning that the sun was no longer referenced at all when figuring out the time of day<ref name=":0" />. The switch to this atomic time has a lot of benefits, as the rotation of the Earth is not the most reliable way to measure time. This is because there are various factors that affect the earth’s rotation, and thus how it would be timed, such as changing tides and tectonic drift. Modern time is kept using Coordinated Universal Time (UTC), which was adopted in the 1960’s<ref name=":0" />.
[[File:Atomic Clock-Louis Essen.jpg|thumb|274x274px|The world's first atomic clock]]
However, in order for sailors to properly keep time, the UTC needed to be synchronized with the earth’s rotation<ref name=":0" />. This introduced the new problem of ‘leap seconds’, which is the necessity of adding a second every so often in order to keep the UTC synchronized with the earth’s rotation. The necessity of leap seconds in this system potentially causes many problems, and in response to these problems comes a push for purely atomic timekeeping. However, since the earth’s spin is slowing, switching to purely atomic time would result in timekeeping that slowly diverges from the time of sunrise and sunset<ref name=":0" />.

In 1971, caesium clocks were used to test Einstein’s theory of relativity, and through use of planes, showed that the speed you are moving at affects how quickly time passes (the faster you move, the more time for you slows)<ref name=":0" />. This could potentially result in going forward in time, if you move quickly enough. However, moving backward in time is currently thought to be impossible. Though this may be true, there is still a way to look backward in time – by looking into space<ref name=":1">{{Cite web|url=http://curious.astro.cornell.edu/about-us/133-physics/general-physics/general-questions/835-why-is-looking-out-into-space-the-same-as-looking-back-in-time-beginner|title=Why is looking out into space the same as looking back in time? (Beginner) - Curious About Astronomy? Ask an Astronomer|last=Masters|first=Karen|website=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

2.     The speed of light in a vacuum is independent of the motion of all observers<ref name=":2">{{Cite web|url=https://www.phy.duke.edu/~rgb/Class/phy319/phy319/node122.html|title=Einstein's Postulates|website=www.phy.duke.edu|access-date=2016-03-14}}</ref>. 

The implications of this are that nothing can travel faster than the speed of light, and the passage of time is different for people travelling at different speeds<ref name=":2" />. According to this theory, if you were in a speeding rocket ship, the passage of time inside that rocket would have to slow down with respect to someone on Earth. Time moves at different rates depending on how fast you move<ref name=":2" />. In fact, if the rocker were traveling at the speed of light, time would apparently stop inside the rocket.

Einstein’s later theory, General Relativity, portrays time as a dimension, just like the three dimensions of space, and combines them all into something called space-time<ref name=":3">{{Cite web|url=http://gizmodo.com/yes-time-travel-is-possible-heres-how-1654958816|title=Yes, Time Travel Is Possible; Here's How|last=Estes|first=Adam Clark|website=Gizmodo|language=en-US|access-date=2016-03-14}}</ref>. Anything with mass or energy warps space-time, creating a gravitational field. Such a field produces a gravitational pull on any matter that traveling through it. The implication of Einstein’s general theory is that space-time is like a fabric that can stretch and shrink<ref name=":3" />. If you were to stretch the space behind you and contract the space in front of you, then you would have the illusion of having moved faster than light. In fact, you would not have moved at all, space has just been deformed<ref name=":3" />.

In his later years, Einstein concluded in his book, Relativity:

“ Since there exists in this four dimensional structure [space-time] no longer any sections which represent "now" objectively, the concepts of happening and becoming are indeed not completely suspended, but yet complicated.”<ref name=":3" />

Einstein claimed that the past, present, and future all exist simultaneously. Because 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 forward. This direction of time is called the “arrow of time”, evidence suggest that this arrow of time can is attributed to the tendency of the universe to move from an ordered state to a disordered state.<ref name=":4" /> A useful measure of order is the entropy of the system, which can be represented mathematically by an equation derived by Ludwig Boltzmann S=k*log(W).<ref name=":4" /> Entropy measures the randomness within a given system. Moreover, there is a tendency for the universe to move from an ordered state to a state of disorder.<ref name=":4" /> Thus, order states become unordered states, which provides a direction of time as this only occurs in one direction. However, before using entropy to explain the arrow of time as we experience it one must understand the beginning of the universe.<ref name=":4" />
[[File:CMB Timeline300 no WMAP.jpg|thumb|Image shows universe expansion where at the beginning during the Big Bang there was low entropy and a state of order, and in the continual expansion, the entropy (disorder) increases.]]
Evidence suggests that the universe began with the big bang, which is regarded as the most highly ordered state in the universe. Following this event the universe has began to expand and the entropy has evidently increased.<ref name=":4" /> This increase in entropy from the big bang results is the arrow of time, as the universe is constantly moving from a highly ordered state to a state of high disorder.[5] Thus, according to this understanding, the big bang is what set the arrow of time in motion, providing a direction to time.<ref name=":4" /> Therefore, occurrences such as the formation of stars to the formation of black holes are all events leading to an increase in disorder set forth by the big bang.
 
Nevertheless, an important question arises. If the universe had a beginning of time, will it have an end? It is theorized that eventually given enough time only black holes will be present within our universe as no more material is present to supply the birth of new stars.<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<ref name=":6">{{Citation|last=Hershell Mery|title=Through The Wormhole - Is Time Travel Possible?|date=2015-03-10|url=https://www.youtube.com/watch?v=tQ9FRiBT5WI|accessdate=2016-03-14}}</ref>.
[[File:Wormhole-demo.png|thumb|317x317px|2-D curved space-time with a 3-D tunnel connecting connecting different points, known as a wormhole.]]
If you take space-time being two-dimensional, a wormhole would appear as a tunnel connecting two points on this two-dimensional plane<ref name=":5" />. A visual representation of this can be demonstrated by taking a piece of paper, bending it in half and sticking a pencil through it so the pencil sticks out of both sides. The pencil in this case is the wormhole, a passage connecting two points in space-time. The distance travelled along the pencil would be significantly short than through through the space between the two points. 

If they exist, wormholes would be smaller than the size of an atom and in order to go through them, the worm holes must be enlarged and stabilized and a special kind of energy known as negative energy will be required to do this. Negative energy is anti-gravity meaning that it repels space-time. This kind of energy would prevent the wormhole from closing on itself<ref name=":6" />.
 
Theories of time travel can be applied to wormholes. One end of the wormhole (the mouth) would need to be accelerated to relativistic velocities while the other mouth remains stationary. This results in the accelerated mouth of the wormhole not aging as quickly as the stationary mouth, leading to a dilation of time to an external observer<ref name=":5" />. This implies that if someone were to enter the wormhole and come back through it, much less time would have passed for them than an external observer<ref name=":5" />.
 
The theory of wormholes can challenge the conventional understanding of how time works. 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<ref name=":5" />. This means that anything which entered the accelerated wormhole mouth would exit the stationary one at a 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" />.
[[File:Time travel hypothesis using cosmic strings.jpg|thumb|Time travel hypothesis using cosmic strings]]
“These interaction could create fields of closed time-like curves permitting backwards time travel” <ref name=":7" />. Physicists have suggested using cosmic strings to “construct” time machines. By manipulating two strings, it is theoretically possible to create closed time-like curves <ref name=":7" />. In theory if one fires two cosmic strings at very high speeds past each other and a space ship were flown in a carefully calculated figure eight, one would be able to emerge at any point in space and time <ref name=":7" />.

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 =