The Reality of Time Travel
Time travel has become a widely studied area in theoretical physics.  Numerous
well known people have provided evidence of the existence of time travel.
However, there are considerable difficulties in the process of constructing a
time machine; there is nothing in the laws of physics that outlaws the existence
of time travel.  Although, the theory of relativity may allow for time travel,
it breaks down in intense gravitational fields, where time travel becomes a
serious possibility.  Neither Einsteins theory of relativity nor the quantum
theory is entirely complete by itself alone.  The quantum theory merely explains
the different mathematical constants, particles and equations.  While Einsteins
theory of relativity is not capable of describing the contents of the enormous
black holes produced by the breaking up of the stars, or the way the big bang
created this universe.  The two theories do not complete each other in this four
dimensional space-time, consisting of length, width, height and time as the
fourth dimension.
Surprisingly, if someone was to examine the two theories in a ten-dimensional
perspective of space-time, they would find that the two theories fit together
properly!  This idea is labeled as the superstring theory on account of the
small disks of string vibrating in the ten-dimensional space-time, regarded as
the fundamental machinery of the universe in this theory.  Here, the various
levels of vibrations would match up with all the different kinds of subatomic
particles.  The big bang instituted the four dimensions of this universe while
the other six dimensions have supposedly collapsed into tiny loops which are
smaller than any known particle.  Although these extra dimensions, in form of
loops, are extremely small, one day they could be detected and used to influence
the inter-dimensional regions.  If this could be accomplished, it would be
possible to generate stable wormholes that could transfer macroscopic objects
instantaneously.  It may even be possible to travel to parallel universes that
function with totally different laws in contrast to this universe.
Among all the different ways that a time machine could become a reality, one
that has been studied the most deals with wormholes.  The idea of distorting
space-time to generate wormholes became prevalent.  These shortcuts in
space-time provide an easy solution to the inability of traveling faster than
the speed of light.  Light is the weirdest thing!  It is a wave and a particle,
which can travel around the earth ten times in one second.  Light travels 670
million miles per hour in deep space.  The speed of an individual traveling
through space toward or away from the direction of light does not effect how
they perceive its passage.  The speed of light is the same for everyone.  So if
the speed of light is not changing, something else must be changing, which
happens to be time.  Therefore, an object moving fast enough through space can
alter its passage through time.
Yet another way to deal with the speed problem is by employing warp drives.
These are traveling fragments of space-time.  The idea is somewhat like a moving
sidewalk in an airport.  This is accomplished through increasing the space-time
at the rear of the spaceship, while reducing it before the spaceship.  The
spaceship would travel along with the fragment of space-time, therefore sort of
doubling its speed.  Together with the fragment of space-time, the spaceships
evident speed would exceed that of the speed of light.  However, there are many
difficulties with these concepts.
Time travel discussions generally begin with Einsteins theories.  First of all,
the theory of special relativity, also referred to as relativity without
gravity, forbids time travel because of its flat space-time.  In addition, the
curved space-time that was acknowledged in the past did not allow for the
existence of time travel.  Before long, in 1949, Kurt Gödel astonished Einstein
by declaring that the universe was a space-time filled with rotating matter and
time loops all over the place.  It is possible to travel into the past or future
in a space-time with time loops as a result of the warping of space-time.
However, Kurt Gödels universe was soon disregarded, he did showed that some
time travel is possible and provided a mathematical basis for it.  Although at
first, the solutions only worked with a cosmological constant, further research
lead to equations that worked without any questionable constants.
Einsteins theories do not correspond to this universe because they are not
accounting for its expansion, they are however useful in discussing time travel.
The more developed theory of relativity may permit time travel.  This theory
assumes that the universes past is definite and devoid of uncertainty.  On the
contrary, matter is fundamentally exposed to uncertainty and undergoes quantum
fluctuations.  Nevertheless, the theory of general relativity is broadly
accepted and taken much more seriously than the special relativity theory.
These equations demonstrate that the distortion of space-time is caused by the
mater-energy content of space.  Advocates of time travel signify that the theory
of general relativity permits time travel in several different ways.  Even
though, the amount of energy needed to bend time into a loop are so enormous
that the equations give way.  Here, quantum theory becomes dominant and
regulates the possibility of time travel.  Therefore, due to the fact that
Einsteins equations fail in immense gravitational fields where quantum theory
becomes prevailing, the vital decision on whether time travel could be made a
reality cannot be established by these equations alone.
In science fiction, crazy physicists construct time machines, which are usually
spectacular looking, and lead to an often humorous story.  In reality, the first
meaningful offer for the construction of a time machine was made by three
physicists.  Their names are Kip Thorne, Michael Morris and Ulvi Yurtsever.  In
June 1988, they persuaded the editors of Physical Review Letters, which only
published serious writings, that their work deserved serious deliberation.  As
knowledgeable scientists they offered their views in established theoretical
language and then cautiously clarified where their weakest assumptions were.
With the intention of overcoming the cynicism of the scientific society, Thorne
recognized that they would have to overcome the usual opposition to using
wormholes as a time machine.
Wormholes are tubes of space-time that may connect two completely different
regions of space and time.  Wormholes would be disputed for numerous reasons.
For instance, the gravitational fields in the middle of a black hole, which is
like a wormhole, are so massive that any spaceship trying to pass through it
would be torn apart.  At the center of black hole is a point, commonly referred
to as the singularity.  Here, space and time cease to exist and matter is
crushed by the intense gravitational forces.  Just about thirty years ago, Roger
Penrose demonstrated that no matter what it is that falls into such a black
hole, it will be dragged into the singularity by its immense gravitational pull
and crushed out of existence.  Although wormholes are mathematically possible,
they are nearly hopeless.
Wormholes are very unstable, small disturbances would cause the Einstein-Rosen
bridge, which acts like a tunnel connecting two regions of space-time, to
disappear.  Consequently, if a spaceship were to enter a black hole, it would
produce a disturbance which in turn will cause the wormhole to collapse.
Furthermore, a wormhole is likely to close by itself just because of quantum
effects.  The opening of the black hole discharges intense radiation, which
would kill anyone who attempted to enter the wormhole or even close the opening
forever.
As Stephen Hawking, an excellent theoretical physicist, has indicated that the
radiation emitted at the wormhole opening will be so vast that it will effect
the matter-energy composition of Einsteins equations.  The input into
Einsteins equations will at the very least adjust the opening to the wormhole,
or might even close it.  Nevertheless, Thorne does not believe that the
radiation will be sufficient enough to seal the opening.  Here, the superstring
theory, which is an entirely quantum-mechanical theory, could be employed to
make modifications to the initial wormhole theory.  However, a host of other
problems follows, the spaceship would have to go faster than the speed of light
to break through the wormhole and arrive at the opposite end.  This is clearly
impossible, for one cannot go faster than the speed of light.  This concept is
explained by the theory of special relativity, which shows how ones mass would
grow to be infinite in the process.  If this was not the case, one could travel
back in time by escalating his or her velocity and therefore causing time to
slow down, stop and go backward.
It became a known fact that time slows down in a wormhole and comes to a
complete stop at the center.  However, the pursuit of knowledge did not stop
here, and in 1960s, a brilliant mathematician from New Zealand demonstrated that
if the black hole was rotating, the circumstances change quite a bit.  This man
was Roy Kerr, who proved that in a rotating black hole a singularity forms in a
form of a ring.  In theory, it is fairly possible to drive into a rotating black
hole, endure the immense but nevertheless limited gravitational forces, travel
through the ring and arrive in a different place and time.  This explanation
became one of the few mathematical models for a time machine.  On the other
hand, if someone was to watch a spaceship traveling through a black hole from
earth, it would seem as if the space traveler slows down drastically while going
through the wormhole and then comes to a complete stop at the center, producing
an image of being frozen in time.  Hence, it takes an endless quantity of time
to pass through a black hole.  If journeying through a wormhole was possible,
and someone could actually go through the center to the other side and back, the
shift of time would be so extensive that conceivably billions of years could
pass on earth during the journey of the spaceship through the wormhole.
Consequently, for all these reasons, the wormhole solutions were never taken
seriously.
Thorne and his colleagues sought to find what was theoretically achievable.  The
laws of physics state what is possible, not what is practical (Kaku 247).
Thorne wanted to determine whether the laws of physics allow for an extremely
advanced civilization to create and sustain wormholes for interstellar travel.
Shortly, they found an astonishingly simple solution that satisfied all their
constraints.  It was not the usual black hole solution.  They named it the
transversible wormhole to set it apart form the other wormholes that, in
reality, were not transversible by a space traveler.  A journey through this
transversible wormhole would actually be as restful as a trip on a regular
plane!  During this trip, the maximum gravitational pull a passenger might
undergo would not surpass 1 g.  Hence, the travelers weight in a spaceship
would not rise above his or her weight on planet earth.  Moreover, Thornes
wormhole is permanently open, so there is no chance of it closing anytime during
the trip.  The overall trip through the transversible wormhole would only take
about two hundred days or less, compared to the millions or billions of years
involved in a trip through black hole.
There was also the question of time paradoxes that arise with time travel.
Thorne simply asserted that the closed timelike curves that may arise in his
wormhole actually fulfill the past, instead of altering it.  At this point, the
three physicists just had to figure out the exact nature of the matter and
energy essential to create this transversible wormhole.  It was soon established
that in the middle of this wormhole should be an exotic sort of matter which
possesses some peculiar properties.  Even so, this exotic sort of matter does
not appear to disobey the laws of physics.  However, in the future, scientists
may perhaps demonstrate that this type of matter cannot be in existence.
Nevertheless, at the moment exotic matter appears to be an entirely reasonable
sort of matter, and could be created if suitable technology was available.
Thorne came up with several versions of a time machine.  The first kind was in a
form of two chambers with each one enclosing two parallel metal plates.  The
powerful electric fields formed among the pairs of plates could only be produced
with extremely advanced technology that does not exist so far.  The electric
fields would tear space-time, producing a gap in space that connects the two
chambers, which are then separated.  The first chamber is positioned on a
spaceship which goes close to the speed of light, whereas the second chamber
stays behind.  Given that a wormhole is capable of linking two areas of space
with diverse times, it can be said that time would pass slower in the first
chamber out in space compared to the other chamber left on earth.  Since time is
going by at different rates on both sides of the wormhole, any individual
entering into one side of the wormhole would instantly appear at the other side
of the wormhole in either the past or future.
If exotic matter could be produced and formed into a metal, a different kind of
a time machine could be constructed.  The time machine could be assembled into a
cylindrical shape, with enough room for an individual to stand in the middle.
The exotic matter would then distort the space-time around it and thus generate
a wormhole that links to a completely different place and time.  In the middle
of the whirlpool is the individual who would undergo about 1 g of gravitational
pull as he or she is being sucked into the wormhole and transported to another
space and time.  This type of time machine resembles the one in Timeline.  Time
machines in the novel were cylinder shaped, however the idea there was to send
an individual to a different time and place like a fax.  The individual was
therefore practically destroyed, send through a microscopic wormhole and rebuild
in a different space-time.  While Thornes time machine would simply, in theory,
open a macroscopic wormhole which would consume the individual who would
instantaneously find himself or herself in a different time and place.  In
Timeline there was no such thing as exotic matter, just much more advanced
technology such as the use of extremely advanced quantum computers.
Thornes mathematical way of thinking seems faultless.  Einsteins equations
certainly demonstrate that wormhole solutions permit time to pass at differently
on either side of the wormhole, so that time travel, in theory, is feasible.
The initial problem is in generating a wormhole from the beginning.  How does
one get enough energy to produce and sustain a wormhole with exotic matter?
Well, one of the fundamental principles of physics is that all substances have
positive energy.  Accordingly, the vacant vacuum of space holds no or zero
energy.  Therefore, if at some point in the future, scientists take on the
challenge of creating various substances with negative energy which would
encompass less energy than the vacuum of space, they could figure out how to
produce exotic arrangements of space and time where time would be twisted into a
loop.  The idea that energy must somehow become at least temporarily negative
for time travel to take place has grown to be a widely accepted principle.
Besides making time travel possible, negative energy would cause antigravity and
various other occurrences which have not been observed experimentally and may
lead to a variety of problems.
However, in 1948, it was discovered that negative energy can be produced via
quantum theory.  It was Henrik Casimir, a Dutch physicist, who recognized that
quantum theory is capable of generating negative energy.  This is done by using
two parallel metal plates, which should be decently large and have no charge.
One would assume that the two uncharged plates would have no force among them,
since they are electrically neutral.  On the contrary, Casimir demonstrated that
the vacuum separating these two plates, because of the Heisenberg Uncertainty
Principle, is actually teeming with activity, with trillions of particles and
antiparticles constantly appearing and disappearing (Kaku 250).  These
particles emerge out of nowhere and vanish back into the vacuum.  Since the
particles only appear for a very brief time, they generally cannot be observed.
However, they do not go against any of the known laws of physics.  The
attractive force produced among the two parallel plates could be measured
according to Casimir.  This phenomenon was soon called the Casimir effect.  The
effect may be employed for the benefit of time travel by placing the two
conducting metal plates at the opposite ends of the wormhole thus producing
negative energy at the two openings of the wormhole.  However, this will not be
the case if the ordinary weak energy state cannot be changed.  In this
situation, time travel would not exist and transversible wormholes would turn
out be a foolish idea.
  No one knows whether a time machine could be built in the distant future.
Nonetheless, it is known that the quantum theory permits time travel to take
place on a microscopic level.  Particles constantly travel back in time; they
might even go around in a circle on a closed loop in space- time.  Particle
detectors cannot monitor the particles that travel in a closed loop.  Yet, they
have been identified and measured in a couple of experiments that detected their
indirect effects.  One of these experiments has found that electrons traveling
in a closed loop cause a tiny shift of light released by the hydrogen atom.
Casimirs parallel metal plates can also be used to demonstrate the existence of
particles traveling in closed loops.  The smaller number of particles making
loops in space-time in the middle of the two plates compared to the outer area
results in a miniature force among the metal plates, thus showing the existence
of these particles traveling in loops in the four dimensions.
The likelihood of producing adequate warping for a macroscopic object to travel
in time is practically zero!  Even so, this idea of time travel on a microscopic
scale was used in the novel Timeline.  Michael Crichton took the concept of
quantum foam and turned it into a time machine.  Therefore, an individual going
back in time was destroyed so that he or she would fit into a microscopic
wormhole.  In real life, this is clearly impossible.  Technology today would not
be able to destroy and furthermore reconstruct a human being at a different
place and time.  In the novel, they had access to highly advanced quantum
computers and all kinds of sufficient technology, while in reality quantum
computers are still in the process of being created.  Although it is hard to
predict how technology will develop in the future, it is highly unlikely that it
would be capable of destroying and reconstructing a human being, therefore this
idea is evidently impossible at the present moment.
Most people who are against time travel cite time paradoxes as the evidence for
the impossibility of time travel.  The paradoxes are too ridiculous and
impossible therefore no such thing as time travel could exist!  Indeed, time
travel may cause various paradoxes, but they can be explained.  First, it is
important to understand that all the different paradoxes can only occur if one
has the free will to do anything while situated in the past.  One approach to
clarify this fascinating phenomenon is through Einsteins world line method.
Einsteins world line illustrates ones entire history.  It is a graph where the
horizontal scale is distance and the vertical scale is time.  An individuals
world line never begins out of nowhere, and never comes to an end.  When the
individual dies, his or her world line separates into billions of world lines of
the molecules that the body consisted of.  As a result, the world line holds
complete information regarding each individuals history.
The world line can help illustrate what takes place when an individual arrives
in the past.  Lets say Bobby goes back into the past where he was not born yet.
He meets his mother as a young lady and she falls in love with him and leaves
his father all by his lonesome.  Does Bobby truly disappear?  Well, on a world
line this evidently cannot happen because his world line would vanish and
according to Einstein, world lines never end.  Consequently, the theory of
relativity proclaims that changing the past is impossible.  However, in Bobbys
circumstances, he may play a significant role in a fight between his parents,
therefore supposedly changing history, which presents further questions.  The
answer to this is that by traveling back in time Bobby is fulfilling the past.
So, one might ask, what happens in this scenario?  Disregarding the multiverse
theory for the moment, clearly, Bobby was born at some point, to come back and
have his mother fall in love with him.  This means that something must have
prevented him from taking his mother away form his father.  Something must go
wrong to keep things the way they happened.  However, this does not mean that
Bobby cannot play a role in history.  Therefore, perhaps an individual cannot
change the past and when this individual travels back in time, he or she is
merely fulfilling that which is already known.
In the novel Timeline, the idea of fulfilling the past is rather implied by the
outcome of the novel.  As the group of historians go back in time to rescue the
professor, they are anxious about altering history.  When Doniger was posed with
the question of time paradoxes and changing history, he simply made an
individual seem too powerless to alter any significant event in history.
However, once in the fourteenth century, the group evidently fulfills the past.
As they worry about changing the past, the professor introduces atomatic fire
to Lord Oliver, giving him an advantage over Arnaut.  Kate and Chris find the
secret passage which is then discovered by Arnaut and gives him an advantage
over Oliver.  Yet in the end everything turns out as it was supposed to, for
according to their history the castle falls to Arnaut because of a traitor who
lets Arnauts men into La Roque.  These traitors turn out to be Kate and Chris
who have therefore fulfilled what was already known!  Also, as Marek stays
behind in the fourteenth century, he is fulfilling what is already known since
at the end of the novel his friends visit the Eltham Castle and the chapel where
they find the burial stones of Lady Claire and Marek, which have always been
there.
Therefore, maybe when someone goes back into the past, they fulfill it, and thus
never change it.  A universe arranged in this way allows for someone to go back
in time and meet themselves when they were little.  In other words, a world line
can perform loops in time without breaking off in any way.  These loops in time
are often referred to as closed timelike curves.  The theory of general
relativity demonstrates how these world lines traveling through time are
acceptable. The world lines themselves originate from the theory of special
relativity.  This theory can predict what happens if someone goes back in time,
but does not determine whether time travel could be made a reality.  The
scientific arguments are centered on whether these world lines are acceptable by
the theory of general relativity and quantum mechanics.
Numerous theories exist concerning time paradoxes.  According to one
interpretation of quantum physics, each and every time a quantum object is about
to make a decision, the universe splits into copies of itself.  In every single
replica of the universe, a different decision is made to allow for every
possible outcome!  So if an electron was faced with a choice of going right or
left, the universe would divide into two copies of itself.  In one replica of
the universe, the electron would go right, in the other version of reality, the
electron would go left.  This analysis of quantum physics, largely pioneered by
David Deutsch, declares that the universe is divided into infinite number of
copies of itself.  In this multiverse, all potential outcomes of all potential
events must take place somewhere in the multiple worlds.  Each and every one of
these parallel worlds would be just as real as any other.  There would be
alternative histories growing out of every decision made in the multiverse,
constantly emerging into infinitely many universes.
How does this resolve the paradoxes?  Suppose Bobby travels back in time to
murder his grandfather when he was a little boy.  According to the multiverse
theory, Bobby has traveled back to a deviation point in a particular universe.
Following the murder, Bobby travels back into his present, which will now be a
different universe.  Here, Bobby was never born and lives happily ever after.
In this case, no paradox forms due to having yet another universe where Bobbys
grandfather is still living!  Bobby is then born in this universe and goes on to
kill his grandfather.  The multiverse idea is explored in Timeline.  During the
plane trip to Black Rock, Gordon presents a simple experiment to the historians,
which proves the existence of the multiverse.  He then explains that since the
universes are infinite in number, they exist at all earlier times and the
professor has traveled to one of these universes.  This idea could also be used
to explain the historians effect on history; however the book does not use it
to explain any changes in history that may have occurred.  The multiverse idea
is often used to explain all the different time paradoxes that may arise.  Many
researchers, including David Deutsch, are now taking this theory more seriously
and considering it as a real possibility.
Time travel is possible in a space-time with time loops, due to the warping of
space-time in these regions.  Stephen Hawking often refers to a time travel
horizon, which is the border segregating the space-time with time loops from the
space-time without them.  There are no motives to believe that this universe has
warped space-time in general, which would mean that it has developed with time
travel in it.  In other words, time travel only occurs in a warped space-time!
Skeptics of time travel often argue that traveling through time is impossible
because there are no visitors from the future.  However, this idea may very well
be counteracted with the fact that warped space-time is needed to travel back in
time.  Therefore, if the first time machine was build in year 2078 and produced
warped space-time, people living in year 2080 would only be able to go back into
the past two years because of the necessary warped space-time.  Different
reasons are given to show that time travel is just a foolish idea.  Stephen
Hawking, being one of the most recognized skeptics, evidently believes that the
laws of physics conspire to prevent time travel by macroscopic objects (153).
Others say it is ridiculous, and contrary to commonsense, therefore it must be
impossible.
There are human beings who have already experienced time travel in a certain
way!  These people are the twentieth century astronauts who have spent
considerable time in space and came back to earth, only to find themselves a
fraction of a second younger.  This means that they have traveled a fraction of
a second into the future!  This is also the case for satellite stations in
space.  Most people assume that time passes at a steady rate, that it is fixed
and universal.  However, this assumption is wrong.  Time passes at different
rates in different places.  People living in places where time slows down get
older less quickly.  Each and every individual travels with his or her own
clock, and this individual can change the rate at which his or her clock is
ticking.
If an individual was to go out in space and travel very fast, as long as
possible, he or she would be traveling into the future.  Space and time are
linked together, therefore when an individual is traveling through space, he or
she is traveling through time with his or her own clock.  When this individual
is traveling through time, he or she is not aware of the change because
everything happening on board of the spaceship, including the individuals
heartbeat and brain waves would slow down by the same amount.  The faster the
individual travels, the slower the time pass for him or her.  So what might be
an hour for this individual, it could be a hundred years for the people on
earth.  Thus, this individual would have traveled one hundred years into the
future.
Time travel is studied all throughout the world.  Ronald L. Mallet is one of the
many people studying time travel today.  Professor Mallet and his colleagues are
planning to at least send subatomic particles into the past.  They have
developed a time machine, consisting of intensely powerful ring of laser light
that twist time into a loop.  Circulating beams of light create rotating region
of space, and since space and time are linked, the twisting of the space causes
the twisting of time!  Time itself is like a line, that goes form past to
future.  This time machine would bend the line into a closed loop, going form
future into the past, therefore allowing for time travel.  The machine itself
consists of stacks of lasers that would create layers of circulating light.
This would then produce circulating loops of time around the flowing light.
How will anyone ever find out if people could really travel through time and
back?  The laws of physics do not outlaw time travel in any way.  However, both
Einsteins theories and the quantum theory are not capable of utterly answering
the question of whether time travel is realistic.  The ultimate verdict on time
travel may perhaps come from the entirely quantized theory of gravity.
Scientists are currently working on this theory, where not only matter, but
moreover space and time would be uncertain and fluctuate.  So perhaps sometime
in the distant future time travel will become a reality, but for now it remains
a mystery.