Time travel

TIME TRAVEL
travelling through time…
Aravind_n_c@yahoo.co.in
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Introduction
 At its most basic level, time is the rate of  Human beings frolic about in the
change in the universe -- and like it or
not, we are constantly undergoing change. three spatial dimensions of length,
We age, the planets move around the sun width and depth.
etc.
 We measure the passage of time in sec,  Time joins the party as that most
min, hrs and years, but this doesn't mean crucial fourth dimension.
time flows at a constant rate. Just as the
water in a river rushes or slows depending  Time can't exist without space, and
on the size of the channel, time flows at space can't exist without time.
different rates in different places. In
other words, time is relative.  The two exist as one: the space-time
continuum.
 Any event that occurs in the universe
has to involve both space and time.

Time Travel Into the Future
Travelling through time…

Time Travel Into the Future
 If you want to advance through the years a
little faster than the next person, you'll
need to exploit space-time.
 GPS pull this off every day, accruing an
extra third-of-a-billionth of a second daily.
 Time passes faster in orbit, because
satellites are farther away from the mass of
the Earth.
 Down here on the surface, the planet's mass
drags on time and slows it down in small
measures.
 We call this effect gravitational time
dilation.

Gravitational Lensing Effect
 According to Einstein's theory of general
relativity, gravity is a curve in space-time.
 When light is moving near a sufficiently
massive object. Particularly large suns, for
instance, can cause an otherwise straight
beam of light to curve in what we call the
gravitational lensing effect.

What does this have to do with time?
 Remember: Any event that occurs in the
universe has to involve both space and time.
 Gravity doesn't just pull on space;
 it also pulls on time.

 You wouldn't be able to notice minute changes in the flow of
time, but a sufficiently massive object would make a huge
difference -- say, like the supermassive black hole Sagittarius
A at the center of our galaxy.
 Here, the mass of 4 million suns exists as a single, infinitely
dense point, known as a singularity.
 Circle this black hole for a while (without falling in) and you'd
experience time at half the Earth rate. In other words, you'd
round out a five-year journey to discover an entire decade
had passed on Earth

 Speed also plays a role in the rate at
which we experience time. Time
passes more slowly the closer you
approach the unbreakable cosmic
speed limit we call the speed of light.
For instance, the hands of a clock in
a speeding train move more slowly
than those of a stationary clock. A
human passenger wouldn't feel the
difference, but at the end of the trip
the speeding clock would be slowed
by billionths of a second. If such a
train could attain 99.999 percent of
light speed, only one year would
pass onboard for every 223 years
back at the train station

 In effect, this
hypothetical
commuter would have
traveled into the
future. But what about
the past? Could the
fastest starship
imaginable turn back
the clock?

Time Travel Into the Past

 We've established that time travel into the future
happens all the time.
 Scientists have proven it in experiments, and the idea
is a fundamental aspect of Einstein's theory of
relativity.
 You'll make it to the future; it's just a question of how
fast the trip will be.
 But what about travel into the past? A glance into the
night sky should supply an answer.

 The Milky Way galaxy is roughly 100,000 light-years
wide,
 so light from its more distant stars can take thousands
upon thousands of years to reach Earth.
 Glimpse that light, and you're essentially looking back
in time.
 When astronomers measure the cosmic microwave
background radiation, they stare back more than 10
billion years into a primordial cosmic age.

Law of Causality
 There's nothing in Einstein's theory that precludes time travel
into the past, but the very premise of pushing a button and
going back to yesterday violates the law of causality, or
cause and effect.
 One event happens in our universe, and it leads to yet another
in an endless one-way string of events.
 In every instance, the cause occurs before the effect.
 Just try to imagine a different reality,
 Say, a person’s birth(cause) and the person (effect)
 Travels to his past, and now we can see the effect is before the
cause which is not possible.
 Exception (BBT)

Could we avoid causality ?
 Some scientists have proposed the idea of using faster-than-light
travel to journey back in time.
 After all, if time slows as an object approaches the speed of light,
then might exceeding that speed cause time to flow backward?
 Of course, as an object nears the speed of light, its relativistic mass
increases until, at the speed of light, it becomes infinite.
 Accelerating an infinite mass any faster than that is impossible.
 Warp speed technology could theoretically cheat the universal
speed limit by propelling a bubble of space-time across the
universe, but even this would come with colossal, far-future energy
costs.

 But what if time travel into the past and future
depends less on speculative space propulsion
technology and more on existing cosmic

Yes, Set a course
phenomena?

for the black hole.

Black Holes and Kerr Rings

 Circle a black hole long enough,
and gravitational time dilation will
take you into the future.
 But what would happen if you
flew right into the maw of this
cosmic titan?
 Most scientists agree the black
hole would probably crush you,
but one unique variety of black
hole might not: the Kerr black
hole or Kerr ring.

 In 1963, New Zealand mathematician Roy Kerr proposed the first
realistic theory for a rotating black hole.
 The concept hinges on neutron stars, which are massive collapsed
stars the size of Manhattan but with the mass of Earth's sun.
 Kerr postulated that if dying stars collapsed into a rotating ring of
neutron stars, their centrifugal force would prevent them from
turning into a singularity.
 Since the black hole wouldn't have a singularity, Kerr believed it
would be safe to enter without fear of the infinite gravitational force
at its center.

 If Kerr black holes exist, scientists speculate that we
might pass through them and exit through a white
hole.
 Think of this as the exhaust end of a black hole.
 Instead of pulling everything into its gravitational force, the
white hole would push everything out and away from it --
perhaps into another time or even another universe.
 Kerr black holes are purely theoretical, but if they do
exist they offer the adventurous time traveler a one-
way trip into the past or future.

Einstein-Rosen bridge

 Theoretical Kerr black holes aren't the only
possible cosmic shortcut to the past or future.
 As made popular by everything from "Star Trek:
Deep Space Nine" to "Donnie Darko," there's also
the equally theoretical Einstein-Rosen bridge to
consider.
 But of course you know this better as a
wormhole .

 Einstein's general theory of relativity allows for the
existence of wormholes since it states that any mass
curves space-time. To understand this curvature, think
about two people holding a bedsheet up and
stretching it tight. If one person were to place a
baseball on the bedsheet, the weight of the baseball
would roll to the middle of the sheet and cause the
sheet to curve at that point. Now, if a marble were
placed on the edge of the same bedsheet it would
travel toward the baseball because of the curve.

 In this simplified example, space is depicted as a two-dimensional plane
rather than a four-dimensional one. Imagine that this sheet is folded over,
leaving a space between the top and bottom. Placing the baseball on the
top side will cause a curvature to form. If an equal mass were placed on the
bottom part of the sheet at a point that corresponds with the location of the
baseball on the top, the second mass would eventually meet with the
baseball. This is similar to how wormholes might develop.
 In space, masses that place pressure on different parts of the universe
could combine eventually to create a kind of tunnel. This tunnel would, in
theory, join two separate times and allow passage between them. Of
course, it's also possible that some unforeseen physical or quantum
property prevents such a wormhole from occurring. And even if they do
exist, they may be incredibly unstable.

 According to astrophysicist Stephen Hawking, wormholes may exist
in quantum foam, the smallest environment in the universe. Here,
tiny tunnels constantly blink in and out of existence, momentarily
linking separate places and time like an ever-changing game of
"Chutes and Ladders."
 Wormholes such as these might prove too small and too brief for
human time travel, but might we one day learn to capture, stabilize
and enlarge them? Certainly, says Hawking, provided you're
prepared for some feedback. If we were to artificially prolong the life
of a tunnel through folded space-time, a radiation feedback loop
might occur, destroying the time tunnel in the same way audio
feedback can wreck a speaker.

Cosmic String

 We've blown through black holes
and wormholes, but there's yet
another possible means of time
traveling via theoretic cosmic
phenomena. For this scheme, we
turn to physicist J. Richard Gott,
who introduced the idea of
cosmic string back in 1991. As
the name suggests, these are
stringlike objects that some
scientists believe were formed in
the early universe.

 These strings may weave throughout the entire
universe, thinner than an atom and under immense
pressure. Naturally, this means they'd pack quite a
gravitational pull on anything that passes near them,
enabling objects attached to a cosmic string to travel
at incredible speeds and benefit from time dilation. By
pulling two cosmic strings close together or stretching
one string close to a black hole, it might be possible to
warp space-time enough to create what's called a
closed timelike curve.

 Using the gravity produced by the two cosmic strings (or the
string and black hole), a spaceship theoretically could propel
itself into the past. To do this, it would loop around the cosmic
strings.
 Quantum strings are highly speculative, however. Gott
himself said that in order to travel back in time even one year,
it would take a loop of string that contained half the mass-
energy of an entire galaxy. In other words, you'd have to split
half the atoms in the galaxy to power your time machine. And,
as with any time machine, you couldn't go back farther than
the point at which the time machine was created.

…. Oh yes, and then


there are the time
paradoxes.

You’ll Never Go Back In Time !!!

grandfather paradox
 For starters, if you traveled back in time 200 years, you'd
emerge in a time before you were born. Think about that for a
second. In the flow of time, the effect (you) would exist before
the cause (your birth).
 To better understand what we're dealing with here, consider
the famous grandfather paradox. You're a time-traveling
assassin, and your target just happens to be your own
grandfather. So you pop through the nearest wormhole and
walk up to a spry 18-year-old version of your father's father.
You raise your laser blaster, but just what happens when you
pull the trigger?

inconsistent causal loop
 Think about it. You haven't been born yet. Neither
has your father. If you kill your own grandfather in
the past, he'll never have a son. That son will
never have you, and you'll never happen to take
that job as a time-traveling assassin. You wouldn't
exist to pull the trigger, thus negating the entire
string of events. We call this an inconsistent
causal loop.

consistent causal loop
 On the other hand, we have to consider the idea of a
consistent causal loop. While equally thought-
provoking, this theoretical model of time travel is
paradox free. According to physicist Paul Davies, such
a loop might play out like this: A math professor
travels into the future and steals a groundbreaking
math theorem. The professor then gives the theorem
to a promising student. Then, that promising student
grows up to be the very person from whom the
professor stole the theorem to begin with.

post-selected model
 Then there's the post-selected model of time
travel, which involves distorted probability close to any
paradoxical situation [source: Sanders]. What does
this mean? Well, put yourself in the shoes of the time-
traveling assassin again. This time travel model would
make your grandfather virtually death proof. You can
pull the trigger, but the laser will malfunction. Perhaps
a bird will poop at just the right moment, but some
quantum fluctuation will occur to prevent a paradoxical
situation from taking place.

Parallel universe
 But then there's another possibility: The future or past
you travel into might just be a parallel universe.
Think of it as a separate sandbox: You can build or
destroy all the castles you want in it, but it doesn't
affect your home sandbox in the slightest. So if the
past you travel into exists in a separate timeline, killing
your grandfather in cold blood is no big whoop. Of
course, this might mean that every time jaunt would
land you in a new parallel universe and you might
never return to your original sandbox.

The Bootstrap Paradox
 The bootstrap paradox is a paradox of time travel in which
information or objects can exist without having been created.
 After information or an object is sent back in time, it is recovered in
the present and becomes the very object/information that was
initially brought back in time in the first place.
 Alternate history is a popular concept of time travel and centers on
the premise of changing history, whether accidentally or
deliberately, while traveling back through it. One counter to this is
the claim that any change a time traveler makes to history is
precisely what was always supposed to happen

Weak Cosmic Censorship
Hypothesis
 Stephen Hawking has spent his career working with black holes, and most of what we know
about them is based on his work. The surface of a black hole is the event horizon, and once any
object crosses this and enters the hole, it no longer exists in our spacetime continuum. It is drawn
by extreme gravity into an infinitely thin strand of energy called a singularity.  
 Hawking’s work theorizes that only the terrific energy of a black hole can create a singularity. The
weak cosmic censorship hypothesis asserts that there can be no singularity unhidden by a black
hole, and thus, no singularity can ever be observed. The singularity is a major talking point of
cosmology, because one theory of black holes paints them as gravitational pulls so strong that
they impart faster-than-light speed to any object entering them. The singularity is the engine of a
black hole’s gravity.  
 So if a spacecraft wanted to break the light barrier, it would need only to travel through a black
hole, and upon emergence from the other side would still be traveling at this speed – namely,
jump-starting a spacecraft past light speed so it can return to Earth at some point in the past.  
 But no object can survive a black hole’s singularity. Here, matter may actually be destroyed,
apparently violating the law of conservation of mass. Hence, until singularities are proven to exist
outside black holes, this method of traveling into the past is impossible.

The Chronology Protection
Conjecture
 This one was dreamed up by Hawking himself, and there is a LOT of mathematics without
numbers involved in it. In a nutshell, the conjecture requires that there be no such thing as a
closed timelike curve. A CTC is the closed path of any object as it travels through 4-dimensional
spacetime; if the path brings the object back to its starting point, the path is said to be closed.  
 No mathematical theory can yet predict if CTCs exist. If their existence is demonstrated,
Hawking’s conjecture is demonstrably false, and travel into the past may be possible, probably
via the next entry. If CTCs do not exist, then the conjecture is true, and “historians throughout the
Universe are protected,” as Hawking says.  
 Our most immediate chance of discovering whether CTCs exist lies in quantum gravity, the
branch of mathematics devoted to combining all four forces of the Universe into a single blueprint
that can describe all physical laws on both the macroscopic and subatomic scales. The four
forces include: the weak force, which holds electrons in orbit around nuclei, initiates hydrogen
fusion in stars, and causes the radioactive decay of all subatomic particles; the strong force,
which holds protons and neutrons together as nuclei; electromagnetism; and gravity. The General
Theory of Relativity reconciles all but electromagnetism; quantum gravity, using a different
approach, reconciles all but gravity. Until quantum gravity is fully explored, CTCs can only be
hypothesized, and in their absence, traveling into the past cannot be done.

Wormholes Disobey the Laws of
Physics
 All our understanding of time travel is based on what we know of the physical properties and interactions of the
Universe. We have devised a branch of mathematics currently separate from physics to describe the laws of
physics on a microscopic scale, and we call it quantum physics. This branch strongly theorizes the existence of
Einstein-Rosen Bridges, named after the two scientists most responsible for our understanding of them.   
 They are more popularly called wormholes, and they are holes that have ripped through the fabric of spacetime. If
we could make use of them, the shortest distance between two points would no longer be a straight line but zero,
caused by puncturing spacetime at the point of origin and at the point of destination, just like poking holes through
a sheet of paper; then spacetime is effectively folded until the two points overlap, and the traveler passes through
from A to B, and spacetime is unfolded to its original state. No physical movement occurs, but the destination may
be at the other end of the known Universe, and the spacecraft would have neither approached, nor surpassed the
speed of light, but simply teleported.  
 This seems to allow the possibility of travel into the past by avoiding the speed of light altogether, but what it does
not account for is what goes on inside a wormhole. Physics has no idea, except to say that the laws of physics do
not exist as we know them, or do not exist at all, inside wormholes. If we attempt to comprehend travel through
wormholes in our terms of physics, then we are not addressing the issue to begin with, and have not yet left
square one.

The Twin Paradox
  This paradox deals more properly with travel into the future. It involves two
newborn, identical twins, one who stays on Earth, and one who travels to Proxima
Centauri, the nearest star, 4 light years away. If the spacecraft travels at 80% the
speed of light, which amusingly seems more realistic, the round trip will take 10
years. That means the twin on Earth will be 10 years old when his brother returns.  
 But on the spacecraft, the crew observes Promixa Centauri and Earth also moving
with relation to the craft, and this causes Points A and B to shorten to a distance of
2.4 light years, not 4. Each leg of the journey will take 2.4 light years divided by the
speed, 80% of the speed of light, for a duration of 3 years one way, 6 round trip.
Thus, the twin onboard will have aged 6 years in the same relative span of time. This
much is not logically impossible.  
 What is impossible is the effect of one twin traveling 101% or more of the speed of
light. This would, at least according to this scenario as we understand it, cause him
to travel into the past and cease to exist, i.e. disappear from onboard, and not return
to his brother on Earth.

E = MC Squared
 The most famous equation in the history of mathematics describes the relationship of energy and
mass. In 1942, it was notoriously seized upon as a great idea for a powerful new weapon.
Einstein had no idea it could be used to build a bigger, better bomb, and wept when Enrico Fermi
and Robert Oppenheimer explained what was going on at Oak Ridge, Tennessee.  
 Aside from explaining how much energy is contained in matter of any size, it also provides an
exploration into what happens to mass when it travels faster. The faster something travels, the
more energy is required to sustain its travel. As an object approaches the speed of light, it
approaches infinite mass, and thus requires infinite energy to continue propelling it forward.  
 This does not prohibit traveling into the future, since all an object has to do is approach the light
barrier. You approach it when you walk into the kitchen to get a beer. The distance into the future
you have traveled is too insignificant to matter. But technically you gain an equally insignificant
amount of mass. The energy required to propel a large object, like a spacecraft, any meaningful
distance into the future, as that meaning relates to our frame of reference, would be greater than
or equal to the energy currently in VY Canis Majoris, the largest star we know of.  
 But to break the light barrier would cause the traveler to go into the past, and this would require
infinite, and then greater than infinite, energy. This is impossible to achieve.

Temporal Causality Loop
 This is a paradox as well, and deals with one specific scenario: the
invention of the first time machine. The inventor travels back in time in an
effort to make his grandfather and grandmother fall in love, only to
accidentally kill his grandfather (see #2). Now, desperate to exist in the
future, he sleeps with his future grandmother and fathers his own father,
thus enabling himself, in the future, to travel back in time and father his
father again.  
 This paradox is illogical because it describes an effect in the future
occurring before its cause in the past. Suppose you were to travel back in
time to before the Big Bang, somehow cause the Big Bang and thus create
the Universe. In terms of fate, this would happen in order to enable you,
13.5 billion years later, to invent the time machine and travel back to create
the Universe so the time machine could be invented. It is fundamentally
insensible.

Temporal Paradox
 This is essentially the negative version of #3, and is also called the Grandfather Paradox.
Traveling into the past must be logically impossible because it would enable you to go back in
time and kill yourself. But if you die, how will you travel into the past from the future to kill
yourself? Critics, especially science fiction fans, are quick to point out that our understanding of
mathematics expands every day thanks to people like Newton, Einstein, Hawking, and Michio
Kaku, and with it comes an expanded understanding of the logic involved in time travel
scenarios.  
 The best current counter to the temporal paradox is the Multiverse, which describes an infinite
number of yous doing an infinite number of things at an infinite number of points throughout your
life. You may be stabbed in a bar fight at 100 years old in another Universe, but die of cancer as a
child in this one. Imagine a Universe without Listverse. Our current understandings of quantum
mechanics and quantum physics lends strong credence to the possibility that the Multiverse is a
reality. It would negate the temporal paradox, and several others, allowing you a future after you
have killed yourself. But there is still no fully formed theory of the Multiverse’s existence, and until
there is, this paradox stands.

No Unified Field Theory
  Frankly, all the previous entries are based more in terms of logic than in pure
mathematics, precisely we can only surmise everything related to time travel according to our
very superficial comprehension of it. Albert Einstein’s life work centered on what we now call
Relativity. He postulated two theories of it, but the next step, an infinitely more important one, is to
unify the General Theory of Relativity with electromagnetism. Einstein died working on this, and
today’s eggheads have taken only baby steps forward. The “highest” form of mathematics to date
is called “M Theory,” which is not even fully described yet. It’s practically a religion to
mathematicians, because so little is understood about it that some don’t believe in it.  
 It identifies 11 dimensions in the Universe, not just 4, and its champions expect that it can unite
the 5 differing string theories that preceded it, and take what may be the only step left beyond: a
unification of the physical properties and laws of all 4 forces of the Universe. M Theory seeks a
common ground between General Relativity and Quantum Gravity with the goal of combining all
4. To do so is to take a mathematical look at how the Universe appeared, and how it acted, when
it was still an infinitely small point packing all the matter and energy that exist in it today. To
comprehend such physics would enable a mathematical comprehension of how to manipulate
spacetime itself and pre-vert to a time in the future or revert to a time in the past. Until someone
unifies all 4 forces into a single physical quantity with a value for each point in spacetime, we
aren’t going any-when.

Confused yet?
Welcome to the world of time travel.

According to Albert Einstein,
 To travel into the future
 we must approach the speed of light.
 To travel into the past
 we must surpass the speed of light.
 The current record holder for time-traveling is Sergei Krikalev.
 He has traveled about 337 million miles in orbit at some 17,450 mph
– reaching a grand total of 0.02 seconds into the future.
 This means that from now on, he takes a step two hundredths of a
second before you see him take it.

Time travel

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