Background:
Whenever we see the word, "time travel", our
mind goes to science fiction and how it is depicted in movies and novels where
time travelers move back and forth in time, trying to rewrite past for a better
future but the question is, Is Time Travel possible according to physics?
Well answer to that question is
YES and no! Laws of physics allow time travel but with some limitation. In this
document I try to provide some overview of some of the existing Time Travel
Theories and their limitations.
Time Travel Theories:
Time Travelers:
Before we launch ourselves into
theories, lets first have a look at some of the time travelers that are actually
travelling in time and we can easily observe them. Everything we see around us
people, things, nature, colors, sky, moon, planets, stars even our sun is
because of light emitted by them and entering our eyes. Since, our perception
of sight is dependent on light and light has a finite speed (3 x 108
m/s – very huge number but still finite), light emitted from a body must take
some time to travel before it reaches our eyes. For things around us this time
is a very small number because the distance light has to travel is very small
compared to its speed but when we look at celestial bodies like moon or sun,
distance are huge or comparable to speed of light. It takes 1 sec for moonlight
to reach earth similarly it takes 8 minutes for light from sun to reach earth.
Now imagine you are looking at the Sun (using some protection or it can cause
serious harm), the current image of Sun you are looking at is a state of Sun in
which Sun was 8 minutes ago and the current state is visible to you 8 minutes
later. Suppose, if Sun suddenly disappears while you were looking at the Sun,
you will see Sun disappearing 8 minutes later than it actually did. In other
words we can say moon and Sun are time travelling into past for 1 second and 8
minutes respectively.
Time Travel in Relativity:
The Special Theory of Relativity
came up with the idea that, “Time is not absolute” but it has a different value
for different observer – giving rise to the question what is time? General
Theory of Relativity gives an answer that time is the 4th dimension
in space-time and no event can be explained without all 4 coordinated (3 coordinated
of space and 1 coordinate of time i.e. when and where that event occurred).
Mathematical formulation of Relativity gives us a window towards time travel as
it claims that time for a moving body moves slower than the body at rest. The
famous twin paradox, where we have two twins and one is sent to a space mission
and when he returns he is still young while the twin on Earth has grown quite
old. Experiments have proven this “Time Dilation” and the most common example
of Time Dilation in daily life is Global Positioning System or GPS – The GPS
systems clocks deviates from clocks on Earth (as calculated by relativity)
therefore the compensation factors are added in GPS system for precision.
Coming back to time travel, the time of moving
bodies is slower than the observing body at rest; if the moving body moves fast
enough it can travel in future as given by formula:
Above equation shows
that as the speed of moving body increases the time slows down. If one is able
to build a space ship that can travel close to speed of light, one can travel
in time in the direction of cosmological time arrow (towards the expansion – or
simply future). But it is not that simple there are indeed some limitations
which are listed:
1. To build a space ship that can travel close to speed of light is out of entire Earth’s resources if we use of current travelling techniques,
1. To build a space ship that can travel close to speed of light is out of entire Earth’s resources if we use of current travelling techniques,
2. As the speed increases, according to the
energy-mass relation by Einstein
,
3. But if somehow, we are able to build such
a time machine with some new technique and overcome energy infinity limitation,
we still have on little problem that are the physical effect of time travel on
human body and human history.
4. Well, at microscopic level, we are
actually speeding particles close to speed of light like in Large Hadron
Collider, where we see relativity effects and verify its predictions, but still
unable to manufacture relativity for macroscopic time travelling.
Time Travel in Wormhole:
Building a time machine that is
strong enough and fuel efficient enough to travel at the speed of light is
still far from concept, while this theory seems quite away from current
scenario there are some other theories one of the most promising and not very
far-fetched is the concept of a WORMHOLE.
Wormholes were first theorized in
1916, derived from Einstein’s equation of relativity under the name
Einstein-Rosen Bridge, 1930 when physicists speculated the existence of
wormholes in the fabric of space-time. Wormhole is a hypothetical concept of a
hole or tunnel, which connects two separate points in space-time in such a way
that resultant looks like a tunnel. It is theorized that, if one enters from
one side of wormhole reaches very quickly on the other side even if
these points are separated by trillions of miles.
You can imagine a wormhole as a
sheet of paper with two points marked on it, one of the top and one on the
bottom, the fastest way from point one to point two is a straight line between
two points, or there could be another possibility for fastest track, that is,
if you fold the paper in such a way that both points come in parallel (instead
of top-bottom, it is now up-down), now you can travel down from one point to
the other much faster than traveling on the paper.
For years this seemed that this
passage or transit is impossible, but recent research by Kip Thorne, suggests
that it is possible with some materials that can tolerate the forces acting
inside a wormhole, but the physical limitations of time are still there, what
happens if you enter and reach a time when that wormhole was not even created
or you entered but the wormhole had an instantaneous life and you couldn’t
exit.
We are still looking for natural
wormholes existing in the universe, also working our way towards creating our
own wormholes, because even if we found one, we need to understand and control
is behavior before we could use it for travel.
Time Travel in String Theory:
What is String Theory?
String Theory is currently the
biggest candidate if we are looking for a complete universal “Theory of
Everything”. According to the String Theory, the fundamentals of the universe
are not particles but strings having lengths but no other dimensions, likely
infinitesimally thin piece of string. These strings some are two ended (open)
while some are closed (looped). Everything is frequency and energy. Particles
interactions are also explained in terms of strings, e.g. the absorption and
emission of particles are explained by joining and separating of strings. “Supergravity” explains the reason
of gravity as a virtual particle called graviton.
The exchange of these gravitons is which cause the effects we call gravity,
the same phenomenon is explained in terms of strings as an H-shaped bridge.
This H-shaped bridge is the result of joining of two strings one from each body
(emission of gravitons in particle theory supergravity).
Time Travel:
Just like the concept of
wormhole, string theory also propose some of the most weird concepts, one of
the most remarkable is multiple dimension (greater than four), these higher
dimension are what causing other effects like multi-verse and multi-inflation
concepts. If all the predictions of string theory are true, then there is not
just another you, but infinite another you, throughout the multi-verse. If you
had an option in selecting your career with two options, you may have taken
both in different dimensions these dimensions may also be separated by another
dimension of time because in imaginary co-ordinates there is no differentiation
between space and time. So, there may be another dimension where you are still
6 years old or 60 years old. String theory puts forwards the concept of
transfer between these dimensions as some of the particles (virtual) are able
to move freely between these dimensions but for bodies like humans should have
a beam of parallel strings passing by at the speed of light, they would bend
the curvature of space-time and will create something similar to a wormhole
into time, here there is no distinction between past or future i.e. you can
travel in either direction. But strings are yet to be observed and scientist
are working not to observe strings but to verify the predictions of string
theory.
Conclusion:
There was a time when time travel
was a bizarre concept, then a time came when it was The Topic for science
fiction, now we are entering into an era where we are exploring time like never
before, we are on the verge of science and technology. Time Travel as depicted
in science fiction is not as simple in real life as it is presented (ignoring
the fact that the human civilization is also shown to be quite advance in most
of them). Time Travel according to the laws of science is possible and not just
possible but also happening right now as a result of finite speed of light, we
see everything in the past, like a galaxy 1 million light years away (it takes
a million years to reach light from that galaxy to our telescopes) what we see
there now, is what there was 1 million years ago. Time Travel for particles is
also possible according to the laws of science, but there are certain
limitations when we try to artificially manipulate space-time, these
limitations includes the huge limitation of resources, the physical laws of
nature of space-time, chemical effect on human mind and biological effects on
human body. At microscopic level, particle smashers are working trying to
produce some useful results and some have observed time travel at microscopic
level according to Einstein’s time dilation and relativity equations.
Currently, for humans time travel is a thing of future but certainly
achievable.
