space-time:
understanding the true nature of time
In my previous blog I discussed about
how Einstein’s theory of relativity changed the perspective of space and time
and discussed about how space and time are not independent but they are
combined as space-time continuum. It was just an introduction to the space-time
concept, in this blog I will discuss about how this unification helped us in understanding
the true nature of time and another mind-blowing concept called time dilation.
Time as I described in my previous
blog is very confusing, each and every person has their own measure of time
which we don’t observe due to the speed with which we are moving is less. What
is nature of time then, how can we describe past, present and future in
space-time? is there any difference between these three or is it just a
persistent illusion? let us find out answers for these questions.
Visualizing a 4-dimensional structure
of space-time is very difficult, so most of the time to understand the
space-time, it is converted to 2-dimensional structure. Before understanding
the graph, knowing what an event is important. An event is something which
happens at a particular place in space and at a particular time, one can specify
it by using four numbers or coordinates.
Maxwell’s equation says that speed of
the light should be constant irrespective of the source. Whenever a pulse of
light is emitted from a point in space at a particular time, it is observed
that as time goes on, the pulse becomes sphere of light spreading across the
space. For ex: when ripples are formed when you throw a rock in the pond. If we
take the snaps of this event at different time periods and kept above each
other we would notice that it forms the shape of a cone. Tip of the cone would
be the place at which pulse of light was emitted and then slowly spreading as
sphere of light just as ripples of water.
The light which is spreading in the
space forms a 3-dimensonal cone in the 4-dimensional space-time structure or
graph. This cone is what we call future cone of the event, similarly we can
also draw another cone called past cone below the future cone.
Now, we can divide the given event
into three classes or three types. The events which can be reached from point P
(P is the point where event takes place) by a particle or wave traveling at or
below the speed of light is said to be in the future of P. whatever takes place
at P has an effect on the future of P. similarly past of P can be defined as
set of all events which leads us to reach the event P traveling at or below the
speed of light. The below diagram shows the light cone in space-time graph
Some of the events neither lie in the
future cone nor in the past cone, these events are said to be in the elsewhere
section of P. whatever happens in this neither affects or gets affected by P.
To understand this let’s take an example which happens daily. when the rays of
sun start its journey at this very moment, it would not affect the things on
earth at this moment, because it would be in the elsewhere event. We can only
observe its affect after 8 minutes, as the light from the sun takes this much
time to reach the earth. Only then we will be in the future light cone of the
event that took place. In the same way the light which we see from the distant
galaxies and stars are actually of past and our earth has now entered the
future cone of the event. This tells us that what we are seeing is actually a
past, a kind of time traveling.
Now, we saw the representation of
past, present and future in the space-time but the real shocking revelation
done by Einstein in his theory of relativity is that, there is no difference
between past, present and future. All of this is just a persistent illusion
which in turn says that time what we see is just an illusion. when I got to
know about this, I was like Archimedes yelling in the streets “Eureka, Eureka”,
but I was doing it inside my mind. Now, you might be thinking how’s it
possible. Let me explain this in a simple way.
Consider all the things happening at
right now moment, for ex: right now, I’m in front of my laptop writing my blog,
my roommate is sleeping, flight is taking off from an airport, a star exploding
somewhere in the universe. These are some of the events which are taking place
at the very moment, in the common-sense point of view we all agree that these
are happening right now. But when motion comes into play this common-sense view
of time goes out of window. Imagine space-time as a loaf of bread. There are
different ways to cut this loaf into slices, in the same way space-time can be
cut into different ‘now’ slices. As motion affects the time and we all are
moving with a particular speed, we cut the space-time loaf in a particular
‘now’ slice. But the person who is far away from us and has his own motion will
have his own perception of ‘now’ slice. Which tells us that his ‘now’ is
different and my ‘now’ is different. Let me give you an example to understand
this confusing thing.
Imagine an alien sitting still some
billion light years from earth. And here on earth I’m sitting on the bench
reading a book. Since we both are still and our clocks are ticking at the same
rate, we both share same ‘now’ time on slice.
Now, imagine alien sitting on bike and
moving away from earth, this makes our clocks ticking at different rates and we
no more share same ‘now’ slice. Alien’s ‘now’ slice will be cut into a
different angle. Even if the angle is small, due to vast distance it creates
such a large difference that Alien will be seeing 200 years past of earth. What’s
even stranger is direction makes the difference too. Imagine alien moving
towards the earth now, this makes alien’s ‘now’ slice cutting into the future.
His ‘now’ would be seeing the future of earth. I would no longer be sitting on
the bench there and the alien would be seeing my great, great, great grand
daughter or son time traveling or teleporting.
This says that your ‘now’ can be my
past, your ‘now’ can be my future and we both are at our own ‘now’ time, then
the past is real and the future is also real. Which concludes that past,
present and future all exist at the same time. there is no difference between
these three and it’s just an illusion. everything that has ever happened or
will happen all exists in this big loaf of space-time bread.
The universe in which we are living is
lot stranger than what we thought, its beyond of our thoughts and imagination.
As my favourite astrophysicist Neil deGrasse Tyson says “we are stardust
brought to life, then empowered by the universe to figure itself out and we
have only just begun”.
