If someone came up and asked you to define your EXACT position, you could give them your
x, y and z coordinates, which would tell them exactly where you are, or more accurately,
your geographical position in space.
Anything can be located with these usual three dimensions of space, which is the system people
use around the world to describe where a particular object is.
However, what if a fourth dimension—time—is added to the mix?
This is the idea on which the concept of space-time is based.
TIME... how do we define it?
And more abstractly, how do we perceive and experience it?
In our daily lives, we perceive time as a constant entity—one that proceeds independently
of everything else.
Whether you are in New York City or Mumbai, time proceeds at the same pace and in the
same direction, i.e., forward.
Indeed, this is how time is characterized in classical physics.
What you might not know is that this definition has some limitations, and as such, there are
many phenomena that the principles of classical physics cannot explain.
This is where quantum mechanics comes in.
Famous physicists Albert Einstein and Max Planck are considered the pioneers of this
branch of physics.
As early as 1905, Einstein proposed a now widely popular theory that the speed of light
is independent of the motion of all observers, and that space and time are interconnected
in a single continuum.
This theory, which is now a cornerstone of modern and quantum physics, is known as Einstein’s
special theory of relativity.
Einstein's proposed idea of a single continuum where space and time are interwoven is what
people call “space-time”.
According to this theory, time—which has traditionally been considered an independent
entity according to the principles of classical physics—is affected when a body moves through
space.
This happens because, according to the theory, time and space are connected and part of a
single continuum—spacetime.
One of the very popular examples of this is that if you, hypothetically, were to move
incredibly fast and approach the speed of light, time would slow down for you.
Einstein also hypothesized that observers in relative motion with each other will disagree
on the timing of simultaneous events.
Put simply, if one observer says that they observed two given events occur at the same
time, another observer, who is in relative motion with the first one, will disagree,
saying that those events happened at different times.
After bringing to the world his ideas about spacetime and how both space and time were
inextricably connected, he continued to work on his theory to study how massive objects
impact the fabric of spacetime, and subsequently introduced gravity into the mix.
As a result, the General theory of relativity was published in 1915—a decade after the
Special theory of relativity.
Classically, gravity has been defined in alignment with Sir Isaac Newton’s three laws of motion—a
force of attraction that exists between two bodies, regardless of how small or massive
they are.
You, watching this video, are currently being pulled by the center of the Earth towards
it.
This is what keeps you from floating off into the air.
According to the general theory of relativity, however, all objects exist in the continuum
of spacetime and distort the spacetime around them.
It is this distortion that pulls in other bodies, giving rise to the force of gravitation.
Think of it this way—imagine spacetime to be a tightly stretched trampoline.
If you put a large object in the center, it will press down on the trampoline and create
a dimple on the surface.
Now, if you roll a marble on the trampoline, it will spiral inwards towards the dimple,
as if it were being pulled in by the dimple of the trampoline.
Although this is a very famous method to visualize the concept of spacetime, remember that it
is a very oversimplified analogy and doesn’t provide the complete picture of gravity or
the general theory of relativity.
Distortions in spacetime give rise to some really fascinating phenomena.
When light in space bends around an enormous object, say, a black hole, the object acts
as a lens.
This is called gravitational lensing, and it helps us see objects that lie millions
of kilometers behind the object operating as a lens.
We have a separate video on gravitational lensing and how it works.
You can find a link to that video in the description.
Even Earth, through its rotational motion on its axes, causes a distortion in spacetime
that affects space objects—natural or manmade—in its vicinity.
Everything causes distortions in space time.
And these distortions help us a lot to enhance our understanding of the incredible universe
we live in.