The Nature of Space-Time
As people of Earth we take space for granted. From our perspectives it's a
zone that occurs about 100 kilometres above the planet, where there is no air to breathe
or scatter light. It's just emptiness after that, right? Time also ticks on incessantly.
Well it's not that simple. One thing that physicists have learned over the past
century is that space and time form a system of such staggering complexity that it might defy our
understanding of reality. So, Let's take a look at what spacetime is composed of? In this video.
Space is really big and also we don't know much about the properties of spacetime itself.
We know it warps and curves but what exactly is it made up of? Astronomers have worked out only 5
percent of the universe which consists of baryonic particles. The rest 95 percent is Dark matter
and dark energy. We still don't know anything about them apart from observing their effects.
Space seems empty to us but that is not the case. The Observable universe is 46.5 Billion
light years in diameter. The age of our universe is 13.8 Billion light years. We know all of this
by observing all the light coming from the distant galaxies reaching as far as the big bang.
The cosmic microwave background radiation is the afterglow of The Big bang which tells us
about the early universe. Further, space is a vacuum, but that doesn't mean it's empty. Gas,
interstellar dust and other bits of matter float around in the “emptier” areas of our universe,
while the more crowded regions host planets, stars and galaxies. However we cannot be sure that our
universe is the only one that exists. We don't know what exists beyond the observable universe.
But what is Dark energy? What is this mysterious force that makes up
about 68% of our entire universe? In the early 20th century scientists were sure
that the expansion of the universe had to slow down over years. They thought so because the
energy density would decrease and gravity would dominate causing the expansion to slow down.
Just like you throw a ball up in the air and it comes down after a period of time.
But when Hubble Turned his telescopes towards the sky in 1929, he observed that distant galaxies
were moving far away from one another. He then gave the famous Hubble law. According to it, every
distant galaxy is moving away from each other with a velocity proportional to its distance.
The farther away a galaxy is, the greater is the velocity with which it moves away. This idea was
baffling for everyone. It's just like throwing the ball up in the air and instead of slowing down
and coming back to the earth it keeps speeding up and moving away from us eventually in space
and away. If we observe something like this with the ball, we would assume there’s an invisible
force behind it. Astronomers called it Dark Energy, discovered by mainly three scientists
Adam Riess, Saul Perlmutter and Australian astronomer Brian Schmidt who won the Nobel
prize in 2011 for their discovery. It is the most important discovery in the history of science.
Discovering the dark energy and that it's causing the universe to expand at a faster rate was
shocking for scientists. We've estimated the rate of acceleration by using the Planck telescope to
map the cosmic background radiation left over by the Big Bang. Final data, released by the Planck
mission last month, reaffirm that dark matter and dark energy must exist, even if we don't know what
they are. In a few hundreds of billions of years, everything but our nearest neighbouring galaxies
will have moved out of reach: no matter how fast future humans might travel, we'll never be able
to reach anything else. More about Dark Energy is unknown than is known. We know how much dark
energy there is because we know how it affects the universe's expansion. Other than that, it is a
complete mystery. One explanation for dark energy is that it is a property of space. Albert Einstein
was the first person to realize that empty space is not nothing. Space has amazing properties,
many of which are just beginning to be understood. People think that space might even be infinite.
But what is in this space? Apart from planets, stars, comets, asteroids, dust and blackholes
and some other objects; almost everything out there is hydrogen. Hydrogen is everywhere,
it's the lightest element to exist. It can be found anywhere from hot, dense stellar nurseries
to the cold and empty voids between the galaxies. As Harlan Ellison wittingly said, “ the two most
common things in the universe are hydrogen and stupidity”. Hydrogen is the most abundant element
in our universe. But what about the space between hydrogen atoms? What is a vacuum of space made up
of? This is where I would have to call quantum mechanics and things get really weird. Richard
Feynman said, "I think I can safely say that nobody understands quantum mechanics." and that's
very true. On the very smallest scales, it appears there is aminum resolution to the universe,
called the “Planck Length”. On these scales, the universe appears to be a strange quantum foam,
with ‘virtual’ particles popping in and out of existence obeying the laws of quantum mechanics.
No matter how counterintuitive this sounds, we can actually measure the effects of quantum mechanics
in the laboratory, for example the Casimir effect. If you place two very thin plates just nanometres
apart in a vacuum, classical physics might tell you that the space between them is empty. But
the sea of virtual particles predicted by quantum physics is affected by the boundary of the plates:
fewer virtual particles can be created in this small confined space than on the outside of the
plates. This vacuum energy might actually be the key to understanding dark energy of the universe
on the largest of scales. Quantum mechanics is our deepest most comprehensive way of
understanding reality. But the problem arises when trying to understand quantum mechanics.
Honest physicists admit that we don't truly understand quantum mechanics. We have a recipe
that we are taught in our textbooks but we don't exactly know what's going on behind the scenes.
But the mysteries of quantum mechanics are a topic for another video!!
That was all about space and what it is composed of. What is space-time? Space and time are like
the opposite sides of the same coin. The famous physicist Albert Einstein had developed the idea
of space-time when he unified these concepts in the theory of relativity. In simple terms we all
know about the three dimensions of our 3d world- Length, breadth and time. Einstein united it with
the fourth dimension, that is, time. We can stay at rest in space, for example, a man sitting at
rest on his chair. But we always move through time in one direction, it never stops. According to our
current understanding of physics, space time explains the unusual relativistic events that
arise from- travelling near the speed of light and the motion of massive objects in our universe.
Before Einstein had united space and time, Newton’s laws described the motion of massive
objects, while Maxwell’s equations described the properties of light. But by the end of the 19th
century there was something special about light that was bugging every physicist. Experiments
showed that the speed of light was always constant no matter from which reference frame you observe.
Around the same time French physicist Poincare speculated that the velocity of light might be an
unsurpassable limit. Ultimately Einstein pulled all of these ideas together in his 1905 Theory
of special Relativity, which postulated that the speed of light was constant. For this to be true,
space and time had to be combined in a single framework that conspired to keep the velocity
of light the same for all observers. Einstein though didn’t reach the conclusion himself. The
idea that space time is a single fabric came from German mathematician Hermann Minkowski in 1908
who said, “ Henceforth space by itself, and time by itself, are doomed to fade away into
mere shadows, and only a kind of union of the two will preserve an independent reality.”
The space-time described by him is known as the Minkowski space-time which serves as the backdrop
of calculations in both quantum field theory and relativity. But how does Space-time work?
Today whenever people talk about space-time they often describe it as a sheet of rubber or
more often a fabric. This comes from Einstein, who realised as he developed his theory of
general relativity that gravity was due to warps and curves in the fabric of space-time.
Massive objects like the sun and earth create distortions in spacetime that cause it to bend.
These bends or curves constrict the ways in which everything moves in the universe, because
objects have to follow the path along the curved spacetime’s curvature. Motion because of gravity
is actually motion along the curved space-time. Nasa actually sent a mission called Gravity Probe
B which measured the shape of the space-time vortex around the earth in 2011 and found that
it closely accords with Einstein's prediction. But still much of it remains difficult for people
to wrap their head around. Though we can discuss space-time as being similar to a sheet of rubber,
the analogy eventually breaks down. This is because the rubber sheet is two-dimensional
whereas space-time is four dimensional. The warp is not just in space but also in time.
Despite its intricacy, relativity remains the best explanation for the physical phenomenon we
observe. Yet there is a problem looming over the horizon for physicists over the past 100 years.
As discussed above, the problem is to combine General relativity with Quantum mechanics,
which explains the properties of subatomic particles with extreme precision but does
not incorporate Gravity. Some theorists speculate that space-time itself comes in quantized chunks.
Some are trying to quantize gravity. Traditionally when people talked about “quantum” gravity,
they were referring to quantum discreteness, quantum fluctuations and almost every other
quantum effect in the book. We hope this problem might be solved in the future by our scientists.
But one thing that all scientists agree on, is that empty space is surprisingly complicated!!
What do you think about space-time? Can you try visualising it? Let us know your thoughts and
questions in the comment section below. And don’t forget to like the video and subscribe to our
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