Oh! Why am I falling?

 

 

A young man, sitting idly in the laps of his garden and an apple strikes his head. The story sounds familiar, right? It indeed is and the young man I speak of is none other than the great Isaac Newton.

Storytellers for centuries have been obsessed by how, in a stroke of brilliant insight, Newton suddenly came up with his theory of gravity after an apple hit his head. The story certainly appears embellished, courtesy generations of narrators who walked the Earth after his demise. Till today, it is one of the most famous anecdotes in the history of science. And that stands testament to the colossal figure that Newton was.

No! This article is not about Newton or that apple which led to one of the greatest theories in science.

Yes! This article is about gravity and tries, at its very core, to help us understand how gravity has shaped everything across the cosmos.

Simply defined, gravity is the fundamental force that binds us to our planet. Gravity keeps everything in place for us to observe and appreciate. It keeps the water in the vast oceans that cover almost three-fourth of our planet; it holds the vast ranges of mountains from the Alps to the Great Himalayas; it even keeps the man-made marvels to stay grounded. It creates stars from the primordial cloud of gas and cosmic dust and sculpts the planets and moons and set them in orbit around these newly formed stars. And gravity connects these star systems together in vast galaxies and steers them on their journey through unbounded space.  It is also responsible for creating shape (and destroying them too!) and order and initiating patterns that repeat across the entire observable universe. Understanding this fundamental force of nature has led us to a far deeper understanding of our place in the midst of vast cosmos. Every second, we experience gravity that is often taken for granted. Yet this force is able to keep us firmly rooted to the ground.

Now, if I were to ask you, “How do you know that there’s gravity around us, everywhere?”  Then you might say, “I can just jump and show that I was pulled back by something back onto the ground. And that something, obviously, is gravity.”

Well, yes, but actually, gravity is a little bit more complicated than that.

In school, we were taught that gravity is typically a force between masses.  When you stand on a weighing machine, the number on the scale represents the pull of the Earth’s gravity on your mass, giving you weight.  It is easy to imagine the gravitational force of the Sun holding the planets or the planets holding their respective moons in their orbits. Well, forces are easy to understand as pushes and pulls.

But we now understand that gravity as a force is only part of a more complex phenomenon. This complex nature of gravity is described by Einstein’s general theory of relativity. While general relativity is a theory of extraordinary importance, it’s a radical departure from the idea of gravity as a force. The key to general relativity lies in the fact that everything in a gravitational field falls at the same rate. Stand on the moon and drop a brick and a paper, and they will hit the surface at the same time. The same is true for any object regardless of its mass, and this is known as the equivalence principle. Since everything falls in the same way regardless of its mass, it means that without some external point of reference, a free-floating observer far from gravitational sources and a free-falling observer in the gravitational field of a massive body each have the same experience. For example, astronauts in the ISS (International Space Station) look as if they are floating without gravity.  Actually, the gravitational pull of the Earth on the space station is nearly as strong as it is at the surface.  The difference is that the space station (and everything in it) is falling. The space station is in orbit, which means it is literally falling around the Earth.

Let me demonstrate another example.

Suppose you are inside a commercial aircraft which is at an altitude of, let’s say, 40,000 ft. Now, by an aviation blunder, the aircraft suddenly suffers fuel starvation. Its engines shut down and the plane starts falling to Earth. But now something extraordinary happens. As a matter of fact, you are now plummeting towards the ground. In fact, you will feel like you are just floating.

By simply falling at the same rate as the plane, you became free of gravity’s hold. On a lighter (or serious?) note, you might not be able to appreciate this experience fully considering that the plane carrying you has run out of fuel and is falling towards the earth.

Now, there’s something very profound here, because although you are falling towards the ground, gravity has completely gone away. Congratulations!  You’ve cancelled gravity out just by falling. If you understand that, then you understand gravity 🙂 It is possible, by the simple act of falling, to get a very different experience of gravity.

And gravity does more than just bring us back down to Earth. Gravity also plays a role on the grandest of stages.  Across the universe, from the insignificant speck of dust to the most massive star, gravity is the artist that created order out of chaos.

Can you recall the definition of gravity that was taught to you during your school days? If you don’t, let’s ask Sir Newton himself.  Back in the 1680s, Newton said that “Gravity is a force of attraction between two objects as a function of their masses and the distance between them.”

Now, force between two small pieces of rocks is very small, because of their small masses, and almost impossible to measure and therefore goes unnoticed. But there is a more massive rock around us. It’s our planet Earth. The mass of our Earth generates a gravitational pull strong enough to sculpt the entire surface of the planet. A pull that is strong enough to shape entire continents and keep the moon in orbit without flying off away from us.

But Gravity does more than what we see around us, in everyday life. Gravity‘s influence can be felt way further than our naked eyes can see…

When you look up into the night sky, then you see the universe as it looks in visible light, with the twinkling stars and in some places, our galaxy as well. But that’s only part of the picture, because the universe is full of dust and gas which you can’t see with naked eye or even with a conventional telescope.

But we can peer deep into space and witness the incredible attractive strength of gravity using a radio telescope. Whilst most galaxies have been rushing away from each other ever since they formed just after the Big Bang, some galaxies formed so close together that they are locked in a gravitational embrace. Milky Way and Andromeda are two such galaxies. Using modern telescopes such as these, it was found that Andromeda, a spiral galaxy roughly over two and a half million light years away, is approaching our galaxy at an incredible rate, roughly at 110 km/s. Every hour, the gap between Milky Way and Andromeda is shrinking by little less than 0.4 million kilometers. In around three billion years’ time, they will collide and all the order would be turned to chaos!

There seems to be no limit to the reach or power of gravity. Its influence can be felt across the vast expanses of space and time. But there’s something peculiarly fascinating about gravity. It is by far the weakest force of nature. When you pick a piece of rock up off the ground, have you ever appreciated the fact there’s an entire planet, Earth, trying to pull it down. And yet, with utter ease, you have picked up that piece off the ground!

Now, you might be thinking that if gravity is so weak, how come it’s so influential? Well, gravity is dependent on mass. So, although gravity feels weak here on Earth, it certainly isn’t weak everywhere across the universe. And that’s because gravity is an additive force. It scales with mass, so the more massive the planet or star, the stronger its gravity. And there are vast array of cosmic bodies where gravity is far stronger as compared to Earth. Jupiter’s surface gravity is 2.5G meaning it is 2.5 times stronger than that on Earth. An exoplanet called OGLE2-TR-L9b, which is around a star in the constellation of Carina has a surface gravity of 4G. Our sun has so much mass packed inside a relatively small space that its surface gravity is an astonishing 28 times that of the Earth!

Throughout a star’s life, there is a constant battle between energy pushing out and gravity pushing in. As long as the star burns, the two forces balance each other out. But when it runs out of fuel, gravity wins and the star collapses and then explodes with the brightness of a billion stars, the size of our sun.

An image of Crab Nebula

 

This is the Crab Nebula, an expanding cloud of gas and dust. It is the remnant of a star, its core crushed by the force of gravity leading to a massive explosion. The explosion was of such scale that the brightness emanating from this star outshone every other star in the night sky for nearly three weeks. The death of this supermassive star was witnessed by the people of Chacoan civilization who, it is believed, recorded the event in a painting made on an overhang circa 1054 AD. While the explosion blew most of the stellar material out into the cosmos to form this vast nebula, there is something very interesting at the center of this nebula. At the center lies the remnant of the star. It has now become a neutron star.

An image of that neutron star, at the center of Crab nebula

 

It is only about 20 kilometers across, but it’s got the mass of our sun. And it’s spinning at a whopping rate of over 30 times per second. As the neutron star spins, jets of particles stream out from the poles at almost the speed of light. These jets are powerful beams that sweep around as the star rotates. When the beams sweep across the Earth, they can be heard as regular pulses, thus the name pulsars. But it’s not these regular pulses and the whopping spinning rate that makes the Crab Pulsar so interesting. It’s the extraordinary nature of gravity on this neutron star. If you were to be on its surface, then the pull of gravity on you would be a hundred thousand million times that that you feel on Earth. Simply put, if you were to jump from a height of two meters, then by the time you hit the ground, you would be travelling at over 11 miles per second! Who on ‘neutron star’ said Gravity is weak?!

Now, what does gravity do to matter at the very smallest scales that brings such extreme cases?

Everything in the universe is made of atoms. Atoms, in turn, consist of an atomic nucleus surrounded by a cloud of electrons. And whilst almost all of the mass is contained in the nucleus, it is incredibly tiny compared to the size of an atom. What I really mean by that is if you were to expand an atom out to the size of Wembley football stadium, then the nucleus would be no larger than the size of a green pea.  And at this exaggerated scale, electrons are incredibly tiny equivalent to specks of dust. Just imagine: a giant sphere centered on the atomic nucleus stretching out all the way to the outer edges of Wembley stadium, with just a few specks of dust in it. That’s an atom!

And that means that matter is almost entirely empty space. Everything you can see in the universe is pretty much empty space, including you and me. Now, if everything in the universe is made up of atoms and atoms are 99.99% empty space, then most of the universe is empty.

But what about the crab pulsar?

Well, in the Crab Pulsar, the force of gravity is so extreme that the empty space inside the atoms is squashed out of existence. So all you’re left with is incredibly dense matter. How dense, you may ask. Well, if you were to take a lump of neutron star matter the size of a sugar cube, it would weigh as much as all of humanity!

But what is gravity?

Well, I’ll explain that in a moment but first let me tell you an interesting story.

Our distant neighbor, Mercury has quite a complex orbit. It’s not a perfect circle but an elongated ellipse. At its closest approach to the sun it’s around 46 million kilometers away and then it drifts out to something just under 70 million kilometers. It’s possible to calculate Mercury’s orbit very precisely using only Newton’s laws of gravity. So, in earlier times, astronomers used to predict the exact time when you could look up into the sky, look at the sun and see Mercury pass across its face. The sad part was they never got it right. They predicted it time and time again, and every time it happened they got it slightly wrong. Rather than questioning Newton, they invented another planet and called it Vulcan. They believed that there must be another planet somewhere in the solar system that wobbled Mercury’s orbit a bit. And so that was the reason their calculations were wrong. For decades, astronomers hunted for Vulcan. A planet that never existed.

They were wrong and Albert Einstein explained them why. The explanation disproving Vulcan’s existence required a modification, in fact, a complete rewriting of Newton’s law of gravity. A new theory, called General Relativity, was published in 1915 by Albert Einstein. The theory proposed that gravity is not a force pulling us towards the center of the Earth. In a sense, gravity isn’t really a force at all!

Not only was the new theory able to explain with absolute precision the strange behavior of Mercury, but it explains to this day everything we can see out there in the universe that has anything to do with gravity. And most important of all, it explains how gravity actually works. Einstein proposed that gravity is the effect that the stars, planets and galaxies have on the very space that surrounds them. According to Einstein, space is not just an empty stage, it’s a fabric called space-time. This fabric can be warped, bent and curved by the enormous mass of the planets, stars and galaxies. All matter in the universe bends the very fabric of the universe itself. But when you get on to the scale of planets and stars and galaxies, then they bend and curve the fabric of the universe  by a very large amount indeed. And here is the key idea. Everything moves in straight line over the curved landscape of space-time. So what we see as a planet’s orbit is simply the planet falling into the curved space-time created by the huge mass of a star. This is able to explain Mercury’s erratic orbit. Because of the planet’s proximity to our sun, the effects of the curvature of space-time are far more visible for Mercury than for any other planet in the solar system. But this idea of curved space is difficult to imagine So one way to think about gravity is that everything in the universe is just falling through space-time. The moon is falling into the valley created by the mass of the Earth. The Earth is falling into the valley created by the sun. And the solar system is falling into the valley in space-time created by our galaxy.

But remarkably, the theory could also predict its own demise.

Our sun takes about 200 million years to make one orbit around the Milky Way. Now, at the center of our galaxy, there are stars known as S stars which take only 15 years to go around the center of the galaxy. They are travelling at a rate of 3000-4000 km/s. And well, the object in the center of our galaxy is 4,000,000 times as massive as the sun and it fits into a space smaller than our Solar System. And there’s only one thing that can be so small and yet so massive. A Black Hole!

Black holes are objects so dense and so powerful that they warp and stretch and bend the structure of space-time so much that they can slow down time. They curve space-time so much that nothing, not even light, can escape. Black holes are the most extreme example of warped space-time. Near a black hole, space flows faster and faster towards the black hole and at the very special point called the event horizon, space is flowing at the speed of light into the black hole. Light itself travelling at 1,86,000 miles per second is not going fast enough to escape the flow. And light itself plunges into the black hole.

As we near the center of a black hole, then our understanding of the laws of physics breaks down. Einstein’s theory of General Relativity, says that space and time become infinitely curved that the center of the hole becomes infinitely dense.

That place is called the singularity, and it is the place where our understanding of the universe stops!

Gravity is a great sculptor, force that can reach out across the vast expanses of space and pull matter together to make the moons, the planets, the stars and the galaxies. But gravity is also the destroyer. And for the most massive objects in the universe, for the most enormous stars and the centers of galaxies, gravity will eventually crush matter out of existence.

Einstein gave us a theory that could explain gravity. From the reason behind the wobbly orbit of Mercury to the existence of black holes, this theory, truly, was a masterpiece that gave answers to questions that had befuddled scientists and astronomers across the planet. But the most surprising thing about Einstein’s theory of gravity is that even this is not complete.

If you are looking for an answer, that is the place to be. On the border between the known and the Unknown…