Dude, where’s my down?

universesandbox
Screenshot of the Solar System from the PC simulator ‘Universe Sandbox’

Every other week my colleagues and I have a creative writing session. This week the first exercise involved answering random questions, some of which are more personal and I’d rather not get into. (Okay, fine. My pet peeve is auto-play videos on the Internet, I have no ex-boyfriends to murmur regrets about; and I ate muesli, kung pao chicken and lasagna that day). The second was a riddle. Here is mine:

I am one of the slightest things you will ever know, but you feel me all the time. I suck you in, I pull you down; I show you up and I whirl you around. The more you are, the more I am. I can bend your light and curve the space around you. What am I?


There is one force that has all these properties: gravity. Let’s take a look at each of these properties. This will reveal where down is. This will also prepare us in understanding the way the awesome black hole in Interstellar looks.

Among the slightest things, but felt all the time

Gravity is one of the weakest forces in nature; it is measlier than the weak force within the nuclei of atoms. Scientists remain baffled by the question of how such a weak force—so weak that detecting its waves constituted the most precise measurement in all of science—can have such profound effects on the face of the universe. Without gravity, matter would not have clumped together to form galaxies, stars, planets, asteroids and other cosmic objects. Nor would elements heavier than lithium have come into existence. There would be no face or character to the universe.

Yet this incredibly weak force is acting on you all the time. You are speeding on a blue ‘little grain of rock’ at 30 km/s around the Sun as you fall around it. More temporally, the Earth is constantly trying to pull you towards its centre, but its surface is in the way, which is the reason you are stuck on it. If you are still not convinced, go to the 22nd floor of a building and jump out of the window. It is not 100% certain that you are a goner, but it’s exceedingly close it. I have personal knowledge of this, but luckily I was strapped, homey.

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Me bungee-jumping at the Orlando Towers in Soweto in November 2015

It sucks you in, it pulls you down

At some point we all agreed that wherever you fall to, we call ‘down’. The only place you can fall to is towards the point where the magnitude of gravitational force is maximum, which for something in the shape of a sphere is the centre. So down on Earth is anywhere towards its centre. The centre lies in a sphere, so falling ‘in’ (like falling into a black hole) is the same as falling ‘down’. Up, conversely, is the point away from the centre. You are ‘up’ in a plane because you are tens of kilometres farther from the centre of the Earth than the surface.

So down on Earth is anywhere towards its centre.

There is one thing I missed in my riddle, however. I should have added this: If you are enough, I make you round, because that’s sound. As I alluded to, the centre is the point of maximum gravitational force. That means every particle in a random clump of matter is pulled towards the centre to arrive at the most compact and thus stable, or sound, shape. This happens to be a sphere, which has of all geometric configurations the most volume compared to surface area. If you were to crush a piece of paper with all your strength until you could not make it any smaller, you would open up your hand to a ball. By the way, this is why the designers of the atomic bomb decided on a spherical plutonium core: to pack as much nuclear fuel into as small a space as possible.

The more there is, the more gravity

Not all cosmic objects get to be round; asteroids don’t have enough mass to self-gravitate and therefore tend toward a sphere. The more matter our random clump has, the more gravity it commands, giving rise to more exotic effects. This understanding is sufficient to explain how the universe looks at the largest scales, even though it’s not the whole story.

Back to our clump of matter. Let’s say it’s made up of hydrogen and a little helium and all this matter clumps together so tightly that it starts fusing and forms a star. At the end of its life, it will become one of three things: a white dwarf, neutron star or black hole. Which one it will become depends on how massive it is and therefore how much gravity it has. The least massive stars form white dwarves, and the most massive stars become black holes. More on black holes later.

It whirls things around

Isaac Newton wondered whether the reason an object on Earth falls towards the ground is the same reason that explains the movement of the Moon. His hunch was right. The picture below shows a fake star system from the simulator programme Universe Sandbox, available on Steam.

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Screenshot of a fake star system from the PC simulator ‘Universe Sandbox’

All of these cosmic objects are in orbit. How much each object moves depends on how massive it is. The blue object is a neutron star, which commands more gravity. It is circled by the less massive star (yellow sphere). The fragments are flying around the centre of mass of the system, which is the point around which two or more objects orbit. An orbit is the path an object takes under the force of gravity around a more massive object. Satellites orbit the Earth, the Moon the Earth, the Earth the Sun and the stars the black hole in the centre of the Milky Way galaxy.

It can bend light and curve space around it

I think one of the most brilliant effects of gravity is gravitational lensing.

First, let us deal with the curving of space. An object that is massive enough commands enough gravity to curve the fabric of space, another prediction of Einstein’s, leaping out of the paper on general relativity. This distortion accounts for orbits of cosmic objects around stars, neutrons, black holes and so on. This is still brilliant.

Now this is my speculation as a way to think about the curving of space. I would love to know whether I’m right or wrong.

When a ray of light travels through space that is not distorted, you can form a right angle with it and a grid line overlaid on the fabric of space, which I show—rather amateurishly, I concede—in the left-hand diagram. But light in curved space also thinks it’s travelling at right angles, even though that grid at some scale is curved. I try to show this in the right-hand diagram, but I had to force some of those angles, only because of my non-existent drawing capabilities. So in this scheme both rays of light, according to them, think they are travelling in a straight line, not curving, through space. This is because, which is true, it is the space itself that is distorted, the three-dimensional extent that objects exist in, not the objects themselves.

End of speculation.

Back to gravitational lensing. Einstein predicted it in the early 20th century and it was proven. This story is depicted in the film Einstein & Eddington, starring David Tennant. Gravitational lensing happens when the position of an object changes because light from it is deflected about an intensive source of gravity such as a star, galaxy or cluster of galaxies. In the Hubble picture below the ring, called an Einstein ring, is actually a distorted image of the anterior part of the galaxy cluster. Light from here bends around the rest of the cluster towards the Hubble camera.

A smiling lens
A galaxy cluster with an Einstein ring (Source: Nasa)

This all leads us to the black hole depicted into the 2014 Christopher Nolan movie Interstellar. It is an interesting and new visual conceptualisation of this mysterious object. You can see it at the 2:14 mark in the trailer below.

This rendering used the latest data and theoretical knowledge on black holes. Kip Thorne, a renowned theoretical physicist at the California Institute of Technology (Caltech), consulted on the movie and explains the form of the black hole featured in the movie in a talk about Einstein held by the Arizona State University Origins Project, which is chaired by Professor Lawrence Krauss, one of my favourite science popularisers.

All the effects we have covered are depicted in this rendering, which you can see here (Figure 9.8). A black hole is round and it pulls towards its centre. Matter that is moving fast enough is not sucked in or down; instead it orbits the black hole, forming what is called an accretion disk, the bright stream of gas whirling around the black empty space in the picture. In doing so, the particles in the disk rub against each other, creating heat and light. The real accretion disk orbits horizontally about the black hole here. The arcs delineating the hemispheres of the black hole are images, caused by gravitational lensing, of the upper and underside of the accretion disk on the far side of the black hole respectively, which you wouldn’t be able to see. The top arc is light coming up over the black hole from the upper side, and the bottom arc from under the black hole towards the observer.

Watch the explanation about the Interstellar black hole below. It starts at about 45 minutes into the video.

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