We are all under its influence and we even exert it on each other yet to come to define it and understand it took us a while. It wasn’t until about three hundred and twenty-five years ago that Isaac Newton put down on paper the laws that govern the force of gravity. Prior to that, others had attempted in vain to explain what gravity is. They had an idea of what it does, in general, but it was Newton who showed the universal nature of this force and described, in the most precise language that is mathematics, how to calculate what the force is and how it acts between objects. Then in 1915, Albert Einstein gave us an even clearer and more accurate definition of the force of gravity. This is not to say that Newton’s picture of gravity is outdated and good only for the history books. We still and we will always use Newton’s definition of gravity to analyse, understand and calculate day-to-day problems involving gravity. It is only when we are required to make very accurate predictions of the behaviour of celestial objects or study gravitational phenomena under extreme conditions that we resort to Einstein’s toolbox for gravity. Extreme conditions here mean super massive or dense stars and galaxies and those that move in a certain peculiar way.
What’s interesting is the way each one of them view gravity. In Newton’s eyes, gravity is considered to be a force just as we generally think a force might be: that is something that can be used to push or pull something. In gravity’s case, it’s a force that pulls objects together. Why aren’t all the objects in the universe clumped into a big lump if gravity’s job is to pull everything together? Well, there are other forces at play that interfere with gravity and influence objects in different ways. But that’s another issue altogether. Gravity, for all intents and purposes, pulls everything together. Also, the fact that gravity is a ridiculously weak force (compared to other known forces in the universe), we don’t fall through the floor even though gravity is what brought us to the floor in the first place. Yes, gravity can be painful, if not lethal (imagine throwing yourself off a cliff), but the reason why we are able to stand on the ground and not fall through it is because of another force called the electromagnetic force. This is what stops the atoms from our feet (or shoes) passing through the atoms of the floor. In perspective, gravity is ten to the power of thirty-eight times weaker than the electromagnetic force! That’s 100 billion billion billion billion times weaker! What a wimp that gravity! Yet it hurts when we fall on the very same floor that the electromagnetic force is stopping us from falling through.
So that’s one way to picture gravity: as a force. Einstein’s description of gravity is more in terms of geometry than force. What Einstein has shown is that this thing we experience as gravity is actually a consequence of how we shape and mold spacetime around us. You see, every object in the universe has this ability to bend and twist spacetime around it much like how our body molds into our mattress as we rest upon our bed or how we deform a cushion if we sit on it. Try this little experiment: press hard in the centre of your bed with one hand so that there is enough of a dent around it; with your other hand place a small round object close enough to the dent and watch how it moves towards your other hand as if it were attracted to it. Similarly, every object in the universe causes the spacetime around it to bend such that it attracts the other objects around it. This attraction is what we call gravity. Here no concept of force is required a priori; all we are dealing with is the geometry of spacetime and how matter affects its curvature. On the flip side, the curvature of spacetime then tells how matter to move. If one object is moving towards another due to this “force” of attraction then it’s because the curvature of spacetime between the two objects is such that it’s bringing the two together. Whether one of the objects is at rest, and the other moving towards the first, it is all relative to the observer. What we should gather from this is that gravity is engendered by the interaction of matter and geometry.
If you press lightly on the sofa or a pillow then the dent will not be that deep. Press harder and you will make more of a valley on that surface you are pushing down. The more mass an object has, the more it curves and bends spacetime around it. The sun curves spacetime more than the moon does because the sun is more massive than the moon. If we take this further, it’s not just the mass that counts towards the curvature but also the volume occupied by the object. A neutron star is a type of star that is extremely dense. It contains a lot of mass in a relatively small volume. It could be as twice as massive as the sun yet be only a fraction of the size of the sun. In that respect, because the neutron star is so dense, it curves the spacetime around it more than the sun does.
Another interesting celestial object that is very, very dense is the black hole. This term is becoming more and more common but it still fascinates scientists and non-scientists alike. A black hole is a star that has shrunk so much it’s become much denser than a neutron star. In fact, it is so very dense it can contain the mass of several suns in a volume so tiny that it could sit on a pin’s tip. This is physics to the extreme! It is difficult to conceive of such an object yet they exist and have been detected. More interestingly, their existence was predicted long before they were discovered. A black hole is therefore so dense that not only does it bend the spacetime around it but it actually pierce through it. Imagine pressing a sharp object, like a pen, down on the mattress with all your weight causing so much of a dent in it, you rip through it. There was all your weight (or most of it anyway) acting down on the mattress and being distributed via this very small area of the tip of the pen. With such a big pressure on the mattress, the only consequence is a hole in it. Likewise, black hole, with its very high density, can poke a hole in the structure of spacetime. A hole in which all surrounding objects fall into and never return. Indeed, even light succumbs to the strong attraction of the black hole. And for this reason, this hole in space, because it eats up all light around it, is in fact black. A black hole is simply that: a hole in spacetime that absorbs absolutely everything, even light rays, and is thus black.
Falling into a black hole would not be something I would recommend even though it might well be the most thrilling and the very last thing anyone could experience.
Strictly speaking, black holes do allow some radiation to escape, as shown by Stephen Hawking in nineteen seventy-four. But this does not take away the fact that they are extreme examples of how gravity can distort spacetime. Yet, it is this very force of gravity that acts upon us here on Earth and is what gives us weight. We weigh what we weigh because of the force of gravity of the Earth is acting upon the mass of our body. We would weigh much less on the moon than on Earth because the force of gravity that the moon would exert on us would be much less. So to think that black holes and other exotic astronomical phenomena are just that, exotic and remote, is to ignore that what governs them and what they influence around them has to do with the same force of gravity which affects our day-to-day life. The fall of an apple from a tree to the ground, the moon going round the Earth, the tides rising and falling, the ground-to-ground missile following a parabolic curve as it shoots towards its target, the weight we feel as we lift a heavy shopping bag, and the rain falling are all due to gravity.