Getting down with gravity; what it is and how we describe it

There are four fundamental interactions in nature, gravitation being one of them. We can’t touch it, or see the force itself, but we can feel it and infer its presence from observation of the motion of objects. We are pulled towards the centre of the Earth which is why we always return to the ground when we jump up. We observe the Earth orbiting the Sun and so infer that there must be a force of attraction between the Sun and the Earth, and vice versa.

Any two old lumps of matter are attracted to each other. You and me are gravitationally attracted to each other, albeit very slightly. In this article we begin to discuss how we describe gravitation, what it is, where it comes from and how our understanding of it has evolved since Sir Isaac Newton changed our understanding of our wonderful universe.

What is gravity?

There are several different ways you can go about thinking about the force of gravity. The first just involves thinking about an invisible tug that everything has on everything else. Think of swinging a bucket round on the end of a length of rope; here the rope is analogous to the force of gravity, keeping the bucket in orbit as you swing it round.

Another way of thinking about it is that gravity is caused by the distortion of space-time by a massive object, such as the Earth or our Sun. Think of a circle of people holding a stretched tarpaulin, or cloth, and then someone dropping a heavy ball onto the cloth. It will roll into the centre and the cloth will bow, or drop towards the ground because of the heavy ball. In this image, the cloth is a ‘sheet’ of space-time while the ball is our Sun, for example. Next, imagine someone releasing a smaller, lighter ball onto the sheet with a small velocity. This ball will go round and round the larger ball at the centre of the cloth, due to the distortion made, until it finally collides with it.

Which ever way you think about gravity, the basic concept is that any object will exert an attractive force on any other object. Another idea to get around is that the gravitational force has an unlimited range. We are experiencing the gravitational pull of our nearest neighbour right now, although the distances involved mean this force is negligible – we don’t really see its effect.

Describing Gravity

In the 17th Century, Sir Isaac Newton proposed the now famous inverse-square of gravitation. This says that the gravitational force between two bodies falls off as one over the square of the distance between them! He also proposed that the force is proportional to the product of the masses, , of the two bodies. To put this into an elegant equation:

Newton’s theory of gravity, and this equation, stood proud and tall for two centuries until the orbit of Mercury around our Sun was found to behave in a way that Newtonian gravity could not account for. This meant that, in some small way, this account of gravity was wrong!

At the start of the 20th Century, Albert Einstein’s theory of general relativity came to the rescue with a different treatment of gravity that accounted for this behaviour, where Newtonian gravity did not.

While we know Newtonian gravity is fundamentally wrong, it is still taught and used in schools from secondary school right up into university. Why, you may ask yourself, do we teach something that is wrong? Well, it actually gives answers that are sufficiently accurate when we are dealing with relatively small masses and speeds. Newton’s theory is also much easier to use than the equations given by general relativity. So those reasons are why we use and teach a four hundred year old equation that is slightly wrong.

Down to Earth

So, this has been a quick account of the tale of our description of gravity but by all means it is not a complete account. There’s much more to discover about gravity. For example, we still don’t really know why gravity is so weak compared to the other natural forces in the universe. We know gravity is weak because we can jump off the Earth’s surface; one magnet can lift another off a table; two atoms are separated at some equilibrium distance instead of colliding with each other.

We also do not really know the true nature of gravity, and why it exists. Experiments at the Large Hadron Collider (LHC), and other scientific institutions, are currently under way which aim to try and figure out gravity’s secret. Remember, without gravity we couldn’t form structures like galaxies or solar systems or the planet on which we live.

Further your knowledge

If you’ve been enthralled by this article, check out the following topics and get back to us with any questions you have!

• The graviton
• Fundamental forces
• Relativistic particles
• What is the gravitational force present between two astronauts with masses of 80kg in empty space at a distance of 1km away from each other, far away from any other mass?