I posted quite some time ago a response to the question ‘how does gravity work?’. It seems my answer was not satisfactory. It is still the question that I most frequently receive via email from this blog. To be fair my prior post was more specifically about how gravity changes across the face of the Earth. all the same here’s another stab at explaining what gravity is, and how it works.

I shall deal with gravity in three easy to swallow steps…

Step One: What is Gravity?

Gravity is the force felt by objects with mass, that pulls them toward other objects of mass. All mass in the universe is pulling on all other mass, all the time. You are attracting the computer screen, I am attracting the keyboard. Jupiter is pulling on your hair right now, as is the Andromeda galaxy and everyone in the room next door. Gravity is pervasive.

The strength of the pull of gravity between two masses (e.g. the mass of me and the mass of you) is determined by a famous equation, derived by Isaac Newton. In fact this equation is the subject of a t-shirt that is sold right here at Orbiting Frog.

F is the force of attraction, m(me) and m(you) are the two masses and r is the distance between the two masses. G is the gravitational constant and is a number that never changes. This kind of equation represents what is known as an inverse-square law. This refers to the fact that the force of gravity gets weaker as r gets larger by a factor of r x r. There are other inverse square laws in physics.

Step Two: How Does Gravity Work? - The Simple Answer

So having briefly covered what we’re taking about when it comes to gravity, we can now ask how gravity behaves and what is causing it. This was famously exemplified by Einstein with his General Theory of Relativity.

It is best to use an analogy here. Think of three dimensional space as a flat surface i.e. let’s ignore one dimension so our heads don’t explode. In this example we shall pretend that space is like a mattress, covering your bed.

If you put a big object on the mattress then other objects will tend to roll toward it if they get close enough. Try this yourself. Put a tennis ball on a bed and then go and put a heavy pile of books down on the bed next to it. The ball will roll toward the books, as if attracted by some force. By bending the fabric of the mattress you have created what looks like an attractive force between the two objects.

This is like the model that is used for gravity. The Sun is a very heavy object and it bends the fabric of spacetime around it, pulling objects toward it. Spacetime is the three dimensions of space plus time. You can think of it as the framework of the universe. Don’t get too carried away though, this is just a model. I don’t want you to think that spacetime actually is like a mattress! It just helps us to understand it better sometimes by making an analogy.

Step Three: How Does Gravity Work? - The Advanced Answer

So what is really happening? Why does spacetime seem like a mattress or a fabric on the face of things?

Quantum mechanics and something called the Standard Model are very good at modelling how the universe works. It tells us how three of the four forces of the universe operate. The electromagnetic force is conveyed via a particle called the photon. It is a massless piece of energy that moves through the electromagnetic field and thus transmits electromagnetic energy from one place to another. e.g. light travels from the Sun to your eyes via photons.

The strong force and the weak force are the two other forces covered by the standard model. Each of these also has a ‘carrier’ particle that transmits them across distances. These are atomic forces that only act on extremely small scales. They hold all your atoms together, so be grateful we figured them out or goodness knows what might happen!

Gravitons are postulated because of the great success of the standard model. In this framework, gravity is transmitted, or mediated, by gravitons, instead of being described in terms of our curved, mattress spacetime as above.

So why the two approaches? Well in on any large scale, the two models give the same result. The problem is that on a microscopic level, the massless graviton creates terrible mathematical issues that cannot yet be experimentally verified or corrected. Thus the graviton remains a theoretical particle… for now. Although people are looking for it.

If it exists then it fits in very neatly with the rest of physics and the graviton can start attending particle physics lunches without embarrassment. If it doesn’t fit in, then there is a problem. It would mean we have something wrong with our model and need to rethink quite a few things.

If you want to read more check out this Feynman book.