My friend Louis asked me the other day whether gravity is different at the equator than at the poles. This was in response to learning that the Earth is wider as the equator and not precisely spherical. Its a good question. He then followed it up with ‘how does gravity work?’ which is an even better question since no one really knows. So here are some thoughts on gravity.

Isaac Newton wasn’t the first to wonder what gravity was. Aristotle and Galileo are just two famous examples of philosophers who tried to address the issue. Aristotle believed that all bodies fell toward the centre of the Earth in proportional to their weight. Galileo famously (or mythically) dropped a cannonball and an apple off the Leaning Tower of Pisa to see which hit the ground first.

Newton was however the first to quanitfy gravity, in his Principia Mathematica. He outlined several of physics’ most fundamental laws in mathmatical form in that text and with his exploration of the understanding of the laws of motion he was able to explain the motions of that mythic cannonball as well as the bodies of the heavens as well.

Newton’s laws are sufficient to explain the motion of objects in out everday lives. From the acceleration of cars on motorways to the flight of planes through the sky to the weight of objects on a tabletop, Newton’s Laws can adequately explain them all. They can almost exactly explain the precise motion of the planets around the Sun.

But not quite.

Albert Einstein outlined his General Theory of Relativity in 1915 which showed gravity as the curvature of the fabric of spacetime. Why he did this isn’t worth going into here, but he did and he was right (so far as we know). We can now successfully predict the motions of the planets to within microseconds. Relativity along with Newton’s laws have allowed to send men to the Moon an spacecraft even further.

So how does it work? Well in Newton’s equation (which is s far as we’ll go here in terms of equations) the force of gravity between two objects is directly proportional to the sum of their masses and inversely proportional to the square of the distance between them. What does that all mean? Well if you double the masses, you double the force and if you double the distance you decrease the force by a factor of four (i.e. two squared).

We can now measure variations of the gravtational field to a high level of accuracy. Spacecraft and satelites have now been measuring the Earth’s delicately varying gravitational field for some time. Below is a map showing the variation of gravity across the globe, taken from the GRACE satellites.

GRACE (Gravity Recovery and Climate Experiment) are two satellites placed into orbit in March 2002 which monitor the Earth’s gravity among other things. It shows in red, regions where the gravitational field is slightly (and I do mean ever-so slightly) higher and in blue where it is lower. Here it is again but in 3D.

So this answers Louis’ question in as much as it can be answered. The answer is that yes, gravity changes across the surface of the Earth, although not perceptably by human standards. How it varies is shown above in the animation and in the map.

As for what gravity really is… well that is the subject if much observational and theoretical debate. It is out there, all pervasive and influencing everything. It shaped the universe from its earliest moments and may well shape its ultimate demise. At the moment it seems to be a good bet that, just as Einstein described it, gravity is the bending of spacetime. It is also a good bet that  the laws of gravity are the same here as they are anywhere else in the universe (although there are many who would argue this point).

If you have any other questions about gravity then I’d be happy to try and find out more for thr blog. Just drop me a line or leave a comment here.