Question: how does gravity work

Cesar Lopez-Monsalvo answered on 12 Jun 2011:

I will try to answer this question in three different levels. You may keep the one that makes you feel more comfortable. However, it will be helpful (hopefully) if you read all the way through. Some of the last bits are things we don’t fully understand yet. So, here we go.

1. The first one (Newton: gravity works as a force).

Gravity is what keeps you on the surface of the earth, what makes things FALL towards the ground. At this point you may have realised that for gravity to work, we need at least two objects: one which falls and one which pulls. In reality, every object pulls and falls at the same time. In the case of you and the earth it is “obvious” that the earth is pulling you towards its centre. However, this is not entirely accurate. The truth is that both, you and the earth, are falling towards a point called the CENTRE OF MASS. Since the earth is much more heavier than you are, that point is actually at its centre and, therefore, it gives the impression that you are being pulled towards the centre of the earth. A slightly more interesting case is that of the earth and the moon. The moon is still much lighter than the earth but is much heavier than you. You may think that the moon ORBITS around the centre of the earth, but again this is not quite true. Both, the earth and the moon, orbit around their centre of mass. In this case, the centre of mass is closer to the earth’s surface than to its centre!

This has been quite factual so far. I haven’t tell you how does gravity work. So here is my first attempt. From the little discussion above you have to notice the following. The way gravity works depends essentially on the masses of the objects. Mass is the key property. Basically, every object you can think of has some mass and, therefore, every object is “falling” towards one another’s centre of mass! The reason I do not see this computer falling towards me, or your neighbour crashing on you (if it happens do not blame on gravity) is because gravity is very weak compared to other effects around. You would need huge masses to be involved to notice anything (like that of the earth keeping you attached to it), but that does not mean that gravity is not there. Gravity is everywhere and affects everything!

You will learn that gravity works according to Newton’s Law of Gravitation, which gives you a formula for the attraction between two objects due to gravity. It took Newton over 20 years to obtain that result. He needed to “invent” a whole new piece of Mathematics to make his physical intuition consistent and solve many problems of the time (the planetary orbits) in a “beautiful and elegant” manner (you may wonder what is beautiful and elegant for a mathematician, but that is another question). This is as far as you can go up to university level.

2. The modern one (Einstein: gravity works as “geometry”).

Gravity is geometry and energy is its source. I will try to explain what I mean by this, although it will be a bit harder. The name of the modern theory of gravity is GENERAL RELATIVITY.

General relativity is our present theory about how gravity works. It explains gravity in terms of the geometry of something we call SPACETIME. Notice I did NOT say space AND time, but SPACETIME as a single word. Spacetime is the “stage” we live in. It is “kind of” a FOUR dimensional fabric with special features and bends wherever some object with mass is. Things move around this fabric according to the bends and the bends change as the objects move around. Everything is in motion!

This may sound quite unbelievable, or that whomever invented it (Einstein) had a lot of creativity. In fact, it took Einstein over 10 years to understand what spacetime actually is and, just like before, some new mathematics were needed to fully describe it. The old idea about the “force” of gravity is contained in this better picture. Basically it says: gravity is not a force, but in our everyday experience behaves AS IF IT WERE a force. To see the real consequences of our modern version of gravity we need even bigger masses than before or, if you have ever seen a SatNav or a GPS device, those work fine because we understand gravity in this “geometric” way, the force picture would give you the wrong answer!

This theory has allowed us to understand a lot about the universe. How planets orbit around the stars, what happens when a star “dies”, what is a black hole, and it has given us very strong clues to understand how the universe itself came to be. However, there are plenty of things we still do not understand, hence the next and last answer:

3. The current one (We don’t know!)

Ok, maybe we do know many things, but when it comes to put a whole picture of the universe together…well, gravity kind of breaks it apart.