What is gravity?

Question: What is gravity?

Answer:

Thanks for asking such a great question! You were right to suspect that your introductory (or perhaps even intermediate) level physics classes did not cover gravity in sufficient detail to give you a "deep" understanding of this phenomenon.

You probably remember that Isaac Newton was the first person to identify all matter as the source of a universal, far-reaching attractive force which he called gravity. Combining his newfound tools of calculus and his brilliant physical insight, Newton summarized his work in the Principia, which for the first time presented, among other things, Newton's law of gravitation. It was the first mathematical theory which successfully described the motions of the planets, in agreement with the painstaking records of the motions of celestial bodies kept by Kepler, Galileo, and others. Newton's equations were notably lacking in one aspect: they did not identify the mechanism by which gravity "arises" from matter. To Newton, this was a non-issue - he famously added a line in the second edition of the Principia which reads "I frame no hypotheses". That is, to Newton, the fact that gravity was an observable phenomenon with a theoretical framework and supporting experimental data was sufficient to explain the phenomenon itself.

It would take almost 300 years for someone to propose a mechanism through which gravity acts. In the early 1900s, Albert Einstein developed his general theory of relativity, through which he deduced that the presence of matter warped spacetime itself. A popular analogy that is used to give a mental picture of what this means goes as follows: if we represent space itself as the fabric of a bedsheet, any massive object placed on the bedsheet will bend the fabric of the sheet in such a way that any other object placed on the sheet will "fall" towards the heavy object. For objects that are comparable in mass with each other, you can imagine that the two objects would fall towards each other, as each deforms the bedsheet by an appreciable amount.

It is not so easy to generalize this visualization to three dimensions, but nonetheless the lesson holds: gravity works because matter itself simply follows the curvature of space time. The bedsheet analogy may not be convincing evidence in and of itself that gravity is actually the curvature of spacetime. It may be a cute analogy, you say, but how do we know that it is more than just a convenient mental picture, but a true description of the physical reality of gravity?

The short answer is that Einstein developed equations that quantitatively describe the degree to which matter curves spacetime. In the case of two massive bodies, his equations describe motion that is very similar to motion described by Newton's law of gravitation. But there are particles in the universe that do not have mass, such as photons - yet Einstein's equations claim that they too should be affected by the curvature of spacetime, whereas Newton's equation makes no such prescriptions.

This of course begs the question, are massless particles such as photons actually affected by gravity, a force arising from the presence of matter? The answer is a resounding yes! Though it would take a couple decades, astrophysicists finally were able to confirm the validity of the Einstein's general theory of relativity by observing a phenomenon known as gravitational lensing. Gravitational lensing occurs when a massive foreground object (typically a large, massive galactic cluster) distorts and, in some cases, magnifies the light coming from an extremely distant background object.

Let's recall the bedsheet analogy and imagine that before we place a massive object on the sheet, we draw a grid of perfectly straight, evenly spaced lines. In the absence of matter, light will always propagate in a straight line, that is, it will follow the grid lines as drawn. If we now place an object on the sheet, we observe that the grid lines become distorted in a region around the object. Since light must always travel in a straight line through spacetime, we would observe the massive object as "bending" the path of the light! In effect, observation of this phenomenon confirmed to physicists that Einstein's theory was correct - that matter does indeed curve spacetime, and that the bedsheet analogy is more than just a convenient mental picture, but in some way illustrates a fundamental truth about how gravity behaves and the mechanisms by which it acts.

And so, in a nutshell, that is the "nature" of gravity as we currently understand it. Any amount of matter deforms spacetime in its immediate vicinity, and this causes other matter to "fall" towards it, and also causes light to "bend" as it propagates forward.

-Matt Brunetti