What Goes Up Must Come Down - What is Gravity?

Gravity is the universal attraction of any two objects with mass. It is the reason we experience weight and stay firmly on the ground. But it’s also so much more than that. Nearly any interaction or process in outer space—from the tides on Earth to the formation of stars to the spirals of galaxies to the inescapable event horizon of black holes—can be traced back to gravity as a cause, whether direct or indirect. If our universe is a sculpted work of art, then gravity is the chisel that carved it. Most people have an intuitive understanding of gravity, but if you dig below the surface, what really is gravity?

The Apple and the Moon

To understand gravity, the first name that would obviously come up is Sir Isaac Newton. We’ve all heard the story about the apple falling on Newton’s head, giving him instant insight into the workings of gravity. This story is an embellishment, as most likely Newton simply observed an apple falling, which led to him questioning why the apple fell straight down, in a line towards the center of the Earth, rather than at any other angle. There was no single moment of “eureka!” Regardless, the question it inspired still led him to what was the greatest breakthrough in the understanding of gravity at the time.

A painting of Sir Isaac Newton as he contemplated the falling apple which lead to his understanding of gravity.

In 1687, Newton realized that the apple wasn’t the only object that seemed to have an invisible string tying it to the center of the Earth. The Moon orbits the Earth in a roughly circular path, meaning that it, too, has some sort of unseen tether to the Earth’s center. Working with this train of thought allowed Newton to deduce that some invisible force must be connecting both the Moon and the apple to Earth. On the surface, the apple and the Moon seem like very different objects, especially since the apple falls while the Moon travels around the Earth. However, Newton realized this difference was simply a matter of forward velocity. It’s like a ball tied to a string. If you swing the string around, the ball will move in a circle. The ball wants to fly off in a straight line, but the string pulls it inward, forcing it into a circle. Newton realized gravity acts as that invisible string, holding celestial bodies in orbit.

Newton’s theory of gravity was that any two objects with mass would have an attractive force between them. The more massive or closer together the objects, the stronger the pull. We typically think of the Earth as having gravity and pulling down on us, but we also have gravity and pull up on the Earth, but we’re so much less massive than the Earth that it doesn’t make any appreciable difference. Newton’s theory didn’t explain what gravity was, but rather explained how gravity functioned with remarkable accuracy. We still use it to this day to describe gravity on an everyday scale.

Warped Expectations

To understand what gravity actually is, we needed to wait over 200 years. In 1915, Albert Einstein published his Theory of General Relativity, which revolutionized our understanding of gravity. Where Newton thought of gravity as an invisible tether between two objects with mass, Einstein instead imagined how an object with mass could warp space. Einstein thought of space as being a fabric, and an object with mass being able to bend that fabric. Imagine a trampoline. The fabric of the trampoline can be thought of as a two-dimensional analogy of the three-dimensional fabric of space. If you place a bowling ball on that trampoline, it pushes down on the fabric and creates an indent. If you then place a marble on the trampoline near the bowling ball, the marble will roll down the indent left by the weight of the bowling ball and be attracted towards it, just like with gravity. Likewise, if you launch the marble with enough velocity across the fabric, it will end up moving around the bowling ball before falling in, just like the Moon orbiting the Earth.

A graphic illustrating how an object with mass can bend space. The grid is a 2D representation of the fabric of space, which is distorted around the Earth due to its mass. The Moon (or any other nearby object) will have the tendency to move down the warped gradient, resulting in the effects of gravity. Image Credit: ThisTimeSpace (CCBY 4.0)

Einstein’s theory was far more robust than Newton’s. It not only explained why gravity exists, but it also better explained interactions between far more massive and/or distant objects. Einstein’s take on gravity also gives us some of the stranger effects gravity can have. Since gravity bends the fabric of space itself, that means that even light is susceptible to gravity, and can be bent by the gravity of extremely massive objects. Get an object with enough mass, and it will be impossible for light to escape. That’s when you get a black hole.

The stretched and curved galaxies seen in this deep field image are actually the result of the light from the (regular looking) galaxies being bent around a massive galaxy cluster at the center of the image by nothing but its gravity. This is called gravitational lensing.

The Future of Gravity

While the Theory of Relativity explains the macroscopic world with astonishing accuracy, it can’t account for the subatomic world. When we switch to the world of the atom and smaller, we instead focus on the field of quantum mechanics. Usually, we don’t need to worry about gravity at such small scales, as something the mass of an atom or smaller will have completely negligible gravity. However, there are extreme cases, such as black holes or the Big Bang where gravity at a quantum scale suddenly becomes important. A unified theory where quantum mechanics and the Theory of Relativity can come together to explain quantum gravity is the next step in our understanding of this fundamental force.

What goes up must come down. Gravity is a part of everyday life on Earth, so much so that we can often forget it even exists, despite it being what literally keeps us grounded. We take it for granted without usually sparing any thought to its inner workings. We may assume what goes up will come down, but we rarely ask ourselves why. It is those few who have dared to ask why that have pushed our understanding further than we ever would have once thought possible.