Gravity isn’t a Force, So How Does it Move Objects?

Posted By: Jolene Creightonon: August 19, 2015In: Physics

 Print Email

Binary system illustration containing a stellar-mass black hole called IGR J17091-3624. The strong gravity of the black hole is pulling gas from a companion star. Image credit: NASA

You may have heard that gravity isn’t a force. This is true. Gravity is not a force; however, this truth leaves us with a number of questions. For example, if gravity isn’t a force (it’s not), then how is it able to accelerate objects? We’re commonly told that gravity “pulls” things towards massive objects, but how can this be, unless gravity is an attractive force?

To begin answering these question, you need to understand that “accelerate” is the proper term, not “pull.”

I know that, when teaching introductory physics (especially in elementary classes), some teachers and textbooks say things like, “Earth’s gravity pulls objects towards the center of the planet,” but such assertions aren’t technically correct. This is a bit of a problem, because it leaves people with a false understanding of how gravity works, and may result in a lot of confusion (and questions) when attempting to tackle more complicated physics concepts.

Of course, there are also a lot of individuals that teach gravity more responsibly, so this shouldn’t be take an an attack on any educators. The fact is,  misinformation happens. And the truth is, gravity does not “pull” objects; rather, gravity warps spacetime, causing objects to follow the bends that are created.

To delve into this a bit more, thanks to Albert Einstein’s Theories of Relativity, we know that energy tells spacetime how to bend. In this case, mass is generally the most important part of the equation i.e., it is an object’s mass energy that bends spacetime.

So n short, mass bends spacetime, and these bends tell energy how to move. In this respect, it is best to think of gravity as the bending of spacetime.

Just like a car travels down a road that has various twists and turns, objects travel along the path of these bends in spacetime. And since objects fall according to the various dips and bends, and since massive objects create extreme bends in spacetime, gravity is able to accelerate objects as they enter (or approach) deep gravity wells.

As an aside, we call these paths—the path that object follow through spacetime—a “geodesic.”

To better understand how gravity works, and how it is able to accelerate objects, take, for example, the Earth and the Moon. The Earth is a rather massive object. At least, it is when compared with the Moon. As such, our planet causes quite a bend in spacetime. The Moon orbits around our planet because of the warps in spacetime that are caused by the Earth’s mass.

So the Moon is just traveling along the bend—the dip, or whatever you want to call it—the downward slope that our planet makes. In this respect, the Moon does not feel any force acting upon it. It it just following a particular path.

But why don’t all the asteroids and meteorites that pass our planet fall into orbit?

Ultimately, the path that an object takes depends on a number of factors, such as velocity, trajectory, and the mass of the respective objects. This is why, every day, there are a plethora of comets and asteroids that go shooting past our planet without being captured and thrust into a particular orbit. Likewise, every day, there are a plethora of objects that are captured by Earth and are pulled down to the planet’s surface (others may orbit the planet for a time). Which will happen depends on the aforementioned factors.

If you want to know more about the relationship between gravity and the path that an object will follow, check out this source. It’s a good starting point that gets into some of the math behind the aforementioned concepts.