Understanding the Big G. What the gravitational constant is and why it's fundamental to our study of the Universe

The gravitational constant, or G, is a way of representing, mathematically, the force that any two objects in the Universe exert on each other due to gravity.

Isaac Newton’s universal law of gravitation, for instance, says that this force can be calculated by the equation:

F_g = G (m₁ m₂) / R²

F_g is the total gravitational force between the two objects (measured in Newtons), m₁ and m₂ are the masses of the two objects (measured in kilograms) and r is the distance between the two objects (measured in metres).

G is usually expressed as 6.674×10⁻¹¹ N m² kg⁻², and is sometimes referred to as the universal gravitational constant, or Newton’s constant.

It’s also often known colloquially as ‘Big G’, which is a handy way of differentiating the gravitational constant G from g, which describes the local gravitational field of Earth.

The value of G and why the gravitational constant is important

Originally defined by Isaac Newton in 1680, the gravitational constant was first calculated accurately by English physicist Henry Cavendish in the late 18^th century.

He used a device called a torsion balance to determine the near-infinitesimal gravitational attraction between two small lead balls, then fed that measurement into the equation above as F_g and worked backwards from there.

Understanding the gravitational constant enables astronomers and astrophysicists to work out all kinds of information, such as calculating the mass of a star based on the mass and orbital speed of the planets around it.

As such, G can be said to underpin much of our modern understanding of how the Universe works.

So it’s a little odd that even our current definition of G is fairly vague, extending only to five decimal places.

That’s partly because any experimental attempts to measure G will themselves be subject to the gravitational pull of nearby objects.

It’s also partly because gravity, though an omnipresent force, is also a very weak one.

Try dropping a box of paperclips on the floor, for instance.

It’s gravity that makes them fall to the floor, sure, but once they have, you’ll find you can easily overcome the entire gravitational pull of planet Earth using nothing more than a tiny fridge magnet!