# Was Newton Wrong About Gravity?

## After standing strong for more than 300 years, Newton’s law of universal gravitation might need an update.

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Newton published the law of universal gravitation in his 1687 Naturalis Principia Mathematica, and it quickly became one of the most important stones on which physics is built. It explains just about everything we can see, from the flight of a ball through the air to the movement of planets. However, some modern physicists have a good reason to question it.

It’s a simple equation. The gravitational force (F) between two masses (m1 and m2) is calculated by their product divided by the square of the distance between them (r) and multiplied by the gravitational constant (G). This means that the gravitational force changes according to the objects’ masses and is heavily dependent on the distance between the objects, according to the inverse square law. When Newton published this equation, although the basics were already known, it was revered as revolutionary. We still use it today, as it is central to classical dynamics.

While Newton’s equation proved mathematically useful out of the gate, it was not until 1798 that it was demonstrated in the laboratory by Henry Cavendish. His now famous, highly-precise experiment consisted of two small balls (h) on the end of a wooden rod suspended by a wire. Two larger balls (W) were placed on opposite ends and on opposite sides. As expected, the two smaller balls were attracted towards the larger balls, causing the rod to rotate. Not only did this experiment prove Newton correct, the results were later used to calculate the gravitational constant (G), the Earth’s mass, and even its density.

However, Newton himself had a problem with his law of universal gravitation. He couldn’t explain the mechanism. He reasoned that some invisible force must be instantaneously acting on the two objects, an idea that violated the understanding of the universe at the time. In a 1692 letter to his friend Bently he wrote, “That one body may act upon another at a distance through a vacuum without the mediation of anything else, by and

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