## What Is Gravity?

Gravity is one of the four fundamental forces in the universe. Although we usually think of gravity as the force that keeps us on the ground and causes things to fall, gravity is a lot more interesting than that. In fact, gravity exists between any two objects that have mass or energy.

The force that causes things to fall to the ground is actually a result the force of gravity caused by the mass of the Earth and the mass of an object. But the Moon also has gravity, stars have gravity, and even we, since we have mass, have our own gravitational fields around us.

Gravity is pretty weak compared to other types of forces, however, and depends how much mass an object has and how close it is to whatever it's acting on. So while you might not be able to sense the force of gravity from your friend or even a star light years away, you might be able to think about another source of gravity that affects us everyday. Stumped? The combined forces of gravity from the Moon and the Sun are responsible for the tidal patterns on Earth, and believe it or not, we even weigh slightly less when the Moon is directly overhead.

## Newton's Law of Universal Gravitation

In the 17th century, Isaac Newton was the first dive deeply into the study of gravity. As legend has it, inspired by an apple falling from a tree, Newton pondered over why objects always fell straight down to the ground. Through his observations and experiments, he came to realize that gravity was responsible for this phenomenon.

What set Newton apart was his use of mathematics to describe gravity. Over 20 years, Newton experimented and formulated mathematical equations known as the laws of motion and the law of universal gravitation. These equations explain how every object in the universe attracts every other object with a force that depends on their masses and the distance between them.

Newton's mathematical approach forever changed our understanding of our universe and the scientific process. It allowed scientists to comprehend and predict the movements of objects in space with great precision. His groundbreaking work became the foundation of modern physics, shaping not only our comprehension of gravity but also numerous practical applications, from space exploration to everyday activities like throwing a ball or driving a car.

## Einstein's Theory of General Relativity

In the early 20th century, Albert Einstein further advanced our understanding of gravity with his theory of general relativity. Einstein proposed an entirely new perspective on gravity, transforming our knowledge of the universe.

According to Einstein's theory, gravity is not merely a force of attraction between objects. He suggested that gravity arises from the curvature of space and time caused by mass and energy. Massive objects, such as stars and planets, "bend" space and time around them, resulting in the force of gravity.

Einstein's theory of general relativity has had a profound impact on our comprehension of gravity. It explains phenomena like the bending of light around massive objects and the behavior of black holes. Many of his predictions from over 100 years ago are still being discovered and proven to this day.

## Top Facts About Gravity for Kids

• #### 1. Black holes are black because of their ultra-strong gravitational pull.

A black hole is a region in space where gravity is extremely intense because of the huge amount of mass packed into a tiny space. The gravitational pull of a black hole is so strong that it creates an "event horizon," which is a boundary beyond which nothing, not even light, can escape.

When an object, including light, crosses the event horizon of a black hole, it gets pulled toward the black hole's center and is forever trapped. This makes black holes appear "black" because no light can be reflected or emitted from them.

• #### 2. Massive objects can actually bend the path of light, a phenomenon known as gravitational lensing.

According to Einstein's theory of general relativity, gravity can bend the path of light as it passes close to a massive object, such as a star, a galaxy, or a black hole. This bending effect is called gravitational lensing. It occurs because massive objects create curves in the fabric of space and time, and light follows those curves. Gravitational lensing has been observed and studied by scientists, providing valuable insights into the nature of gravity, the structure of the universe, and helping us discover and study distant celestial objects.

• #### 3. Astronauts are never totally in zero gravity. In the International Space Station (ISS), astronauts experience about 90% of the Earth's gravitational force.

This is because the ISS orbits the Earth at a height of about 400 km, where the gravitational pull of the Earth can still be felt. However, because the ISS orbits Earth, everything inside the station, including the astronauts and their equipment, appears to float. This is why it is often said that astronauts experience "microgravity" rather than "zero gravity." Despite the small amount of gravity, the effects of microgravity on the human body and scientific experiments are still significant and require special considerations.

• #### 4. As predicted by Einstein over 100 years ago, in 2016 it was proven that all matter emits gravitational waves, or "disturbances in the curvature of spacetime" (yikes).

In his theory of general relativity in 1907, Albert Einstein explained how gravitational waves are like ripples in space and time, caused by the movement of massive objects like planets, stars, and black holes. These waves are incredibly small, and detecting them required the most sensitive instruments ever built. In 2016, the Laser Interferometer Gravitational-Wave Observatory (LIGO) detected gravitational waves for the first time using a system of lasers and mirrors. The signal was caused by the collision of two black holes over a billion light-years away!