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What are the four fundamental forces of nature?

Gravity Getting You Down?
This little guy is about to find out what gravity is all about.
This little guy is about to find out what gravity is all about.
Steve Puetzer/­Getty Images

­The first force that you ever became aware of was probably gravity. As a toddler, you had to learn to rise up against it and walk. When you stumbled, you immediately felt gravity bring you back down to the floor. Besides giving toddlers trouble, gravity holds the moon, planets, sun, stars and galaxies together in the universe in their respective orbits. It can work over immense distances and has an infinite range.

Isaac Newton envisioned gravity as a pull between any two objects that was directly related to their masses and inversely related to the square of the distance separating them. His law of gravitation enabled mankind to send astronauts to the moon and robotic probes to the outer reaches of our solar system. From 1687 until the early 20th century, Newton's idea of gravity as a "tug-of-war" between any two objects dominated physics.

But one phenomenon that Newton's theories couldn't explain was the peculiar orbit of Mercury. The orbit itself appeared to rotate (also known as precession). This observation frustrated astronomers since the mid-1800s. In 1915, Albert Einstein realized that Newton's laws of motion and gravity didn't apply to objects in high gravity or at high speeds, like the speed of light.

In his general theory of relativity, Albert Einstein envisioned gravity as a distortion of space caused by mass. Imagine that you place a bowling ball in the middle of a rubber sheet. The ball makes a depression in the sheet (a gravity well or gravity field). If you roll a marble toward the ball, it will fall into the depression (be attracted to the ball) and may even circle the ball (orbit) before it hits. Depending upon the speed of the marble, it may escape the depression and pass the ball, but the depression might alter the marble's path. Gravity fields around massive objects like the sun do the same. Einstein derived Newton's law of gravity from his own theory of relativity and showed that Newton's ideas were a special case of relativity, specifically one applying to weak gravity and low speeds.

When considering massive objects (Earth, stars, galaxies), gravity appears to be the most powerful force. However, when you apply gravity to the atomic level, it has little effect because the masses of subatomic particles are so small. On this level, it's actually downgraded to the weakest force.

Let's look at electromagnetism, the next fundamental force.