Everyone knows about gravity - it's what keeps us from flying off of the Earth. But gravity is only one of four fundamental forces in the universe that keeps things running smoothly.

Gravity

Apart from being one of the most widely known of the fundamental forces of the universe, Gravity is the weakest of the four. Even though it is the weakest force, it is the only force that can work over vast distances.

Gravitation is the force of attraction that exists between all particles with mass in the universe.[1] When you are standing on Earth the gravity of the planet attracts the mass of your body toward it. Because you observe this as being in a downward direction, you may say that gravity "pulls you down", but it really pulls you toward the center of the Earth.

A Common Mistake

Some people think that it is the spin of the Earth that causes us to stay rooted to the ground. This is an understandable conclusion, but incorrect. The action of the Earth spinning gives us something on which we can incorrectly blame our attraction to it, but really gravity is an invisible force that acts on everything.

Even things that are on Earth exert the force of gravity on each other. Every item you can see is drawn to you by gravity, but your mass and the mass of those items are too small to force you together. This is why we say that gravity is weak.

Although you may not feel it, all masses in the solar system exert the force of gravity on you. Even planets as far away as the Neptune exert some gravitational force on you, even if you can't perceive it.

Gravity doesn't seem like a very important force because we live with it every day, but it would be inconvenient for us if gravity were to stop working: We would all fly off of the Earth into space. The planets would leave their orbits around the sun. The loss of gravity would even stop the sun from burning.

Electromagnetism

Electromagnetism describes how charged particles interact with one another. During the course of a day, you manipulate electromagnetism more than any other fundamental force.

When you turn on a light switch, you control the flow of energy through the wires to the light bulb. When the light bulb creates light, the light that is emitted is governed by laws of electromagnetism. Imagine how much control over electromagnetism you exert when you're running a television!

Particles of matter can have positive or negative charges. Like charges attract and different charges repel each other. Motion of these particles through a space creates a field of invisible energy that affects all of the particles in its wake.

When you turned on that light switch, the motion of the particles moving through the wires caused a small, invisible magnetic field to form around those wires. You can reveal this invisible force by holding a compass near the wire. The needle of the compass should turn, affected by the magnetic field that the wire generates.

Albert Einstein's Nobel Prize

Albert Einstein is a pretty famous guy. When most people think of Einstein, they think of E=Mc² and Relativity, even if they don't know what any of that means. People generally aren't surprised by the fact that Einstein won a Nobel prize, but they usually are surprised to learn of the research that won him the prize - The photoelectric effect.

Light contains energy. When light hits a surface, the energy in the light can cause electrons to leave the atoms of the material which it strikes. This is the photoelectric effect. What use does this have?

You may not have realized it, but today's digital cameras are a small laboratory of the photoelectric effect in action. Light is focused through the lens of the camera onto a special piece of silicon. When the light hits the silicon, it causes electrons to leave its surface. This electron loss is detected by the camera via electromagnetic impulses in the silicon. By knowing which parts of the silicon were struck by what light, the camera can store the impact of the light as an image.

Electromagnetism's Significance to Astronomy

Electromagnetism is one of the most important tools for astronomers to understand the universe, because electromagnetism governs light, and without light you wouldn't be able to see this book, much less planets and stars.

Astronomers can examine electromagnetic radiation from distant stars to determine their essential properties. All four forces may be essential to the workings of the universe, but electromagnetism is essential to our understanding of it.

The other forces

• Weak Nuclear Force
• Strong Nuclear Force

What causes these forces?

Who knows?