Special relativity facts for kids

Einstein in 1905, the year he published his amazing papers, including the one that started special relativity.

The theory of special relativity was explained by Albert Einstein in 1905. He wrote about it in his paper On the Electrodynamics of Moving Bodies. Einstein said that special relativity is based on two main ideas:

• Relativity: This idea talks about how motion looks different depending on who is watching. The laws of physics are the same for everyone who is not speeding up or slowing down.
• Speed of Light: This idea says that the speed of light is always the same. It does not matter how fast you are moving. When things travel very close to the speed of light, distances can look shorter. Also, clocks seem to tick more slowly.

What is Relativity?

Imagine you are in a car moving at a steady speed. You and the car are in an "inertial frame." To you, inside the car, you feel like you are sitting still. But to someone standing outside, you are moving at the car's speed.

If you toss a ball inside the car, it moves normally for you. But for the person outside, the ball's path looks different. This is because your view of motion depends on your own movement. People in the same "frame" measure time the same way. But for people in different "frames," time can seem to pass at different rates.

The Amazing Speed of Light

Before Albert Einstein, scientists tried to measure the speed of starlight. They thought light would reach Earth faster if Earth was moving toward the star. They also thought it would be slower if Earth was moving away.

But no matter what, the speed of light in empty space was always the same. It did not matter who measured it or how they were moving. Einstein said this constant speed of light is a law of nature. It is always about 186,000 miles per second.

This idea has some strange effects. Things that happen at the same time for one person might not for another. This is especially true if they are moving at very different, very fast speeds. For example, an object's length might look different to two observers. This depends on how fast each observer is moving.

Special relativity also connects energy and mass. This is shown in Albert Einstein's famous formula: E=mc². Here, E is energy, m is mass, and c is the speed of light.

Cool Facts About Relativity

• Einstein once explained relativity like this: "When you are courting a nice girl, an hour seems like a second. When you sit on a red-hot cinder, a second seems like an hour."
• Special relativity is used for "special" situations. This means it is used when talking about huge amounts of energy, super-fast speeds, and giant distances. It does not include gravity.
• The word "relativity" was not in Einstein's famous 1905 paper. He liked the term "invariance theory" better.
• Special relativity helps explain why events might seem to happen at the same time for one person. But they might happen at different times for another person.

Related pages

• General relativity

Images for kids

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  This picture shows how events can happen at different times for different observers.

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  This shows how a cube might look shorter when it moves very fast.

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  The M87 galaxy shoots out a powerful jet. This jet moves at nearly the speed of light!

See also

In Spanish: Teoría de la relatividad especial para niños