# Theory of Relativity: Definition & Example

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• 0:27 The Speed of Light
• 1:43 Mass-Energy Equivalence
• 4:33 Lesson Summary

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Lesson Transcript
Instructor: Yuanxin (Amy) Yang Alcocer

Amy has a master's degree in secondary education and has taught math at a public charter high school.

Watch this video lesson, and you will learn about the theory of special relativity and how it applies to you. Learn how you can use one of the most famous equations from this theory to calculate the power inside just one tiny object.

## The Theory of Special Relativity

The world hasn't changed, but theories about the way the world works have changed over time. Our calculations are getting more and more accurate, and we are able to take into consideration things such as the speed of light. It is all thanks to Einstein's special theory of relativity, which tells us how objects appear to behave as they approach the speed of light and also how mass and energy are related to each other.

## The Speed of Light

According to Einstein, as objects approach the speed of light, time seems to slow down for them when measured by an observer that is not moving. Also, when measured by an observer, objects traveling at a faster speed will measure shorter than if the objects weren't moving with respect to the observer. For example, I'm sitting on my front porch, and I'm watching cars drive by me. The faster the car is driving by, the shorter the car will look to me. Also, the faster the driver is driving, the slower time will seem to pass for him when compared to me. As an object gets close to the speed of light, time will seem to stop, and its length will seem to shrink to nothing.

The biggest equation to come out of Einstein's theory of special relativity is E = mc^2, which tells us how E, the energy, m, the mass, and C, the speed of light, are related to each other. This mass we are using is not the same as weight. An object's mass is how much stuff is inside of it, while its weight is how hard gravity is pulling on it. This is why when you measure your weight in outer space, it is close to zero.

## Einstein's Mass-Energy Equivalence

This special equation, E = mc^2, is called the mass-energy equivalence equation because the speed of light is a constant equal to 299,792,458 meters per second. This equation by Einstein allows us to calculate the amount of energy an object has inside of it. Because the speed of light is such a large number, you can see just how much power objects can possess. This energy is released when the mass of an object - its matter - is converted into energy. This conversion of mass to energy is what happens inside our sun. The units of this energy are g * (m/s)^2.

So think of our sun and its big size. Think about how far away it is. Even though it is so far away, because it is so huge, it has enough energy to heat up our Earth and grow all our plants, and we are only using just a tiny fraction of the energy the sun produces. We can see and feel the energy that is produced by our sun. We see it in the form of light, and we feel it in the form of heat.

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