# Mass and Energy: Description and Interchangeable Relationship

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• 0:07 Theory of Relativity
• 0:47 Albert Einstein and Relativity
• 2:03 Energy and Mass Are Relative
• 2:41 Evidence for the…
• 4:22 Mass to Energy
• 5:15 Lesson Summary

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Lesson Transcript
Instructor: John Simmons

John has taught college science courses face-to-face and online since 1994 and has a doctorate in physiology.

Einstein used E = mc^2 to prove that mass and energy are relative to each other. This lesson describes how energy can be converted into mass and mass into energy. Experimental results from particle accelerators are used to demonstrate the relative nature of mass and energy.

## Relativity of Mass and Energy

The theory of relativity allows observers to agree on what they see from different perspectives. For example, an object appears larger close up than it does from a distance, yet we agree on the size of the object. Additionally, two points can appear close together or farther apart depending on the observer's angle of perspective. Yet, we can agree that they are separated by the same space, regardless of our perspective. Furthermore, an object can appear to be stationary or moving depending on the observer's state of motion. We accept velocities are relative to the velocities of other objects.

## Albert Einstein and Relativity

While Albert Einstein was not the first to suggest relativity, he certainly expanded its application. Before Einstein, scientists struggled with how the speed of light could be constant regardless of the source of the light or the perspective of the observer. To resolve this problem, Albert Einstein proposed the components of speed - space and time - to be relative to the state of motion of the observer. As it turns out, he was correct.

As objects increase speed, both time and length decrease. The relativity of space and time allows different observers to agree on the same speed of light. Einstein took relativity even further. He suggested we reevaluate how we look at other properties as well - for example, mass and energy. This questioning led to perhaps the most popular scientific equation of all time, E = mc^2, where E = energy, m = mass, and c = the speed of light.

## Energy and Mass Are Relative

The equation E = mc^2 states that the amount of energy possessed by an object is equal to its mass multiplied by the square of the speed of light. Since the speed of light is an incredibly high number, almost 300,000 km/sec, a small amount of mass contains a lot of energy. Additionally, the equation suggests that energy and mass are interchangeable with each other. In other words, energy can be converted to mass and mass to energy.

## Evidence for the Relativity of Mass and Energy

Scientists have proven that mass and energy are interchangeable properties. Mass can be converted into energy, and energy can be converted into mass. This phenomenon can be demonstrated with particle accelerators. Particle accelerators are used to, well, accelerate particles. For example, scientists can use particle accelerators to make protons approach the speed of light. Energy must be applied to accelerate the proton, just like a car needs gas to move and we need food to run. As the energy is added, the proton accelerates - it moves faster. However, not all of the applied energy is used to make the proton accelerate.

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