How to Understand the Theory of Relativity

Reviewed by Anne Schmidt

Last Updated: May 8, 2023 References

When people hear the phrase "Theory of Relativity," they generally think of Albert Einstein and complex mathematical equations like $e=mc^{{2}}$ . But many scientists played a part in developing the theory. By learning of the history and practical applications of relativity, you can get an understanding of this complicated subject.

Method 1

Method 1 of 4:

Learning the History of Relativity

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1
Start with Galileo. The 16th century scientist Galileo Galilei is considered one of the founders of modern science. ^{[1]
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Research source} His research into the mechanics of falling objects and moving projectiles led to his formulation of the first modern theory of relativity, and raised the question known as "the problem of relativity." So how to understand the problem of relativity?
- Imagine two people observing the same event. For example, two people at a baseball game sitting on opposite sides of the stadium watch the batter hit a home run. The time of the home run will be the same will be the same for both observers while the distance from them will differ. Both fans witnessed the same event relative to each other.
- Imagine a person driving a car going 60 miles per hour. The driver is traveling 0 miles per hour with respect to the car, but to an outside observer the driver is traveling 60 mph. The speed of the driver changes relative to the observer's point of view.
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Continue with Sir Isaac Newton. In the 17th century, Isaac Newton was a student at Cambridge University. When Cambridge closed for two years due to the Black Plague, Newton continued to study complex math, physics, and optics on his own. During this time he developed the concept of infinite-series calculus, and laid the groundwork for his three laws of motion. ^{[2]
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Research source} Eventually, Newton would study how the laws of motion related to the motion of the Earth, Sun, and Moon, a concept he would call "gravity." ^{[3]
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Research source} What are some of the practical applications of the laws of motion?
- Experience the first law of motion at the playground. Newton's first law of motion is known the law of inertia, which states that every object will remain at rest or in uniform motion in a straight line unless acted on by an external force. ^{[4]
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  Research source} For example, a person at the top of a sliding board will stay there until they push themselves (or are pushed) down the board. They will stay in motion until they are stopped when reaching the bottom of the slide. ^{[5]
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- Do the math for the second law of motion . In the first law, Newton presented the theory that an object in motion stays in motion, and an object at rest stays until an outside force affects them. Newton's second law takes this a step further by determining how much force is need to change the state of the object. It states that an object subjected to an external force will accelerate and that the amount of the acceleration is proportional to the size of the force. For example, a 40-ton tractor trailer will require more force to reach a velocity of 60 miles per hour than a 2-ton compact car will need. Exactly how much force can be determined by the mathematical formula force=mass x acceleration, abbreviated as $f=ma$ .
- Observe the third law of motion . Newton's third law of motion states that for every action there is an equal and opposite reaction. ^{[6]
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  Research source} Simply stated, an object pushes against another object, the second object pushes back just as hard. Sometimes the third law is not obvious, as when you are standing still. Gravity pushes down on the ground, while the ground pushes back with equal force. Since there is no motion, the forces cancel each other out. ^{[7]
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  Research source} With greater force and more massive objects, the third law is more apparent, as when a rocket is launched. As the engine burns fuel, the downward thrust pushes the rocket upward.
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Travel through the ether.
- Segue to the 19th century. Since Isaac Newton's time, scientists theorized that the universe was filled with a medium they called ether. Light and radio waves traveled through the ether in the same manner as sound waves travel through the air. ^{[8]
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  Research source} By the 19th century scientists had worked out ways to measure the properties of the ether, and hoped to create a theory describing the universe.
- Measure the light. In 1887, physicists Albert Michelson and Edward Morley attempted to prove the existence of the ether using an instrument designed by Michelson known as an interferometer, consisting of a half-silvered glass plate, two mirrors, and a telescope. ^{[9]
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  Research source} By aiming a beam at the glass plate, the beam would be split and the two beams would reach the two mirrors at different times, depending on which direction they were traveling with respect to the ether. The unexpected result was both beams reached the mirrors at the same time, failing to prove the existence of the ether. Michelson regarded his experiment as a failure. ^{[10]
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  Research source} But it would be a key piece in the work of a young clerk in the Swiss Patent Office.
In 1905 Albert Einstein worked at the Patent Office in Bern, Switzerland. During that time, Einstein published four papers determining the speed of light was constant in a vacuum, which also disproved the existence of the ether. This discovery led to the first of Einstein's two theories of relativity: Special relativity and general relativity.

Method 2

Method 2 of 4:

Understanding Special Relativity

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1
Discover your frame of reference. Einstein's research showed there was no "absolute" frame of reference in the natural world. As long as an object is moving in a straight line at a constant speed (with no acceleration), the laws of physics are the same for everyone. ^{[11]
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- Imagine being on a train. Looking out the window, you see another train that appears to be moving. Based solely on this observation, it is impossible to tell if your train or the other train is moving. The same is true for anyone in the train you are observing.
The Michelson-Morley experiment failed to prove the existence of the ether, but proved that light travels at a constant rate of speed, regardless of an observer's frame of reference. ^{[12]
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Research source} Einstein further postulated that as an object approached the speed of light, its mass would increase, eventually becoming infinite as it reached the speed of light. ^{[13]
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Understand space-time. As Einstein researched the properties of light, he realized that if the speed of light was an absolute constant, then time and space must be variables. In the everyday world, time seems to be a single entity that flows at a constant rate, when in fact it is actually part of a more complex system linked with space. Therefore, when an object moves in space it also moves in time, which slows down in direct proportion to the rate of speed at which the object is moving. This property is known as time dilation. ^{[14]
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- In October 1971, the relationship between time and space was demonstrated by an experiment conducted by physicist Joseph C. Hafele and astronomer Richard E. Keating. Taking four atomic clocks, they flew around the world on a commercial airline and compared the time shown on the clocks with others that had remained at the United States Naval Observatory. The two sets of clocks showed different times, consistent with the predictions of the space-time theory. ^{[15]
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From these two principles, Einstein theorized that matter and energy were connected in ways scientists never previously realized. ^{[16]
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Research source} Eventually, Einstein concluded matter and energy were the same thing in different forms, and by accelerating matter sufficiently, it would become energy. This resulted in the famous mathematical formula $e=mc^{{2}}$ , or energy=mass x the speed of light squared.

Method 3

Method 3 of 4:

Understanding General Relativity

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Einstein's theory of special relativity is called that because it applies to the special instance of objects moving at a constant speed. But objects do not always maintain a constant speed. It took ten years for Einstein to expand his theory to include acceleration, a theory that came to be known as the General Theory of Relativity.
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Define gravity. When Sir Isaac Newton first defined the theory of gravity, he believed it to be an innate force that could exert an influence across distances. The force of gravity would be stronger for a massive object like the sun, which explained why it attracted smaller objects like Earth orbiting around it. ^{[17]
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Research source} However, as Einstein attempted to explain gravity mathematically, he discovered that gravity was not a force that traveled through space, but was a distortion of space-time. The more massive an object, the more it warped space-time. ^{[18]
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- Imagine the universe as a trampoline. If you put a bowling ball on the trampoline, it will cause the trampoline to bend. Smaller objects such as a baseball will roll towards the bowling ball due to the distortion it caused in the trampoline. This has been proven to apply to space-time as well. ^{[19]
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Method 4

Method 4 of 4:

Applying Relativity to Everyday Life

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It has been that the faster an object moves, the more time slows down. GPS satellites measure time at a small but measurably slower rate than time on Earth. By calculating the time it takes for a signal sent from the GPS satellites orbiting the earth to your device, it is possible to determine your location on the planet.
Most metals are shiny because their electrons jump to and from different levels known as orbitals. With gold, the electrons closest to the atom's nucleus must move at a high rate of speed, approximately half the speed of light, to avoid being absorbed by the nucleus. In order to move to a different orbital, the electrons must absorb light. Most of the light absorbed is towards the blue spectrum, while light closer to the yellow spectrum is reflected, resulting in the metal's luxurious yellow color.
Like gold, mercury is a heavy atom whose inner electrons travel at a high rate of speed. As their speed increases their mass increases proportionately. This results in there being a weak bond between mercury atoms, and the metal being in a liquid state at average temperatures.
Thanks to the mathematic principle of $e=mc^{{2}}$ , solar and nuclear energy are possible. Without energy and matter being interconnected, there would be no energy and no light.

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How to Understand the Theory of Relativity

Steps

Learning the History of Relativity

Understanding Special Relativity

Understanding General Relativity

Applying Relativity to Everyday Life

Expert Q&A

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