In 1905 Albert Einstein wrote a paper titled “On the Electrodynamics of Moving Bodies” which would drastically transform our understanding of the relationship between space and time. This paper introduced the world to the concept of special relativity. What makes this topic “special” is that the behaviors described in the paper apply to objects traveling in an inertial frame of reference. An inertial frame of reference is a frame in which the object being observed is moving at a constant or non accelerated speed. As an example, If I am standing still then I am in an inertial frame of reference. If a passerby is traveling by me in a vehicle at 55 miles per hour they are in a separate inertial frame of reference. Relativity is all about how objects move relative to one another. In order to describe the behavior of bodies moving in an accelerated frame of reference general relativity is needed. One of the major revelations of special relativity is the unification of space and time. Until Einstein published his now famous paper space and time were thought of as two independent coordinates. In 1908 a mathematician named Herman Minkowski developed a mathematical model based on Einstein’s paper and was the first to use the term space-time.
Postulates of Special Relativity
As noted above the effects of special relativity occur only in inertial frames of reference. If an object accelerates or changes direction then special relativity no longer applies and an object is subject to general relativity. There are two postulates of special relativity which provide the foundation for the entire theory. The first postulate is that there are no preferred inertial frames of reference and that all inertial frames of reference are equally valid and useful. This postulate is the basis for the idea that events that are simultaneous for an observer in one inertial frame of reference need not be simultaneous to an observer in another inertial frame of reference. The second postulate states that the speed of light in a vacuum is a constant and invariant quantity. This idea may sound simplistic but is very different than the way other things work in our daily life. If I were riding in a car traveling at 50 miles per hour relative to the ground and were to throw a ball at 10 miles per hour relative to the car than a bystander at rest on the sidewalk would see the ball traveling at 60 miles per hour relative to the ground. Light, on the other hand, travels only at the speed of light. If you were to measure the speed of light from the headlights of my vehicle at rest, you would measure the speed of light to be 670,616,629 miles per second. If you were to measure the speed of light on an object traveling at the speed of a man made satellite, 16,800 miles per hour, you would still measure the speed of light as 670,616,629 miles per hour. The speed would not be added to the speed of the satellites the way the speed of the ball was added to the speed of my vehicle. This fact will become significant in understanding the idea that time is not absolute and can change depending upon one’s frame of reference.
Consequences of special relativity
There are three effects or consequences that occur as a result of special relativity. These effects do not become discernible until an object is traveling at some significant portion of the speed of light. The first effect is that the faster an object moves through space the slower it moves through time. This is called time dilation and has been repeatedly verified experimentally. Tests have been done using atomic clocks, one on the ground and one flown around the world in a plane. When they compare the clocks after the flight there is a slight disagreement between the clocks. This is not merely an issue of the clock not functioning properly, time is moving differently for each of these clocks. There is a simple mathematical equation that can be used to predict the time dilation between an object traveling at some significant portion the speed of light as compared to the time passage of an object in a rest frame of reference.
This equation allows you to calculate the difference between special relativity and classical relativity of an object in a rest frame as compared to an object traveling at a high rate of speed. If you were traveling aboard a space ship traveling at say 60% percent the speed of light for one year, you would experience the passage of one year while people back on earth would experience 1.25 years. This means that you would have traveled 3 months into the future! As of now we have no ability to accelerate a space ship anywhere close to that rate of speed.
Effect number two is length contraction. As an object travels at relativistic speeds the object contracts in the direction of its motion. A passenger on the a space ship would not notice nor experience any difference in time or length in his frame of reference. An observer watching the space ship travel would, in fact, notice the length being contracted in the direction of its motion.
Effect number three is an increase in mass of an object as it approaches the speed of light. The faster an object moves the more mass it gains. Consequently, it is impossible for an object to reach the speed of light because it would become infinitely massive and require an infinite amount of energy to travel at that speed.
The World’s most famous equation!
Everyone is familiar with the famous equation E=mc^2 but what does it actually mean and why is it important? The “E” represents energy in this equation while “m” represents the mass of an object and c represents the speed of light squared. The equations states that the amount of energy you can obtain from an object is equal to the mass of the object multiplied by the speed of light squared. Even if the mass is extremely small the amount of energy available will be extremely large due to the large value of the speed of light squared. https://www.youtube.com/watch?v=hW7DW9NIO9M
It sounds simple-we can get large amounts of energy from everyday objects so we should never have to worry about energy ever again. The difficult part is how to obtain this energy from everyday objects. As it turns out we can harvest energy from very small objects in a process called nuclear fission. In nuclear fission a large amount of energy that can be obtained from a small amount of uranium contained in fuel rods. The splitting of the nucleus releases a large amount of energy energy. The release of energy in which is used to heat water and is eventually converted to electricity. Currently 29 nuclear reactors across the United States generate 20 percent of the country’s electricity by taking advantage of Einstein’s famous equation.
Neville’s Phantastic Physics Phun 