Embedded Files
Competency 7: Explain the Consequences of the Postulates of General Relativity
Einstein's Theory of Special Relativity
Einstein's Theory of Special Relativity
The theory of special relativity explains how space and time are linked for objects that are moving at a consistent speed in a straight line. One of its most famous aspects concerns objects moving at the speed of light. The theory of relativity is two theories that Albert Einstein came up with within the early 1900s. One is called "special" relativity and the other is called "general" relativity.
Special Relativity
Special Relativity
Two main ideas make up Einstein's theory of special relativity.
The principle of relativity: The laws of physics are the same for any inertial reference frame.
The principle of the speed of light: The speed of light in a vacuum is the same for all observers, regardless of their relative motion or the motion of the source of the light.
What does "relative" mean?
The first principle listed above is pretty confusing. What does this mean? Well, before Albert Einstein, scientists thought that all motion occurred against a reference point called the "ether". Einstein claimed that the ether did not exist. He said that all motion was "relative". This meant that the measurement of motion depended on the relative velocity and position of the observer.
A Relative Example
One example of relativity is to imagine two people on a train playing ping-pong. The train is traveling at around 30 m/s north. When the ball is hit back and forth between the two players, the ball appears to the players to move north at a speed of around 2 m/s and then south at the speed of 2 m/s.
Now imagine someone standing beside the railroad tracks watching the ping-pong game. When the ball is traveling north it will appear to travel at 32 m/s (30 m/s plus 2 m/s). When the ball is hit in the other direction, it still appears to travel north, but at a speed of 28 m/s (30 m/s minus the 2 m/s). To the observer by the side of the train, the ball always appears to be traveling north.
The result is that the speed of the ball depends on the "relative" position of the observer. It will be different for the people on the train than for the person on the side of the railroad tracks.
E = mc2
One of the results of the theory of special relativity is Einstein's famous equation E = mc2. In this formula E is energy, m is mass, and c is the constant speed of light.
An interesting result of this equation is that energy and mass are related. Any change in an object's energy is also accompanied by a change in mass. This concept became important in developing nuclear energy and the nuclear bomb.
Length Contraction
Length Contraction
Another interesting result of special relativity is length contraction. Length contraction is when objects appear shorter the faster they are moving about the observer. This effect only occurs as objects reach very high speeds.
To give you an example of how objects moving very fast appear shorter. If a spaceship 100 feet long was flying by you at 1/2 the speed of light, it would appear to be 87 feet long. If it sped up to .95 the speed of light, it would only appear to be 31 feet long. Of course, this is all relative. To people on board the spaceship, it would always appear to be 100 feet long.
Speed of Light
Speed of Light
One of the important concepts of the theory of relativity is the speed of light. You may think that light is instantaneous, that when you turn on the light in your room, the light instantly illuminates your entire room. However, it just seems that way, light travels at a given speed. That speed is just really fast.
The speed of light is 299,792,458 m/s. To put it another way, light travels 186,000 miles in a second. The speed of light is always the same. In physics, it is a constant and is usually represented by the letter "c" in formulas.
It turns out that there is nothing faster than light. It was this constant speed of light that inspired Albert Einstein to come up with the theory of relativity. Einstein found that when objects started to approach the speed of light, then the laws of physics that Isaac Newton came up with started to break down. Einstein's theory of relativity helped to explain why this happened.
Light Year
Light Year
Astronomers use the speed of light to measure how far away stars and other galaxies are from Earth. A light year is a distance that light travels in one year. It is equal to 5,900,000,000,000 miles. That is five trillion, nine hundred billion miles.
Some stars and galaxies are so far away that the light we are seeing left the stars billions of years ago. Light can not only travel fast, but it can travel great distances too.
Time Dilation
Time Dilation
One of the most interesting results of the theory of relativity is time dilation. Time dilation says that time will pass slower for someone traveling near the speed of light relative to someone standing still.
As an example of time dilation, let's imagine you left on a space flight today. On that space flight, you flew around outer space near the speed of light for around three years according to your clock. However, the clock on Earth was moving much faster. In fact, more than sixty years would have passed on Earth. You would only be a few years older, but everyone on Earth would be much older.
Time and Gravity
Time and Gravity
In Einstein's theory of general relativity, he proposes that time will run more slowly the stronger the gravity. This means that time will pass faster on top of a mountain than at the beach. You will age slower at the beach than in the mountains. This is a very small difference, however, nothing you would ever notice.
Light and Gravity
Light and Gravity
Another concept from general relativity is that rays of light will bend in a gravitational field. The Sun bends light enough is that we can see stars that are barely behind the Sun. However, it takes a huge amount of gravity to bend light significantly. As big as the Earth is, it hardly bends light at all.
A black hole is a place in space where gravity pulls so much that even light can not get out. The gravity is so strong because matter has been squeezed into a tiny space. This can happen when a star is dying. Because no light can get out, people can't see black holes. They are invisible.
Is this science fiction?
A lot of the theory of relativity seems like science fiction, especially concepts like time dilation. However, scientists have been able to run experiments with very accurate clocks to prove the theory true. This is not science fiction, but real stuff!
Interesting Facts about the Theory of Relativity
● Theoretically, if you could travel faster than the speed of light, time would go in reverse.
● Scientists think that the universe is expanding and that parts of it are moving away from us faster than the speed of light.
● Albert Einstein once said, "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. That's relativity."
Many aspects of the theory of relativity have been confirmed by experiments run with particle accelerators. Particle accelerators can accelerate electrons and protons to nearly the speed of light.
Page updated
Report abuse