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David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.
Einstein was a genius and his work extremely complicated. It would be impossible to fully explain special relativity in a short lesson. But contrary to popular belief, it is possible to understand the basic ideas behind it without having a degree in physics. By the end of this lesson, you will be able to show off to your friends by saying that you actually understand the basics of Einstein's famous theory: a theory that changed physics forever.
Special relativity is a theory in physics that concerns the relationship between space and time and says that they're two sides of the same coin: spacetime. Like all scientific theories, it is backed by a large body of evidence and is widely accepted as being accurate.
The two main postulates of special relativity are:
A reference frame is just a particular point of view. An observer on a moving train is in a different reference frame to an observer beside the tracks. These two postulates might seem obvious, but they have some baffling and hard to understand consequences.
Einstein liked to explain his theories using thought experiments, so let's try one. Imagine for a moment that a person is sitting in the exact middle of a moving train car. This person releases a pulse of light in two directions: one towards the front of the train car, and the other towards the back. Which pulse of light reaches the edge of the train car first?
It turns out that the answer is relative. The answer depends on your point of view. To the person on the train, the light reaches both ends of the train car at the same time. This seems obvious to them - they're in the exact middle of the train car, and so, each pulse of light travels the same distance in the same time.
But someone watching the train from alongside the tracks would see something quite different. To them, because the train is moving, it catches up with the light pulse moving towards the back of the train car. So the backward-facing pulse gets to the back of the train first. For this to be noticeable, the train would have to be going a significant percentage of the speed of light, but it is still the case even if you can't see it at normal speeds. These might seem like contradictory answers, but according to special relativity, both observers are absolutely correct.
The train thought experiment shows how strange the consequences of special relativity can be, and there are many such consequences. The ones we will discuss in this lesson are length contraction, time dilation and mass-energy equivalence. Most of these consequences only apply when objects are moving at a significant percentage of the speed of light, otherwise they are unnoticeable.
Length contraction says that a moving object will appear shorter than an object that is stationary relative to the observer. This effect can be summarized by this equation:
Time dilation says that time passes at different rates depending on whether objects are moving relative to each other. As humans, we are used to time being a constant, but special relativity tells us that the passage of time actually varies depending on your point of view. This equation describes this difference in the amount of time that will have passed:
As strange as this may seem, time dilation has now been conclusively proven. In the 1960s, two atomic clocks (extremely accurate clocks) were synchronized, and one was put on a jet plane. The jet plane was flown through the air and returned to its original destination. When the clocks were reunited, they were no longer synchronized. The clock that went on the jet plane had traveled into the future! While the difference was less than a second, the number fit perfectly with Einstein's equations.
Mass-energy equivalence is the idea that mass and energy are really the same thing. If you increase an objects speed (and therefore energy), you also increase its mass. This is why it is impossible for an object with mass to go faster than the speed of light: as you get closer to this speed, you increase the object's mass, and if you increase the object's mass, you make it even harder to give it more speed. This happens at such a rate, that you can never quite reach the speed of light - you can only get infinitely close to it. The most powerful particle accelerators in the world can only accelerate particles to 99.9999% of the speed of light due to this limit. The equation that tells us mass and energy are the same thing is Einstein's most famous equation:
Special relativity is Einstein's most famous theory. It is based on two ideas:
Those consequences include length contraction, time dilation, and mass-energy equivalence. Length contraction says that objects moving relative to you appear thinner. Time dilation says that time moves at different rates for moving objects. Mass-energy equivalence says that mass and energy are really the same thing.
Special relativity explained a lot of our observations about the world around the turn of the 20th century. Einstein's answer had such incredible consequences that it changed the face of physics forever.
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Back To CourseFundamental Physics
22 chapters | 254 lessons