Speed of light and special relativity

Nearly everyone has heard of relativity, but not many people know what it means. There are two different forms of relativity, both proposed by Albert Einstein. The theory of special relativity connects velocity, time, and space. The theory of general relativity presents a totally different way to understand gravity. Both theories may seem like science fiction, but they are well tested and part of the technology you use every day. GPS satellites incorporate general relativity, and virtually all optical fiber data networks must take account of special relativity. Read the text aloud
Speed of light in a vacuum
The speed of light, c = 299,792,458 m/s, is the fastest speed in our universe. Around 1900, Einstein asked himself what light would look like if he could see it when it wasn’t moving. Instead of making the light stop, Einstein thought about traveling beside light at the same speed. You may have driven next to someone going the same speed on the highway. When you look out the window, the other car seems motionless relative to you. Einstein thought about whether it might be possible to travel at the same speed as light and about how light would appear if you could. Read the text aloud
Two different observers see different speeds for the dartRecall the dart thrown on a train from earlier in this chapter. If the person throws the dart at 10 m/s relative to the train, and the train is approaching you at 10 m/s, the dart gets 20 m closer to you every second. As far as you are concerned, you see the dart traveling 20 m/s toward you. The speed of the dart in your reference frame is the speed in its reference frame (the train) plus the speed of that reference frame with respect to you. Read the text aloud
Two different observers see the same speed for lightEinstein considered what the speed of light would be if the person on the train shined a flashlight at you as in the case of the thrown dart. Physicists at that time believed that the ground observer should measure a speed equal to the speed of light in the train’s moving reference frame plus the speed of the train. But that is not what happens. The speed of light is 299,792,458 m/s for both observers no matter the speed of the train or its direction, forward or backward! Read the text aloud
In 1887, Albert A. Michelson and Edward W. Morley wanted to determine whether the speed of light changed as a result of the Earth’s motion. Michelson and Morley made sensitive measurements of the speed of light both parallel and perpendicular to the orbital motion of the Earth. In effect, they used the Earth as a “train” moving at 29,800 m/s—Earth’s orbital speed. They found the speed of light to be exactly the same for both directions! This result is not at all what they expected. How could it be explained? Read the text aloud Show Einstein and the experiment

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