Hi. I am new to the forums and I am hoping to learn a few things. I have been interested in cosmology and its theories for a while, but I have only recently picked up a book on it. It is Brian Greene's The Fabric of the Cosmos. In the third chapter, Greene gives an example of how the speed of light is a constant 670 million MPH, no matter if one is going towards it or moving away from it. In his example, Bart Simpson and Lisa Simpson are conducting an experiment in which Bart uses a 500 million MPH skateboard to chase after a beam of light. While Bart is chasing the beam, Lisa is observing the light to be speeding away from Bart at 170 million MPH (since Bart is going at a constant 500 million MPH; 670 million MPH - 500 million MPH = 170 million MPH). When Bart returns from his trip, he tells Lisa that the light was speeding away at a constant 670 million MPH, not 170 million, while from Lisa's viewpoint, the light was speeding away at 170 million MPH. As far as I understand it, Greene explains this paradox in this way: "...the input that [Bart] uses to figure out how fast the light is receding from him, are different from Lisa's measurements... experimenters who are moving relative to each other, like Bart and Lisa, will not find indentical values for measurements of distances and durations. The puzzling experimental data on the speed of light can be explained only if their perceptions of space and time are different." It seems as if the explanation that he gives explains the next concept about the two different motions - motion through space and motion through time, where the faster you are traveling the more energy you are diverting towards movement through space instead of time, thus going through time more slowly from the perspective of another person who is stationary. But how does this relate to the paradox about the light traveling at a constant speed of 670 million MPH in relation to a traveler who is speeding at 500 million MPH? I still do not understand why this happens; I am going through the text, and I still don't get it for the second day of reading it through. How and why does this 670 million MPH number remain constant in relation to a traveller going at 500 million MPH? Thank you for your help!