Tricky stuff... Hi Blueshift, Darkness cannot travel at the negative of the speed of light ; when generally talking about speeds, they are always assumed to be positive (negative speeds are only used to indicate that an object is moving in the opposite direction of what you choose to be "forward motion"). Now for your question: how fast does darkness (or a shadow) travel. The answer depends on the situation, but you can say that in most situations you encounter, it travels with the speed of light. An example would be when you turn off the lights in a room. Since the lights stop emitting photons, you would say that the darkness (or absence of light/photons) propagates as fast as the last photon that leaves the lights. In this example, darkness would propagate with the speed of light. Now there are some wierd thought-experiments (gedankenexperiments) where you can let light or a shadow travel faster than the speed of light (and this is not a violation of the theory of special relativity), but I don't think these situations are the ones you meant in your question. So to conclude: darkness (mostly) travels at the speed of light. Bye! Crisp
um... That made more sense than I expected it would, but I have a question, I was hoping could be answered. You see the way light relates to dark is like the way heat and cold relate, right? So, cold is nothing more than absence of heat. Which means that when someone says,"Close the window. You're letting all the cold air in", they should really be saying, "Close the window. You're letting all the warm air out". Now I know this much. The problem is that my brother and I have debates like this quite often while we smoke ( I KNOW you didn't need to know that) and this is one that hasn't ever been resolved because he's too bullheaded to even try to understand. His argument being that since you can feel cold, or cold air to be specific, that cold is an actual thing that exists like water or something. I've tried several times to explain to him that cold air on your skin is just the air around you having a much lower temperature than you and it's taking your body heat because there is an absence of it in the air. So ultimately cold is nothing more than a heat void. Could anyone help me better explain this in a manner that would knock his socks off and MAKE him listen to reason?!
Dear Crisp & willakitty, Crisp is correct. Darkness cannot travel at the negative of the speed of light. Speed is a scalar, not a vector. Speed: The car goes 60mph Vector: The car goes North at 60 mph Therefore it (speed) does not have a direction and can not be considered negative. Crisp gets a B+. Here's why: Light is the absence of darkness, so if you think in reverse, darkness is the absence of light. So if you remove darkness by shinning a light, and then suddenly turn off the light, the speed (scalar) at which darkness proprogates throughout the room will indeed be the speed of light. Darkness cannot move slower than light because where darkness hasn't moved to yet there would still be light. Light cannot move slower than darkenss for the same reason. Light and dakness are both vectors (direction), so the velocity (also a vector) of darkness would have the same direction (vector) as light. To use Crisps' example of a light bulb: Light travels from the light bulb and moves toward the walls of a room. When the light bulb is turned off, darkness first appears around the light bulb and moves toward the walls of the room. To answer your question about Warm/Cold Air. Cold air is more dense (heavier) than Warm air which is less dense (lighter). If your in a warm room and open the window, the cool air from the outside will first move to the floor in waves. So if you lived on the ceiling, and your brother lived on the floor, he would get cold before you even if you were directly above him. Believe it or not, cold air has heat. When cold air condenses, it releases heat. So if your body temperture is warmer than the surrounding air your in, the warm air from your body will rise and be replaced by the cooler air which will condense release heat evaporate and you will feel cool. Don't believe it? Check out a Cold Front Occlusion. -blueshift
