[ARCHIVED THREAD] - Light speed (Page 1 of 2)
Posted: 10/21/2008 6:51:46 PM EDT
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Another thread caused me to remember a question I once had on a physics test. Its really one of Albert little thought experiments but here goes. You are standing in a train in the last car. The train is traveling at the speed of light. You walk forward towards the front of the train. Are you going faster than the speed of light as you do so?  Enjoy. 
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Nope. Time is warped. The light is traveling light speed away from you, and it is also traveling light speed away from a stationary observer, even though you and the stationary observer are traveling at different speeds. |
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If the train was going at the speed of light and you moved foward inside the train then yes. Your relative speed compared to the train added to the speed of light = faster than the speed of light. Basic mathematics. The conundrum is getting the train to go the speed of light in the first place, which is impossible. Essentially the question is invalid because the premise is false. |
Basic mathematics don't work when dealing with relativity. |
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A different question with light speed. Why are people worried that the Large Hadron Collider, which spashed two sets of particles together moving near the speed of light, would create black holes and destroy the planet? Wouldn't pointing two flashlights at each other produce the same thing? The light from one flashlight is moving at the speed of light, the light from the other flashlight is moving at the speed of light. Where the light beams meet the net result is a collision that is twice the speed of light! Why doesn't that cause the world to implode upon itself? |
299 792 458 m/s + 1 m/s still equals 299 792 459 m/s. Math doesn't magically change. Since the train could never reach the speed of light the question is moot. |
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nope, the train decreases in velocity by an undetectable amount. for every action theres an equal and opposite reaction Think of it as a momentum balance (mass times velocity). Train is moving at CxTmass, Tmass=Tyou+Ttrain, therefore CxTmass=VtrainTtrain+VyouTyou. At least until you arrest your forward motion and cause the train to accelerate back to light speed... unless you cause a deformation of the tread which converts forward momentum into heat.... bwhwhahaha |
Assuming the train could travel the speed of light, for you to take a step forward would be impossible due to the fact that as things close in on the speed of light they gain mass eventually requiring an infinite amount of energy to get it to a higher speed. Not to mention time would stop completely as you attempt go beyond your existence in time. If that makes any sense. |
Time is not a constant. |
 That's why it's called "relativity". In fact, you would not be exceeding the speed of light, even to a stationary observer. I have no idea why or how, but that's the theory, and it seems to withstand scrutiny. |
Since I don't understand could someone explain this then? I would rather turn this into a learning experience than, "wow I was a dumbass." |
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Just look up Theory of Relativity. There are PLENTY of videos and sources of info on it. After thinking about Light and how it work for years and years Einstein realized that light was the cosmic speed limit and that time was actually a variable produced by acceleration, speed of travel, mass. What came out of that is a very strange phenomena where there is not one actual truth to the way a series of events happens, it is dependent on the observer's speed relative to other possible observers that create different accounts of the order things happened in. You get it now? LOL Look up Time Dilation, it can tell you what you might see if you were on the train going near light speed and then walked forward. |
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Time and space compress near the speed of light. In fact time and space are part of the same "thing" It is all about where you are observing this event from. A theoretical fixed point observer watches the train (and you on it) go by at the speed of light. From their perspective the "length" of the train is 0 (or very near zero) Therefore your movement inside the train is irrelevent as you are moving along a zero length. Time has also slowed WAY down from this perspective. So you walking 2 paces forward that you think takes only a few seconds, to the observer takes perhaps decades. |
Read Einstein's Relativity: The Special and General Theory. It's public domain, and you can get it from Project Gutenburg. The basic concept of time, mass, and length dilation is very approachable and easy to understand.z |
i'll do my best. it is pretty much proven that light speed is ALWAYS, constant. if you accept that, then it becomes a pretty simple theory. once again, light speed IS ALWAYS THE SAME. it doesn't matter if the light coming from a train doing 60mph, or the sun which is traveling at a ridiculous speed RELITAVE the universe. THE SPEED OF LIGHT IS ALWAYS THE SAME. just to make it simple, let's say light can travel 1000 miles a second. so light from a relativly stationary street light (relativly because even the street light is moving with the earth, etc) will be 1000 miles away in 1 second. Edited this paragraph for clarity- now, if your sitting on a train that's moving 1 mile a second, the light thats on that train with you will COVER THE DISTANCE of 1000 miles, plus the one mile that the train has moved. so that light has traveled 1001 miles in one second. Speed is defined as distance traveled over time. Since we know that the SPEED of light of light is always the same, then the TIME of one second must be altered to account for the extra distance traveled. hope that helps. ETA: fuck i'm not making any sense. using the figures from my example, think of it this way, 1 second is the time it takes light to cover 1000 miles. if the source is moving relative to you, then light seems to cover more distance in the same period of time. since this is impossible (light speed is constant), then a second must be shortend to however long it takes light to travel 1000 miles. |
I have actually thought of this myself as well. I gave up. |
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Actually all of you who state nothing can go faster than the speed of light are incorrect according to current theories (i.e. last 30+ years or so). Really, nothing moving slower than the speed of light can accelerate to the speed of light or faster. Particles already moving faster than the speed of light (i.e. tachyons as one example) can not decelerate to a speed slower than the speed of light. There is quite a bit of study regarding using tachyon theory to create communications devices which could allow communications over great didtances such as from one solar system to another within our galaxy. Pretty interesting stuff if you're so inclined. |
See, I understand all of that, and I get time dilation, etc... However, what I DON'T get is WHY light speed is constant. Light behaves both like waves and also like particles (photons). I don't understand how something that has particle behavior can have a constant speed relative to the viewer i.e. if there are two people, with one "standing still" (relative to the other), and the other is moving at say 1/4 light speed, and they both observe the same burst of light, each would see the light moving along at 186K miles/sec relative to their own position and velocity. This scrambles my brain. |
If you really want to blow your mind, read up on Frame Dragging. |
You forgot one important fact. This guy is the engineer.  
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Forget the observer. why does there always have to be an observer? Forget the observer for a moment. Point A to point B, object moving a constant speed with an object on said object moving slightly faster. Would the second object still technically be going faster? If they were to both continue along to a destination and both were allowed to continue their speeds, wouldn't object #2 get there sooner? |
Without an observer, they would not exist. |
I always wondered WHY we are SO CERTAIN that 'c' is an absolute?? It's only because we don't have any instrument that can measure any faster. Hell, think about it; in times past people thought that if a car, plane, train, etc went fast we would die in some horrible fashion. I like to have an open mind about things like this, so I say maybe, maybe not. We don't really know. Can't prove it one way or the other. Remember, we used to be CERTAIN the earth was flat and that the universe rotated around us! |
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Here's one to bust your noggin: quantum entanglement. Basically quantum entanglement binds the state of two particles such that a change in the state of particle A causes the opposite change in the state of particle B. The cool part is that this affect is not impacted by distance. So, given two entangled particles, leave one in a lab on Earth and (somehow) send the other on a spaceship to Vega 25ly away. Modifying the state of the particle on Earth should cause a corresponding change to the state of the particle at Vega. Thus, information has traveled 25ly instantly. |
I am not observing my car in my driveway right now but I am damn sure it exists. You want observers here ya go. How about the guy driving the train and the guy walking. Guy driving light speed train "damn that guy is walking faster than us. I guess he will get there before us." Guy walking a little faster "wow I am passing the train a little, I guess I will beat that train to the destination" |
Cant gravity affect the speed of light or what about a black hole? isn't the light going slower trying to get out? |
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I'm old and it's late and I took my last quantum physics class 44 years ago so I don't remember the formula; but here's how it works. Additive velocities are always less than the sum of the two individual velocities; but until one of the velocites is at or approaches C the effect is too miniscule to be noticed. Example: You're on a "train in a perfect vacuum" going 60MPH and you throw a baseball 60 mph in the exact direction that the train is moving. The sum of the velocity of the baseball isn't actually 120 MPH it's something like 119.999999999999999999999999999999999999999999999999.......MPH. Example 2: Your on the same train under the same conditions and there's a head light on the damn thing. The speed of the light emited by the headlight isn't C + 60 MPH; it's still only C. ETA: The speed of light is only 186,000 miles per second in a vacuum. In a lab, some female scientist slowed the speed of light down to only a couple of miles per hour in an experiment where she created something that I believe was called a Bohrs-Einstein condensate through which light moved at a crawl of about 22 miles per hour. |