Hubbs Posted April 14, 2011 Share Posted April 14, 2011 Light is an entirely different than a spaceship. The spaceship would not impart any extra velocity to a light it produces. The speed of light is the speed of light. Just like if you had a bullhorn yelling from a supersonic jet fighter, your words, starting in the same location as the jet, would not just automatically be travelling Mach 2. People on the ground would not hear it any sooner than they hear the jet, because the speed does not impart any extra velocity to the sound either. So how come you can talk to someone in front of you on a supersonic jet? If, hypothetically, a stationary person was able to float in the sky and also let an airplane pass through him, and someone yelled from the back of a Concorde right as it passed through the Floating Man, wouldn't the FM perceive the sound from the yeller to be moving faster than the plane itself, and therefore faster than the speed of sound? Link to comment Share on other sites More sharing options...
Spec138 Posted April 14, 2011 Share Posted April 14, 2011 So how come you can talk to someone in front of you on a supersonic jet? If, hypothetically, a stationary person was able to float in the sky and also let an airplane pass through him, and someone yelled from the back of a Concorde right as it passed through the Floating Man, wouldn't the FM perceive the sound from the yeller to be moving faster than the plane itself, and therefore faster than the speed of sound? As far as I know sound always moves at the speed of sound relative to the medium it's traveling in. Therefore, when the sound changes medium it will slow down the wave to the speed of sound in stationary air and increase the frequency, making it a higher pitch. #NotIntendedToBeAFactualStatement Link to comment Share on other sites More sharing options...
Ancalagon the Black Posted April 14, 2011 Share Posted April 14, 2011 @Spec138: Hubbs is actually right here. Sound isn't like light. If sound moves through a rail at x speed, and the rail is moving at y speed in the same direction relative to you, you will perceive the speed of sound as x+y (with a SLIGHT adjustment for relativity, but ignorable). Link to comment Share on other sites More sharing options...
China Posted April 14, 2011 Share Posted April 14, 2011 What if it's a cop? Where do you pull over? ~Bang If it's a cop you tell him that because of the Doppler shift the red light appeared green to you. Just hope he doesn't realize that means you were going about 1/3 the speed of light and give you a speeding ticket instead. Link to comment Share on other sites More sharing options...
Spec138 Posted April 14, 2011 Share Posted April 14, 2011 @Spec138: Hubbs is actually right here. Sound isn't like light. If sound moves through a rail at x speed, and the rail is moving at y speed in the same direction relative to you, you will perceive the speed of sound as x+y (with a SLIGHT adjustment for relativity, but ignorable). That's what I thought I was saying by "As far as I know sound always moves at the speed of sound relative to the medium it's traveling in" but I don't think it was very clear and I still might be mistaken. I was under the impression that the wave would be traveling at the speed of sound relative to the Concorde but if it left the Concorde it would slow down to the speed of sound relative to outside and just increase its frequency. How far off am I here? Maybe I don't understand what Hubbs was saying. Link to comment Share on other sites More sharing options...
AsburySkinsFan Posted April 14, 2011 Share Posted April 14, 2011 well each of your questions is about how everyone observes the speed of light. The answer to each, assuming my first post was correct, is 186,282.397 miles per second You're right that light continues moving that fast but if you are traveling almost as fast then the perception of the light extending from in front of your space craft would be as incrementally slow as a car that's passing you on the highway that's doing only 1mph faster than you are. In other words, the light is still going the speed of light but you're going almost as fast so it doesn't light up everything in front of you like it does in a car, in fact it would quite useless to have headlights on a spaceship that can travel nearly the speed of light. As for the guy traveling in front depending on how far in front he is it could take months or years to even realize that the guy behind him even turned on his lights. I love relativity. ---------- Post added April-14th-2011 at 01:01 AM ---------- This doesn't answer your question either, but when I was a kid, I once asked my mom "If you're on the Concorde, going faster than the speed of sound, can you talk to the person in front of you?" Yes, you can because the air that is carrying the sound waves of your voice is also moving at faster than the speed of sound, now if you were able to sit on the wing of the airplane then no they wouldn't hear you, because by the time the sound waves reached the person's ear they would be well out of range of your voice. Link to comment Share on other sites More sharing options...
Spec138 Posted April 14, 2011 Share Posted April 14, 2011 I love relativity What you described was the exact opposite of relativity. Light travels at the same speed for all observers, even those moving at .9999999c. Link to comment Share on other sites More sharing options...
AsburySkinsFan Posted April 14, 2011 Share Posted April 14, 2011 in that case, I do believe they'd observe the speed of light itself to be the same, as the speed of light is a constant regardless of your velocity. HOWEVER, I do believe they will observe a Doppler shift resulting in a different color of light. Nope, if they are traveling the speed of light then the light from the other craft will remain the same distance behind them which would be equal to the distance apart that the two crafts were when the lights were turned on, and neither light beam would ever catch the opposite craft as long as their speeds remained constant now...if say those crafts were able to travel faster than the speed of light, and they turned in a circle to place themselves on their same position then they could actually watch themselves turn on their lights. ---------- Post added April-14th-2011 at 01:11 AM ---------- What you described was the exact opposite of relativity. Light travels at the same speed for all observers, even those moving at .9999999c. Relativity is about the relativity of position. You're right that light travels at the same speed but if they are traveling as fast or just slower than relative to their position the light appears to be traveling more slowly...much more slowly...that's relativity. ---------- Post added April-14th-2011 at 01:12 AM ---------- They would redshift, but not actually be able to measure any light faster than 186,288,397 mps. They wouldn't see anything, because the light wouldn't catch them. The same way that two cars on cruise control doing 55mph down the interstate won't catch each other nor pull away from each other. ---------- Post added April-14th-2011 at 01:19 AM ---------- So how come you can talk to someone in front of you on a supersonic jet? You're thinking wrong because sound is carried on the air and the air in the ****pit is moving at the same speed as the speakers, it too is being carried. Light on the other hand is not carried on the air. If, hypothetically, a stationary person was able to float in the sky and also let an airplane pass through him, and someone yelled from the back of a Concorde right as it passed through the Floating Man, wouldn't the FM perceive the sound from the yeller to be moving faster than the plane itself, and therefore faster than the speed of sound? No, because again the sound is being carried on the air which is moving at the same speed as the speaker, as such their voice isn't traveling faster than the speed of sound relative to the inside of the jet. Now, if you were to clock it from outside factoring in the two speeds then yes it would be moving at twice the speed of sound, but that measurement would be irrelevant because the sound is still only moving at the speed of sound inside the jet which is the only place that it matters. Link to comment Share on other sites More sharing options...
Spec138 Posted April 14, 2011 Share Posted April 14, 2011 Nope, if they are traveling the speed of light then the light from the other craft will remain the same distance behind them which would be equal to the distance apart that the two crafts were when the lights were turned on, and neither light beam would ever catch the opposite craft as long as their speeds remained constant now...if say those crafts were able to travel faster than the speed of light, and they turned in a circle to place themselves on their same position then they could actually watch themselves turn on their lights. ---------- Post added April-14th-2011 at 01:11 AM ---------- Relativity is about the relativity of position. You're right that light travels at the same speed but if they are traveling as fast or just slower than relative to their position the light appears to be traveling more slowly...much more slowly...that's relativity. ---------- Post added April-14th-2011 at 01:12 AM ---------- They wouldn't see anything, because the light wouldn't catch them. The same way that two cars on cruise control doing 55mph down the interstate won't catch each other nor pull away from each other. Einstein weeps in his grave. The Principle of Invariant Light Speed – "... light is always propagated in empty space with a definite velocity [speed] c which is independent of the state of motion of the emitting body." (from the preface).[1] That is, light in vacuum propagates with the speed c (a fixed constant, independent of direction) in at least one system of inertial coordinates (the "stationary system"), regardless of the state of motion of the light source. Light is observed to travel at the same speed for all observers at all velocities. Link to comment Share on other sites More sharing options...
AsburySkinsFan Posted April 14, 2011 Share Posted April 14, 2011 Einstein weeps in his grave. Light is observed to travel at the same speed for all observers at all velocities. Ok, one more time. Two cars in a line traveling at 55mph, a guy in the back car launches a rocket that flies at 55mph (assume endless propulsion) that rocket while doing it's actual speed will NOT reach the car ahead...ever. The same goes with light. The ship in front won't see the light from the ship behind for a very long time, since in the OP the ship in front was NEAR the speed of light. The only way the ship in front sees it instantaneously is if the light travels FASTER than the speed of light, which as we have already established is constant. I have NEVER once said that the speed of light actually slows...it just appears to slow to the people in the ships because they are approaching the speed of light. Which is the same as race cars appear to pass each other slowly on the track because their speeds relative to one another are only marginally different. Link to comment Share on other sites More sharing options...
DCSaints_fan Posted April 14, 2011 Share Posted April 14, 2011 Things can get pretty confusing when talking about special relatvity (SR) but if you remember the two postulates 1) LIght travels at the same speed in all inertial reference frames and 2) The laws of physics are the same in all inertial frames. Because SR is based on those two postulates, by rejecting them you have to reject all of the logical consequences of SR. Keep this in mind to avoid any mental gymnastics ("light appears to travel slower" = we observe a slower speed for light = not possible under SR) Link to comment Share on other sites More sharing options...
AsburySkinsFan Posted April 14, 2011 Share Posted April 14, 2011 Things can get pretty confusing when talking about special relatvity (SR) but if you remember the two postulates 1) LIght travels at the same speed in all inertial reference frames and 2) The laws of physics are the same in all inertial frames. Because SR is based on those two postulates, by rejecting them you have to reject all of the logical consequences of SR. Keep this in mind to avoid any mental gymnastics ("light appears to travel slower" = we observe a slower speed for light = not possible under SR) So with Special Relativity the light will catch the ship ahead a speed that is faster than light can travel? The rocket from the car will actually travel faster than the rocket can travel? The whole point of relativity is that the faster we travel the slower the light moves away from us thus slowing time relative to the viewer. Link to comment Share on other sites More sharing options...
DCSaints_fan Posted April 14, 2011 Share Posted April 14, 2011 Ok, one more time.Two cars in a line traveling at 55mph, a guy in the back car launches a rocket that flies at 55mph (assume endless propulsion) that rocket while doing it's actual speed will NOT reach the car ahead...ever. I'm assuming you mean 55 mph relative to a stationary observer(with respect to the other cars)... if so this is true. However no physical object can reach c, only very close to c, so the above scenario isn't possible wrt to light. The same goes with light. The ship in front won't see the light from the ship behind for a very long time, since in the OP the ship in front was NEAR the speed of light. The only way the ship in front sees it instantaneously is if the light travels FASTER than the speed of light, which as we have already established is constant.I have NEVER once said that the speed of light actually slows...it just appears to slow to the people in the ships because they are approaching the speed of light. Which is the same as race cars appear to pass each other slowly on the track because their speeds relative to one another are only marginally different. It won't "appear" slower ... when light reaches the observers in the front car, they will measure c. Remember that time is also relative under SR ... the observers cannot detect when the other spaceshift actually turned on their lights until the light actually reaches them ... so the appearance of slowness or some type of delay does not make sense. Observers in the first spaceship will not even agree with observers in the second spaceship about what time the first spaceship turned on its lights. Link to comment Share on other sites More sharing options...
AsburySkinsFan Posted April 14, 2011 Share Posted April 14, 2011 I'm assuming you mean 55 mph relative to a stationary observer(with respect to the other cars)... if so this is true. However no physical object can reach c, only very close to c, so the above scenario isn't possible wrt to light. Well what fun are you? The whole thing is a hypothetical and you want to throw it out because no physical object can reach c? Heck no ship can reach even close to c, but we're still playing with it. It won't "appear" slower ... when light reaches the observers in the front car, they will measure c. Remember that time is also relative under SR ... the observers cannot detect when the other spaceshift actually turned on their lights until the light actually reaches them ... so the appearance of slowness or some type of delay does not make sense. Observers in the first spaceship will not even agree with observers in the second spaceship about what time the first spaceship turned on its lights. When the light does hit them it will appear that the ship behind instantly turn on the lights....only it will be several light years into the future, but the actual turning on of the lights in universal history will have occurred several light years in the past. The same way we can watch a super nova occur in real time on Earth, even though in universal history that super nova occurred millions of years ago. The only issue is that if we were able to move at c then the image for us would freeze, and if we were to push past c then the image would reverse. Link to comment Share on other sites More sharing options...
DCSaints_fan Posted April 14, 2011 Share Posted April 14, 2011 So with Special Relativity the light will catch the ship ahead a speed that is faster than light can travel?The rocket from the car will actually travel faster than the rocket can travel? The whole point of relativity is that the faster we travel the slower the light moves away from us thus slowing time relative to the viewer. Alice is traveling at 0.9c relative to some stationary observer, Bob, and turns on her headlights. She will observe light moving away from her at speed c. Bob will also observe the light moving away from Alice at speed c. They will not, however, agree on the time that Alice turned on her headlights ... Yes, it appears to be a contradiction. But all the observations we've made so far, and the consequencie (most notably, time dilation, length contraction, relativety of simultaneity) say its correct. A more interesting aspect of SR, is the Twins Paradox .... Link to comment Share on other sites More sharing options...
PokerPacker Posted April 14, 2011 Share Posted April 14, 2011 So with Special Relativity the light will catch the ship ahead a speed that is faster than light can travel?The rocket from the car will actually travel faster than the rocket can travel? The whole point of relativity is that the faster we travel the slower the light moves away from us thus slowing time relative to the viewer. consider this: There is no such thing as speed without it being relative to something. There is some frame of reference for, say, the speed you travel in a car; that reference is the earth. So person A is moving 25 mph with reference to point REF, and person B is moving -25 mph with reference to point REF. Person A is thus moving at 50 mph with reference to person B. However, light is traveling at the same speed in reference to all three. Light is crazy like that. Link to comment Share on other sites More sharing options...
Spec138 Posted April 14, 2011 Share Posted April 14, 2011 Ok, one more time.Two cars in a line traveling at 55mph, a guy in the back car launches a rocket that flies at 55mph (assume endless propulsion) that rocket while doing it's actual speed will NOT reach the car ahead...ever. The same goes with light. The ship in front won't see the light from the ship behind for a very long time, since in the OP the ship in front was NEAR the speed of light. The only way the ship in front sees it instantaneously is if the light travels FASTER than the speed of light, which as we have already established is constant. I have NEVER once said that the speed of light actually slows...it just appears to slow to the people in the ships because they are approaching the speed of light. Which is the same as race cars appear to pass each other slowly on the track because their speeds relative to one another are only marginally different. All I can say is that light doesn't behave like our slow objects, and that we observe light traveling at the same speed regardless of our velocity or reference point. This is a key concept to time dilation IIRC. As for slowing light... Light, which normally travels the 240,000 miles from the Moon to Earth in less than two seconds, has been slowed to the speed of a minivan in rush-hour traffic -- 38 miles an hour.An entirely new state of matter, first observed four years ago, has made this possible. When atoms become packed super-closely together at super-low temperatures and super-high vacuum, they lose their identity as individual particles and act like a single super- atom with characteristics similar to a laser. Such an exotic medium can be engineered to slow a light beam 20 million-fold from 186,282 miles a second to a pokey 38 miles an hour. "In this odd state of matter, light takes on a more human dimension; you can almost touch it," says Lene Hau, a Harvard University physicist. [/Quote] http://news.harvard.edu/gazette/1999/02.18/light.html Link to comment Share on other sites More sharing options...
AsburySkinsFan Posted April 14, 2011 Share Posted April 14, 2011 Alice is traveling at 0.9c relative to some stationary observer, Bob, and turns on her headlights. She will observe light moving away from her at speed c.Bob will also observe the light moving away from Alice at speed c. They will not, however, agree on the time that Alice turned on her headlights ... Yes, it appears to be a contradiction. But all the observations we've made so far, and the consequencie (most notably, time dilation, length contraction, relativety of simultaneity) say its correct. A more interesting aspect of SR, is the Twins Paradox .... Appears to be a contradiction? It IS a contradiction. You can't say that something is constant, c, and then say that something moving at nearly that speed will see something moving at c happens at the speed of c. Link to comment Share on other sites More sharing options...
Spec138 Posted April 14, 2011 Share Posted April 14, 2011 Appears to be a contradiction? It IS a contradiction. You can't say that something is constant, c, and then say that something moving at nearly that speed will see something moving at c happens at the speed of c. You should give this a watch: w_DenvIA9gQ Link to comment Share on other sites More sharing options...
AsburySkinsFan Posted April 14, 2011 Share Posted April 14, 2011 consider this: There is no such thing as speed without it being relative to something. There is some frame of reference for, say, the speed you travel in a car; that reference is the earth. So person A is moving 25 mph with reference to point REF, and person B is moving -25 mph with reference to point REF. Person A is thus moving at 50 mph with reference to person B. However, light is traveling at the same speed in reference to all three. Light is crazy like that. Then light is traveling faster than the speed of light, which is supposedly constant. I get that it's not like sound, but light is either wave or particle and if it is wave then you can catch up to a wave, if it is particle the same applies. The wave still is moving at the same rate, but so are you. This idea would say that light waves/particles are moving away even from each other at the speed of light. Link to comment Share on other sites More sharing options...
DCSaints_fan Posted April 14, 2011 Share Posted April 14, 2011 Well what fun are you? The whole thing is a hypothetical and you want to throw it out because no physical object can reach c? Heck no ship can reach even close to c, but we're still playing with it. At c you get singularities .... the gamma factor becomes infinity, so you can't make any observations that can be confirmed. When the light does hit them it will appear that the ship behind instantly turn on the lights....only it will be several light years into the future, but the actual turning on of the lights in universal history will have occurred several light years in the past. Thats the key here ... there is no universal history ... time is also relative and there is no preferrred reference frame. "Several years in the future" ... according to one observer .. but not the other. The same way we can watch a super nova occur in real time on Earth, even though in universal history that super nova occurred millions of years ago. The only issue is that if we were able to move at c then the image for us would freeze, and if we were to push past c then the image would reverse. SR says no such thing ... in fact SR says that its impossible to reach c or even go past it.... thats ths stuff of sci-fi writers. Link to comment Share on other sites More sharing options...
AsburySkinsFan Posted April 14, 2011 Share Posted April 14, 2011 Fine the ship behind shoots a laser gun at the ship ahead, the laser beam travels at the speed of light...when does the ship in front explode? And will the ship in front see the laser approach at the speed of light or less? Link to comment Share on other sites More sharing options...
DCSaints_fan Posted April 14, 2011 Share Posted April 14, 2011 Appears to be a contradiction? It IS a contradiction. You can't say that something is constant, c, and then say that something moving at nearly that speed will see something moving at c happens at the speed of c. It helps to think in terms of what people can actually measure ... everyone will measure c, no matter how fast they are travelling relative to each other. To many there appear to be contradictions, because they are still thinking in terms of absolute frames of reference and absolute time ... but the consequences of a unversally measured speed of light mean that there cannot be a preferred frame of reference, and that time must also be relative. Link to comment Share on other sites More sharing options...
AsburySkinsFan Posted April 14, 2011 Share Posted April 14, 2011 At c you get singularities .... the gamma factor becomes infinity, so you can't make any observations that can be confirmed.Thats the key here ... there is no universal history ... time is also relative and there is no preferrred reference frame. "Several years in the future" ... according to one observer .. but not the other. SR says no such thing ... in fact SR says that its impossible to reach c or even go past it.... thats ths stuff of sci-fi writers. Yet, if the Universe is not infinite but contained then from the perspective of the whole from outside would determine when something happened apart from the relative time it happened from within. ---------- Post added April-14th-2011 at 02:05 AM ---------- It helps to think in terms of what people can actually measure ... everyone will measure c, no matter how fast they are travelling relative to each other. To many there appear to be contradictions, because they are still thinking in terms of absolute frames of reference and absolute time ... but the consequences of a unversally measured speed of light mean that there cannot be a preferred frame of reference, and that time must also be relative. But, I'm not thinking in respect to time being absolute, in fact I'm thinking quite the opposite in that time is relative to one's speed. Link to comment Share on other sites More sharing options...
DCSaints_fan Posted April 14, 2011 Share Posted April 14, 2011 Fine the ship behind shoots a laser gun at the ship ahead, the laser beam travels at the speed of light...when does the ship in front explode? And will the ship in front see the laser approach at the speed of light or less? The ship will see the laser approach at the speed of light .... observers on the front ship and observers on the rear ship will not agree when the ship in front exploded. Link to comment Share on other sites More sharing options...
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