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Author  Topic: Purple dosn't beleve in relativity  
Raymon Inactive Member 
You got a lot of replies to this statment that basically said that the math works out, as well as posts that pointed out that your picture implies a 3rd observer. But there is an interesting paradox going on here that is taught to students learning relativity. Of course, it's not a real paradox. Just as the Twins paradox can be resolved, so can this notion of A's time is shorter then B's time while the converse is also true. As long as you know that length contracts at relatavistic speeds just like time slows down it's pretty straighforward. The way it is usually presented is like this Suppose the Flash, a man who can run at relatavistic speeds, wants to test relativity. He reasons thus: "If I have a barn that's 10m long and has doors on both ends that can be closed very quickly and a 15m long pole I can show relativity doesn't work. If I run fast enough that length contraction is 50% then I can hold the pole horizontally, run through the barn, and have the farmer try to close both doors on while I'm inside the barn. From the farmers perspective it should work because my pole is length conracted to 7.5 m. So he should be able to close both doors while I'm inside. Of course he'll have to open then again to let me out so I don't undergo acceleration and go into General Relativity's realm. But for me, the barn is only 5m long, so there is no way I'm going to get my entire pole in there. So when the doors close at the same time they are going to hit my pole. So if the doors are able to close then the farmer's reference frame is the correct one and if they hit my pole on the way down then mine is." This is basically the same scenario as you proposed, except instead of two times, you are comparing two lengths. So what happens? From the farmer's reference frame, he closes the doors at the same time and Flash's pole fits inside. From Flash's reference frame, as he is running into the barn the far end closes before he gets there and then opens again to let him pass though. Then, after the entire length of the pole has passed through the near end of the barn, the near end door closes and opens. Thus we have another lesson to learn from relativity. Simultanius events in one reference frame aren't simultanius in another. Suppose a farmer is sitting on his porch, overlooking a long stretch of road with syncronized clocks placed every mile. Say the clocks are just striking noon as the Flash wizzes by him. To the Flash, the clocks far in front of him read to be 1pm while the ones far behind him read to be 11 am. Personally, I think it's one of the wierdest aspects of relativity. I hope this cleared up the "paradox". This message has been edited by Raymon, 12082004 07:59 PM
 
Hangdawg13 Member (Idle past 3071 days) Posts: 1161 From: Texas Joined: 
Put another way, If both observers do believe that time is moving slower for the other one, why do atomic clocks tell us which one is moving and which one is stationary? This question is really confusing to me. Many times I've heard people explain how a space ship traveling at close to the speed of light for many years (ship time) and returning to the earth would find that many many more years had passed on earth. I had no problem with this, but how can this happen if both observers believe time is moving slower for the other one????
 
NosyNed Member Posts: 8842 From: Canada Joined: 
Two points: One minor; you'd have to travel at near .99c to have time slow down by half. It is not linear and it is not time moving at half speed if you do half light speed. Doesn't really matter to the overall question it is just a nit. There is a difference between Zorg and Viola or the shuttle and the earth. Once the trip out and back is complete you can distinguish between the two. One of them has undergone accelerations and decelerations. That is the one whose clock will show up as slow. That is when the shuttle returns (at some speed so that it's clock runs at half the rate when observed from earth) then it will show less time passing. I don't actually know if this is a purely special relativisitc effect (which I think is the case) or if general relativity is involved. The reason I wonder about that is because time is dilated in a gravitational field as well as by speed. And GR says that acceleration is exactly equivalent to a gravitational field. For this reason the clocks on the shuttle will be slowed by the accelerations. What I don't know is whether the numbers work out. Remember that the shuttle would have to undergo very large accelerations or very long times accelerating to get to .5 (or more correctly .99c). That would definitely have a general relatavistic effect. Does anyone know if the numbers work out?
 
jar Member Posts: 30984 From: Texas!! Joined: Member Rating: 4.6 
The standard is totally arbitrary and exists only withing a single POV. Aslan is not a Tame Lion
 
The Dread Dormammu Inactive Member 
I've gone through a few drafts of this post and I think the best way to explain it is to use yet another story. Ok so Zorg and Viola are drifting away from eachother and both of them see the other as moving slower through time. When Zorg says he has "had enough" and accelarates towards Violas ship. When Zorg turns his ship around and acccelerates back TOWARDS Viola what does he see? Does he see time dialation? Yes he does but she seems to be moving through time FASTER than him. As he approaches her ship on his return trip he sees her undergo BLUEshift, Her voice sounds HIGHpitched and squeaky on the radio (due to the same doppler effect etc. When he finaly pulls up to a stop next to her ship she is OLDER than him (slightly more rusted etc.). The exact same thing happens in the classical twin paradox. The only difference is that instead of two ships, witch we tend to view as symetrical, they use something we tend to think of as relatively stationary (the earth) and something we think of as nonStationary (the accelarating ship).
 
Lammy Member (Idle past 20 days) Posts: 3607 From: Chicago Joined: 
http://www.phys.unsw.edu.au/~jw/twin.html Hate world. Revenge soon!
 
Lammy Member (Idle past 20 days) Posts: 3607 From: Chicago Joined: 
http://www.phys.unsw.edu.au/~jw/twin.html Hate world. Revenge soon!
 
The Dread Dormammu Inactive Member 
Added by edit: in praticular look at the seeming flurry of responces from Joe, in Janes perspective after she turns around. This message has been edited by The Dread Dormammu, 12092004 09:50 PM This message has been edited by The Dread Dormammu, 12092004 09:51 PM
 
The Dread Dormammu Inactive Member 
http://www.phys.unsw.edu.au/~jw/pole.html
 
Sylas Member (Idle past 3425 days) Posts: 766 From: Newcastle, Australia Joined: 
Strictly speaking, the factor for time dilation is sqrt(1v^{2}/c^{2}). This means you can travel at about 0.87c, and time slows down by half.
You want numbers... we got numbers! Both velocity and gravity effects contribute to time dilation of an astronaut, but by comparison with ground control the velocity effects dominate, because a shuttle orbit is not that much higher than the surface, by comparison with Earth's radius. Hence the shuttle astronauts age fractionally less than ground controllers, simply because they move faster. But astronauts in a very high orbit will age fractionally more, since the relative velocity has little effect, and ground controllers are deep inside a gravitational well, so their clocks run more slowly. The effect of gravity is sqrt(12GM/Rc^{2}). M is mass of the Earth, and R is distance from the center. For a circular orbit, velocity is sqrt(GM/R). When you put it all together, the time dilation for a body in circular orbit is sqrt(1  3GM/Rc^{2}). For the ground controller, the effect of velocity can be ignored, and they experience only sqrt(1  2GM/Rc^{2}). This means that the orbit clock matches the ground clock at an orbit with a radius of 3/2 times earth's radius; an altitude of 3200 km. Astronauts at that orbit experience no time dilation with respect to ground control; those above age more. We can try some numbers here. G is 6.7e11, M is 2e30, c is 3e8 and R is 6.4e6. (R being radius of the earth.) We can approximate sqrt(1x) by (1x/2) for small x. Hence the factor sqrt(12GM/Rc^{2}) is close enough to running slower by GM/Rc^{2} seconds every second. This is about 7e10 Put another way, time dilation for a ground controller with respect to an observer way out in space is a bit more than two hundredths of a second every year. A shuttle astronaut, at an orbit of 200 km, is a bit more than three hundredths of a second per year younger with respect to the distant observer. Hence the shuttle astronauts age about one hundredth of a second less than the ground controller over a year, or about 200 micro seconds in a one week mission. This effect is measurable, and confirms the relativistic calculations. Hi gang. I'm back... for a bit anyway. Cheers  Sylas
 
NosyNed Member Posts: 8842 From: Canada Joined: 
sure beats remembering off the top of my head (and wrong).
 
Sylas Member (Idle past 3425 days) Posts: 766 From: Newcastle, Australia Joined: 
You age more slowly by moving faster, and also by being deeper down in a gravitational well. Both effects contribute to making your watch and also your body clock run more slowly, and as a result you end up being younger than those who moved more slowly or lived at higher altitudes. Cheers  Sylas (who always wanted to live in the mountains anyway) PS. I had a special hello for you in my original post Ned! But it somehow got lost before I posted it. Nice to see you again! This message has been edited by Sylas, 12102004 01:35 AM
 
The Dread Dormammu Inactive Member 
What about someone in the center of mass between two nutron stars orbiting eachother? (I think frame dragging might influence this one) the net is still 0g's or am I making a mistake somewhere? This message has been edited by The Dread Dormammu, 12102004 06:05 AM
 
Sylas Member (Idle past 3425 days) Posts: 766 From: Newcastle, Australia Joined: 
This is out of my league; I'm reduced to guessing. Time dilation should always be measured as a difference between two observers; not as a comparison against an absolute. I am pretty sure that the center of the earth would have time synchronous with an observer at infinite distance from the earth. I have no idea about frame dragging, however. Cheers  Sylas
 
RAZD Member Posts: 19871 From: the other end of the sidewalk Joined: Member Rating: 5.3 
there is zero net gravitational attraction in any one direction, but there is still massive gravity affecting the universe fabric. we are limited in our ability to understand by our ability to understand RebelAAmerican.Zen[Deist



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