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JocK
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 Einstein proven wrong? -- physics --   « on: Jul 23rd, 2005, 7:50am » Quote Modify

Let's consider the following 'gedanken experiment':

Two identical cars fitted with a powerfull rocket engine are parked on a long straight road. One car is positioned slightly ahead of the other. You are standing next to the road at a position such that both cars are at the same distance to you. You have a gun that you fire. On that sign, both cars start accelerating at the same constant rate, one chasing the other. You watch them gaining speed. Since they have the same acceleration, you will observe their speeds being equal at all times and therefore you will observe them staying a constant distance apart.

But after a time the cars will acquire a large velocity, approaching the speed of light. And so - according to Einstein - the distance between them should suffer Lorentz contraction...

Howwzat?

Did we finally prove Einstein to be wrong? Have we been fooled for 100 years?

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solving abstract problems is like sex: it may occasionally have some practical use, but that is not why we do it.

xy - y = x5 - y4 - y3 = 20; x>0, y>0.
EzisEz
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 Re: Einstein proven wrong? -- physics --   « Reply #1 on: Jul 23rd, 2005, 8:40am » Quote Modify

Watching them from stationary position relative to them. You WILL observe them contracting in the direction of their motion by a factor of sqrt(1-v^2/c^2) which will not be noticble until they gain a large velocity approcing the speed of light , this is due to the change in the way the space and time are measured relative to them and to the observer....... SO you are just using a different coordinate system than them one that is sqrt(1-v^2/c^2) longer..
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JocK
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 Re: Einstein proven wrong? -- physics --   « Reply #2 on: Jul 23rd, 2005, 8:54am » Quote Modify

on Jul 23rd, 2005, 8:40am, EzisEz wrote:
 Watching them from stationary position relative to them. You WILL observe them contracting in the direction of their motion by a factor of sqrt(1-v^2/c^2) [..]

Yes, but in my coordinate system (the one at rest) I do observe both cars starting at the same moment in time and accelerating at the same rate, hence driving at the same speed, and therefore keeping at same distance...

How would that be compatible with a contraction?

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solving abstract problems is like sex: it may occasionally have some practical use, but that is not why we do it.

xy - y = x5 - y4 - y3 = 20; x>0, y>0.
EzisEz
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Posts: 10
 Re: Einstein proven wrong? -- physics --   « Reply #3 on: Jul 23rd, 2005, 9:10am » Quote Modify

ok. I see your point....this is more of a general relativity issue , but i will try to approach it from a special relativity point of view....in special relativity accelerating frames of reference do not apply, but i would assume that you believe in contraction if the were moving at constant speed ( for example tyhey start accelrating at then they keep constant velocity at .8 c)...then u believe in contractiob by applying relativity on ur frame of reference and their frame of reference...however because they are accelrating , u can think of them being at constant speed for a very small time epsilong and apply relativity and every instant of time , as if they were moving at constant speed....so pretty much , u cant keep a constant frame of reference when there is acceleration because it changes at every instant of time , i hope that makes any sense
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rmsgrey
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 Re: Einstein proven wrong? -- physics --   « Reply #4 on: Jul 23rd, 2005, 9:54am » Quote Modify

Whose opinion are you taking on whether they're gaining speed at the same rate? If you observe them both constantly moving at the same speed at any given time, then the trail car will observe the other car gradually increasing its acceleration as their opinion on simultaneity gradually diverges from yours. Likewise the lead car will see the other car reduce its acceleration as their idea of "now" becomes different from yours, in effect making the distance appear to grow for both cars, while remaining constant for you.

On the other hand, if the two cars appear to each other to be maintaining constant acceleration/separation, you will see the lead car accelerate less than the trail car, and the gap appear to close up.
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