Physics Help Forum Why did Einstein need thought experiment?

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 Nov 2nd 2017, 05:34 AM #1 Senior Member     Join Date: Feb 2017 Posts: 205 Why did Einstein need thought experiment? Newton believed time to be absolute. One reason may be that there were no technology to test time dilation. If there were spaceships during newtons time he might have been able to compare the time of the clocks, one inside the spaceship and one on a stationary space station. He could'nt have tested time on a train even as trains did not exist during newtons time. During Einsteins time there was no spaceship so he did a thought experiment with a spaceship using twin paradox. The first airplane wasinvented in 1903, two years before the theory of special relativity was proposed by Einstein. So we could understand that the general public did not have access to a aeroplane at time of Einstein and he could not place a clock on an aeroplane and compare the time in an airport clock. But why did Einstein need a thought experiment to prove time dilation? During his time there were trains and the train's clocks run at 4/5 the rate of the station clocks. So here we have a proof, an actual experiment. So why need a thought experiment? Last edited by avito009; Nov 2nd 2017 at 05:52 AM.
Nov 2nd 2017, 06:18 AM   #2

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 Originally Posted by avito009 Newton believed time to be absolute. One reason may be that there were no technology to test time dilation. If there were spaceships during newtons time he might have been able to compare the time of the clocks, one inside the spaceship and one on a stationary space station. He could'nt have tested time on a train even as trains did not exist during newtons time. During Einsteins time there was no spaceship so he did a thought experiment with a spaceship using twin paradox. The first airplane wasinvented in 1903, two years before the theory of special relativity was proposed by Einstein. So we could understand that the general public did not have access to a aeroplane at time of Einstein and he could not place a clock on an aeroplane and compare the time in an airport clock. But why did Einstein need a thought experiment to prove time dilation? During his time there were trains and the train's clocks run at 4/5 the rate of the station clocks. So here we have a proof, an actual experiment. So why need a thought experiment?
There may be a better answer for this but as far as I know he didn't do a thought experiment on time dilation. It falls out of the equations of SR directly. So it wasn't so much of a thought experiment as it was a prediction made by the theory that needed to be tested.

-Dan
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Nov 2nd 2017, 08:20 AM   #3
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 During his time there were trains and the train's clocks run at 4/5 the rate of the station clocks. So here we have a proof, an actual experiment.
0.8C ???

Where do you find trains that run at that speed?

Nov 3rd 2017, 06:38 AM   #4
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 Originally Posted by avito009 During Einsteins time there was no spaceship so he did a thought experiment with a spaceship using twin paradox.
That's not quite correct.

 Originally Posted by avito009 But why did Einstein need a thought experiment to prove time dilation?
He didn't. Einstein invented and published relativity in 1905. In that paper Einstein stated two postulates (here stated in modern terminology) (1) the laws of physics are the same in all inertial frames of reference and (2) the speed of light is the same in all inertial frames, i.e. independent of the motion of the source (i.e. c is invariant). After he stated those two postulates he then derived the relationships between spacetime events in two inertial observers. There is no mention of a twin paradox in that paper.

 Originally Posted by avito009 During his time there were trains and the train's clocks run at 4/5 the rate of the station clocks. So here we have a proof, an actual experiment. So why need a thought experiment?
First off physics is not about proof. That's a common misconception. A friend of mine at MIT Explains if in a video he did for me.See:

http://www.newenglandphysics.org/com...an_Guth_04.mp4

Second, thought experiments help people understand nature. Einstein's first thought experiment was about what he would see if he was riding a bicycle.

The twin paradox wasn't published until 1911 where he co-authored a paper with physicist Robert Resnick. The paper didn't talk about spacesshjps or trains or bicycles etc. The spoke merely of a traveling observer.

Nov 3rd 2017, 07:13 AM   #5

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 Originally Posted by studiot 0.8C ??? Where do you find trains that run at that speed?
 Originally Posted by Pmb ...Einstein's first thought experiment was about what he would see if he was riding a bicycle.
Greeeeat. Not only do we have a train going at 0.8c, now we have a bicycle??

-Dan
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 Nov 3rd 2017, 10:31 AM #6 Senior Member   Join Date: Apr 2015 Location: Somerset, England Posts: 1,035 My apologies, I'm surprised no one noticed but a time dilation of 0.8 corresponds to a velocity of 0.6C. Wikipedia has a nice table and graph. https://en.wikipedia.org/wiki/Lorentz_factor Sorry about that, but still faster than any train we have in Somerset.

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