Physics Help Forum Question about length contraction

 Special and General Relativity Special and General Relativity Physics Help Forum

 Dec 4th 2015, 01:03 AM #1 Junior Member   Join Date: Dec 2015 Posts: 1 Question about length contraction If someone is moving horizontally close to the speed of light his observation of the space around him looks thinner. In contrast people's observation of the object moving close to the speed of light also looks thinner, but that only applies to the space occupied by the moving object but not the space around based on this thread. My question would be, how come we get two different observation of the space around us, 1st one where the space looks thinner, 2nd one where the space looks normal, is there an explanation for this?
 Dec 4th 2015, 10:45 AM #2 Physics Team     Join Date: Jun 2010 Location: Morristown, NJ USA Posts: 2,347 Under Special Relativity if two objects have a velocity relative to each other then each observes the other object is fore-shortened compared to what they would measure if their relative velocity was zero. It is not that space is somehow "made thinner," but rather that the object itself is observed to be shorter due to its relativistic velocity.
Dec 5th 2015, 01:27 AM   #3
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 Originally Posted by fredreload ... but that only applies to the space occupied by the moving object but not the space around based on this thread.
That's not true. Think of a ruler which is moving parallel to its length as reckoned in the inertial frame S. Observers at rest in S will reckon that both the ruler and the space that it's in are both shorter, not just the ruler. After all, think about what a ruler is. Its used to measure distances between two points in space! That page you referenced doesn't say what you think it does.

 Originally Posted by fredreload My question would be, how come we get two different observation of the space around us, 1st one where the space looks thinner, 2nd one where the space looks normal, is there an explanation for this?
That's simply a result of relativity. Relativity tells us that something's are relative to the observer. In this case, length.

 Dec 7th 2015, 02:43 PM #4 Senior Member     Join Date: Apr 2008 Location: Bedford, England Posts: 668 It is a definite brain twister, As I interpret what I have read, it goes something like this: When two objects are traveling at different speeds relative to each other, the time and space dimensions they experience are "rotated" with respect to each other such that what one experiences as a purely space-wise direction the other experiences as being partially time-wise and similarity the dimension that the first object experiences as being purely time, the second experiences as being partially space-wise. (and visa versa). Thus they are literally experiencing space and time from different perspectives. I don't know if this helps or just confuses more, it sure confuses me! __________________ You have GOT to Laugh !
Dec 7th 2015, 04:19 PM   #5
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 Originally Posted by MBW It is a definite brain twister, As I interpret what I have read, it goes something like this: When two objects are traveling at different speeds relative to each other,
This makes no sense. If I approach you at 100 m/s, then you approach me at -100 m/s. The magnitude of relative velocity between two object is the same (though the direction is different).

 Dec 8th 2015, 07:44 AM #6 Senior Member     Join Date: Apr 2008 Location: Bedford, England Posts: 668 oops Yes ChipP I agree, it is poorly worded. I was trying to avoid having the stationary observer verses moving observer scenario, because an isolated stationary observer is impossible. They are either stationary with respect to each other or they are moving with respect to each other. It was the moving with respect to each other that I was trying to convey. But as you point out they can't be moving at different speeds with respect to each other. __________________ You have GOT to Laugh !

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