Length Contraction - altitude?

[debroglieman]

An observer on Earth sees a meteor approaching rapidly, heading directly for the Earth's surface. At a certain instant, the Earth observer sees the meteor at 650m altitude. If the meteor is traveling at 0.92c, as recorded by an Earth radar, what would be the altitude as measured by the alien resting on the meteor?



L=Lo (ɣ)



I know the solution is to plug in Lo = 650 and solve for L, but not know why this is so. It seems that the two inertial frames are identical, and as such the answer should be 650m? Advice/solution appreciated.

 

[Whovian]

No. The inertial frames are (not) identical (but symmetric), but measurements from them disagree anyway, such as those of distance. Remember that lengths appear* to change from frames of reference moving WRT the object whose length is being measured.

*for lack of better wording

 

[debroglieman]

Okay, so how do I know what is Lo and what is L?