# A question in the Feynman Lectures on physics,chapter 15-4

1. Jan 25, 2012

### Haixu

In 15-4,Feynman tried to explain why the rod perpendicular to the motion does not shrink."How do we know that perpendicular lengths do not change? The men can agree to make marks on each other's y-meter stick as they pass each other.By symmetry, the two marks must come at the same y- and y'- coordinates,since otherwise when they come together to compare results,one mark will be above or below the other,and we could tell who was really moving."

The last sentence means that we can not tell which system is moving by doing experiments except looking outside.But is comparing the y'- with y- a kind of "looking outside"?? As y- is lying outside actually. So why?

Last edited: Jan 25, 2012
2. Jan 25, 2012

### ghwellsjr

It took me awhile to find your quote because it is on page 15-6, not 15-4.

The point of "looking outside" is to determine who is really moving and who is really stationary by seeing a difference. But for this comparison, even when they do look outside, since it is a symmetrical situation, they still cannot determine which is the one that is moving and which is the one that is stationary.

An example of determining which one is really moving by not looking outside would be if they each had some kind of identical apparatus that worked one way for one of them and worked a different way for the other one. Then you could use this apparatus as an absolute speedometer that could tell you how fast you were going without looking outside. Of course you would look outside to compare all the space ships with these speedometers to show that it was not symmetrical and you would have to show that the readings were consistent with the relative speeds of the different space ships. But if everyone's apparatus registers the same result even when you know there is a relative speed, then it's not working as a speedometer.

3. Jan 25, 2012

### Haixu

Thanks for your help, you explained it very clear!

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