# Relative Velocity

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Interesting method. Two points. First, would this mean that X was measuring his relative velocity by reference to the part of the ruler next to him (like where X's clock is) and thus Y (by extension!) would effectively be present at X ? Second, how could this be made to work if the distance between X and Y is large - maybe light years?

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First, would this mean that X was measuring his relative velocity by reference to the part of the ruler next to him (like where X's clock is) and thus Y (by extension!) would effectively be present at X ?

X is measuring his relative velocity by reference to the part of the ruler next to him, yes, but the:

"thus Y (by extension!) would effectively be present at X"

i do not follow... when you say by extension, if you are referring to the ruler being part of Y then yes Y is present at X, but that is if the ruler is part of Y, which it isnt, i dont think!

Second, how could this be made to work if the distance between X and Y is large - maybe light years?

you'd need a way of measuring that distance, there must be a way, how else would scientists know that the neareast star (except our sun) is however many light years it is from us, i dont know how they'd work it out... it must be possible.

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i do not follow... when you say by extension' date=' if you are referring to the ruler being part of Y then yes Y is present at X, but that is if the ruler is part of Y, which it isnt, i dont think!

[/quote']

I imagined that your ruler was "attached" to Y in the sense that Y had hold of the ruler and extended it towards X. Then X would be able to see the marks on Y's ruler. X could then compare any changes in his relation to the marks with those on his clock. There would be real problems if the ruler wasn't attached to Y. X wouldn't be sure whether his motion was relative to Y or relative to the ruler which had no connection with Y. Seems connection is the key.

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yeah ok, so there'd have to be a connection between Y and the ruler, i mean, if i hold a ruler the ruler doesnt become part of me, although there is a connection.

do you understand it all now?

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do you understand it all now?

Some' date=' not all. Thanks for your help. If you come up with any thoughts about the [b']big[/b] distance problem let me know. Perhaps you'll be able to throw some light on it.

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The ruler represents the rest frame of y.

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The ruler represents the rest frame of y.

What do you mean. What is a rest frame? Please don't say that it's a frame at rest!

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What do you mean. What is a rest[/b'] frame? Please don't say that it's a frame at rest!

Isn't it a non-accelerating frame?

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