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Old Aug 11th 2011, 07:34 AM   #1
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Question A probably stupid question.

I've got a fairly new founded interest in Relativity, so although I'm keen I know very little. Therefore this is probably an excruciatingly obvious question, but...

Imagine that two objects are fired past each other, each at a fraction of the speed of light higher than a half - let's say three quarters of c. They are both in steady motion (not accelerating or decelerating), so according to Relativity are both perfectly entitled to believe that they are at rest and that the other object is moving. If this were true, it would appear to each object that the one fast approaching them was going at the sum of their speeds, i.e. one and a half times the speed of light.

I'm clearly thinking about this the wrong way, as one and a half c makes no sense, so what am I missing?
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Old Aug 11th 2011, 10:15 AM   #2
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I'm not well versed in it, but that thread might interest you:

Do you think gravity shells, around a mass ...
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Old Aug 12th 2011, 07:37 AM   #3
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Hi and welcome to the forum!
Originally Posted by ToTheWareMobile View Post
I've got a fairly new founded interest in Relativity, so although I'm keen I know very little. Therefore this is probably an excruciatingly obvious question, but...

Imagine that two objects are fired past each other, each at a fraction of the speed of light higher than a half - let's say three quarters of c. They are both in steady motion (not accelerating or decelerating), so according to Relativity are both perfectly entitled to believe that they are at rest and that the other object is moving. If this were true,
so far so good, all is ok.
it would appear to each object that the one fast approaching them was going at the sum of their speeds, i.e. one and a half times the speed of light.

I'm clearly thinking about this the wrong way, as one and a half c makes no sense, so what am I missing?
Your reasoning here is "Galilean" or the one of classical mechanics. See http://www.physicshelpforum.com/phys...er-than-c.html. And why not Einstein velocity addition.
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