Physics Help Forum compartive speed of light

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 Jan 8th 2009, 12:16 PM #1 Junior Member   Join Date: Jan 2009 Posts: 1 compartive speed of light Hello a question has been vexing me and I would love some feedback. If two beams of light passed each other by going in exactly the opposite direction then surely the speed difference between them would be twice the speed of light? If I follow this through in my mental experiment and have 2 space craft traveling at 80% the speed of light collide into each other, then the impact is at greater than the speed of light ? Any ideas?
Jan 18th 2009, 07:28 PM   #2
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 Originally Posted by megloman Hello a question has been vexing me and I would love some feedback. If two beams of light passed each other by going in exactly the opposite direction then surely the speed difference between them would be twice the speed of light? If I follow this through in my mental experiment and have 2 space craft traveling at 80% the speed of light collide into each other, then the impact is at greater than the speed of light ? Any ideas?
Speed of light should be a constant. There should not be any speed difference even light travels in opposite directions.

 Jan 30th 2009, 09:18 AM #3 Junior Member   Join Date: Jan 2009 Posts: 1 i have been thinking the exact sam ething for a long time. my physics teacher hasnt answered me yet. they way i thought of it is that according to Einstein's general theory of relativity nothing can travel faster than the speed of light. in the period after the big bang called inflation, the universe expanded exponentially. an object in relation to the big bang may be travelling just over half the speed of light (at least), and in the opposite direction an object is traveling just over half the speed of light. in relation to the big bang singularity these objects are only traveling over half the speed of light. but in relation to each other, they are traveling faster than the speed of light. althoguh these objects would niot be able to see each other, they would be traveling well over the speed of light in relation to each toher. maybe the answer is that object can travel faster than the speed of light, but anything that travels faster than the speed of light can not observe any other object. i didnt explain that very well. for example. no two objects, which in relation to each other, have a speed of more than the speed of light can see each toher. if we measure the speed of something travelling perpendicular to me towards my right, travwelling at 50% the speed of light, and an object travelling to my left at 51% of the speed of light i can see them both. but neither of the objects can see each other. the objects can only see each toher when their combined speed in relation to each other is less than the speed of light. maybe there are objects in space which travel much faster than the speed of light (tachyons) but we are unable to ever see them because our speed plus their speed is more than the speed of light. sorry i dont think that made sense. pleaaaaaase pleaaaaase reply

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