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SleazyD23
2008-Mar-15, 11:16 PM
Alright, if light was seen (by an outside observer) traveling just behind the expanding edge of space, would it appear to be traveling faster than "the speed of light." Would the velocities of the light and the expanding edge of space add together? Or does special relativity cover it and the light is seen as traveling no faster than usual?

antoniseb
2008-Mar-15, 11:24 PM
How are you thinking an outside observer could be outside the universe and see things from the universe?

Ken G
2008-Mar-16, 01:55 AM
Would the velocities of the light and the expanding edge of space add together? Or does special relativity cover it and the light is seen as traveling no faster than usual?Special relativity doesn't cover that, the speeds do effectively add, and can exceed c in the total. This is not true for "outside observers", which we know nothing about, but it is true for "inside obververs", i.e., for us. Note that this presumes a certain way of talking about "space" such that space is imagined to be in free fall along with the matter. This is just a picture for space, not a physical entity, which is part of why special relativity should not be expected to work without also including the "expansion of space" that is built into this description. Physical descriptions of space, stemming from direct measurement, have to be local, and so does special relativity, so there's a degree of arbitrariness in the words when you extend that to a global description of cosmology.

SleazyD23
2008-Mar-16, 06:02 PM
Sorry about the "outside observer" complication, i did in fact mean as viewed in the position of an "inside observer" as Ken G. put it. I'm still a little confused though. I would notice a change in the speed of the light (assuming I could detect fluctuations at those speeds) even though I, along with that light is traveling relative to the expanding universe. Does that sound correct? I'm sort of confusing myself :o

Ken G
2008-Mar-16, 06:06 PM
You won't notice any change in the speed of light as it passed you, because your local version of space isn't moving relative to you. But light that comes to you from distant places will have covered much greater distances than you would think possible, given your measures of distance and time.

SleazyD23
2008-Mar-18, 11:47 PM
Alright, I understand how the light's travel would appear to me as the observer. As you can tell, I don't have a full understanding of some of these concepts, so bear with me. When you said "your local version of space isn't moving relative to you," is it that space doesn't move relative to any one object? And if so, does any object actually have motion relative to space? I am just throwing these questions out there as they pop up in my head. Thanks for the thorough responses

Ken G
2008-Mar-18, 11:58 PM
When you said "your local version of space isn't moving relative to you," is it that space doesn't move relative to any one object? It's that you build the concept of space the way it makes sense to measure it, so typically that is stationary with respect to you. The universe doesn't hand us "space", we conceptualize it and use it to turn our measurements into what are called "coordinates." I know a lot of physics is taught the other way around, as if the coordinates were there first and amazingly your measurements conform to them, but that's not really what is happening.


And if so, does any object actually have motion relative to space? It can have motion relative to you, and you build a concept of space around yourself (typically), so that is what gives the object its motion relative to (your concept of) space. Of course, another observer riding on that object thinks you are moving relative to (their concept of) space.