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Author Topic: How can galaxies receding at faster-than-light-speed be observed?  (Read 8596 times)

Offline Phractality

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Quote from: Phractality
That's not how velocities add in SR.
Itís unclear from what he wrote who was doing the measuring/observing of the motion. Thatís why I ignored it. He very well could have meant that as measured in frame S there is an object moving in the Ėx direction with speed v = -2c/3 as measured in S and one moving in the +x direction with speed v = 2c/3 as measured in S and the relative speed of one body with respect to the other is 4c/3 as measured in S. Since no body is moving FTL then thereís no conflict with relativity.
While the distance between the two, in S, is growing at a rate of 4/3 c, that is not a velocity as velocity is defined in SR or in GR.
 

Offline SimpleEngineer

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Sorry I framed my questions badly, as I was interested in the same thing with my own thoughts..

Yes, the two objects travelling at 2c/3 measured from a single frame.. and it was meant to describe how unless something was travelling at c and the observer was travelling at c in the opposite direction (-x and +x) you would always be able to observe anything else.

I dont really understand the article in wikipedia, my question was simply the definition of expansion.. is 1 'piece' of space becoming 2'pieces' of space across the board (including space between 2 stationary subatomic particles?) or is it space is just being added on to the 'outside' of everything.. in essence, is there no space until energy gets there and energy is travelling outwards. The article seems to suggest both.. which makes very little sense as it describes 2 conflicting theories on the universe.. a) that there's an 'outside' and b)that there's only 'in here'

I am sure we all understand inertial expansion (moving into the 'outside') but the expansion from within says theres no 'outside' as the 'skin of the balloon' is the 'edge' of space 

 

Offline Pmb

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Quote from: Phractality
While the distance between the two, in S, is growing at a rate of 4/3 c, that is not a velocity as velocity is defined in SR or in GR.
And that's why nobody said it was and that's why I didn't say that either body had that speed.  I said that one body has that speed relative to the other.

That's like an obserer at rest in S who is observing one car moving to the right with speed 50mi/hr and one moving to the left with speed 50mi/hr. In this sense one can say that in this sense that it is the relative speed of one car with respect to the other is moving apart with speed 100mi/hr. And when the reader understands that nobody is suggesting that this read as "X is moving at speed V" then we're all set.
 

Offline Pmb

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Quote from: SimpleEngineer
Sorry I framed my questions badly, as I was interested in the same thing with my own thoughts..
No worries. I understood what you meant and have no problem with it. Anything else is too nitpicky for my taste. :)
 

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