From: Anders Sandberg (asa@nada.kth.se)
Date: Sat Jul 12 2003 - 05:54:24 MDT
Just a question here: could anybody point me to the frame dragging
explanation of galactic arms? The more I think about it, the less sense
it does make. To my knowledge galactic arms are usually ascribed to
waves of star formation (and the older density wave theory), not a
general relativistic effect - if it was GR, then we would get them in
elliptic galaxies too.
On Fri, Jul 11, 2003 at 07:52:48PM -0400, ABlainey@aol.com wrote:
> By this I mean that if you are at fixed position and you emit a beam of laser
> light into space. The light can only travel at the speed of light (duh). Then
> if that laser is driving a light sail craft which has attained a speed of a
> quater the speed of light (it has been out there for quite a while). Then the
> light sail craft can emit a beam of light forwards also at the speed of light.
> Relative to our fixed position, this beam from the craft will be traveling at
> 1.25 times the speed of light.
This is not what is observed in reality.
Velocities do not add like v3=v1+v2 in relativity, they add as
v3=(v1+v2)/(1+v1*v2/c^2). For v1=0.25 c and v2=c, you get v3=c.
Light *always* moves at c in relativity, that is one of the two basic
postulates (the other one is that experiments done in inertial
laboratories give the same results regardless of the speed of the
laboratorium; from these two you can derive all of special relativity!).
-- ----------------------------------------------------------------------- Anders Sandberg Towards Ascension! asa@nada.kth.se http://www.nada.kth.se/~asa/ GCS/M/S/O d++ -p+ c++++ !l u+ e++ m++ s+/+ n--- h+/* f+ g+ w++ t+ r+ !y
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