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View Full Version : Light - gains or doesn't gain speed from source


Alex Kudrasev
July 21st 03, 03:52 PM
There seems to be some conjecture whether we see light gaining a direction
component from a moving star.

Is that correct?

Surely, if it does, we will see a 'streak' of light in the sky as light from
various sides of the star gets to us?

Cheers,

Alex.

Bill Nunnelee
July 21st 03, 08:18 PM
Observers in all reference frames measure light in a vacuum traveling at the
same speed, c. This goes all the way back to Special Relativity, and I'm
not aware of any serious conjecture to the contrary. Light from objects
moving towards us is blue shifted, while light from objects moving away from
us is red shifted, but the speed is the same in both cases.

Because of the great distances involved, the motion of stars moving
tangentially to us can only be measured over time. None of them streak.
The faster moving ones, like Barnard's star, can be seen to move over a
period of years. Otherwise, detection of proper motion is a slow and
tedious process.



"Alex Kudrasev" > wrote in message
...
> There seems to be some conjecture whether we see light gaining a direction
> component from a moving star.
>
> Is that correct?
>
> Surely, if it does, we will see a 'streak' of light in the sky as light
from
> various sides of the star gets to us?
>
> Cheers,
>
> Alex.
>
>
>

Bill Nunnelee
July 24th 03, 02:25 PM
The speed of the star is much less than the speed of light, so there's no
possibility that photons arriving from different points along the star's
trajectory would arrive at the same time.


"Alex Kudrasev" > wrote in message
...
> Precisely my question - Why don't stars travelling tangentially streak? If
> light inherited direction from a body's motion, different direct rays from
> the spherical surface would appear to us from different times in the
body's
> history.
>
> So what is the answer?
>
> Alex Kudrasev
>
>
> "Bill Nunnelee" > wrote in message
> rthlink.net...
> > Observers in all reference frames measure light in a vacuum traveling at
> the
> > same speed, c. This goes all the way back to Special Relativity, and
I'm
> > not aware of any serious conjecture to the contrary. Light from objects
> > moving towards us is blue shifted, while light from objects moving away
> from
> > us is red shifted, but the speed is the same in both cases.
> >
> > Because of the great distances involved, the motion of stars moving
> > tangentially to us can only be measured over time. None of them streak.
> > The faster moving ones, like Barnard's star, can be seen to move over a
> > period of years. Otherwise, detection of proper motion is a slow and
> > tedious process.
> >
> >
> >
> > "Alex Kudrasev" > wrote in message
> > ...
> > > There seems to be some conjecture whether we see light gaining a
> direction
> > > component from a moving star.
> > >
> > > Is that correct?
> > >
> > > Surely, if it does, we will see a 'streak' of light in the sky as
light
> > from
> > > various sides of the star gets to us?
> > >
> > > Cheers,
> > >
> > > Alex.
> > >
> > >
> > >
> >
> >
>
>
>

Alex Kudrasev
July 25th 03, 07:14 AM
"Greg Neill" > wrote in message
...
> "Bill Nunnelee" > wrote in message
> arthlink.net...
> > The speed of the star is much less than the speed of light, so there's
no
> > possibility that photons arriving from different points along the star's
> > trajectory would arrive at the same time.
> >
>
> True. On the other hand, jets from active galaxies are
> moving at a significant fraction of the speed of light,
> and if the geometry is right with respect to source and
> observer, will appear to be moving superluminally.
>

So I take it that we can observe 'streaks' (they appear as lines) in fast
moving light sources?

Alex Kudrasev
>

Greg Neill
July 25th 03, 12:44 PM
"Alex Kudrasev" > wrote in message
...
>
> "Greg Neill" > wrote in message
> ...
> > "Bill Nunnelee" > wrote in message
> > arthlink.net...
> > > The speed of the star is much less than the speed of light, so there's
> no
> > > possibility that photons arriving from different points along the star's
> > > trajectory would arrive at the same time.
> > >
> >
> > True. On the other hand, jets from active galaxies are
> > moving at a significant fraction of the speed of light,
> > and if the geometry is right with respect to source and
> > observer, will appear to be moving superluminally.
> >
>
> So I take it that we can observe 'streaks' (they appear as lines) in fast
> moving light sources?

Well, jets are more or less continuous in nature, sometimes
with lumpy inclusions. But I can't think of a geometry for
the source and observer that would have light from different
points on the trajectory of the same "lump" arrive at the
same time at the observer.