From: aguirre@wigner.physics.upenn.edu (Anthony Aguirre)
Newsgroups: alt.alien.visitors,alt.sci.physics.new-theories
Subject: Re: Elapsed Time near c (was Re: Q&A on Extraterr
Date: 22 Jul 92 14:31:12 GMT
Followup-To: alt.alien.visitors

In article <1992Jul22.131739.1@cc.newcastle.edu.au>, ccasm@cc.newcastle.edu.au writes:
|> In article <1992Jul21.002755.17265@news.eng.convex.com>, swarren@convex.com (Steve Warren) writes:
|> > Nope.  The fact is that nothing can travel faster than light, in any reference
|> > frame.  If it wasn't for the foreshortening effect, you would literally see
|> > the universe passing by faster than the speed of light.
|> 
|> I disagree. One cannot see anything passing faster than the speed of light, but
|> that does not stop objects travelling faster than c.
|> 
|> The speed-of-light is the speed of that light transmitted or reflected by any
|> object. It is a constant, no matter what the speed of the object is -
|> whether it is stationary, moving at 100 mph or 186,000 mph (all relative to
|> the observer on Earth). Like watching a car - stopped or going.
|> 
|> BUT LIGHT SPEED IS INDEPENDENT from THE SPEED OF THE OBJECT.
|>                    ^^^^^^^^^^^                     ^^^^^^
|> 
|> If an object travels SLOWER than the speed of light it would be observed as
|> being `where it is'. BUT it is only NEAR where it is (the difference is so
|> little). It takes time for the light to travel to our eye, during which the
|> object has moved on. But this is not obvious at relative slow speeds such as
|> occur in everyday life (like catching a ball or watching a car). This is
|> "convenient" else we would be run over by a car that was not there at the
|> time. :-)
|>  
|> If an object travelled FASTER than the speed of light, it would not be observed
|> as being where it actually is. This would not be significant until it travelled
|> at much higher speeds than light. So what? Sometime after it stopped its image
|> would catch up with it. Thus the object would not appear to have exceeded the
|> speed of light, to the stationary observer (who is only observing the image). 

	You really have things backwards.  The statement "nothing can travel
faster than the speed of light" as applied to a massive object means, for
example, that is a spaceship sets out for a star four light years away, it CANNOT
return in less than 8 years, as measured on earth.  Now if the traveler is going
near the speed of light, he will measure that less time has passed, thus will
claim that he went "faster than light" but Lorentz distance contraction, as was
pointed out, takes care of this.  
	However, it is easy to make "something" travel faster than the speed of
light.  For example, shine a laser onto the moon.  Flick yourwrist, and watch the
spot of light move across the moon's surface at much > speed of light.  This does
not violated relativity because all you are really sesing is a locus of light
emissions (reflectiosn from the surface)  Likewise, if something COULD travel
faster than the speed of light, it would not "out-run" it's image.  I'm not sure
what would be seen exactly (since the object would be > infinitely short and
massive) but this has nothing to do with "outrunning" an image.


|> Bad luck if the object is travelling straight at you. All that would be seen is
|> a black hole until well after it passed (or hit) you.
|> 
|> Compare this to the speed of sound. We can see an event such as a gunshot
|> before hearing it. The speed of the bullet is not effected by the observation.
|> 
|> To the observer aboard the object/craft, other objects around it would appear
|> to be the ones moving. Again they would not appear to be travelling at above
|> the speed of light but in fact they would not be where they appear to be.
|> Collisions would be unavoidable if navigation was based on sight. Computers
|> could adjust for known objects, but uncharted small particles would be
|> disastrous unless the craft was protected by a "forcefield", as they would hit
|> before they were seen. Using some means of detection other than light, it might
|> be possible to navigate ad-hoc at above-light speed. Is there any such method?
|> An expendable "pathfinder" might be possible - send another craft on the same
|> path slightly before your flight. The distance (time?) between the two flights
|> would be the reaction time of the second craft to navigational changes. I don't
|> think this will work. What if you ran into another pathfinder?
|> 
|> Inside the craft the observer would not notice any difference as all objects
|> would be travelling at close to the same speed, including the observer. However 
|> movements needing timing, such as eye-hand coordination, would they be clumsy?
|> 
|> Looking out of the craft the observer would in fact be looking forward/back in
|> TIME. Looking to the rear while overtaking the light that had been reflected
|> from the Earth the observer would see into the past. Or would he see nothing -
|> a black hole? Looking forward you would see into the future. All these
|> observations are of course relative to the observer on earth.
|> 
|> This thought exercise has been fun, but the practicalities are another thing.
|> How does one get all those molecules to accelerate enormously at the same time.
|> This is necessary if the craft is to avoid either dragging its tail or
|> squashing its nose.

This would not be a problem, since the acceleration would not be that great. 
Assuming no realtivistic effects (which you seem to do), one could reach the
speed of light in about 1 year accelerating at only 9.8 m/s/s, for example.

 Or what if the destination is no longer there - how would
|> you know until you were under way.
|> 
|> Personally I don't see the point of travelling at these speeds. We don't need
|> to send humans anywhere around the universe - we can build observers for us,
|> but they can only communicate at c or less, unless the message is "attached" to
|> an object travelling above the speed of light. Our biology is not designed for
|> it. The Earth is our only home for some time, I'm afraid.
|> 
|> Big Al