From: [d--m--y] at [cats.ucsc.edu] (David Empey)
Newsgroups: rec.games.frp.advocacy
Subject: Re: Realistic FTL
Date: 15 Oct 1993 20:14:48 GMT

In <29mkth$[d n q] at [TAMUTS.TAMU.EDU]> [f t s 5792] at [cs.tamu.edu] (Frank T Sronce) writes:

[ quoting me ]

>>
>>Unfortunately, it just aint so.  You can't evade causaulity problems
>>caused by FTL travel by "going outside the universe".  I'm trying to
>>think up a pithy way to say why this is without being just plain
>>wrong.  The problem lies with the fact that STL observers will observe
>>the same set of events in different time-orders, and each observer's 
>>account must be considered equally valid.  Remember, this is _not_
>>a result of time-lag due to the finite speed of light; even after
>>correcting for the speed of light, the same events will seem to
>>happen in different orders.  If you travel FTL, you will end up
>>going backwards in time relative to _some_ observer, no matter _how_
>>you do it.
>>
>>-Dave

>===========================================================================
>Hm. Yes, I can see that.  If I point a powerful flashlight at a man on the 
>moon (yes, a very powerful flashlight) and then teleport to him somehow, the
>light from earth will appear to keep on shining after I arrive.  

>But, at the risk of sounding facetious, so what?  If I turn OFF my flashlight
>after arriving, it doesn't cause a paradox because the light has already been
>emitted. It just seems to me to be a photon-based version of the same kind
>of optical illusion, like hearing a jet after it passes by, or thinking that
>a wheel is travelling backwards because it's spinning too fast for my eyes
>to follow.  

>I'm hoping that someone can enlighten me here- I can see arguing that FTL can't
>work because I could go back in time and stop myself from leaving, but what
>does it matter that I can't trust my eyes because the object is moving FTL?

Nothing.  But that's not the point I was making in the paragraph you 
quoted.  The fact is, FTL travel causes _real_, not just apparent, violations
of causality. (Unless you restrict the FTL travel in certain ways which
are hard or impossible to justify given the assumptions of Special Relativity.)
Why is this true?  I'll try to explain, using an example I posted a
few days ago, which has been vetted by Real Physicist John Kim.

Imagine a spaceship leaving Earth at a large fraction of the speed of
light.  As we all know, according to relativity, the spaceship experiences
'time dilation'; that is, to observers on Earth, the spaceships clocks
seem to run slow, say by a factor of 2-1.  So when a year has passed
on Earth, the Earth astronomers will observe that the spaceship's
clocks say only six months have passed.  Remember, this effect is
_real_; is will appear even after correcting for propagation delays
caused by the finite speed of light.  

But according to relativity,
the spaceship's reference frame is just as valid as Earth's.  In the
ship's frame, the _ship_ is standing still, and Earth is travelling
away from them at a large fraction of lightspeed.  So the ship
observers will observe _Earth_'s clocks as slowed by the same factor
of 2-1.  Again, this is a _real_ effect (to the extent that the
word 'real' has any meaning)_not_ an illusion caused by the finite
speed of light.  

How can the ships clocks and the Earths clocks run slower than each
other?  This is one of the paradoxes of relativity, and I can't
really explain it myself.  But if you do the math, that's the
result you get.  Due to the finite speed of light, we never get
causality violation in this situation.  But what if we can
communicate, or teleport, _faster_ than the speed of light?
For ease of argument, let's suppose we have an instantaneous
communicator, an 'ansible'.

Now, a year after the spaceship leaves Earth, Mission Control fires up
their ansible and sends an instantaneous message to the spaceship.
'Hi, how are you?'
Due to time dilation, the message arrives after six months have
passed on the ship.  The ship uses their ansible to reply instantaneously.
'Were fine, how are you?'
But according to the ship's observers, only three months have passed
on Earth in the six months they've been in space.  So the ship's
reply arrives three months after the ship leaves Earth.  

The reply arrives nine months before the message was sent!

I hope this made some sense.  The problem is that the ship and
the Earth have different definitions of what 'instantaneous' is.
With a little more work, you can get the same paradox if you
just travel faster than light, say at twice lightspeed.

-Dave

>ie- an incoming spaceship would generate a double-image. Once it slows down
>to dock, you'll see it as you would normally- but there will also be a 
>an image of it travelling backwards away from you as the light that bounced
>off of it as it approached finally reaches your eyes. (Although that would
>probably be too brief for the human eye to see- it would disappear into the
>distance in a few milliseconds, I expect)

>Kiz

>[Wanted: Theoretical Masochist... er, Physicist, to explain General Relativity
>to 6 billion clueless members of the public]