Newsgroups: sci.physics

mcirvin@husc8.harvard.edu (Matt McIrvin)
Keywords: FTL SR causality Special Relativity
level res
but I think that the FTL communication you describe would have,
to say the least, some bizarre properties if you require that it
not violate SR or causality. 

markc@smsc.sony.com (Mark Corscadden) writes:

>Let's say you are with NASA mission control and are remotly controlling
>a robot on Mars.  If you can have two-way communication between you on
>Earth and the robot on Mars and the signal delay is, say, only a few
>seconds, then *that* is FTLC, isn't it?  I wouldn't even ask this question,
>but this is *the* essential point that I'm introducing that I haven't seen
>before in sci.physics or in the literature, so it's worth belaboring.

>Here is the correct formulation of what it means to have a general FTLC
>capability.  Let O1 be an observer and let E1 and E2 be two events which
>take place at the origin of O1's reference frame, where E2 happened at
>a later time.  BTW, all observers will assign a larger time coordinate
>to E2 than to E1.  Let O2 be any other observer.  Then there must exist
>some event E which took place at the origin of O2's reference frame, such
>that E1 caused E, which in turn caused E2.  Here I'm using "caused" in the
>inclusive sense of having partially caused an effect.  This is the
>formulation of what it means to have FTLC that I believe to be correct.

If you just examine E1, E2, and E, there's no trouble.  But consider this:

Suppose that there is apparatus at rest in O2's frame that is just like
O1's, and vice versa.  O2 can set up events F1 and F2 which are both at
the origin of O2's spatial coordinates; F1 follows E and F2 follows F1,
according to O2's time coordinate.  F1 causes event F which is at the
origin of O1's spatial coordinates, which in turn causes F2, all of this
reckoned using O2's time coordinate.  This must be possible if SR
applies and the apparatus works everywhere. 

Now if O2's frame is moving at a sufficiently high speed relative 
to O1, it's easy to see that F can *precede* E1 according to O1's time
coordinate.  They're at the same spatial position according to O1, so
all observers will agree that F precedes E1.  If the relaying apparatus
working at events E and F can record the information that gets 
received and retransmitted, then O2 can easily take the message received
at E and send it back from F1 to F to F2, whereupon it gets recorded at F--
which was *before* E1!  So by this means O1 can send a message into O1's
own past, with O2's cooperation. 

The only way to defeat this while retaining SR is to specify that somehow
information gets "forgotten" at E and/or F so that the message received
and retransmitted can have no physical effect at that point.  This would
seem to exclude any control over the remote robot-- or any activity
other than bouncing a message from Earth back to Earth!
 
This seems hardly useful as communication, and would furthermore
involve some rather bizarre physics, to say the least (though no more
bizarre than EPR correlations, of which this is reminiscent).  
It's hard to see how you'd even get evidence that such a "communication"
had taken place.

I could, of course, be hallucinating as well...

-- 
Matt McIrvin
 by
     
ink you w
hero" app
>
> In <1992Dec10.031927.18409@augean.eleceng.adelaide.edu.AU> dabbott@augean.eleceng.adelaide.edu.AU (Derek Abbott) writes:
>
> >Does anyone konow how the creation of sci.phys.solid-state is
> >coming along?
>
> Since there is not enough interest for creating this subject alone, a
> compromise has been reached to create sci.electronics.micro-elec, which 
> covers that plus the chemical means to produce semiconductors, lasers
> LCDs. The group is under the vote now.  Make sure to vote.

Why not try an existing group, like comp.lsi, and see what the volume
is before rushing off to create yet another newsgroup?  I'm not alone
in thinking that it is silly to create new groups before establishing
that there will be reasonable volume (with one notable exception...).

- John
--
hagerman@ece.cmu.edu
uld not 
NNTP-Posting-Host: punisher.caltech.edu
Summary: Planets are bigger and dimmer
bout 5-6
> Can anyone give a good explanation why the stars (even the very bright
> ones) twinkle and not the planets.

Being a theorist, I'll jump right in with a hypothesis before even
verifying your observation, hopefully beating my colleagues to publication!

I'd guess it's because the planets subtend a larger solid angle in the sky
(i.e. you can resolve a planet's disk with a small telescope, but not a
star's), so it's harder for atmospheric fluctuations to distort all the
light coming from the planet to your eye at once.

--
Staccato signals of constant information,  |  David Brahm, physicist
A loose affiliation of millionaires and    |  (brahm@cco.caltech.edu)
  billionaires and Baby ...                |---- Carpe Post Meridiem! --
These are the days of miracle and wonder,  | Disclaimer:  I only speak
And don't cry, Baby, don't cry, don't cry. | for the sensible folks.
 land we
Distribution: sci
res Phd

>Hello,
> I have a rather trivial question. Can anyone give a good 
>explanation why the stars (even the very bright ones) twinkle
>and not the planets.

Planets and even the sun and flashlights twinkle too but they are bright
enough and span a big enough arclength that you don't notice it.
 large
  
rested in
>In article <1992Dec9.182243.4113@novell.com>, dseeman@novell.com (Daniel Seeman) writes:
>> In article <1992Dec8.171857.1@sscvx1.ssc.gov> draper@sscvx1.ssc.gov writes:
>>>(By the way, this brings up a mildly interesting freshman problem. To see
>>>your complete reflection in a mirror, the mirror has to be half your height
>>>exactly. The surprising part is that this is independent of how far you stand
>>>from the mirror.)
>> The above cannot be true.  If you stand so close to the mirror that it is nearly
>> touching your knees (assuming you have a relatively planar profile) the light 
Newsgroups: sci.physics,alt.sci.physics.new-theories