Orchid XP v8 <voi### [at] devnull> wrote:
> On 25/01/2011 08:11 PM, Warp wrote:

> >    My take: If there is life in the Universe (which happens to be the case),
> > some of that life must have been the first one to form. Well, we might just
> > be that first, don't you think?

> My understanding of general relativity is limited, but I was under the 
> impression that "first" is not a meaningful concept at relativistic 
> velocities.

  That would be paradoxical.

  Assume that intelligent life evolves in planet A, and it starts sending
radio signals to outer space. These radio signals eventually reach planet B.

  Some time later life evolves in planet B to a point where they can receive
these signals from A. Clearly, intelligent life evolved in A first, and in
B after that.

  If there was an external frame of reference where the intelligent life
forms in B before it forms in A, from this frame of reference it would look
like B is receiving radio signals from A before any intelligent life in A
has evolved (and started sending those signals). In other words, from this
frame of reference it would look like B is receiving radio signals from the
future. This would be paradoxical.

  (Obviously if the external observer can see the radio signals arriving
at B, and hence see that B has evolved to the point of being able to receive
and interpret those signals, this external observer can trace the source of
those signals to A and see what is sending them.)

-- 
                                                          - Warp

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Warp wrote:
> Darren New <dne### [at] sanrrcom> wrote:
>> I always enjoyed thinking about such things. What if we really *are* in the 
>> middle of the universe, and it's not that space is expanding, but only that 
>> everything really is rushing away from us because it all started right near 
>> here? :-)
> 
>   What's the difference?

Well, if it was my way, we'd be special. Or at least in a special place.

If you assume we aren't in the middle, then the only reasonable conclusion 
is that space itself is expanding, i.e. that everything is the middle.  If 
we're actually in the middle, it's possible space is completely static, and 
everything is just moving away from the center, without any additional space 
being created between galaxies.  It kind of changes the whole viewpoint of 
the Big Bang. (Yes, I know it's nonsense, but think of it in a sci-fi novel 
kind of way.)  All of space already existed, then there was this big 
explosion right about where we are, and everything is still rushing away 
from that.

Or perhaps space isn't expanding, but since we're in the center, time is 
slowing down as you move farther away. The galaxies aren't red-shifted 
because they're receding, but because their time is actually running slower 
the farther you get from the center. Which, again, supports us as being the 
first species to have evolved to sapience, since we happen to be in the area 
of the universe where time is running fastest.

-- 
Darren New, San Diego CA, USA (PST)
  "How did he die?"   "He got shot in the hand."
     "That was fatal?"
          "He was holding a live grenade at the time."

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Warp wrote:
> Orchid XP v8 <voi### [at] devnull> wrote:
>> On 25/01/2011 08:11 PM, Warp wrote:
> 
>>>    My take: If there is life in the Universe (which happens to be the case),
>>> some of that life must have been the first one to form. Well, we might just
>>> be that first, don't you think?
> 
>> My understanding of general relativity is limited, but I was under the 
>> impression that "first" is not a meaningful concept at relativistic 
>> velocities.
> 
>   That would be paradoxical.
> 
>   Assume that intelligent life evolves in planet A, and it starts sending
> radio signals to outer space. These radio signals eventually reach planet B.
> 
>   Some time later life evolves in planet B to a point where they can receive
> these signals from A. Clearly, intelligent life evolved in A first, and in
> B after that.

If A and B are separated by space-like distances, neither is first. Here 
you're postulating that A and B (where A and B are the evolution of initial 
radio-emitting life) are separatd by time-like distance.

As you say, if one is in the past light-cone of the other, there's a first 
and a second. It only gets ambiguous where neither of the two 4D events are 
in the direct past light-cone of the other.

-- 
Darren New, San Diego CA, USA (PST)
  "How did he die?"   "He got shot in the hand."
     "That was fatal?"
          "He was holding a live grenade at the time."

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Darren New <dne### [at] sanrrcom> wrote:
> Or perhaps space isn't expanding, but since we're in the center, time is
> slowing down as you move farther away. The galaxies aren't red-shifted
> because they're receding, but because their time is actually running slower
> the farther you get from the center. Which, again, supports us as being the
> first species to have evolved to sapience, since we happen to be in the area
> of the universe where time is running fastest.

Sounds reminiscent of Vernor Vinge... :)

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>> My understanding of general relativity is limited, but I was under the
>> impression that "first" is not a meaningful concept at relativistic
>> velocities.
>
>    That would be paradoxical.

Yes. It's general relativity. It bemuses everybody.

>    Assume that intelligent life evolves in planet A, and it starts sending
> radio signals to outer space. These radio signals eventually reach planet B.
>
>    Some time later life

Your language is still assuming that "earlier" and "later" are valid terms.

A small thought experiment: There are two parallel train tracks and a 
train station in the middle of the tracks. There is a train sitting at 
one end of one line, and another train at the opposite end of the other 
line. The station and both trains synchronise their clocks, and at 
exactly noon the two trains accelerate down the track to relativistic 
speeds. Each train emits a flash of light at 1PM, according to the 
on-board clock.

Applying the Lorentz transform makes my head spin. But the general 
result is this:

- Train A emits a flash at 1PM. Since train B is travelling at 
relativistic speed with respect to train A, from train A the clock on 
train B appears to have slowed down, and so it appears that flash A 
happens /before/ flash B.

- Train B emits a flash at 1PM, and by the same argument it appears that 
flash B occurs /before/ flash A.

- Standing on the station platform, both trains' clocks appear to have 
slowed down by the same factor, and hence both flashes appear to happen 
at /the same time/.

Thus, depending on who you ask, flash A occurs either before, after, or 
at the same time as flash B. QED.

Now replace speeding trains with moving planets, add in the influence of 
gravity and perhaps the negative curvature of space, and call me back 
when you get over the searing pain in your brain...

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Darren New <dne### [at] sanrrcom> wrote:
> If you assume we aren't in the middle, then the only reasonable conclusion 
> is that space itself is expanding, i.e. that everything is the middle.  If 
> we're actually in the middle, it's possible space is completely static, and 
> everything is just moving away from the center, without any additional space 
> being created between galaxies.

  In that situation, why would someone in a galaxy not in the center of the
universe not see all the other galaxies receding?

  (Anyways, I think that the reason why the universe must be expanding
rather than all galaxies receding from us simply because they got an
initial velocity away from the center is that in the latter case all the
galaxies would be receding from us at the same speed regardless of
distance, while in an expanding universe the farther away from us the
galaxy is, the faster its recession speed, which is what is observed.)

-- 
                                                          - Warp

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Invisible <voi### [at] devnull> wrote:
> >> My understanding of general relativity is limited, but I was under the
> >> impression that "first" is not a meaningful concept at relativistic
> >> velocities.
> >
> >    That would be paradoxical.

> Yes. It's general relativity. It bemuses everybody.

  General relativity does not introduce paradoxes.

> >    Assume that intelligent life evolves in planet A, and it starts sending
> > radio signals to outer space. These radio signals eventually reach planet B.
> >
> >    Some time later life

> Your language is still assuming that "earlier" and "later" are valid terms.

  Clearly if B receives a radio signal from A, the radio signal was sent
before it was received. The signal being received before it had been sent
would be paradoxical (it would mean that the signal travels back in time,
which would require FTL travel, and as we know, radio signals don't).

  If we define the concept of "intelligent life" at "capable of sending
and receiving radio signals to/from space", clearly intelligent life
developed in A before it developed in B.

  I'm not saying there aren't situations where whether A evolves before B
or the other way around depends on the frame of reference. What I am saying
is that when A's light cone (which started when intelligent life appeared
on A) reaches B (before intelligent life appears on B), it becomes
unambiguous. An ambiguous situation in this case would require FTL (and
hence time) travel.

  Hence you can unambiguously state that intelligent life developed in A
before it developed in B.

-- 
                                                          - Warp

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Bill Pragnell wrote:
> Darren New <dne### [at] sanrrcom> wrote:
>> Or perhaps space isn't expanding, but since we're in the center, time is
>> slowing down as you move farther away. The galaxies aren't red-shifted
>> because they're receding, but because their time is actually running slower
>> the farther you get from the center. Which, again, supports us as being the
>> first species to have evolved to sapience, since we happen to be in the area
>> of the universe where time is running fastest.
> 
> Sounds reminiscent of Vernor Vinge... :)

While he is one of my heros, that idea is my own. :-)

-- 
Darren New, San Diego CA, USA (PST)
  "How did he die?"   "He got shot in the hand."
     "That was fatal?"
          "He was holding a live grenade at the time."

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Warp wrote:
> Darren New <dne### [at] sanrrcom> wrote:
>> If you assume we aren't in the middle, then the only reasonable conclusion 
>> is that space itself is expanding, i.e. that everything is the middle.  If 
>> we're actually in the middle, it's possible space is completely static, and 
>> everything is just moving away from the center, without any additional space 
>> being created between galaxies.
> 
>   In that situation, why would someone in a galaxy not in the center of the
> universe not see all the other galaxies receding?

They would, but it would be asymetrical. The galaxies between them and us 
would be receding from them more slowly than the galaxies on the other side 
of them from us.

>   (Anyways, I think that the reason why the universe must be expanding
> rather than all galaxies receding from us simply because they got an
> initial velocity away from the center is that in the latter case all the
> galaxies would be receding from us at the same speed regardless of
> distance, while in an expanding universe the farther away from us the
> galaxy is, the faster its recession speed, which is what is observed.)

Unless the galaxies all exploded out at different speeds, so of course the 
ones farther away are receding at higher speeds. ;-)

-- 
Darren New, San Diego CA, USA (PST)
  "How did he die?"   "He got shot in the hand."
     "That was fatal?"
          "He was holding a live grenade at the time."

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Darren New <dne### [at] sanrrcom> wrote:
> >   (Anyways, I think that the reason why the universe must be expanding
> > rather than all galaxies receding from us simply because they got an
> > initial velocity away from the center is that in the latter case all the
> > galaxies would be receding from us at the same speed regardless of
> > distance, while in an expanding universe the farther away from us the
> > galaxy is, the faster its recession speed, which is what is observed.)

> Unless the galaxies all exploded out at different speeds, so of course the 
> ones farther away are receding at higher speeds. ;-)

  However, in an expanding universe the recession speed accelerates
(because the more the galaxy recedes, the larger the distance from us,
and hence the larger the recession speed). In a situation where the
galaxies simply got an initial speed the recession would be constant.
(Well, in fact that's not true: The recession speed would actually be
decelerating because they are slowed down by the gravity of all the other
galaxies.)

  It then comes down to whether we can measure whether the recession speed
of a galaxy is accelerating or not. (Can we?)

-- 
                                                          - Warp

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