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starship-design: Relativity is not measurement-based
Whoa - lots of stuff here to respond to. Don't have time to do it all now,
but I do need to make one point.
Lee sends a web item:
>So, as you probably suspected, there is a way out of the paradox. The pro=
>blem, as with most of modern Physics, comes in making the measurement. Su=
>ppose that when I passed you the first time, and we synchronized our watc=
>hes, we were right next to each other. That means that sixty seconds late=
>r (your frame), when you checked my watch, I was a long way away. How do =
>you look at my watch a long way away? Your eyes take in light that bounce=
>d off it; your ears take in sound coming from it; whatever you do, you're=
> using something that travelled from me to you. And it took time to do it=
>The point is, you can't say "I'm looking at his watch now." You have to s=
>ay "I'm looking at light that came from his watch a while ago," and I hav=
>e to say the same thing when I look at you. So when you and I are in diff=
>erent places, whatever we see about each other is old news. And we have t=
>o take that into account when we say "This is what I'm seeing on his watc=
>h," admitting that this is simply what his watch used to say. When we tak=
>e that into account, we can plug through the math of Einstein's equations=
> and we wind up without a paradox.=20
This is simply incorrect. I'll explain the real way out of the paradox
And Timothy asks a question along the same lines:
>Do I understand correctly:
>Is this because the light takes a finite time to reach the observer?
>And if you would reckon with the time it took for the light to reach the
>observer, would you be able to configure the right order of the events?
>If so, I don't understand the fuzz about it.
>If not, I don't understand at all (and can you explain in a different
This is a common misperception of relativity; that it's only an effect of
taking measurements at light speed. This is completely incorrect. For
example, if a train's coming toward you at near lightspeed, the light from
the beginning and the end of the train arrive at different times, making
the train appear longer than it really is. That's a MEASUREMENT problem.
As you may know, objects travelling fast actually get SHORTER, not longer.
So IN REALITY, the fast train gets shorter, although it APPEARS longer to a
stationary observer -- because he's measuring the ends of the train train
at two different moments. These are two different effects; the first is
relativity, the second comes from the measurement process. The first is a
fundamental fact of nature, the second can be accounted for and removed.
In my earlier Paradox example, all of these secondary, "measurement"-based
effects have already been removed.
This is a way to separate out the two effects -- in a thought experiment
anyway. (This is a bit too impractical to do in real life). String out a
bunch of cameras, with synchronized video, all the way between here and
Alpha Centauri. Put them all in the rest frame of both planets. Call this
Detector system #1. Then string out another set of cameras and call that
Detector system # 2. Accelerate #2 to 0.4c moving away from Earth. You
now have a detector system in two different frames.
Now, although an observer at Earth can't see all the camera information at
once, after everything happens an observer at Earth can patiently collect
all of the camera information, complete with time-stamped frames, and
recreate events AS THEY ACTUALLY HAPPENED, removing any effects caused by
the finite speed of information. What's left is the actual situation
produced by relativity, with all of the measurement problems removed.
And this is the situation I was describing. All of my (space,time)
coordinates are REAL; they're deduced after the fact with a 4.3 light-year
detector system. So now, according to Timothy, I guess I'll have to
explain it in a different way.
Still, no matter how I explain it, the reason it will always seem
counterintuitive is the issue of simultaneous events. There's no such
thing. You see, every way I try to explain it will result in the (seeming)
impossibility of Dwight thinking his journey takes him forward in time,
while an observer at Earth seeing him travel backwards in time. To most of
us, that makes no sense. You can't both go forward and backward in time
with the same trip. Time, to us, is a stable medium. It makes intuitive
sense to us that two different events that happen at the same time are
somehow SPECIAL; that it is an objective fact that they DID happen at the
But this is not true. When we see events A and B happen at the same time,
that is a subjective perception from our frame of reference. One observer
(travelling one way) sees A happen before B. Another observer (travelling
the other way) sees B happen before A. Who's right? Everyone. But how
can this be? What if A causes B; how can the cause happen after the
effect? That's the beauty of it. This "uncertain" time order can only
happen for "space-like" points in space-time, as I defined them in my first
iteration. This means they are separated by such a distance that a light
beam can't get from one to the other in time. There can be no cause or
effect; A and B can't exchange information. If A and B are close enough to
exchange information, then they are "time-like", and the time-order becomes
fixed; it's the spatial order that is now obeserver-dependant.
But two "space-like" points CAN exchange information, of course, if you
have FTL. Then you can travel from A to B, as Dwight does on the way home.
But if some people see A happen first and some people see B, which way does
the travel "really" go? Both ways. Everyone's always right. But unless
our minds can come to grips with the idea that two points in space-time are
not necessarily ordered in time (i.e. A does not always come "before" B),
we'll never "believe" that this is true.
This, btw, is the solution to the twin paradox. You can set up the string
of cameras in that case as well. And when one twin is saying "how old is
the other right NOW", his idea of "now" is not the same as the other twin.
The two points in space time that are simultaneous for one twin are not
simultaneous for the other. A tough concept, but once it's understood, all
of relativity falls into place. And not just in the equations, as the web
quote seemed to state. It can actually make sense in one's mental picture
of reality. It's a different picture than the one we normally have, but
considering it seems to bean accurate reflection of reality I think it's
well worth the trouble.
Let me know if that helped clear things up. Hopefully it will at least
give us a place to focus our discussion: whether or not the time-order of
any two points in space-time can be fixed. I'll try again tomorrow...