Re: OT: Aliens in the heavans (was Re: seti@home)
Ron Johnson wrote:
[snip]
> Yes, yes, all those TV, radio stations, satelite uplink stations
> (the vast majority all of which are in the Northern Hemisphere)
> are radiating outwards.
>
> However, since you know full well the, well, astronomical,
> distances involved, and given that radiation strength drops as
> a cubic (since waves go out in 3 dimentions) function as distance
> increases, I have a question:
This would be true were it an electric field. Actually electromagnetic
radiation falls off inversely with distance, not with the cube of
distance, since no-one has yet devised an antenna which radiates very
well in all directions and there is some fancy explanation of Maxwell
that shows that it's 1/r, now 1/r^2 or 1/r^3.
> If most stars need telescopes (even the Hubble) to see them,
> and they radiate jillions^3 of watts on energy, and still look
> like pin-pricks, how could the signal from a 50,000 watt radio
> station, or a 250,000 watt TV station (both of whose signals
> are absorbed somewhat by the air) reach an antennae 20 light
> years away, while passing through all that background noise?
>
> Since I've been wrong more than once, please tell me what I'm
> missing.
This is still a good point, even if your thoughts on wave energy density
are wrong.
> > Not only that, but there's probably a lot of omni-directional antennas
> > in use for surface-to-surface broadcasts as well. This means they send
> > as much signal straight up as they do to surface targets. I'm
There is not really any such thing as a 3D omni-directional antenna (not
in common use, anyway). Antennas are pretty much all 2D
omni-directional or better (or worse, depending how you look at it).
Think of a simple monopole antenna, like you have on your car; the
electrons in the monopole only have one degree of freedom, ie. they can
only move up and down. The direction of propagation is perpendicular to
the direction of motion of the exciting charges (aren't they exciting?
;-) and so the wave propagates in the horizontal plane (assuming that
your antenna is oriented that way. Keep in mind also that a monopole
actually forms a dipole by virtue of it's reflection from the earth -
this makes the monopole look even more like an infinitely long line and
helps you to believe what I'm saying ;-)
> > simplifying it a lot (mainly because it's been so long since I studied
> > this stuff), but suffice it to say we "leak" a *LOT* of radio signal
> > out into space. It's safe to assume that any other civilizations out
> > there, with of the same technological means, do the same and I'm sure
> > that's what SETI is targeted at finding.
> >
> > There are plenty of reasons to question the worth of the SETI project,
>
> What are they?
I can think of a number of better ways of spending the money/time/spare
CPU cycles.
> > just not on the grounds that an alien civilization would have to
> > intentionally broadcast a signal to us for us to detect it.
>
> Not to US, but just out in _EVERY_ direction, since that alien civ
> doesn't know in what direction the receiving civ is.
I agree with you (except about wave energy density); I believe a 1 MW
RF plane radiator is very difficult to demodulate with our technology
from the edge of our solar system, and not many people are licensed to
operate at 1MW. You could consider the whole earth as one large
radiator, but it would not appear intelligible; most of the EM spectrum
gets reused in each country in the world, often for very different
things (just go have a look at how mobile phone specifications line up -
GSM frequencies are used for ISM and millitary purposes in the US), but
from a significant distance from the earth (the moon, say) it would be
quite difficult to resolve the 1MHz signal coming from the UK from the
1MHz signal coming from the US. FM signals are easier, but...
> > Of course if you're being sarcastic then I just wasted too much time
> > replying, and you need to use smileys! :)
>
> Well, no, not sarcastic, just of a different religion. I prefer to
> waste my cycles trying to brute-force crack a 64-bit cypher. Since
> it's been going for ~3 years now, any rational person should deduce
> that brute-force cracking RC5-64 is a BIG waste of time...
Hey, if you have the time there then it's up to you how you use it ;-)
Tom
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