Interstellar travel
Christopher Neufeld
>[on the practicality of interstellar travel]
>
>1. The goal is to find intelligent life and/or a habitable planet. It
>seems to me that both are extremely
> rare events. Though I've no doubt that there is life somewhere else
>in the universe, I'm sure it is
> an exceedingly rare event.
>
Maybe, maybe not. This is one of the things which we might find out
by sending automated probes to the stars.
>2. I don't see mankind inventing practical anti-matter propulsion or
>whatever the current idea is
> in the next century or two.
> One
>related question is- are there
> thermodynamic efficiency considerations which limit the maximum
>possible speed? Would
> these depend on the method of propulsion used?
>
No thermodynamic constraints appear.
> The point is, due to relativistic mechanics, wouldn't it be a 1-way
>trip? If you travelled to a
> star 5 light-years away at .9c, wouldn't a few centuries or
>millenium pass on Earth?
>
Certainly not. If you go 5 light years at 0.9 c, it will take 5.555
years in the Earth frame for you to complete the journey. In the ship's
frame, ignoring acceleration and deceleration time, it will take 2.42
years.
>How severe is the time dilation effect? Again, I'm not a physicist.
>What happens to time during
> the acceleration and deceleration- when the velocity is not
>constant.
>
To good approximation the instantaneous relativistic corrections for
constant velocity situations can be used here.
> Does anyone know how to figure that all out
>realistically, including all the
> non-constant terms.
>
Yes.
>How long would a trip take at .1c, .5c and .9c
>in both ship's and Earth's
> time?
>
In the Earth's frame the time taken is just distance/velocity. In the
ship's frame this number is multiplied by the quantity sqrt(1-v^2/c^2).
>3. Assuming all the fuel is burned during the acceleration and
>deceleration phases, how would
> the craft return?
>
You can bet a lot of money that there will be a gas giant in the
solar system where you land. Such things will show up on the HST, and so
the aspiring traveler will know before leaving whether there is such an
entity. This is a good source of hydrogen, which can be reaction mass,
or fusion fuel, or energy source for an antimatter factory.
>4. Even if it had the fuel to return, how would it find the Earth again,
>since it is not exactly
> standing still, and may have moved unpredictably due to passing
>stars, black holes, etc. in
> the path of the Sun.
>
Certainly not. Such unpredictable behavious is on the scale of
millions of years, not decades. Most easily, the Sun has to be one of
the stars in a certain range of distances (you know how far you've come)
and the spectral lines of the Sun would identify it unambiguously. Also,
the Earth is a prominent radio star which should be obvious several
solar systems away. Of course, this last fact may change as we begin to
pipe our signals through fiber optics to replace the "RADIATION" which
carries our radio and television signals.
Personally, I am confident that humans will send manned slowships to
the stars within the next five hundred years, barring some sudden
disaster. Of course, if FTL shows up it could be much sooner. General
relativity makes so many contradictory statements on the subject of FTL
that there are actually a few different blueprints for FTL drives in the
scientific literature which are fully consistent with general
relativity. Don't bet that FTL will ever appear, though. I believe it
might, but there seems to be a deficiency in our present understanding
of the universe and causality.
>On a related note- can't we dismiss UFO reports out of hand? My
>reasoning is this:
>
>1. There are no little green men on the moon. Therefore, E.T.'s would
>have to come from
> outside the solar system = other stars.
>
Non sequitur. How about creatures floating in the atmospheres of the
gas giants, colonizers from other solar systems who live in the asteroid
belt (when you think about it, once you've arrived at another solar
system there seems little reason to drop yourself down a deep gravity
well), or half a dozen other possibilities.
>2. If something is coming from another star, it would have to have near
>light-speed travel.
>
Why? What if they hibernated, or had naturally extended life spans,
or used generation ships, etc.
Well, this has been fun....
--
Christopher Neufeld....Just a graduate student | "Like most
neu...@helios.physics.utoronto.ca | intellectuals he is
cneu...@pro-generic.cts.com Ad astra! | intensely stupid."
"Don't edit reality for the sake of simplicity" | Marquise de Merteuil
John F Nixon
-I noticed a lot of talk about interstellar travel a while back, and
-I'm a little confused. Am I wrong, or is there a point to thinking
-about such things? It seems to me that there a a couple principles
-which make it completely impractical- but I'm not a physicist, so
-please expand or correct me if I'm wrong.
There is a point to thinking about such things. It exercises the mind,
and it is rare that something can be completely dismissed out of hand.
-1. The goal is to find intelligent life and/or a habitable planet.
Exploration for the sake of observing the Universe is also a very
valuable thing to do.
-2. I don't see mankind inventing practical anti-matter propulsion or
-whatever the current idea is in the next century or two.
Leonardo didn't let a couple of centuries bother his technological
speculation.
-The point is, due to relativistic mechanics, wouldn't it be a 1-way
-trip?
If you simply go real fast (c - epsilon) then yes, from the point of
view of the people on earth, the trip is one way. The spacetravelers
could make a two way trip, returning to earth decades to centuries to
millenia later, depending on the distance. This is a nifty way to do
time travel while you see the Universe. I'll sign up...
-3. Assuming all the fuel is burned during the acceleration and
-deceleration phases, how would the craft return?
Refuel at the other end.
-4. Even if it had the fuel to return, how would it find the Earth again,
-since it is not exactly standing still, and may have moved unpredictably
-due to passing stars, black holes, etc...
Good question, although in trips of less than tens of millenia, nothing
much is likely to disturb our galatic orbit. Space is BIG, and things change
S L O W L Y.
-What I'm saying is basically this: We don't have the technology to
-do it.
Today, or for the next few years. But I hope not forever.
[stuff about the same applying to Space Aliens travelling to Earth]
This assumes they haven't made the Big Discovery, which enables you to
traverse vast interstellar distances in a wink. You can say it is highly
improbable, and we cannot do it, nor do we understand how it could be done
without violating causality, but you cannot flatly state no chance. Also,
the Space Aliens could bring their home with them; they just happen to be
passing by at the moment.
The probability is near zero, though...
----
jni...@atl.ge.com ...steinmetz!atl.decnet!jnxion
Henry Spencer
>1. The goal is to find intelligent life and/or a habitable planet. It
>seems to me that both are extremely
> rare events...
This is unproven, and there is reason for doubt. It looks as if planets
are normal around small stars like the Sun, and there is a reasonable
chance of them being habitable. (The evidence for the first part is
fairly strong, the second part is the current best guess.) The chances
for intelligent life are highly controversial: theoretical arguments
suggest that it should be common, but the Fermi Paradox observes that
we should have had visitors by now in that case. That's an unsolved
puzzle at present.
> ...Though I've no doubt that there is life somewhere else
>in the universe, I'm sure it is
> an exceedingly rare event...
Can you elaborate on the reasoning that leads you to this conclusion?
>2. I don't see mankind inventing practical anti-matter propulsion or
>whatever the current idea is in the next century or two.
If one believes Robert Forward, antimatter propulsion in particular could
be practical within decades if a concerted effort were made. A number of
starship-propulsion schemes have been proposed, and several of them are
not too far beyond our current abilities. Barring disaster, the odds
are good that at least one will be practical within a hundred years.
Interstellar travel is expensive, but it's not all that difficult.
>related question is- are there
> thermodynamic efficiency considerations which limit the maximum
>possible speed?...
No. For example, if you assume very large quantities of antimatter and
solutions to some related problems, fairly close approaches to the speed
of light are possible. Similarly, a well-designed Bussard ramjet could
approach the speed of light very closely. (Periodically someone discovers
the "ramjet speed limit" and trumpets interstellar ramjets as an obvious
fallacy. Then he reads the ramjet literature and, if he's honest, makes a
public apology for his blunder.) The energy requirements are formidable
but there are no truly fundamental barriers.
> The point is, due to relativistic mechanics, wouldn't it be a 1-way
>trip? If you travelled to a
> star 5 light-years away at .9c, wouldn't a few centuries or
>millenium pass on Earth?...
No. Travelling five light-years at roughly the speed of light takes roughly
five years, Earth time. Rather less aboard ship, due to time dilation.
>What happens to time during
> the acceleration and deceleration...
More complicated versions of the same thing.
>3. Assuming all the fuel is burned during the acceleration and
>deceleration phases, how would
> the craft return?
Refuelling in the target system is the easiest way, assuming it's possible
with the propulsion system used. Ambitious antimatter-based systems might
be able to just take along the fuel for the return trip. Or you can simply
accept that it's a one-way trip, either for colonization or as a scientific
expedition. (There is no shortage of people who would be willing to spend
the rest of their lives exploring a new solar system.)
>4. Even if it had the fuel to return, how would it find the Earth again,
>since it is not exactly
> standing still, and may have moved unpredictably due to passing
>stars, black holes, etc...
This becomes a problem only when voyages of many thousands of years are
being considered. That's not very likely to happen. Interstellar flight
at sub-light speeds is much more likely to proceed by short hops between
nearby stars than by enormous leaps out into the farther reaches.
>... So who would fund this research, when the astronauts
>would be returning
>(if they ever returned) 20,000 years into the future...
Under our current setup, nobody is likely to fund it; it's hard enough
getting funding for projects that last longer than one US presidential
term. In general, it will be difficult to rouse much enthusiasm for
starflight until it can get results within a human lifetime. That was
one of the constraints of the old Daedalus study: flight time (of an
unmanned Voyager-style probe) to Barnard's Star to be no longer than
40-50 years, so the younger participants would be alive when results
came back. That study concluded that the technology to do it, while
not here yet, was not that far off.
>4. Wouldn't something decelerating from near light-speed in our solar
>system have to shed all
> its relativistic mass as energy? In that case, it would probably be
> [detectable]. Since we haven't seen this, doesn't
>that alone prove that there are no UFO's?
UFOs unquestionably exist; remember that those initials stand for Unidentified
Flying Object, note the "U" part in particular. It seems very unlikely that
they are extraterrestrial spaceships, for a number of reasons. The above
argument actually isn't a very good one, because all it says is that the
little green men have to know more about physics than we do, which would
seem quite plausible. (Actually, even we know of theoretical ways to go
places by means other than raw velocity -- e.g., general-relativistic
space warps -- even though right now we have no idea how such a thing
could be made practical.)
>... I can't imagine him
> just buzzing some farmer in Arkansas and heading off into the sunset.
This, now, is one of the stronger arguments against the little-green-men
theory of UFOs: their behavior fits atmospheric and optical phenomena --
plus the occasional fake -- much better than it fits spaceships.
>would say the other 1%
> (from reliable sources, Air Force pilots, etc.) are military craft
>or experimental weapons, etc.
> that don't officially exist...
Actually, the other 1% are almost certainly explainable in mundane ways
just like the first 99%, after discounting observer error, out-and-out
hoaxes, etc. There is a prevailing myth that folks like professional
pilots are expert observers, which they are not: they are trained to
try to figure out what's going on, so they can react accordingly, not to
simply observe as accurately as possible without introducing errors by
trying to interpret what they see. There are documented cases of pilots
reporting UFOs pacing the plane, etc., when it is about as certain as
it can possibly be that what they were really seeing was a meteorite a
hundred miles away.
--
With features like this, | Henry Spencer at U of Toronto Zoology
who needs bugs? | uunet!attcan!utzoo!henry he...@zoo.toronto.edu
Jeffrey Kirk Bennett
a little confused. Am I wrong, or
there a a couple principles which
make it completely impractical- but I'm not a physicist, so please
expand or correct me if I'm wrong.
1. The goal is to find intelligent life and/or a habitable planet. It
seems to me that both are extremely
rare events. Though I've no doubt that there is life somewhere else
in the universe, I'm sure it is
an exceedingly rare event. But even if it's 1 in a trillion, the
sheer number of stars makes it
statistically likely. However, the probability of finding life or a
good planet in any PARTICULAR
star system is vanishingly small. Therefore, even if we could build
a ship to visit a few nearby
stars, we would probably have to investigate a few MILLION stars-
several hundred thousand
of which would be very remote- in order to find anything useful.
2. I don't see mankind inventing practical anti-matter propulsion or
whatever the current idea is
in the next century or two. But, since there are no theoretical
reasons why a craft cannot fly
arbitrarily close to c, assume that something will be developed
which is capable of
accelerating to, say, .9c within a reasonable (ship's) time. One
related question is- are there
thermodynamic efficiency considerations which limit the maximum
possible speed? Would
these depend on the method of propulsion used?
The point is, due to relativistic mechanics, wouldn't it be a 1-way
trip? If you travelled to a
star 5 light-years away at .9c, wouldn't a few centuries or
millenium pass on Earth? How
severe is the time dilation effect? Again, I'm not a physicist.
What happens to time during
the acceleration and deceleration- when the velocity is not
constant. And what happens
when the ship stops accelerating and starts decelerating, when even
the acceleration is
not constant? Does anyone know how to figure that all out
realistically, including all the
non-constant terms. How long would a trip take at .1c, .5c and .9c
in both ship's and Earth's
time?
3. Assuming all the fuel is burned during the acceleration and
deceleration phases, how would
the craft return?
4. Even if it had the fuel to return, how would it find the Earth again,
since it is not exactly
standing still, and may have moved unpredictably due to passing
stars, black holes, etc. in
the path of the Sun.
What I'm saying is basically this: We don't have the technology to
do it. Even if we did, it
would be a 1-way trip. Even if we accepted that and went anyway, how
would we have fuel
enough to get back? Even if we had fuel enough to get back, how would we
even FIND the Earth
again? Even if we surmounted all these difficulties, the probability of
finding something useful is
almost nil anyway. So who would fund this research, when the astronauts
would be returning
(if they ever returned) 20,000 years into the future when the Earth is
ruled by dinosaurs or mutants
or something, and most likely empty-handed? With AIDS, drugs and the
deficit, such a Don Quixote
exercise in futility wouldn't have a chance.
So I don't see the point in even talking about it, but comments are
welcome.
On a related note- can't we dismiss UFO reports out of hand? My
reasoning is this:
1. There are no little green men on the moon. Therefore, E.T.'s would
have to come from
outside the solar system = other stars.
2. If something is coming from another star, it would have to have near
light-speed travel.
3. The aliens would never do this, since they would be faced with all
the problems discussed
above. Even if we assume a totally alien value system which would
cause them to do it
anyway...
4. Wouldn't something decelerating from near light-speed in our solar
system have to shed all
its relativistic mass as energy? In that case, it would probably be
the brightest star in the
sky, visible even in the daytime. Even if it's not so dramatic (I'm
not a physicist- any estimates
on this?), I'm sure we would pick up something, with all the
tracking devices and telescopes
we have pointed into space. Since we haven't seen this, doesn't
that alone prove that there
are no UFO's?
5. Even if I'm wrong there- if an alien makes a 1-way trip to another
star and finds advanced life
on a planet, wouldn't he at least LAND or something? What else can
he do? He can't go home-
everyone he knows is centuries dead at least- what does he have to
lose? I can't imagine him
just buzzing some farmer in Arkansas and heading off into the sunset.
6. 99% of the UFO reports can be explained by strange and rare
meteorological events which are
unfamiliar to most people. Nature is very strange indeed at times. I
would say the other 1%
(from reliable sources, Air Force pilots, etc.) are military craft
or experimental weapons, etc.
that don't officially exist and that we are not supposed to know
about, and which the government
covers up. I have no problem with that- they have to keep some secrets.
These are things which have bugged me forever. I believe interstellar
travel is probably impossible
and certainly impractical, and UFO's don't exist. Am I missing something here?
Jeff
Al Bowers
I noticed a lot of talk about interstellar travel a while back, and
I'm a little confused. Am I wrong, or is there a point to thinking
about such things? It seems to me that there a a couple principles
which make it completely impractical- but I'm not a physicist, so
please expand or correct me if I'm wrong.
I also noticed this disscussion and it seemed to me there was alot
of sci-fi type of mentality. I pointed out the near term solution of
a Daedalus type of one-way unmanned probe and was immediately
_flamed_ for lack of foresight so I withdrew from subsequent
disscussion. As an engineer living in the here and now such probes
interest me as they will provide the only answer in my lifetime as to
whether our solar system evolution is unique or not (Hubble will
answer some of the questions, but not as many as a probe).
1. The goal is to find intelligent life and/or a habitable planet. It
seems to me that both are extremely rare events. Though I've no doubt
I disagree. Finding anything at all will tell us a lot. Just by
visiting another star we double our experimental sample ;-).
2. I don't see mankind inventing practical anti-matter propulsion or
whatever the current idea is in the next century or two. But, since
Nuclear drives are available now, and with a limited development of
technology it appears that .12c is possible. This gives a flight time
of 30 to 50 years to various stars (Proxima Centauri, Barnard's star,
Alpha Centauri, etc.). If I live to be 90 or so it could be done
before I die, but we gotta start now!
3. Assuming all the fuel is burned during the acceleration and
deceleration phases, how would the craft return?
Just crawl first, worry about walking later. Do the fly-by mission.
You gotta start somewhere, right?
4. Even if it had the fuel to return, how would it find the Earth
again, since it is not exactly standing still, and may have moved
unpredictably due to passing stars, black holes, etc. in the path of
the Sun.
See above. There are some time dilation effects even on this short
mission, but readjusting the data rate isn't a _major_ undertaking.
So I don't see the point in even talking about it, but comments are
welcome.
On a related note- can't we dismiss UFO reports out of hand? My
reasoning is this:
I leave this to the National Enquirer and Star crowd to argue with
you. I would not dismiss ETI completely (I do believe SETI is
important) as it is utter arrogance to outright assume that we are
unique in the Universe. I do discount UFO's as preposterous.
These are things which have bugged me forever. I believe interstellar
travel is probably impossible and certainly impractical, and UFO's
don't exist. Am I missing something here?
I think interstellar travel will begin in the next few generation (my
grandchildren or great-grandchildren?) at an unmanned probe level. As
for flying people, I don't know and I would not hazard a guess (one
Einstein in a life time can turn physics upside down on you ;-).
--
Albion H. Bowers bow...@elxsi.dfrf.nasa.gov ames!elxsi.dfrf.nasa.gov!bowers
`In the changing of the times, they were like autumn lightning, a
thing out of season, an empty promise of rain that would fall unheeded
on fields already bare.'
attributed to Abe Shosaburo by Dave Lowery
Nick Kline
to estimate various factors which relate to the chances of intelligent life,
like say % of suns with planets, % with life, % with intelligent life,
% with life that doesn't destroy itself, similar in scope to the equation in
Cosmos, by Carl Sagan?
-nick
Henry Spencer
To sum up *very* briefly, the Fermi Paradox is that interstellar flight at
substantial fractions of the speed of light doesn't seem too difficult for
a civilization not much older than ours, a single civilization can thereby
fill the entire galaxy in an eyeblink of geological time, and such
civilizations ought to be common in the galaxy by everything we know now...
so, in Fermi's words: "Where are they?". Why haven't we been visited?
Why has native intelligence (ours) been able to develop undisturbed on a
planet that has been ripe for colonization for a billion years or so?
Why aren't the large engineering works of *really* advanced civilizations
visible in the galaxy?
It is not difficult to concoct explanations for why a single civilization
would fail to be visible. The hard part is making the explanations really
inevitable, so that even oddball civilizations well off the normal
development path cannot escape them, while making them natural enough so
that *we* don't have to be inordinately lucky or extremely unusual to
have gotten as far as we have.
I've seen many proposed explanations of the Fermi Paradox; I researched
the subject a bit for a series of articles on it for the Canadian Space
Society newsletter a while ago. I find none of the proposals believable
enough to account for the evidence. It's unsolved.
Tom Neff
> I'm sure that people tried to discourage Columbus with much
^^^^^^^^
>the same arguments (with obvious adjustments for available technology,
>of course).
On what basis.
Certainly not anything historical!
I just reviewed Jeff's arguments (which I wish he'd posted in under 80
columns; read Eugene's Reminders posting folks! Not everyone has an
ultrawide window in a fancy workstation) and none of them have anything
to do with the situation prevailing in Columbus' time.
I hate the kind of facile nonsense quoted above. It damages the debate.
Study Columbus or leave him out of the discussion, sez I.
Gregory Milligan
I'm sure that people tried to discourage Columbus with much
of course).
Greg Milligan
Jim Meritt
Why make this into a physics problem? Make it a biology problem.
If you can't shorten the trip, lengthen the lifespan/patience of the
travelers!
That that is is that that is. That that is not is that that is not.
That that is is not that that is not. That that is not is not that that is.
And that includes these opinions, which are solely mine!
j...@aplvax.jhuapl.edu - or - j...@aplvax.uucp - or - meritt%aplvm.BITNET
Peter Scott
Nope, that's the Drake equation. The Fermi paradox basically says,
"Where is everybody???" The Drake equation suggests that the galaxy should
be teeming with life, given the number of stars we have. *Our* presence
as a technological society can readily be detected within a 50ly radius;
yet we have found no signs, similar or other, that life exists elsewhere in
the universe. Speculation as to why this is the case runs rampant and I
refer you to some excellent pop science discussions of same in _Analog_.
This is news. This is your | Peter Scott, NASA/JPL/Caltech
brain on news. Any questions? | (p...@aristotle.jpl.nasa.gov)
Ron Prine
_EXPANDED UNIVERSE_ pg. 369
ROUNDTRIP BOOST
COMPARISON OF ELAPSED TIME
Thrust Earth-Mars-Earth Earth-Pluto-Earth
@ 1 gee 4.59 days 4.59 weeks
@ 1/10 gee 14.5 days 14.5 weeks
@ 1/100 gee 45.9 days 45.9 weeks
@ 1/1000 gee 145.0 days 145.0 weeks
He states that he added the 1/1000 so that you could see that even
with Solar Sails you could make the trip.
Now you can tear this apart and see how this works.
My idea is that we need to get our constant acc. drives in better
shape.
Later
Ron Prine
Dan Tilque
>
>Nope, that's the Drake equation. The Fermi paradox basically says,
>"Where is everybody???" The Drake equation suggests that the galaxy should
>be teeming with life, given the number of stars we have.
The Drake equation is an attempt to quantify how many technologically
advanced civilizations there are in the galaxy. If they (i.e. Drake,
Sagan, et al.) could come up with a high enough estimate, they could
justify doing what they wanted to do anyway: a radio search for alien
radio signals.
What the Drake equation really does is to identify our areas of
uncertainty a little more precisely. Instead of one big uncertainty,
we now have a couple of reasonably certain numbers, one or two numbers
we can guess at with some degree of certainty, and a lot of numbers
which are totally unknown.
What this means is that the Drake equation didn't suggest anything.
With equal certainty, you can substitute numbers into the equation to
get less than one such civilization per universe or a civilization for
every 100 stars.
>*Our* presence
>as a technological society can readily be detected within a 50ly radius;
>yet we have found no signs, similar or other, that life exists elsewhere in
>the universe.
Since we've been listening to the sky at radio frequencies for > 40
years now and haven't discovered any alien generated signals, it's
probably safe to say that there are no civilizations like ours within
that 50 ly radius. This seems to be better limiting data than the
results of the Drake equation.
---
Dan Tilque -- da...@mrloog.WR.TEK.COM
Mark Gellis
I've been reading the discussion of ETs and interstellar travel. Here are
my two cents, for what they are worth:
Interstellar travel might be accomplished by our civilization, if we have
a large enough industrial base to support it. The various methods I have
heard include:
Matter-antimatter drives (could reach relativistic speeds, like .6 c)
Laser-powered lightsails (if the power is available, the limit here might
be something like .99 c)
(In a book called INTERSTELLAR MIGRATION AND THE HUMAN EXPERIENCE, an
excellent anthology, there is a discussion of boosting entire habitats to
very high speed over a long period of time; doubtless they employ fusion
or matter-antimatter power rather than solar)
Fusion drives might work, but I believe the theoretical limit on nuclear
fusion is a specific impulse of about 2.6 million, which means that you
could not get a spacecraft moving much faster than about 10%-15% c. This
might work if you are using unmanned probes or have perfected suspended
animation technology, but otherwise it is a bad idea for manned probes.
If there is no way around the lightspeed limit, we might well colonize
other solar systems, very slowly, over the next several centuries, but
these solar systems would be isolated civilizations. They would receive
messages, and be able to send them, but they would be politically
autonomous; no one tries to run an empire with a multi-year delay on
responses to political situations!
There is also, by the way, the possibility of colonizing the cometary
halo in our own or other solar systems. Remember, once you have a space-based
economy/industry, those 5 km. chunks of ice and rock = lots of raw materials
(and hydrogen for fusion power plants!) While playing around with some
numbers for an sf story I was working on, I figured that a 10 km. cometary
nucleus (larger than average, but not rare) would easily support, even
with my conservative figures, a population of 200,000 in raw materials
for building a large habitat, water, organics, etc.
As for those pesky aliens, we have a few options:
1) There are none. (Oooo...creepy, could that silly book with all the
hard-to-pronounce names begetting one another be right?)
2) There are some, but they are a LOT further away than 50 light years,
which means they have no idea we are around.
How far, by the way, can normal radio transmissions be detected? After
all, an advanced technological civilization might be relatively "radio
quiet," having moved much of its communications to lasers and other direct
channel (cable, etc.) technology.
3) There are some, and they know we are here, but they have decided not
to talk to us because...
a) They know that if they do, our civilization will self-destruct
trying to deal with all the too-advanced technology we will steal even if
they don't give it us
b) They do not feel we have anything worth buying from us (and they
are not going to just give us disinto-ray technology out of the goodness
of their seven-valved hearts)
c) They are not sure if we are intelligent, yet
d) They don't like us (probably becuase they have intercepted some
of our radio transmissions and listened to things like Michael Jackson
or Barry Manilow or Milli Vanilli or whatever, so they have a good idea
of what our culture is like).
Enjoy.
Douglas Harper
] >So what is the Fermi Paradox? Is this the equation where you try
] >to estimate various factors which relate to the chances of intelligent life...
]
] To sum up *very* briefly, the Fermi Paradox is that interstellar flight at
] substantial fractions of the speed of light doesn't seem too difficult for
] a civilization not much older than ours, a single civilization can thereby
] fill the entire galaxy in an eyeblink of geological time, and such
] civilizations ought to be common in the galaxy by everything we know now...
] so, in Fermi's words: "Where are they?". Why haven't we been visited?
] Why has native intelligence (ours) been able to develop undisturbed on a
] planet that has been ripe for colonization for a billion years or so?
] Why aren't the large engineering works of *really* advanced civilizations
] visible in the galaxy?
What's always puzzled me about the Fermi Paradox is why it's considered
paradoxical. Since Fermi's argument is a strong one, it strongly
suggests that "what we know now", just isn't so. This is hardly
paradoxical (unless you go by the "demonstrated proposition which is
contrary to received wisdom" sense of "paradox"). So what if
intelligent life is rare in the galaxy? Why not?
I don't know about the rest of the human race, but I find the prospect
of an unpopulated galaxy for us to explore and populate to be
completely within reason, and very exhilarating besides.
--
Douglas Harper oravax!har...@cu-arpa.cs.cornell.edu
"Finally, in conclusion, let me say just this." -- Peter Sellers
Henry Spencer
>What's always puzzled me about the Fermi Paradox is why it's considered
>paradoxical. Since Fermi's argument is a strong one, it strongly
>suggests that "what we know now", just isn't so. This is hardly
Of course it suggests that our current knowledge is inadequate -- but
*which* current knowledge? The paradox is that two things we think we
can make fair guesses about -- the incidence of intelligent life elsewhere
in the galaxy, and the likelihood that such life would leave visible
evidence that we should be able to see -- contradict each other. Either
we are vastly overestimating the probability of intelligent life, or we
are very seriously wrong about what an advanced civilization would do
in the way of interstellar expansion. But which, and why?
>So what if intelligent life is rare in the galaxy? Why not?
Why not? Because by what we know now, it ought to be everywhere. This
is like saying "so what if 2+2=5? why not?". (Well, okay, it's a bit less
drastic than that, but you get my point, I hope...) If intelligent life
is rare, something is *seriously* wrong with our theories on the subject.
It would be comforting to know what.
>I don't know about the rest of the human race, but I find the prospect
>of an unpopulated galaxy for us to explore and populate to be
>completely within reason, and very exhilarating besides.
Personally, I'd postpone the exhilaration until we understand *why* the
galaxy is unpopulated. One unpleasant group of ideas says that the galaxy
is unpopulated because it is much more hostile to intelligent life than
we think. If we've simply been lucky, great, but that doesn't mean we'll
stay lucky.
Thomas Cervera
>In article <4a8uilO00...@andrew.cmu.edu> jb...@andrew.cmu.edu (Jeffrey Kirk Bennett) writes:
>2. I don't see mankind inventing practical anti-matter propulsion or
>whatever the current idea is in the next century or two. But, since
> Nuclear drives are available now, and with a limited development of
> technology it appears that .12c is possible.
I'd just like to know how one feels when interplanetary matter (meteors) or
interstellar matter (gases and dust) collides with the spacecraft at .12 c ;-)
I'm not really sure, but if my memory serves me right, Sirius (Canis Minor
alpha), for example, is somewhat 11 ly apart. A trip to that star (it's not
the closest, I know) would take decades (spacecraft's time) at .12 c.
This implies a high probability of such collisions, IMHO.
-thomas
--
Thomas Cervera | UUCP: alde...@tubopal.UUCP
SysMan RKOFBI (PDP/VAX)| ...!unido!tub!opal!alderaan (Europe)
D-1000 Berlin 30 | ...!pyramid!tub!opal!alderaan (World)
Motzstrasze 14 | BITNET: alderaan%tub...@DB0TUI11.BITNET (saves $$$)
Brian or James
Perhaps the local solution to the Fermi paradox is that, *if* other
technologically inclined species tend to fill their galaxies to the
saturation point in a few hundred millenia, and *if* the conditions
caused by the saturation of the galaxy preclude new technological
species from evolving, then our present existance requires that
the industrialisation of our galaxy has not yet occured. This model
is fairly hard to disprove, so it probably not useful. If other galaxies
do have wide spread civilisations, I wonder if we could detect them
remotely. Perhaps that's the explaination for the 'Great Wall'. The
universe is more or less identical [in terms of mass/volume], but
the techno types are blocking the visible light with all those darn Dyson
spheres :). I'd expect the big dark areas to be sources of infra-red
radiation, if this were true.
JDN
Henry Spencer
> I'd just like to know how one feels when interplanetary matter (meteors) or
>interstellar matter (gases and dust) collides with the spacecraft at .12 c ;-)
> I'm not really sure, but if my memory serves me right, Sirius (Canis Minor
>alpha), for example, is somewhat 11 ly apart. A trip to that star (it's not
>the closest, I know) would take decades (spacecraft's time) at .12 c.
> This implies a high probability of such collisions, IMHO.
The Daedalus report, which planned a nominal mission to Barnard's Star,
six LY, at about that speed, concluded that this was manageable. In
their design, about one-tenth of the interstellar cruise mass was the
bow shield, designed to severe worst-case assumptions; a good bit of that
would be lost to ablation by dust grains during the mission, and it
would run moderately warm due to gas and dust throughout. Hitting
something large would vaporize the whole probe, of course, but that is
relatively unlikely -- interstellar space is very empty.
Serious fractions of the speed of light start to pose larger problems.
In particular, the protons and electrons composing the interstellar gas
start to look like high-energy radiation in substantial quantities.
Tom Neff
is that when you achieve interstellar travel, somebody comes and kills
you. Let's hope that's not it.
--
Stalinism begins at home. }{ Tom Neff }{ tn...@bfmny0.UU.NET
Mark SOKOLOWSKI
nature, let's talk about the martians!!!
I think that the fact that our galaxy could be colonized in a time span
ranging from 3 million (high speed at 0.1c) to 30 million years
(chemical propulsion at 0.01c) proves at least to me that no other
advanced civilization exists in our galaxy, since this later object
is at least 12 billion year old. This is more than enough time for
hundreds of planets having some form of life to reach some kind
of "maturity" comparable to our own technical level...
Therefore, I will have this DEFINITE conclusion: We are alone, and I
am very pleased with that (We're already 6 billion here!!!!),
notwithtanding the fact that space is VOID, COLD, DEAD in general,
which means that any INTERSTELLAR civilization (including us in a few
hundred years...) will be so imperialistic and so energy thirsty that
I won't want to live in there (but some others surely will...) and if
they come here, we're all dead, PERIOD!
Fraering Philip
at the possibility of using a dust or gas cloud put in front of you while you
are coasting to cut down on the amount of gas or dust likely to collide
with you.
Imagine the dust in interstellar space colliding with an exhaust plume or
cloud of ice particles put out by a lowly RL-10. Of course, it depends on how long the cloud holds together. Doubtlessly something could be done
with electrostatics or a magnetic field holding the dust in place.
Scotty, raise the shields... :-)
Philip Fraering
dlbr...@pc.usl.edu
"I'm trobled, I'm dissatisfied, and I'm Irish."- Marianne Moore
Forrest Gehrke,2C-119,7239,ATTBL
>
> Since we've been listening to the sky at radio frequencies for > 40
> years now and haven't discovered any alien generated signals, it's
> probably safe to say that there are no civilizations like ours within
> that 50 ly radius. This seems to be better limiting data than the
> results of the Drake equation.
Who is this "we" that has been listening? And with what? These
signals are not going to be picked up on a boombox with the
announcement the signal is from outer space.
I have seen these postings saying that our radio signals have been
going out for 40 or 50 years. That puts us back to 1940 or 1950.
Anyone heard of Heinrich Hertz or Marconi? 80 or 90 years would
be more accurate. But the point is, we really haven't been
listening. It is only in the last decade or two that there have
been sufficiently sensitive receivers and antennas available to
listen. But if these installations picked up such a signal it
would be mostly accidental.
On another tack--let's assume there is an intelligent life out
there about 50 ly. Let's also even assume their evolution was on
the same time track. So they discover radio. The easiest wavelength
is the same as with us: long waves. Who is listening below 200
meters? Besides which, our earth's atmospheric racket would
drown out any signals that might possibly be on those frequencies.
(Assuming these signals were coming at us straight on so that
there would be a minute chance of getting through our ionosphere).
But even at UHF and microwave frequencies (which can make it through
our ionosphere), this is a dauntingly wide spectrum to be
attempting to cover simultaneously (let alone doing it in all
directions). It is also a daunting problem to discriminate against
the signals being caused to be transmitted by ourselves.
I don't believe it has been proved there is no intelligent
life within a radius that our radio signals have been going
out, simply because we haven't heard anything come back we
could identify as non-earth related. I pick this time period
only because we would have to have been detected by some
intelligent life for this life to direct its antennas at us.
We really haven't been listening.
Forrest Gehrke f...@dodger.ATT.COM
Mike Van Pelt
>One possible explanation for the Fermi paradox that nobody can disprove
>is that when you achieve interstellar travel, somebody comes and kills
>you. Let's hope that's not it.
Or, when you achieve radio somebody comes and kills you. I think Dr.
Benford did an analysis of this possibility (plus a novel or three):
Some really paranoid BEM out there decides that allowing other
technological life forms to develop is an Unacceptable Risk. So they
build self-replicating robots which listen for radio signals of
intelligent origin, and when they detect them, go and destroy the
senders.
Of course, the BEMs had better be *real* sure the technology of their
"deadly probes" is sufficient to deal with said technological life
forms when they finally get there, because an unsucessful attempt at
planetcide is bound to get the victims upset -- and they might come
looking for the perpetrators.
If so, what should we look for?
(My random signature of the day seems especially appropriate...)
--
Mike Van Pelt | What happens if a big asteroid hits Earth?
Headland Technology | Judging from realistic simulations involving a
(was: Video Seven) | sledge hammer and a common laboratory frog, we
...ames!vsi1!v7fs1!mvp | can assume it will be pretty bad. -- Dave Barry
Daniel Briggs
been made, but I think I can add a bit to the SNR on how radio astronomy
is "normally" practiced.
For those who don't care to read the whole thing here, the short
answer is that I agree with Forrest Gehrke. We haven't heard anything
yet because for the most part we haven't been listening.
Dan Tilque writes:
>Forrest Gehrke writes:
>>Dan Tilque writes:
>>>
>>> Since we've been listening to the sky at radio frequencies for > 40
>>> years now and haven't discovered any alien generated signals, it's
>>> probably safe to say that there are no civilizations like ours within
>>> that 50 ly radius. This seems to be better limiting data than the
>>> results of the Drake equation.
>>
>>Who is this "we" that has been listening? And with what? These
>>signals are not going to be picked up on a boombox with the
>>announcement the signal is from outer space.
>
>There's this obscure branch of science which you are probably unaware
>of. It's called Radio Astronomy. One of the things they're good at is
>aiming large radio antennae at the sky.
Reality check here. It is *hard* to catch internal communications
over interstellar distances. Here are a couple of rough numbers I
just pounded out. You can assume that I've done the sums right, or
check them yourself.
Assume a 1 GW transmitter 50 lyr away. Since this is supposedly an
evesdropping situation, we had better assume that this is radiating
isotropically. If you want to assume a beamed transmitter, you can.
That will raise the power of the beam by whatever geometric factor you
assume, and also cut down the chance we will see it by the same
factor. If you want to stay inside of 50 lyr, you are talking modest
number statistics. I don't happen to know the exact numbers, but we
are probably talking tens of thousands of eligible suns within that
radius. If you start assuming a geometric beaming factor of tens or
hundreds, then you are reducing the number of "eligible" suns by that
factor --- the rest of them we won't see. We wouldn't consider the
fact that we didn't detect life in a sample of 10 suns a definitive
result that there is no life within 50 lyr. A sample of 10000 suns,
and the assumption that the XTs use a moderately directional
communications beam, reduces to the same case. (But the beam itself
is much easier to detect.)
Enough digression. Assume that we can only evesdrop on the
extraterrestrial Lawrence Welk Show if we can detect the isotropic
transmission. I have no good idea of what to assume for a transmitter
power. I think that 1 GW is pretty damned big by today's standards.
(Correct me if I am badly off here, BTW. I may be.)
1 GW spread over a sphere of 50 lyr is 3.6*(10)^-2 (10^-26 W/m^2)
The reason for the funny choice of units is that radio astronomers
like to talk about Janskys. 1 Jy = 10^-26 W/m^2/Hz. To make it
easier to detect, we will assume that the aliens are using a
monochromatic transmitter. In this case, to go from the above power
flux to Jy, we divide by the bandwidth we observe at. *For a
monochromatic source*, the smaller bandwidth you use, the better. For
the the case where you have power spread all over the spectrum, more
bandwidth is better. Anyway,
Our transmitter looks like
3.6*(10)^-2
----------- = 7.1*(10)^-4 microJy (the normal continuum case)
50(10)6 Hz
and also like
3.6*(10)^-2
----------- = .2 microJy (the most extreme spectral line
.20(10)6 Hz case that we can observe at the
VLA)
To give you some feeling as to what we can actually detect, twelve
hours of observation at 1.4 GHz (the 21 cm line that you have heard so
much about), will get us down to an RMS noise of 15 microJy in the
continuum case, and down to 240 microJy in the spectral line case.)
Assume that we need at least 5 times the noise level to get anyone's
attention, and we find that the ratio of detectable signal to actual
signal is about 100,000 in the continuum case, and 6,000 in the
spectral line case. This is staring straight at it with one of the
most sensitive instruments in the world! To get a serendipitous
discovery, you generally need a signal that is a good deal more
powerful than that.
>Also there have been
>some programs to listen to a number of nearby stars for unusual
>signals.
Quite true. If you streched Arecibo to the limit, you might be able
to get this sort of signal from some of the very nearest stars.
That's about the only antenna on earth that could, though. (Not
excepting us, BTW). If you are really streched to the limit like
this, it means that you don't get a very large sample, though. I
think that many of the SETI experiments have to assume that the other
guys are deliberately sending stuff, and then guess where to look, and
what a deliberate signal might look like. This is to dodge the
problem that isotropic radiation is so damned hard to pick up. When
you get into this sort of guessing game, then you have to sample an
enormous paramter space. That's what these million channel receivers
are all about. Normal radio astronomers aren't interested in this:
you don't use a SETI specialized receiver by accident! I should
probably shut up about SETI, since I don't really know that much about
it. Anyone care to enlighten us with SETI details?
>>It is also a daunting problem to discriminate against
>>the signals being caused to be transmitted by ourselves.
>
>One of the steps in processing radio astronomical signals is to filter
>out locally generated signals.
Sorry Dan, but I have to agree with Forrest here too. At the lower
frequencies, interference is a hell of a problem. You don't just
"filter it out". Generally you do your best to observe where it
isn't, but this is really tough. At the low frequencies (say less
that a GHz or two) where ordinary civilian electronics can contribute
significant stuff, it's terrible. (Not just transmission equipment,
but anything electronic or electromechanical. Cars, computers, &
microwave ovens are all detectable!) At the middle frequencies, (say
a few GHz to a few tens of GHz), we only have to fight with civilian
radar and the military. This isn't as bad, since there isn't as much
of it. Also, it is usually fairly narrow band and directional. From
several 10s of GHz to maybe a hundred or so there is not a whole lot
other than military projects and space projects. Above a hundred GHz
we pretty well have the spectrum all to ourselves. (Or at least so it
appears to us. I know that the spectrum has been allocated by the FCC
well past this point. Most of the high end stuff seems to be labeled
"space" or "mobile communications". I just haven't personally run
into any of this as a problem. It's a fact, though, that it is much
harder to build receivers at these kind of frequencies. This too will
change....)
-----
This is a shared guest account, please send replies to
dbr...@nrao.edu (Internet)
Dan Briggs / NRAO / P.O. Box O / Socorro, NM / 87801 (U.S. Snail)
Dan Tilque
>da...@mrloog.WR.TEK.COM (Dan Tilque) writes:
>>
>> Since we've been listening to the sky at radio frequencies for > 40
>> years now and haven't discovered any alien generated signals, it's
>> probably safe to say that there are no civilizations like ours within
>> that 50 ly radius. This seems to be better limiting data than the
>> results of the Drake equation.
>
>Who is this "we" that has been listening? And with what? These
>signals are not going to be picked up on a boombox with the
>announcement the signal is from outer space.
There's this obscure branch of science which you are probably unaware
of. It's called Radio Astronomy. One of the things they're good at is
aiming large radio antennae at the sky.
>I have seen these postings saying that our radio signals have been
>going out for 40 or 50 years. That puts us back to 1940 or 1950.
>Anyone heard of Heinrich Hertz or Marconi? 80 or 90 years would
>be more accurate.
We didn't really start to send signals in very high strength until TV
and radar were in widespead use in the 40's. But our sending out
signals is not necessarily connected to our ability to detect an alien
civilization.
>But the point is, we really haven't been
>listening. It is only in the last decade or two that there have
>been sufficiently sensitive receivers and antennas available to
>listen. But if these installations picked up such a signal it
>would be mostly accidental.
True they would get it accidentally. I would assume that a relatively
nearby star which produces unusual amounts of radio frequency for it's
spectral type would be studied by an astronomer. Also there have been
some programs to listen to a number of nearby stars for unusual
signals.
>On another tack--let's assume there is an intelligent life out
>there about 50 ly. Let's also even assume their evolution was on
>the same time track.
I never made the assumption that the aliens were exactly even with us
in development. If they are ahead of us, they are probably easier to
detect, if they are behind us, then your objections are valid.
>The easiest wavelength
>is the same as with us: long waves. Who is listening below 200
>meters? Besides which, our earth's atmospheric racket would
>drown out any signals that might possibly be on those frequencies.
There's only so much bandwidth at long wave lengths. Just like
us, they would soon start using other wavelengths. Also, long
wavelengths are unsuitable for many uses such as radar.
>But even at UHF and microwave frequencies (which can make it through
>our ionosphere), this is a dauntingly wide spectrum to be
>attempting to cover simultaneously (let alone doing it in all
>directions).
I'm assuming that, like Earth, the aliens produce a large amount of
"waste" radio emissions at many frequencies. This is the weakest part
of my argument. If the aliens do little or no general broadcasting, we
may have trouble detecting them.
>It is also a daunting problem to discriminate against
>the signals being caused to be transmitted by ourselves.
One of the steps in processing radio astronomical signals is to filter
out locally generated signals.
>I don't believe it has been proved there is no intelligent
>life within a radius that our radio signals have been going
>out, simply because we haven't heard anything come back we
>could identify as non-earth related. I pick this time period
>only because we would have to have been detected by some
>intelligent life for this life to direct its antennas at us.
I wasn't talking about intelligent life, I was talking about a
civilization similar to ours. Let me be slightly more precise: a
civilization which has been using radio extensively for at least 50
years. But even with these restrictions, you're right, it hasn't been
proven. It's just strongly indicated.
Why do you assume that we can only detect them if they try to signal us?
The 50 light year limit was somewhat arbitrarily chosen and is not meant
to coincide with the length of time we've been producing strong signals.
>We really haven't been listening.
Actually, Drake and a few others have been listening off and on since the
early 60's.
Forrest Gehrke,2C-119,7239,ATTBL
> >da...@mrloog.WR.TEK.COM (Dan Tilque) writes:
> >>
> >Who is this "we" that has been listening? And with what? These
> >signals are not going to be picked up on a boombox with the
> >announcement the signal is from outer space.
>
> There's this obscure branch of science which you are probably unaware
> of. It's called Radio Astronomy. One of the things they're good at is
> aiming large radio antennae at the sky.
>
for limited power source signals, not that of the Big Bang or the
radiation resulting from energy released by a supernova.
To pick up the signals of Voyagers at the outer limits of the
solar system we need 70 meter parabolic antennas, carefully
aimed, tuned, and internally noise-reduced within the limits
of our technology. Ok, our Voyagers don't start out with very
much transmitting power and their antennas are not very large.
But extend that out 50 ly and then tell me how much effective
radiated power will be required to achieve the Voyager signal.
We could do that, but we are not doing it; nobody would agree
to the expense.
Voyager level signals are what I would expect to be listening for
and those means required to make the search. I don't know of any
such search going on. If some intelligent life happened to be
out beyond a Voyager on a straight line, on the same frequency,
we might hear them. Else forget it. I don't expect any
intelligence in this galaxy to initiate a supernova and then
sentiently modulate it to get our radio astronomers' attention.
>
> We didn't really start to send signals in very high strength until TV
> and radar were in widespead use in the 40's.
None of our TV broadcast signals, despite often using effective
radiated power of a few megawatts, are aimed at outer space. Those
canny transmitter owners spend a lot of money making sure that
energy is directed pretty close to the earth's surface where the
paying customers are. So their signals are not going to be very
strong out at 50 ly. That intelligence at the other end will have
to aim their 70 meter dishes very carefully, tuned to channel 6,
and listen hard. Let's see, that will be about the year 2010.
Maybe if our grandchildren are listening about 2060, they will
hear that long-awaited "CQ Earth--what happened to Mr. Ed?". (;-))
Forrest Gehrke f...@dodger.ATT.COM
Brian or James
Most of the discussion I've seen assumes that that communication
we're trying to 'eavesdrop' is only by chance detectable by us. If
you are trying to spot other technological cultures, it gets a
lot easier if they are trying to be heard [I have this mental image
of forty million civilisations listening carefully to the radio
bands while trying to be perfectly silent themselves]. While it
is probably true that older cultures could be using 'exotic' means
of communication that we, in our technologically primitive and
intellectually fallen state, cannot imagine, there's not much point
in trying to listen for means of communication that are currently
inconceivable to us :). If you assume [and I realise that there is no
binding reason to do so] that there might be a culture which *wants*
to be heard from, the EMR is a good choice to listen to, since it seems
likely to be used for communication for at least some point in a
technological culture's history. It's like the wheel; a likely
although not inevitable tool for sapients to discover, and if you
want to be heard, it's a good guess at a communications technology
that's likely for most technological cultures to be aware of.
Now, the *motives* for a culture to broadcast to other cultures
are open to question, and while currently, humans find it hard to
justify any activity that lasts longer than a decade [Let's hear it
for the Harvard Business School of thought!], even we lowly primates
have had projects which took lifetimes to finish [cathedrals in Europe,
for example], so it doesn't seem unreasonable to speculate about another
species having activities that last multiple centuries or millenia. As
someone else pointed out, even though broadcasting in all directions is
inefficient, there are groups, like Voice of America or Radio Moscow
that have motives for doing so. I hope this doesn't mean that most
detected SETI signals are things like 'Quantification of Angel Packing
in a Infinitesimal Volume' :)
I wonder if there other ways of detecting ETIs at range. We have had
an impact on our planet's atmosphere, but I have no idea if that kind
of thing is detectable given *big* telescopes [100 km mirrors out beyond
pluto, :) ] or obvious that humans caused the change. It could be that
life forms like plants are much easier to spot than other high tech
species [Any world with lots of O2 in its atmosphere has got to have
something constantly replacing the oxygen]. Back in the '70s, there
was speculation that a nuclear war is detectable for ~50ish LY, but
that's not the kind of technique that is useful for long term detection
of signals. I suppose in a few decades, we could do things like dumping
a megatonne of rare elements [like Plutonium] into the sun and hope
that if any ETI notices the weird emission lines in Sol's spectrum,
they don't just create a model of stellar evolution that includes a
class of 'stars that really should not have plutonium emission lines
but do anyway'. Back when they found LGM 1, and after they realised
that it looked much more like a natural source than a technologically
produced signal, someone commented that, while pulsars are a stupid way
to try and communicate, he knew of no binding reason why high tech
types couldn't also be stupid.
JDN
lawrence.m.geary
which repeats every 30 minutes. Even if alien listeners cannot recognize
the signal as being of intelligent origin, it's odd periodicity will
give it away. It will also make the signal easier to detect at distances
of many light years, even through the cacophony of other RF sources.
When the aliens do come in their vast starships, they will land in an
open field near the source of the signals - possibly Central Park in
New York, or the wetlands outside Seacaucus NJ. In order to attempt
communication with us, they will play back to us the signal we have
been sending toward the stars these many decades. A portal will open
in the side of the starship, and a large flat screen will erect itself.
Rhythmic music will be heard to come from the ship, and the screen will
begin to display a stylized human heart over which will be superimposed,
in a flowing script font, the following words:
I Love Lucy
--
--Larry: 74017...@compuserve.com
Henry Spencer
>I don't have references, but studies on Interstellar travel have looked
>at the possibility of using a dust or gas cloud put in front of you while you
>are coasting to cut down on the amount of gas or dust likely to collide
>with you.
Maintaining such a cloud during the long cruise phase is costly in mass.
Project Daedalus just used armor on the front for cruise, but did in fact
use a smoke cloud for the actual encounter with the target solar system.
Solar systems are very dusty places at 15% of the speed of light...
Tom Neff
>No, but if a supernova went off by chance, it would make sense to start
>sending signals in the _opposite_ direction. Any ETs who studied the SN
>closely might notice the signal (or may be smart enough to look for it).
Right, now what might we send to make it through the SN signal? Hmm, how
about a rapidly, regularly pulsating beacon... yeah I like it :-)
--
If the human mind were simple enough to understand, =)) Tom Neff
we'd be too simple to understand it. -- Emerson Pugh ((= tn...@bfmny0.UU.NET
Ted_An...@transarc.com
as an obvious fallacy" I would be interested in specific references to
the "ramjet literature" you allude to.
My understanding of the problems of intersellar ramjets make them
extremely dubious without invoking new principles of nuclear physics.
The only variant that seems barely plausible is bringing along your own
energy (anti-matter is the only thing with enough energy density) and
using the scooped hydrogen as reaction mass. And still this only buys
you slightly more than a factor of two in propellent mass, unless your
ram scoop works very well even at "low" speed.
Ted Anderson
Dan Tilque
>I can't speak for any of the deliberate attempts at SETI that have
>been made, but I think I can add a bit to the SNR on how radio astronomy
>is "normally" practiced.
>
>For those who don't care to read the whole thing here, the short
>answer is that I agree with Forrest Gehrke. We haven't heard anything
>yet because for the most part we haven't been listening.
Ok, you convinced me for the most part. The rest of this article is few
technical nits which will probably not modify your argument in the
least.
>I don't happen to know the exact numbers, but we
>are probably talking tens of thousands of eligible suns within that
>[50 ly] radius.
There are about 50 star systems containing about 70 stars within 16 ly
of Earth. If this is a representative sample (a reasonable assumption)
that gives about 1500 star systems in that 50 ly radius.
>Enough digression. Assume that we can only evesdrop on the
>extraterrestrial Lawrence Welk Show if we can detect the isotropic
>transmission. I have no good idea of what to assume for a transmitter
>power. I think that 1 GW is pretty damned big by today's standards.
By today's standards 1 GW is huge for a single transmitter. I was
thinking of the collective power of all sources. Typical radars emit
about 5 MW and TV stations about the same, I think. The largest single
emitters are probably certain phased array radars run by both the U.S.
and Soviet military which are designed to detect ICBMs, but I don't know
what they typically emit. I'm not sure how many TV stations and radar
systems there are so I can't say what the collective output is. A
horseback guess would say that it's on the order of 10 GW.
Of course, this is just our current output. However, your figures
(which I've deleted) seem to indicate that our output would have to go up
by several orders of magnitude before being detectable by our current radio
telescopes at 50 ly.
Andrew Pettifer
What about all those 50Hz power grids?
They would cause some beat frequencies, and i'm sure that the phase
relationships are drifting all the time, which would help to make the signal(?)
seem more random.
Henry Spencer
> As one of the people who intermittently "trumpets interstellar ramjets
> as an obvious fallacy" I would be interested in specific references to
> the "ramjet literature" you allude to.
Much of what I'm aware of has been in the "Interstellar Studies" issues
of JBIS over the years.
The "obvious fallacy" part is when somebody discovers the "speed limit"
supposedly imposed when exhaust velocity equals intake velocity. A quick
look at the literature reveals that this was noticed, and circumvented,
long ago: it's a problem only if the kinetic energy of the incoming gases
is lost (e.g. in heating them) rather than stored (e.g. by decelerating
the protons against an electric field, so the exhaust can be re-accelerated
as an "afterburner").
There *is* a problem with the fuel, as you point out. Unless some devious
way can be devised to make an ordinary-hydrogen fusion reaction burn much
more quickly than the natural ones, ramjets don't work well. Devious ways
are not impossible, as are complete departures from fusion, like monopole-
catalyzed annihilation reactions. They just aren't at the stage where we
can definitely say they're possible. (For example, monopole-catalyzed
annihilation works only if monopoles exist, really do catalyze annihilation,
and have a useful catalysis cross-section... and there is indirect evidence
that at least one of these requirements probably isn't met.)
> The only variant that seems barely plausible is bringing along your own
> energy (anti-matter is the only thing with enough energy density) and
> using the scooped hydrogen as reaction mass. And still this only buys
> you slightly more than a factor of two in propellent mass...
Sure about that? With a ramjet, you don't optimize for maximum exhaust
velocity any more -- you get maximum momentum transfer out of a given
supply of antimatter by using it to accelerate lots of reaction mass,
since the reaction mass is free. That's how it looks to me at first
mathematical glance, anyway... it's not an area I'm really up on.
Phil Nelson
>Sorry Dan, but I have to agree with Forrest here too. At the lower
>frequencies, interference is a hell of a problem. You don't just
I hope this is not one of those often asked questions, what about 3000
miles of wire sending gigawatts at 60Hz? I don't know if it's all in phase,
but big chunks of it are. I guess it would be better if all the wire was
running parallel, maybe a little further from the ground, but it is a lot
of power. Does anyone know what the propogation of say, 20-200Hz is?
Assuming these freqs propagate well, it seems like a good place to look.
On the other hand, if the the ETs think as I do, they will bury all their
cable quick, in case there are any bad guys listening. If I were an ET, I
wouldn't want EARTH listening, at least not until I knew for sure that it
wasn't going to turn in to a one-world government determined to spread
socialism to the stars.
I guess I could assume that any one-world bureaucracy would be so self
absorbed and mired in red tape that they would not make it to the stars,
but what if I were wrong?
>-----
>This is a shared guest account, please send replies to
>dbr...@nrao.edu (Internet)
>Dan Briggs / NRAO / P.O. Box O / Socorro, NM / 87801 (U.S. Snail)
Phil Nelson . uunet!pyramid!oliveb!tymix!pnelson . Voice:408-922-7508
The lips of the righteous feed many,
but fools die for lack of sense. -Proverbs 10:21
Steve Nuchia
> I hope this is not one of those often asked questions, what about 3000
>miles of wire sending gigawatts at 60Hz? I don't know if it's all in phase,
>but big chunks of it are. I guess it would be better if all the wire was
>running parallel, maybe a little further from the ground, but it is a lot
>of power. Does anyone know what the propogation of say, 20-200Hz is?
It propagates pretty well, and the overall grid will be phased to
radiate in one direction or another at pretty much any time. The
problems I can see are:
1) It is very regular. There is nothing special about the
frequency. How will the BEMs know it isn't a pulsar?
They could be scratching their ersatz-heads now and churning
out PhD theses proposing models to explain the strange low
frequency emitting, low-mass binary of that otherwise unremarkable
yellow star. Just imagine how they'd ridicule you for suggesting
it might be a planet.
2) The power companies work very, very hard to balance the
currents in the lines, and the voltages are always well balanced.
The field of a balanced pair (or triple) of point sources
falls off as the _qube_ of distance. By the time you get to
the far-field, where you can start considering the grid a
point source (say, 1 AU or so) there will be surprisingly
little power left.
Another way to check the last point is to ask youself how
long the power companies would stay in business if they
were radiating a large fraction of the gigawatts they
move around into space. Engineers make sure most of
it gets where its going, not to the BEMs. Power costs money.
Still, it could easily be the case that the earth's brightest
spectral lines are at 50 and 60 Hz (remember, not all the
world is north america -- some places came even closer to
choosing the optimally fatal frequency than we did). Those
lines are certainly among our narrowest.
The most distinctive signals, the easiest to associate with
intelligent life, are probably the ones from the early
warning and ballistic missile defence radars. Gigahertz
pulse trains, much narrower beams than broadcast entertainment,
steered and deliberately propagated outside the atmosphere.
If they happened to intercept a command sent to a far-off
space craft, like the "wake up" messages occasionally
sent out at very high power levels, that might get their
attention too.
Radio navigation beacons are a candidate too, and will remain
so long after most entertainment broadcasting has gone to cable.
Eventually they will be replaced by satellite systems, whose
transmitters all point down.
Maybe if we all turned on our microwave ovens at the same time ...
Arnold G. Gill
Gehrke,2C-119,7239,ATTBL) says:
>
>Voyager level signals are what I would expect to be listening for
>and those means required to make the search. I don't know of any
>such search going on.
hemispheres with million channel analyzers being operated by The Planetary
Society. I can't remember what size dishes they use, but it is being done, and
all by private funding.
-------
-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=- -=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
| Arnold Gill | |
| Queen's University at Kingston | If I hadn't wanted it heard, |
| BITNET : gilla@qucdn | I wouldn't have said it. |
| X-400 : Arnol...@QueensU.CA | |
| INTERNET : gi...@qucdn.queensu.ca | |
-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=- -=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
Tim Bedding
> I don't expect any
> intelligence in this galaxy to initiate a supernova and then
> sentiently modulate it to get our radio astronomers' attention.
>
sending signals in the _opposite_ direction. Any ETs who studied the SN
closely might notice the signal (or may be smart enough to look for it).
off. As far as I know, nobody has tried looking for radio signals in
the direction of it.
Tim Bedding
Dept of Astrophysics
Uni of Sydney
Daniel Briggs
I remember the suggestion that you're thinking of. It was a little more
recent than 1987, though. About a year ago in the letters section of either
Nature or Science? Anyway, wasn't the point that if a culture was unlucky
enough to be in the immediate vicinity of a SN, (and hence doomed), that
it might want to save the ammassed knowledge of the race. If it did that,
the logical direction to send it would be away from the SN. (BTW, I don't
know how close you have to be to a SN to get cooked, yet far enough away to
have some time to do anything about it. If there is interest, we could
probably figure it out, but it would take some work.) This idea got
generalized to the idea of using SNs as beacons, although I don't see that
the incentive is as strong as in the first case.
To the several people who have pointed out that we are probably radiating
like crazy at 50 & 60 Hz: I didn't think of that. We certainly don't have
anything like a decent telescope at those ultralow frequencies. Still, I
can't think of a good reason why such a thing couldn't be built. My inital
thought is that such a frequency would probably be strongly attenuated by
any sort of plasma. Maybe the solar wind would be a problem? I really
don't know. On a slightly more practical side, you would need an antenna
of rough order the wavelength to see anything. The wavelength of such a
frequency is slightly less than the diameter of the earth. To get any sort
of spatial resolution, you probably would want to fly several of these things
as an interferometer. An Earth-Moon distance gives you a little less than
degree. Maybe you would want to fly a whole bunch of these things in solar
orbit. Not impossible, but rather unlikely in the immediate future. I
remember reading a science fiction story at one point that involved an ultra
low frequency antenna, but can't for the life of me remember what it was.
Daniel Briggs
It turns out that 60 Hz loses, guys, because of galactic background
radiation. There are a lot of natural things out there that are putting
out a fair bit of energy at those frequencies. It seems that as far as
trying to send an intelligible signal is concerned, you are best off
choosing a wavelength between a few millimeters and maybe 30 centimeters.
The rest will get swamped by the natural stuff. (This is, after all, why
we built radio telescopes in the first place.)
I looked in _Radio Astronomy_, 2nd Ed. by John Kraus, and he has a small
interoduction to SETI. It's only about 5 pages long, but convers the basic
mechanics of an interstellar communications link. (He assumes a density of
.01/lyr^3. Which gives about 5000 stars within 50 lyr.) He also points out
that there was a 1985 IAU Symposium #112 entirely devoted to SETI. I'd say
that if you're interested in the subject, this would be a darned good place
to start looking.
J.D. Taylor
All this talking about the Arecibo message and communicating over vast
interstellar distances made me think about one pretty big problem. How do we
know that a totally alien culture could even distinguish and interperet
anything we send to them anyway? Take the Arecibo message as an example. They
have to be listening to the EM spectrum at the frequency which hydrogen
resonates (I think thats right) and have just 27 seconds (or so) to pick up the
message. They then have the problem of decoding it. We hope that these aliens
can order the message as an m by n array of binary digits, look at it and say
"Oh look, there is a nice picture of their solar system and thats what one of
them looks like and yes, that looks like a parabolic dish, probably the one
they used to send the message...."
Lots of chance of this happening eh? We are basically assuming an alien
culture which thinks along the lines we do. They may not. Even if they study
the signal for years and years and arrange it into all possible formats their
culture may well omit a format which we find totally natural. They may be
looking for a particular kind of message ("We come in peace") and might dismiss
the Arecibo message as some sort of unexplainable noise when they cannot decode
it.
Don't get me wrong, I'm all for SETI, space travel and all the rest of it,
but I just think that the whole thing is too full of problems which we may
never be able to overcome. But I still hope I am wrong ;-)
John D. Taylor, Dept. of | "I'm the one who's got to die when its time for me
Elec Eng, Newcastle univ. | to die, so let me live MY life, the way I WANT to"
J.D.T...@uk.ac.newcastle | - Jimi Hendrix.
Christopher Neufeld
>dbr...@nrao.edu (Daniel Briggs) writes:
>
>>I don't happen to know the exact numbers, but we
>>are probably talking tens of thousands of eligible suns within that
>>[50 ly] radius.
>
>There are about 50 star systems containing about 70 stars within 16 ly
>of Earth. If this is a representative sample (a reasonable assumption)
>that gives about 1500 star systems in that 50 ly radius.
>
is a very interesting scale model of the stars within 50 light years.
The recorded voice claims that "all 787 stars within 50 ly are present"
in the spherical model. Beacuse many stars are doubled or tripled up,
this gives considerably fewer than 1500 star systems.
>Dan Tilque -- da...@mrloog.WR.TEK.COM
--
Christopher Neufeld....Just a graduate student | "Like most
neu...@helios.physics.utoronto.ca | intellectuals he is
cneu...@pro-generic.cts.com Ad astra! | intensely stupid."
"Don't edit reality for the sake of simplicity" | Marquise de Merteuil
Forrest Gehrke,2C-119,7239,ATTBL
> And one more followup to my own article.
>
> It turns out that 60 Hz loses, guys, because of galactic background
> radiation. There are a lot of natural things out there that are putting
> out a fair bit of energy at those frequencies. It seems that as far as
> interoduction to SETI. It's only about 5 pages long, but convers the basic
> mechanics of an interstellar communications link.
> darned good place to start looking.
John Kraus is also a well-known antenna expert, especially in the
field of arrays. A study of his book "Antennas" (not real positive
of the title) will help to explain why much of the MW being radiated
by TV stations is NOT being wasted on space.
The W8JK rotatable array, famed among radio amateurs and a forerunner
of the Yagi array, is attributable to John Kraus.
Hi Power pulsed microwave radars are the best source today for a
signal that might be detected at +LY distances. Unfortunately,
these do not operate continuously and when they are operating they
seldom stay on one path for very long. In any military
operation it's downright dangerous to do so for more than a few pulses.
Forrest Gehrke f...@dodger.ATT.COM k2bt
Eric Pepke
anybody who receives those signals. We know that they have figure out
radio. If they hadn't, they wouldn't be listening. There are a whole lot
of things that you have to figure out before you can figure out radio.
Mathematics, for instance. There will no doubt be differences in the way
we percieve major areas of mathematics, but there should be many
similarities.
The most commonly suggested message that would take advantage of this
common knowledge is a sequence of prime integers, encoded as simply as
possible--uniformly spaced pulses counting the integer followed by a
pause. This is easily comprehensible, and such sequences are not
generated by any known natural process.
Now, there are a number of possible extreme theoretical objections to
this, if you really want to reach. Perhaps the BEMs do not understand the
concept of integers. However, even if they naturally think in continua
and live in the liquid phase as part of a group mind, they would have to
figure out that things are countable in the study of physics and chemistry
and, failing that, if only to be able to wind the wire the right number of
times around the ferrite core. Perhaps the BEMs have no interest in prime
numbers and wouldn't recognize one if it kissed them (or whatever it is
that BEMs do). This is also very unlikely. Prime numbers are so
staggeringly useful, and have such tight connections with fundamental
problems in mathematics and algorithm design that it is almost beyond
belief that they would never stumble onto them.
Finally, perhaps C.M. Kornbluth was right, and at some point the machines
they built would acquire self-sustaining capability and the beings would
evolve into happy little technologically advanced morons, forgetting even
the basic principles that once they needed to survive. This one is a
little bit harder to laugh off.
Eric Pepke INTERNET: pe...@gw.scri.fsu.edu
Supercomputer Computations Research Institute MFENET: pepke@fsu
Florida State University SPAN: scri::pepke
Tallahassee, FL 32306-4052 BITNET: pepke@fsu
Disclaimer: My employers seldom even LISTEN to my opinions.
Meta-disclaimer: Any society that needs disclaimers has too many lawyers.
Steve Nuchia
The 10 Hz beat would be interesting. The phase relationship
probably doesn't change very much, since most grids are phase-locked
to a coordinated time base. The time bases are stable enough that
a simple AC line-synchronous mechanical clock is more accurate than
most quartz crystal time bases (ie, watches). As long as they don't
lose power.
Lord Snooty @ The Giant Poisoned Electric Head
> Arthur C. Clarke (amazing how often his name comes up in this group,
> isn't it?). That proposal was for one of fairly modest wavelength -- a
> few kilometers, if I remember correctly.
This ULF antenna - it wouldn't have been a conducting version of the
Space Tether, another of Arthur's bright ideas, would it?
Just curious...
--
...........................................................................
Andrew Palfreyman and...@dtg.nsc.com Albania during April!
Dan Tilque
>I
>remember reading a science fiction story at one point that involved an ultra
>low frequency antenna, but can't for the life of me remember what it was.
You're probably thinking of a proposal for one in IMPERIAL EARTH by
Arthur C. Clarke (amazing how often his name comes up in this group,
isn't it?). That proposal was for one of fairly modest wavelength -- a
few kilometers, if I remember correctly.
---
Dan Tilque -- da...@mrloog.WR.TEK.COM
KIRSTEIN DALE
What ever happened to the barium drop experiment launched on
Pegsat? Was the first cannister dropped on schedule, did anyone see it,
and what is the schedule for the second cannister drop
i---------------------------------------------------------------------------
this .sig left intentionally blank
Steve Nuchia
>It's not entirely that grim. We can determine some things for sure about
>anybody who receives those signals. We know that they have figure out
>radio. If they hadn't, they wouldn't be listening. There are a whole lot
Maybe they are really big, or really small, and can see at those wavelengths.
Maybe they build radiotelescopes like we build optical ones. Aren't
maybes fun?
>Now, there are a number of possible extreme theoretical objections to
>this, if you really want to reach. Perhaps the BEMs do not understand the
>concept of integers. However, even if they naturally think in continua
If you understand radio, you understand integers. Harmonics are
just too basic a pheonomenon to miss. The significance of primes
might take a little longer, but if they are looking at the signal
in the time domain they will eventually suspect it is of sentient
origin. Fibonacci numbers are about the most non-obvious number
sequence to occur naturally here, chances are they won't have
any biology or geology that generates more than, say, a dozen primes.
Geometry is probably a better place to look, nature isn't particularly
fond of right angles but anyone doing hard-matter engineering will
recognize Pythagoryan triples, and probably the face-orders of the
regular solids. Could try the atomic number/atomic weight pairs
for the stable isotopes.
>Finally, perhaps C.M. Kornbluth was right, and at some point the machines
>they built would acquire self-sustaining capability and the beings would
>evolve into happy little technologically advanced morons, forgetting even
>the basic principles that once they needed to survive. This one is a
>little bit harder to laugh off.
Like, what planets do we know of where that's happening right now?
Sigh.
Comment on carbon/silicon life: Don't forget that other temperature
and pressure regimes are possible. We know staggeringly little
about really high pressures, and less than we'd like about very
low temperatures. Exotic combinations? Forget it. Just about
anything could happen out there, but if it did we'd have a heck
of a time finding anything in common on which to base a meaningful
relationship.
One new theme to emerge in science in the last few decades is
that self-organizing systems aren't as unusual as we thought.
The fact that all four of our gas giants have bands and great
spots is a good example. (I think -- Neptune does, anyway.)
At one of my clients' offices there is a small poster, apparently
once part of a magazine ad. It is a picture looking across a
mountain valley from the top of a very high, very rocky slope,
looking over the handlebars of a motorcycle. The caption:
"Never rule anything out."
Words to live by.
Jim Meritt
Opposite direction relative to what?
Why use radio waves? You have a very powerful emitter centrally located
already. Why not just dump a mass into the central star of something
that clearly does not belong there? A line for technium in the
spectroscopic examination of a star should be a REAL indicator of
a technological civilization nearby.
Any ideas on how much of what could be used?
"In these matters the only certainty is that nothing is certain"
- Pliny the Elder
These were the opinions of :
j...@aplcen.apl.jhu.edu - or - j...@aplvax.uucp - or - meritt%aplvm.BITNET
Dan Tilque
>
>Hi Power pulsed microwave radars are the best source today for a
>signal that might be detected at +LY distances. Unfortunately,
>these do not operate continuously and when they are operating they
>seldom stay on one path for very long. In any military
>operation it's downright dangerous to do so for more than a few pulses.
You're confusing wartime operation with peacetime operation. In
peacetime, large military radars are fixed in location just like
civilian radars and operate continuously. These are basically the ones
that watch borders for violations as well as those used for routine
training missions. However, since some of these are natural targets
(NATO assumes that all it's radar sites in W. Germany will be lost in
the first few minutes of war) there are backup systems which would
operate like you say.
Now the largest military radars (as I said in an earlier post) are
large phased array systems which watch for ICBMs. The U.S. has five of
them (or did that last I heard). Three of these, located in Alaska,
Greenland and England, look for missiles launched from the Soviet
Union. The other two are located on the U.S. east and west coasts and
look for submarine launches missiles. These last two use over the
horizon radar (using a troposcatter effect, I think) so much or most of
their emissions probably do not escape to space.
The Soviet Union has even more large phased array radars than the U.S.
In order to detect ICBMs, these radars would have to have ranges of
about 2 or 3 times that of regular radars. That means that (assuming
similar receiver sensitivity) they would use 16 to 81 times the power
of ordinary radar (which typically run about 2-5 MW).
Source: U.S. Air Force training classes (unclassified) plus thinking
about what they told us (not necessarily an encouraged activity).
Robert W. Spiker
>
>Why use radio waves? You have a very powerful emitter centrally located
>already. Why not just dump a mass into the central star of something
>that clearly does not belong there? A line for technium in the
>a technological civilization nearby.
>
>Any ideas on how much of what could be used?
>
atmospheres of certain supergiant stars, known as S stars. Technetium
(which has a half-life of oh, 100,000 years) is produced by the
s-process in the star's atmosphere. The s-process forms elements
heavier than iron by slow capture of free neutrons onto iron and similar
elements. It's something we know a good bit about, without having to
resort to ETs to explain it.
(There's also something called the r-process which is a little bit
trickier, but hey, no one's got all the answers.)
Robert W. Spiker, UVa Dept. of Astronomy
--------------------------------+ It is truly written that a man has five
rw...@astsun.astro.virginia.edu | times as many fingers as ears, but only
or @bessel.acc.virginia.edu | twice as many ears as noses.
John Sahr
More thoughts on a: why we don't hear anything, and b: why no one may be
hearing us, and c: a possible explanation to Fermi's "paradox."
Humans have been radiating a fair amount of electromagnetic power for
the last 50 years or so. As our electronics art has improved, we have
begun to rely more and more on "tricky" schemes to efficiently use
bandwidth and power.
For example, AM radio is a mode that carries around its own local
oscillator, in the sense that the carrier frequency component has a
substantial fraction of the transmitter power. It also has two
technically redundant sidebands. Because better, stable oscillators
can be built, and tricky filter or mixing circuits can pass 2.4 khz at
several MHz operating frequency, single sideband (no carrier, one
sideband) has become a popular mode for some kinds of radio
communication.
Another relatively new idea is to use "spread spectrum" techniques to
aid interference reduction (and perhaps to impede detection). This is
roughly related to code-domain multiplexing techniques as well.
Even in radar waveforms, which may have repetition times of a few
milliseconds, the actual transmitted wave may not be a simple pulse.
Incoherent scatter radars use fairly elaborate techniques of frequency
hopping, "random" pulse spacing, and "pseudo-random" codes to improve
their operation, while increasing the bandwidth and complexity of the
signal. I don't know much about military radars, but I can imagine
several reasons for using complex waveforms there as well.
The point is, there are advantages to using modulation techniques
which make our radio emissions look more and more noiselike, which is
one way of expressing a fundamental result of Claude Shannon's work.
The abundant radio power we emit, while growing larger in absolute
size, is also growing in complexity. Our own signals may not seem
particularly interesting, because they look like everything else.
Clearly this is not literally the case, but on the average, I suspect
that our "energy per radiated bit" is decreasing, even if our total
radiated power is increasing. That the "energy per bit" is decreasing
makes the detection of Earth-origin signals harder, and is ultimately
more relevant than the total power we radiate.
Basically, my postulate is this: A civilization "discovers" radio, and
spends fifty years learing how to occupy the spectrum and make high
power transmitters. As their electronic prowess increases, so does
their net irradiated power, but the "energy per bit" drops
dramatically, so that two hundred years after the discovery of radio,
the civilization is transmitting signals that are largely
indistinguishable from noise, rendering them rather invisible against
the background of stars. Thus, rather than having "enlightened"
civilizations continuously radiating signs of their presence, they may
in fact only emit a two hundred year thick shell of "detectibility."
This would allow some very old civilizations to be quite near to us,
without being spotted by us, and also without any specific attempts to
hide from us. Suppose that at 50 ly there is a 10,000 year old "radio
capable" civilization. Their "shell of detectibility" will have long
passed us, while ours will be just reaching them.
Pure speculation; food for argument, that's all.
--
John Sahr, | Electrical Engineering - Space Plasma Physics
jo...@alfven.spp.cornell.edu | Cornell University, Ithaca, NY 14853
John Sahr
>
To the extent possible, the power grids are "in phase" across the USA.
However, 60 Hz probably isn't such a great place to go looking for
radio sources.
1) 60 Hz is much lower than the "plasma frequency" of the ionosphere
(greater than about 1 MHz everywhere on the Earth).
2) High power lines come in triples for three phase power, and the
separation is much less than the wavelength at 60 Hz, so that the
"radiated" energy field probably falls off faster than inverse square
(inverse cube or maybe even fourth) even in the absence of an
ionosphere.
3) It is very noisey at low frequencies beneath our ionospheric canopy.
4) It is also fairly noisy above, as natural geophysical processes generate
"TMR," or Terrestrial Myriametric Radiation above the ionosphere.
Dan Tilque
>Still, it could easily be the case that the earth's brightest
>spectral lines are at 50 and 60 Hz (remember, not all the
>world is north america -- some places came even closer to
>choosing the optimally fatal frequency than we did).
Is this a figure of speach or is some low frequency actually fatal?
John Sahr
[]
>Now the largest military radars (as I said in an earlier post) are
>large phased array systems which watch for ICBMs. The U.S. has five of
>them (or did that last I heard). Three of these, located in Alaska,
>Greenland and England, look for missiles launched from the Soviet
>Union. The other two are located on the U.S. east and west coasts and
>look for submarine launches missiles. These last two use over the
>horizon radar (using a troposcatter effect, I think) so much or most of
>their emissions probably do not escape to space.
OTH radars use ionospheric refraction to see "over the horizon."
Tropospheric scatter has its uses, but the big OTH radars don't rely
on it. The one I know about is several MW continuous power, using an
FM ramp to determine range (and other techniques to determine Doppler
shift). It has the ability to pick an operating frequency from 3-20
MHz or so to pick optimum "skip."
[]
>In order to detect ICBMs, these radars would have to have ranges of
>about 2 or 3 times that of regular radars. That means that (assuming
>similar receiver sensitivity) they would use 16 to 81 times the power
>of ordinary radar (which typically run about 2-5 MW).
Not necessarily true. For an OTH radar, the signal return is
currently limited by clutter, notably ionospheric irregularities.
Thus, increasing the transmitter power just increases the clutter.
Even for the DEW-line radars, it may be pointless to increase the
power. Consider that the "E-region horizon" is only 1100 km away (you
can see a 100 km tall mountain from 1100 km), and ICBM trajectories
are fairly low. Also, there can be a lot of ionospheric junk
happening during magnetic disturbances, and these radars are bothered
by such clutter. The thermal fluctuation of the ionospheric plasma is
enough to cause noticable backscatter ("incoherent scatter"), even
when the ionosphere is "calm."
High altitude bombers are only up about 15 km or so, and low altitude
bombers can be very low indeed. It is pointless to increase the
transmitter power to see them any better (unless you want more detail,
perhaps).
>of ordinary radar (which typically run about 2-5 MW).
2-5 MW is a pretty boss transmitter, no matter what. There are
certainly radars with much lower power. The CUPRI radar, for upper
atmospheric irregularity studies, has a transmitter (peak) power of
about 40 kW, and I have heard of two radar systems that used only
about 1 kW.
Radio amateurs familiar with "meteor scatter" accompish hard target
scatter at hundreds of kilometer ranges with a few hundred watts of
transmitter power.
Henry Spencer
>>In order to detect ICBMs, these radars would have to have ranges of
>
>...Even for the DEW-line radars, it may be pointless to increase the
>power. Consider that the "E-region horizon" is only 1100 km away (you
>can see a 100 km tall mountain from 1100 km), and ICBM trajectories
I think you're a little confused. First, DEW-line radars, ICBM-warning
radars, and OTH radars are three entirely different systems. The DEW
line is an old bomber-warning network. Second, both DEW and the BMEWS
ICBM-warning radars are ordinary microwave line-of-sight radars that
largely ignore the ionosphere. Third, ICBM trajectories are quite high
in most cases, as "depressed trajectory" paths reduce payload (although
they are considered in defense planning because they also reduce warning
time).
--
If OSI is the answer, what on | Henry Spencer at U of Toronto Zoology
Earth could be the question?? | uunet!attcan!utzoo!henry he...@zoo.toronto.edu
Paul Dietz
>To the extent possible, the power grids are "in phase" across the USA.
>However, 60 Hz probably isn't such a great place to go looking for
>radio sources.
>
>1) 60 Hz is much lower than the "plasma frequency" of the ionosphere
>(greater than about 1 MHz everywhere on the Earth).
The plasma frequency of a plasma with electron density N (in units of
eletrons per cubic meter) is 56.35 N^{1/2} Hz.
The electron density of the solar wind is roughly around 10^6 m^-3; of
interstellar space, perhaps 10^4-10^-6 m^-3.
Looking for low frequency leakage from alien power grids is,
therefore, futile -- space is opaque at those frequencies.
Paul F. Dietz
di...@cs.rochester.edu
Paul Dietz
> interstellar space, perhaps 10^4-10^-6 m^-3.
I meant 10^6 m^-3.
Paul F. Dietz
di...@cs.rochester.edu
Steve Nuchia
>>spectral lines are at 50 and 60 Hz (remember, not all the
>>choosing the optimally fatal frequency than we did).
>Is this a figure of speach or is some low frequency actually fatal?
One thing about power grids -- they reach into homes, where they
occasionally come into contact with children, pets, electricians,
whatnot. Different frequencies of AC, in contact with the body,
differ in probability of serious injury. 60 Hz is just above
the saddle in the 50% fatal voltage level curve, 50 Hz is closer.
The fact that they also use twice the voltage we do for standard
residential service is probably a more important factor in the
different electrocution statistics, which I don't have handy
(and which are skewed by the fact that the Europeans *know*
getting shocked is more dangerous than we do.)
If we had it to do over again, 400 Hz would have been a much better choice.
At the time though, the extra complexity in the rotating machinery
placed a low upper bound on the frequency. Today's utility generators
are plenty complex; wouldn't be a big deal. Changing over would.
Nothing about low frequency *radiation* is particularly hazardous.
To include a shred of space relevance, which power distibution
scheme is currently in vogue for the space station? I wonder
if shock hazard is one of the things they are considering in
making the choice? The RF they were proposing is (somewhat)
less hazardous in direct contact, but it is a lot more likely
to find a leakage path to the case, than DC or 400 Hz. I don't
know which is the best choice, but I suspect it's 400 Hz.
Given that the human specimens selected for space duty are in
superb condition, and that they are isolated from communicable
diseases once on station, electrocution has to rank pretty high
on the list of mission-critical medical contingencies. Contamination
of the air supply, eg by a small fire, and burns from experiments
or fires are about the only other things that are likely (ignoring
EVA for the moment). Minimizing electrocution risk would seem to be
worth a few kilograms mass.
Arnold G. Gill
>
>whatnot. Different frequencies of AC, in contact with the body,
>differ in probability of serious injury. 60 Hz is just above
>the saddle in the 50% fatal voltage level curve, 50 Hz is closer.
guess I was always under the mistaken impression that the dangers of
electricity have a lot more to do with voltage and the amount of current that
that voltage can pump through a person (0.1A is supposedly the fatal current
level). What effect can frequency have?
Henry Spencer
>To include a shred of space relevance, which power distibution
DC. For a while they were planning an insane 20kHz system, which would
have meant custom-building everything from wiring to lightbulbs. When
they abandoned that as a cost-cutting measure, I guess it would have been
humiliating to admit the mistake, so they went straight to DC instead of
using the aviation standard of 400Hz AC.
Paul Dietz
>>The plasma frequency of a plasma with electron density N (in units of
>>eletrons per cubic meter) is 56.35 N^{1/2} Hz.
>
>The formula above yields the radian-frequency. The plasma frequency
>in Hz is
>
> 9 sqrt(N) Hz
Ack; quite right. However, my point stands: the plasma frequency
in space is somewhere in the kilohertz, so 60 Hz radiation doesn't
propagate.
Paul F. Dietz
di...@cs.rochester.edu
John Sahr
[]
>>1) 60 Hz is much lower than the "plasma frequency" of the ionosphere
>>(greater than about 1 MHz everywhere on the Earth).
>
>The plasma frequency of a plasma with electron density N (in units of
>eletrons per cubic meter) is 56.35 N^{1/2} Hz.
The formula above yields the radian-frequency. The plasma frequency
in Hz is
9 sqrt(N) Hz
(The "9" is approximate, should be 8.977297....)
Radian frequencies should be marked as 1/sec or sec^-1, which is not
the same as Hz. For a nice intro to plasma physics, see the text
"Introduction to Plasma Physics and Controlled Fusion" by Francis
Chen (2nd edition is MKS, 1st edition is cgs).
Steve Nuchia
>>the saddle in the 50% fatal voltage level curve, 50 Hz is closer.
> You are going to have to explain this one to me a little bit better. I
No problem. I'm working from memory though.
>guess I was always under the mistaken impression that the dangers of
>electricity have a lot more to do with voltage and the amount of current that
>that voltage can pump through a person
The critical value, at low power level, is the current through the heart.
At high power levels it just cooks flesh, and you may die of shock even
if the current path doesn't involve the nervous system. Shocks not involving
the heart can also cause convulsions, which may be dangerous in their
own right.
> (0.1A is supposedly the fatal current level)
A tenth of that through the torso will usually be fatal, less sometimes is.
Your skin has a DC resistance in the megohms when dry, but it is
rather thin. Once inside the bulk resistance varies depending on
the tissue involved, but is rather low. The skin resistance can be
broken down by puncture, charring, capacitive coupling, or just by
having enough voltage.
> What effect can frequency have?
Frequency has an effect on how well the energy is coupled to you and
how much current is associated with your capacitance relative to ground.
At very high frequencies the skin effect (referring to the outer
portion of a conductor, not people-skin) causes most of the current
to flow near the surface of the body, avoiding the heart. Neither
of these effects is significant in the 10-400 Hz region. In that
region the important effect seems to be the match between the
frequency of the shock and the natural frequencies of the heart
heart's control "circuits". It is a second-order effect, and
the curve I alluded to is rather shallow, and it moves around
depending on individual factors, but it is statistially significant.
If anybody happens to know of a good reference I'd appreciate
a pointer too.
Forrest Gehrke,2C-119,7239,ATTBL
> The 10 Hz beat would be interesting. The phase relationship
> probably doesn't change very much, since most grids are phase-locked
> to a coordinated time base.
If you think a beat could be detected any distance out beyond the
earth, build yourself a low frequency receiver and listen for it
right here on Terra Firma. You will be rewarded with more racket
than you ever dreamed possible, but no 10 hz beat. I have read
somewhere that at any moment there are several thousand lightning
storms going on somewhere on earth. You will think you are
hearing them all.
I know that receivers that operate below the standard broadcast
band are not plentiful, but if you ever have the chance to
listen to one you will learn that those wavelengths are a
far piece from what you hear in the FM band. A lightning bolt
has to be within visible distance to hear it at these
short wavelengths.
As to radiation from high power transmission lines, consider for
moment how much radiation you would get from an antenna array whose
element current vectors at all points add up to virtually zero. Why
would you expect to get any substantial far field radiation from such
an antenna?
Forrest Gehrke f...@dodger.ATT.COM
Dan Tilque
>da...@mrloog.WR.TEK.COM (Dan Tilque) writes:
>[]
>>Now the largest military radars (as I said in an earlier post) are
>>large phased array systems which watch for ICBMs. The U.S. has five of
>>them (or did that last I heard). Three of these, located in Alaska,
>>Greenland and England, look for missiles launched from the Soviet
>>Union. The other two are located on the U.S. east and west coasts and
>>look for submarine launches missiles. These last two use over the
>>horizon radar (using a troposcatter effect, I think) so much or most of
>>their emissions probably do not escape to space.
>
>OTH radars use ionospheric refraction to see "over the horizon."
That's right. I was confusing radar with an over the horizon microwave
comm system.
>>In order to detect ICBMs, these radars would have to have ranges of
>>about 2 or 3 times that of regular radars. That means that (assuming
>>similar receiver sensitivity) they would use 16 to 81 times the power
>>of ordinary radar (which typically run about 2-5 MW).
>
>Not necessarily true. For an OTH radar, the signal return is
>currently limited by clutter, notably ionospheric irregularities.
>Thus, increasing the transmitter power just increases the clutter.
Henry Spencer has already pointed out that the ICBM detection radars are
line of sight called BMEWS (for Ballistic Missile Early Warning System).
I couldn't remember the acronym when I made my post (thanks, Henry).
It's these guys which are the large phased array systems. There are three
of them which cover a huge arc of territory from the Gulf of Alaska,
across the North American Arctic and over to Europe. They have to be
powerful to do that.
The OTH systems were called "Pave Paws" the last I heard. That's a code
word, they may have assigned an acronym to them by now. These look for
submarine launched missiles.
>Even for the DEW-line radars, it may be pointless to increase the
>power.
The DEW line radars are probably about as powerful as other USAF airplane
detection radars, i.e. 2-5 MW.
To get this back on the topic of aliens detecting these from out there --
most of these powerful radars (BMEWS, DEW, etc and Soviet counterparts)
are primarily located in the northern hemisphere, in fact, many are well
north of 50 degrees north. Thus aliens from stars in that section ofthe
sky are much more likely to detect us than other aliens.
>>of ordinary radar (which typically run about 2-5 MW).
>
>2-5 MW is a pretty boss transmitter, no matter what.
2-5 MW is what most long range Air Force search radars typically operate
at. You must remember that the enemy planes are not necessarily going to
have their IFF/SIF turned on.
>Radio amateurs familiar with "meteor scatter" accompish hard target
>scatter at hundreds of kilometer ranges with a few hundred watts of
>transmitter power.
Remember, radar operates under an inverse 4th power function.
Henry Spencer
>Radio amateurs familiar with "meteor scatter" accompish hard target
>scatter at hundreds of kilometer ranges with a few hundred watts of
>transmitter power.
Unless they're doing something very different from the meteor-bounce
communications systems in use for other purposes -- no, meteor-bounce
is not all amateur these days -- they are getting bounces off the
ionized trails, not the objects themselves.
Fraering Philip
> Remember, radar operates under an inverse 4th power function."<
Yes and no.
In terms of the radar set looking at something else, it is an inverse
4th power function, unless the radar wave hits an edge, at which point
it is an inverse square function (I forget whether there is a factor of
1/sqrt(2) in there or not.)
For being detected elsewhere, the radar can be considered a transmitter
with the power dropping off under an inverse square function, focusing
and phase steering permitting, of course.
Philip Fraering
dlbr...@pc.usl.edu
Avery Ray Colter
>At one of my clients' offices there is a small poster, apparently
>once part of a magazine ad. It is a picture looking across a
>mountain valley from the top of a very high, very rocky slope,
>looking over the handlebars of a motorcycle. The caption:
> "Never rule anything out."
Or, as my former roommate, a mountain bike racer in the team called FAT
(Finish Alive Team) would say:
FAT FACT #1:
The fastest way to accelerate:
DESCEND WITH CONVICTION!
-----------------------
--
Avery Ray Colter Internet: av...@well.sf.ca.us | {apple|hplabs}!well!avery
o/~ Mama, mama, mama, keep those skinny girls at home,
o/~ `Cause this skinny boy wants a BIG FAT BLONDE! - The Rainmakers
Dan Tilque
>da...@mrloog.WR.TEK.COM (Dan Tilque) writes:
>>
>>Remember, radar operates under an inverse 4th power function.
>>
>radar detectors here, so only a r^2 law should apply, *nicht wahr*?
Nicht. We were talking how much power a radar emits. However, someone
else claimed that reflection off corners of the target reflected changed
the entire power function as inverse square. I'm not sure what causes
this unless he meant an inside corner, i.e. corner reflector.
Badger BA 64810
>
>Remember, radar operates under an inverse 4th power function.
>
radar detectors here, so only a r^2 law should apply, *nicht wahr*?
----
Bernard A. Badger Jr. 407/984-6385 |"Get a LIFE!" --- J.H. Conway (Just joking! :-)
bba...@x102c.ess.harris.com |Buddy, can you paradigm?
bbadger%x1...@trantor.harris-atd.com |'s/./&&/g' Tom sed expansively.
Paul Johnson
>>>the saddle in the 50% fatal voltage level curve, 50 Hz is closer.
>> You are going to have to explain this one to me a little bit better. I
>>electricity have a lot more to do with voltage and the amount of current that
>>that voltage can pump through a person
>
>The critical value, at low power level, is the current through the heart.
>At high power levels it just cooks flesh, and you may die of shock even
>if the current path doesn't involve the nervous system. Shocks not involving
>the heart can also cause convulsions, which may be dangerous in their
>own right.
>
>> (0.1A is supposedly the fatal current level)
>
The following is abstracted from an Safety Guidance Note on the Safe
Use of Electricty in my company. Any errors, spelling mistakes etc
are mine, and the following does not necessarily represent company
policy or opinion.
When an electric current passes through the body, muscles are
stimulated and may react in one of two ways:-
1) A violent reaction in which leg or arm straightens and throws the
person across the room.
2) Muscles may contract and become rigid, itghtening the grip and
reducing contact resistance. If chest muscles or nervous system is
involved, breathing can be stopped and the heart affected.
In a series of experiments, 150 male volunteers had various currents
passed from hand to hand.
Below 1 mA, 50Hz A.C., the current is virtually undetectable. Above
this level pain increases to be very unpleasant at 9 mA. Most healthy
men can let go at this point, but none could release above 24 mA.
At 20mA or greater, lung muscles contract and breathing stops,
although it may restart naturally if the current is stopped quickly.
By this stage there is also great pain with burns.
Experiments with animals from guinea-pigs to cows have revealed a
linear relationship between weight and current required to cause
ventricular fibrillation. There was also a relationship between the
time between current (I, milliAmps) and time (T, Seconds). Assuming a
body weight of 50 kg (8 stone) this is
I = 116/sqrt(T)
In other words, 50mA for 5 seconds would probably cause fibrillation.
It was easier to cause fibrillation than to stop the heart completely.
(Note: a fibrillating heart is harder to restart than a completely
stopped one: the fibrillation must be stopped and the heart then
restarted.)
With frequencies either side of 50 Hz there is a marginal imprevement
in the let-go current. With DC, muscular control is not lost so that
the victim has a better chance of escape, althought the greatest pain
is experienced when breaking the circuit.
The resistance of the human body varies considerably. A good contact
with bare skin, hand to hand or hand to foot gives about 2500 ohms at
25 volts, reducing to 1000 ohms at 250 volts with a minimum asymptotic
value of 650 ohms. This suggests that a maximum safe voltage is only
35 volts, although of course contacts are usually glancing rather than
full grip, and shoes and clothing increase safety considerably. Never
the less, potential danger is present.
Reference:
M.P. Smith: Electrocuiton, Its Effects and Prevention. J. Naval
Science, Vol 3, No 1, Jan 1977, P39-44.
--
Paul Johnson UUCP: <world>!mcvax!ukc!gec-mrc!paj
--------------------------------!-------------------------|-------------------
GEC-Marconi Research is not | Telex: 995016 GECRES G | Tel: +44 245 73331
responsible for my opinions. | Inet: p...@uk.co.gec-mrc | Fax: +44 245 75244