On Sun, Feb 4, 2018  Lawrence Crowell <goldenfieldquaterni...@gmail.com>

*​> ​a fairly weak signal in the megawatt range that we generate in most
> transmissions they will only reach 10 or a few 10s of light years out.*

The Arecibo transmitter itself may have been in the megawatt range but when
used in conjunction with the 300 meter dish it produces a radio beam
equivalent to a omni
directional signal of 2*10^13 watts; and this energy is concentrated in a
frequency range far far narrower than any natural radio
; in that
​ beam and in that​
very narrow frequency band it would be the brightest thing in the galaxy.
You might argue that the reason we haven't seen such a thing is that ET
just doesn't have the resources to do
, and that could indeed be the explanation
​, ET is just too primitive​
. And that is exactly what worries me.

2*10^13 watts
is not a impressive amount of power, even we can do it (in a narrow beam)
and we've only been at it for about a century. After all just one star, our
sun for example, produces 4*10^26 watts, so for a civilization hundreds of
years ahead of us, not to mention millions or billions of years ahead,
2*10^13 would should be a ridiculously tiny amount of power.

And don't forget lasers, I was just reading in the January 26 2018 issue of
Science about a Laser in China called "SULF" that has produced brief pulses
of light that contain 5.3*10^15 watts of power, they hope to ramp it up to
10^16 watts in a few months and 10^17 watts by 2023.

> ​> ​
> *The magnetic field of a pulsar has as much energy per cubic meter as the
> sun produces in light within a few minutes.*

​Interesting fact: according to the Science article mentioned above the
2023 Chinese Laser should be able to produce a light intensity of 10^24
watts per square centimeter; ​of course it can only do that for a
trillionth of a second or so and only over a spot 3*10-6 meters across, but
that intensity over a square about 5 inches on a side would equal the
entire power output of the sun.

​> ​
> A gigawatt signal then reaches about 3 to 4 times the distance. The Debye
>  attentuation is exponential, so increasing power by 10 will roughly only
> double the distance.

​I was under the impression ​
Debye attenuation
​concerns X rays ​not radio waves.

​> ​
> A terrawatt signal could then reach 100s of light years, and so forth.

​I think a terrawatt signal could do a lot better than that. We can still
communicate with​

​the Pioneer spacecraft, it is about one light day away and it only has a 8
watt transmitter connected to a tiny low gain dish only 9 feet across.
That's 8 watts, not megawatts or even kilowatts, just watts.​ And being
that close to a star the signal had to punch through more gas and dust than
in a typical volume of interstellar space, and yet we can still hear it.

>  if ETI is say 100 light years away and they are casually emitting
> megawatt radio frequency radiation

If megawatts is the best they can do then ET is slightly less advanced than
we are, and in a 13.8 billion year old universe having 2 such civilizations
​ ​
100 light years away and being that close technologically would be a
astronomically unlikely coincidence unless intelligence is
​very ​
common in the universe but it always destroys itself when it advances to
our level.

​> ​
> It could also mean the nearest ETI is on the other side of the Virgo
> cluster some 25 million light years away. I am not an exponent of ideas
> about faster than light travel and at those distances radio transmitters
> are simply too weak.

​Forget radio, if a civilization a million years ahead of us was only 25
million light years away if would be easily detectable with a telescope you
could buy at the toy section at Wall-mart, but even our largest telescopes
can't find a hint of large scale engineering. I must conclude that no such
super advanced civilization exists and I can only come up with 2 credible
theories to explain why. I hope the explanation is simply that we're the
first. I fear the explanation is civilizations always destroy themselves
when they get to our level.    .      ​

> ​> ​
> So at a distance of that sort and on our past light cone we will never
> hear them. We can't get there and they can't get here.

Now you're talking about tens of billions of light years, when I say we're
the first I mean the first in the observable universe.

> ​> ​
> The broadcast and detection of radio signals between collectives of
> intelligent life on different planets I think will have to be due to
> deliberate actions.

I agree, eavesdropping on communication is not likely to be important. And
actually it became harder for
 to detect a civilization on Earth
June 12 2009. On that date all TV
transmitters in the USA switched over from analog to digital. This will
make things more difficult for a hypothetical ET for two reasons:

1) Digital transmitters use far less power than the analog variety.

2) A analog TV signal obviously came from a technological civilization, but
if you don't know the particular compression codec used by digital
broadcasters in the USA the signal would almost look like white noise. The
compression is not perfect that's
why I said "almost".

> *​>​ If we receive something it will be intentional, and if we want to
> hail putative ETIs out there we have to be constantly sending collimated
> high powered radio signals.*

​The best signal we could sent ​
​ET that Earth is an interesting place would be for us to do some large
scale engineering, a ​Dyson sphere might be a good place to start.

John K Clark


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