APOD: A Message From Earth (2012 Feb 19)

[skyhound]

"They say" that everything anyone ever said is still out there resounding in the atmosphere. But can we still hear it? No. Can we hope to build a machine that could detect what was said a thousand years ago above the noise? No. This message is just like those sounds. By the time it reached any being capable of detecting it, the signal would be just as lost as the words spoken last week in Springfield are. This is the way SETI is. Its all pie in the sky and they never own up to the limitations. When pressed, they invoke the vague idea that some super beings will have a solution that makes it all work. I have nothing against what these people do, in principle, but I do think they are rather less than honest in how they sell it to the public.

[neufer]

"They say" that everything anyone ever said is still out there resounding in the atmosphere. But can we still hear it? No. Can we hope to build a machine that could detect what was said a thousand years ago above the noise? No. This message is just like those sounds. By the time it reached any being capable of detecting it, the signal would be just as lost as the words spoken last week in Springfield are. This is the way SETI is. Its all pie in the sky and they never own up to the limitations. When pressed, they invoke the vague idea that some super beings will have a solution that makes it all work. I have nothing against what these people do, in principle, but I do think they are rather less than honest in how they sell it to the public.

Both sending and attempting to receive stimulates the imagination
and it is no more harmful or wasteful of man hours than prayer.

[Chris Peterson]

"They say" that everything anyone ever said is still out there resounding in the atmosphere. But can we still hear it? No. Can we hope to build a machine that could detect what was said a thousand years ago above the noise? No. This message is just like those sounds. By the time it reached any being capable of detecting it, the signal would be just as lost as the words spoken last week in Springfield are.

Not so. While the Arecibo transmission was largely symbolic, most of the actual SETI work has surveyed fairly close stars- typically within a few hundred light years. At that range, any directed radio transmission (which is what SETI hopes for) could be above the noise floor of our instruments. Directed transmissions- whether from some other star, or made by us- are likely to be above the background noise for tens of thousands of light years. The limiting noise source will be the detector itself, and that can be arbitrarily low, depending only on technology, not physical limitations.

That is very different from vibrations in the atmosphere, which are fundamentally undetectable regardless of technology.

[neufer]

<<Directed transmissions- whether from some other star, or made by us- are likely to be above the background noise for tens of thousands of light years. The limiting noise source will be the detector itself, and that can be arbitrarily low, depending only on technology, not physical limitations.

But eventually the limiting noise source will be the 3K background.

What Is the Water-Hole?

<<The electromagnetic spectrum, as viewed from Earth, is a noisy place. Low frequencies are plagued by galactic noise, primarily due to synchrotron radiation (charged particles spiraling through our Sun's and our planet's magnetic fields). High frequencies are subject to quantum-effect emissions, and the whole continuum experiences a 3 Kelvin background radiation level from the residual radiation of the Big Bang. These natural radiation sources limit our ability to detect artificial emissions. In addition, the Earth's own ocean of air generates spectral absorption and emission lines which draw a further curtain across our sky. Fortunately, there are a few relatively clear windows on the cosmos. Our eyes evolved to operate in one such window, the optical spectrum. It is this window which first allowed us to observe the stars and planets. Another clear spot is in the microwave region, between about 1 and 10 GHz.
----------------------------------------------------------------------------

• Water-Hole spectrum

[Note: Galaxies are "radio quiet" at frequencies above 2 GHZ but
earthbound radio telescopes must avoid the broad tropospheric 22 GHZ water vapor line.]
----------------------------------------------------------------------------
Within this so-called Microwave Window, photons (the substance of electromagnetic communication) travel relatively unimpeded through the interstellar medium, at the speed of light. This is, as far as we know, the fastest possible speed, making photons the fastest spaceships known to man. Thus the Microwave Window, where natural noise is at a minimum, is a favored region for conducting radio astronomy research, including SETI.

Toward the bottom of the microwave window, radiation from the precession of interstellar hydrogen is clearly heard in our receivers at a frequency of 1420.40575 MHz (corresponding to a wavelength around 21 cm). The Hydrogen Line, first detected by Ewen and Purcell at Harvard University in 1951, provided us with our first direct evidence that space is anything but an empty void -- it is a veritable chemistry set. We hypothesize that any civilization in the cosmos which possesses radio astronomy knows about the Hydrogen Line. Since there is roughly one hydrogen atom per cubic centimeter of space, the combined voices of countless hydrogen atoms produce a raucous chorus. The very first SETI studies were conducted near the Hydrogen Line, and today it still looks like a logical place to seek deliberate beacons from beyond.

Just a little way up the spectrum, near 1660 MHz (a wavelength of 18 cm), a team of scientists at MIT Lincoln Labs detected in the 1960s a cluster of radiation lines from interstellar hydroxyl ions (OH). Like the Hydrogen Line, the Hydroxyl Lines occur near the very quietest part of the radio spectrum. They too should be known to other civilizations which have studied the cosmos at radio frequencies.

The chemist looks at H and OH and recognizes them as the disassociation products of water, the solvent essential to the very existence of life as we know it. During the landmark Cyclops study of 1971, Dr. Bernard M. Oliver, then vice-president of engineering for Hewlett-Packard Company (and later the chief of the NASA SETI program) hypothesized that the Hydrogen and Hydroxyl lines constituted obvious signposts to a natural interstellar communications band, one which would likely occur to other water-based lifeforms who had some knowledge of the radio sky. Since the H and OH lines are visible from anywhere in the cosmos, in the quietest part of the spectrum, they are markers which are by no means geocentric.

It was Barney Oliver who dubbed the spectral region between H and OH the Cosmic Water-Hole. "Where shall we meet our neighbors?" he asked. "At the water-hole, where species have always gathered."

Although other regions of the spectrum hold much promise, The SETI League and other organizations concentrate a part of their resources on the Water-Hole, in hopes that there we might detect signs of other life.>>

That is very different from vibrations in the atmosphere, which are fundamentally undetectable regardless of technology.>>

http://voyager.jpl.nasa.gov/spacecraft/sounds.html

[Chris Peterson]

<<Directed transmissions- whether from some other star, or made by us- are likely to be above the background noise for tens of thousands of light years. The limiting noise source will be the detector itself, and that can be arbitrarily low, depending only on technology, not physical limitations.

But eventually the limiting noise source will be the 3K background.

Yup... there's always some physical limit. The point in the case of SETI, however, is that the limits are technological, not physical, for a very long distance. That means it is technically feasible to detect other civilizations by their radio transmissions- especially if those transmissions are directed.

[alter-ego]

<<Directed transmissions- whether from some other star, or made by us- are likely to be above the background noise for tens of thousands of light years. The limiting noise source will be the detector itself, and that can be arbitrarily low, depending only on technology, not physical limitations.

But eventually the limiting noise source will be the 3K background.

Yup... there's always some physical limit. The point in the case of SETI, however, is that the limits are technological, not physical, for a very long distance. That means it is technically feasible to detect other civilizations by their radio transmissions- especially if those transmissions are directed.

I made a rough estimate for the propagation distance for the Arecibo message at 2380 MHz. For the conditions listed below, this distance worked out to be ~1000 pc.

There are two main sources of background noise that are considered in this estimate: Galactic Synchrotron Radiation and CBR. From multiple sources, I found a roughly consistent CBR-to-GSR brightness ratio at 2380 MHz ≈ 5 (assuming an isotropic GSR background), and the CBR brightness @ 2380 Hz ≈ 2x10^-21 Watts/[m² Hz sr]. The Areceibo message had an effective bandwidth of 10 Hz, 1 MW of power, and a rough full-width divergence 3 arc minutes. I assumed ET uses a 1000 meter dish having field of view ≈ 1 arc min. With these parameters, I calculated the distance where the directed Arecibo intensity (Watts/m²) = GSR + CBR, i.e. within a 10 Hz bandwidth, the total signal is 2x the nominal background noise level.

If I did the simplified calculation correctly, the result suggests that M13 may be a bit far for our present-day capabilities, but we don't really care about this case anyway. Although one can easily change the numbers to get a 10x larger propagation distance, it is certainly seems clear that communication is possible within our local galactic neighborhood.

[neufer]

Click to play embedded YouTube video.

I made a rough estimate for the propagation distance for the Arecibo message at 2380 MHz. For the conditions listed below, this distance worked out to be ~1000 pc. The Areceibo message had an effective bandwidth of 10 Hz, 1 MW of power, and a rough full-width divergence 3 arc minutes. I assumed ET uses a 1000 meter dish having field of view ≈ 1 arc min. With these parameters, I calculated the distance where the directed Arecibo intensity (Watts/m²) = GSR + CBR, i.e. within a 10 Hz bandwidth, the total signal is 2x the nominal background noise level. If I did the simplified calculation correctly, the result suggests that M13 may be a bit far for our present-day capabilities, but we don't really care about this case anyway. Although one can easily change the numbers to get a 10x larger propagation distance, it is certainly seems clear that communication is possible within our local galactic neighborhood.

[skyhound]

"They say" that everything anyone ever said is still out there resounding in the atmosphere. But can we still hear it? No. Can we hope to build a machine that could detect what was said a thousand years ago above the noise? No. This message is just like those sounds. By the time it reached any being capable of detecting it, the signal would be just as lost as the words spoken last week in Springfield are.

Not so. While the Arecibo transmission was largely symbolic, most of the actual SETI work has surveyed fairly close stars- typically within a few hundred light years. At that range, any directed radio transmission (which is what SETI hopes for) could be above the noise floor of our instruments. Directed transmissions- whether from some other star, or made by us- are likely to be above the background noise for tens of thousands of light years. The limiting noise source will be the detector itself, and that can be arbitrarily low, depending only on technology, not physical limitations.

That is very different from vibrations in the atmosphere, which are fundamentally undetectable regardless of technology.

You seem to have completely missed my point. I was complaining about the mythology that various SETI researches have allowed to persist with the public because it is in their interest to keep it going. They don't put much effort into explaining the assumptions they make, such as a signal beamed directly at us or vice versa, or that advanced alien technology is required, and they seem more comfortable if the whole inverse square law issue is simply ignored--at least when it comes to the public. There is a reason they looked at the closest stars first. Truth be told, they have already scanned all of those and found nothing. I feel the text of the APOD was far too uncritical. Is SETI science or is it something so meaningless that it is not even worth questioning?

What I'd like to see is every SETI-related story contain a calculation that turned the question around in terms of today's technology as a starting point. If we beam a signal, at what distance would we reasonably be expected to detect and identify this same signal ourselves? If we point our radio telescope at a particular star, would we be able to detect a mirror image of the earth that was using the exact same technology we are using?

[Guest]

"They say" that everything anyone ever said is still out there resounding in the atmosphere. But can we still hear it? No. Can we hope to build a machine that could detect what was said a thousand years ago above the noise? No. This message is just like those sounds. By the time it reached any being capable of detecting it, the signal would be just as lost as the words spoken last week in Springfield are. This is the way SETI is. Its all pie in the sky and they never own up to the limitations. When pressed, they invoke the vague idea that some super beings will have a solution that makes it all work. I have nothing against what these people do, in principle, but I do think they are rather less than honest in how they sell it to the public.

I should have said something like, "recognize and decode" rather than "detect." The word "detect" can become far too theoretical. The problems are practical. For success, assumptions must be made such as that the listener is targeting us directly or that they have some technology that we can only imagine.

[neufer]

This is how far human radio broadcasts have reached into the galaxy
The Planetary Society Blog By Emily Lakdawalla Feb. 24, 2012

[b][color=#0000FF]Extent of human radio broadcasts Humans have been broadcasting radio waves into deep space for about a hundred years now, since the days of Marconi. That, of course, means there is an ever-expanding bubble announcing Humanity's presence to anyone listening in the Milky Way. This bubble is astronomically large (literally), and currently spans approximately 200 light years. But how big is this, really, compared to the size of the Galaxy in which we live (which is, itself, just one of countless billions of galaxies in the observable universe)? To answer that question, Adam Grossman put together this diagram. It's not the black square; it's the little blue dot at the center of that zoomed-in square. Credit: Adam Grossman / Nick Risinger[/color][/b]

---

<<The other day, I was playing around with stumbleupon and came across this photo, which -- well, it speaks for itself. Wow.

Gives you perspective, doesn't it? Actually, I'm a little surprised that the dot shows up on this image at all. Some people describe this as humbling, but for me, I see it as just the beginning. I'm very grateful to be a member of the very few generations of humanity that have ever lived who are (a) capable of creating radio broadcasts and (b) realizing how much more of the universe there is beyond what we've experienced.

I tweeted a link to it, and while I expected some retweets, I was surprised to see its spread -- I think it's probably the single most retweeted tweet I have ever written. There's probably several reasons for that. Links to photos are more likely to be retweeted than others. Very short but still substantial tweets are more likely to be retweeted, because it gives the repeater scope for their own commentary. And this is the sort of thing that can make just about anybody who is capable of operating a cell phone go "hmm," so it has wide appeal.

The one thing I feel bad about is that stumbleupon sent me directly to the photo on somebody else's website, and I didn't bother looking up its origin before I tweeted the link. So now I have, and I can tell you that the diagram was made by Adam Grossman on the jackadamblog, using an artist's concept of the Milky Way by Nick Risinger that he took from Wikipedia. They have a neato-looking iOS app, Dark Sky, that provides very short-term weather predictions. My apologies, Adam, for sending so much traffic directly to the photo rather than to your blog! Hopefully this post will correct that error.

A special note to the pedants: yes, I do realize that the signal from our radio and TV broadcasts is so attenuated by that 100-light-year boundary as to be undetectable except by some kind of magical alien technology. That's not the point. Don't be so literal!>>

[jessy27]

Let me get this straight, are we trying to get them to believe there is intelligent life here