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Tom Mazanec
2013-Apr-23, 06:45 PM
Suppose there is a civilization technologically identical to ours in the Alpha Centauri system. Would they know we are here?
I seem to recall that the only radio signals that could be detected more than a light year or two from the Earth with our present level of technology are the Distant Early Warning radar signals (which says something about our species). I do not know if these are still in use, and even if they are they are in the wrong hemisphere to be seen from Alpha Centauri. And I understand broadcasting is actually getting less prominant as it goes to energy efficient coding and beaming.
If they are there, do they know about us?

beskeptical
2013-Apr-24, 12:39 AM
It's ~4.5 light years from here, so yes, our transmissions have likely gone that far.

Radio waves are on the electromagnetic spectrum* (http://www.darvill.clara.net/emag/emagradio.htm) just like light is. They travel at the same speed but the waves are different frequencies.


(*I don't think the cancer danger notes at the bottom of the page are necessarily valid.)

redshifter
2013-Apr-24, 12:49 AM
I'm not sure about the propagation of radio waves through interstellar space -- how far they can travel vs. how powerful the transmitter is or if it's even possible to detect radio waves at interstellar distances. There are folks here much more knowledgeable than I on that subject (or on a lot of subjects for that matter..)

However, if this hypothetical civilization had the human equivalent of Kepler or other extrasolar planet discovery methods, and assuming they have been searching for extra solar planets for the same # of years as we have, they might know about the Earth or Venus, though I believe Earth and Venus sized planets are on the very edge of our detection capabilities as far as extra solar planets go. It takes 3 transits to flag a planet for confirmation, but we've only been observing with Kepler for ~4 yrs, thus gas giants in our solar system would be unobserved at this point (or maybe a single transit would have been observed). Unless they could detect our radio transmissions, they might not be able to tell if there was any life in our solar system. Perhaps they could analyze our atmosphere during transits and infer our existence through greenhouse gases (esp. if they could measure increases of them) or pollutants, though that might not be enough for confirmation. And, extrasolar planetary atmosphere analysis is still beyond our capabilities AFAIK.

My pie in the sky dream: Advances in extra solar planet detection that allows us to image them directly. How cool would it be to see lights of an alien civilization on the dark side of their world?

beskeptical
2013-Apr-24, 12:57 AM
I'm not sure about the propagation of radio waves through interstellar space -- how far they can travel vs. how powerful the transmitter is or if it's even possible to detect radio waves at interstellar distances. There are folks here much more knowledgeable than I on that subject (or on a lot of subjects for that matter..) ...We can detect radio waves from space.

The main difference is the wave shape and energy levels. Speed, propagation would be the same, even photons are the same, though the energy in the photon differs:

A photon is a quantum of electromagnetic energy. (http://www.wa1mba.org/micros.htm)
Physicists think of electromagentic energy as having a "dual nature", in that some experiments reveal its nature as a particle which we call a photon and other experiments reveal its nature as a wave. When it comes to lower frequencies (longer wavelengths), such as microwaves, VHF, and the like, it becomes much less convenient to think of energy in the form of photons, but there is no specific reason to decide that only one nature exists at these longer wavlengths. Sometimes photons are reffered to when describing an RF interaction with matter. The author does not know of any other word to describe the particulate nature of a propogating RF energy field except "photon". When the interaction with matter converts the energy into a mechanical form, we sometimes refer to the energy packets as "phonons". This is not a propogating Electro-Magnetic (EM) field, but rather a sound wave, and at the most minute level, even mechanical energy is quantized.

Zwart Gat
2013-Apr-24, 09:15 AM
It should be possible to estimate the number of photons from a particular radio trasmission on Earth which would hit an Earth sized planet at Alpha Centauri. And then speculate about the likelyhood that it could be distinguished from the noise as "unnatural". Since their enormous fusion explosion radio transmitters, their stars, appear as only a tiny dot here, it seems impossible for us to make a dent. But intuition and sense of proportions has never been useful in astronomy, so who knows.

MaDeR
2013-Apr-24, 12:39 PM
Suppose there is a civilization technologically identical to ours in the Alpha Centauri system. Would they know we are here?
I seem to recall that the only radio signals that could be detected more than a light year or two from the Earth with our present level of technology are the Distant Early Warning radar signals (which says something about our species). I do not know if these are still in use, and even if they are they are in the wrong hemisphere to be seen from Alpha Centauri. And I understand broadcasting is actually getting less prominant as it goes to energy efficient coding and beaming.
AFAIK undirected radio broadcasts are lost in static noise of Universe pretty fast, already at Oort cloud. Directed beams would be detectable without problems, if both sides have aligned their broadcaster and receiver (ours pointed at alpha centaur, theirs pointed at our sun).


If they are there, do they know about us?
Assuming they exists and have same tech level - almost certainly no.

beskeptical
2013-Apr-24, 05:03 PM
AFAIK undirected radio broadcasts are lost in static noise of Universe pretty fast, already at Oort cloud. Directed beams would be detectable without problems, if both sides have aligned their broadcaster and receiver (ours pointed at alpha centaur, theirs pointed at our sun).


Assuming they exists and have same tech level - almost certainly no.Then why are we bothering with SETI?

The Straight Dope (http://www.straightdope.com/columns/read/3058/can-alien-beings-listen-in-on-tv-and-radio-broadcasts-from-earth) addresses your points.

Alien listeners would be likely to detect passive radiation only in certain frequencies, generally 10 to 200 GHz, where the background noise of the cosmos is weaker. They’d need a huge antenna, and they’d have to listen for a long time before gathering enough traces of signal to confirm intelligent origin.Believers contend it can be done. An antenna similar to the 1,000-foot-diameter Arecibo radio telescope in Puerto Rico could probably detect our passive radiation from 30 to 50 light years away. With a giant array of 1,000 100-meter dishes linked together, that distance could be extended to 500 light years.
Just one problem. The aliens would be able to hear us at those enormous distances only if they already knew where we were and could point their telescope at us.(bold is mine)
Surely we've pointed the Arecibo array at the closest star to us. Wouldn't they do the same if they were trying to find life in the galaxy?

neilzero
2013-Apr-25, 03:50 AM
Many of our signals are arriving in the Centarii system, but they are very weak and very brief with rare exceptions, so they may not guess they are sentient instead of noise. The governments have some satelites listening to communications on Earth's surface, but the results are classified, so we are mostly guessing. Neil

MaDeR
2013-Apr-25, 09:56 AM
Then why are we bothering with SETI?


Directed beams would be detectable without problems, if both sides have aligned their broadcaster and receiver (ours pointed at alpha centaur, theirs pointed at our sun).
Any other questions?


The Straight Dope (http://www.straightdope.com/columns/read/3058/can-alien-beings-listen-in-on-tv-and-radio-broadcasts-from-earth) addresses your points.
Nice cherrypicking. Rest of article practically destroys notion of detecting passive signals from ET. Forget about it.

Zwart Gat
2013-Apr-25, 12:24 PM
Double post.

Zwart Gat
2013-Apr-25, 12:32 PM
If it's not off topic, I wonder what ideas there are around about what traces of civilization which might be detectable from other stars, except purposeful radio waves.

For example, I've heard of the (certainly very bad) idea to send used nuclear fuel into the Sun. And that this would change the Suns radiation in a way measurable from afar. And with no potential natural explanation, other than that uneconomic intelligent-kind-of life was around, doing crazy stuff. Could we tiny guys really tint the sunlight?

Also, with extreme foresight, it has been suggested that we push an asteroid to a solar orbit just outside that of Earth. This would pull Earth outwards over millions of years and compensate for the coming process of the Sun turning into a red giant, and keep our temperature rather stable. Are strange exoplanet configurations a sign of ETI?

Finally, I wonder if a neighboring civilizaton wouldn't use their star as a gravitational lens for a telescope pointing towards us, and if they then wouldn't be able to gather some of our everyday radio transmissions? (search FOCAL and maccone for more about that idea).

cjameshuff
2013-Apr-25, 02:54 PM
However, if this hypothetical civilization had the human equivalent of Kepler or other extrasolar planet discovery methods, and assuming they have been searching for extra solar planets for the same # of years as we have, they might know about the Earth or Venus, though I believe Earth and Venus sized planets are on the very edge of our detection capabilities as far as extra solar planets go. It takes 3 transits to flag a planet for confirmation, but we've only been observing with Kepler for ~4 yrs, thus gas giants in our solar system would be unobserved at this point (or maybe a single transit would have been observed). Unless they could detect our radio transmissions, they might not be able to tell if there was any life in our solar system. Perhaps they could analyze our atmosphere during transits and infer our existence through greenhouse gases (esp. if they could measure increases of them) or pollutants, though that might not be enough for confirmation. And, extrasolar planetary atmosphere analysis is still beyond our capabilities AFAIK.

Earth doesn't come anywhere close to transiting the sun from Alpha Centauri's viewpoint. Transmissions from our southern hemisphere would be about their only chance.



It should be possible to estimate the number of photons from a particular radio trasmission on Earth which would hit an Earth sized planet at Alpha Centauri. And then speculate about the likelyhood that it could be distinguished from the noise as "unnatural". Since their enormous fusion explosion radio transmitters, their stars, appear as only a tiny dot here, it seems impossible for us to make a dent. But intuition and sense of proportions has never been useful in astronomy, so who knows.

Stars aren't explosions (except for those occasional brief periods where a star is actually exploding). Ones like ours fuse hydrogen very slowly, and are only hot because of their enormous volume for their surface area. Human metabolism actually has a higher power density...

The sun is an approximate black body. It's actually not difficult to outshine the sun with a narrow-band signal, especially a directional one. A listener/watcher would still be receiving far more EM radiation from the sun in total, but spread across the entire spectrum with abroad peak in the visible range. The problem is that the receiver has to know what slice of the spectrum to look at.



Surely we've pointed the Arecibo array at the closest star to us. Wouldn't they do the same if they were trying to find life in the galaxy?

Arecibo isn't an array, it's a single dish, and it can only target portions of the sky in a range from -1 to 38 degrees declination. The Centauri system is far out of its field of view. We could put up similar scale dishes in orbit, but haven't done so for astronomy. (though we've gone up to at least 100m for Earth surveillance purposes (http://www.globalsecurity.org/space/systems/trumpet.htm))

beskeptical
2013-Apr-27, 04:01 PM
I guess I was under the wrong impression about the search for radio transmissions. Thanks for the corrections.

Article on planet searches. (http://www.huffingtonpost.com/2012/10/17/earth-sized-planet-alpha-centauri-b-solar-system_n_1973059.html)
Alpha Centauri was the first place the private Search for Extra Terrestrial Intelligence program looked in its decade-long hunt for radio signals that signify alien intelligent life. Nothing was found, but that doesn't mean nothing is there, said SETI Institute astronomer Seth Shostak.

Talking Back from Alpha Centauri

(http://www.centauri-dreams.org/?p=26219)
But lasers can help in a major way. Dispersion of the signal is negligible compared to radio, and optical signals can carry more information. Lesh is not a propulsion man so he leaves the problem of getting to Alpha Centauri to others. But his point was that if you could get a laser installation about the size of the Hubble Space Telescope into Centauri space, you could send back a useful datastream to Earth.
The probe would do that using a 20-watt laser system that would lock onto the Sun as its reference point and beam its signals to a 10-meter telescope in Earth orbit (placed there to avoid absorption effects in the atmosphere). It’s still a tough catch, because you’d have to use optical filters to remove the bright light of the Alpha Centauri system while retaining the laser signal.

Githyanki
2013-May-03, 11:37 PM
If Australia showed reruns of the Eastenders, the Centaurians are surely going to destroy us.

mapguy
2014-Feb-12, 09:58 PM
Recently there was an intriguing UT article (http://www.universetoday.com/109219/50-years-after-taking-over-earth-the-beatles-space-invasion-is-well-underway/) noting that the 1964 VHF broadcast of the Beatles on the Ed Sullivan show has now traveled 50 light years from Earth. Interesting to think about the number of star systems that we've "reached" so far, and how that number is increasing exponentially.

But then I wondered: if "anyone" is out there listening, could they actually detect those signals? That's what led me to this thread... and I think what you all are saying is: because our broadcasts are un-directed, chances are pretty slim that anyone listening could distinguish our intelligent* signals from the noise of the galaxy. Am I stating that correctly?

*debatable