Catch up on stories from the past week (and beyond) at the Slashdot story archive


Forgot your password?
Australia Communications Space Science

No Intelligent Aliens Detected In Gliese 581 239

astroengine writes "Using an Australian very long baseline array (VLBA) of three radio antennae, the first very long baseline interferometry (VLBI) campaign has been carried out on a SETI target star: the famous Gliese 581 red dwarf. However, after 8 hours of observing the star — thought to play host to six exoplanets, two of which are in the star's 'habitable zone' — no alien signals were detected. This result isn't surprising, as the likelihood of us stumbling across intelligent aliens living in the Gliese 581 system transmitting radio is extremely slim, but it does validate VLBI as a very exciting means of using the vast amount of exoplanetary data (coming from missions such as the Kepler space telescope) for 'directed SETI' projects."
This discussion has been archived. No new comments can be posted.

No Intelligent Aliens Detected In Gliese 581

Comments Filter:
  • by silentcoder ( 1241496 ) on Saturday June 02, 2012 @05:58PM (#40196357)

    Erm... do you even know what SETI is ? Or the concept of a round-trip ?

    1) If we had sent a signal, it would take 20 years to get there, and we could expect a reply in no less than 40 years. Twice your estimate.
    2) That doesn't actually matter however since we didn't send a signal at all. All we did was listen for signals coming from there. In other words - we were hoping maybe they sent us a signal 20 years ago - or more likely just generally sent out a signal in case *anybody* answers, or even more likely that we could catch a listen-in on a signal that was never intended for outsiders to begin with. If an alien race has satelite television - then any of the beamed-up signals that don't quite hit the target keeps going through space - SETI is really about trying to pick up any that may have come our way.

    The most likely result we could get from SETI would probably be accidental signals - on the basis that any aliens running a seti project is most likely to get such signals from us (we sent a lot of signals into space, none of it is actually intended for alien consumption).
    A signal intending to be picked up by other species (as in Sagan's Contact) would be a bonus.

    That is actually the biggest problem I have with SETI. We're just listening hoping somebody else is bothering to talk or we pick up some stray signal. Truth is, we may well end up in a situation where half a dozen intelligent species are doing the same. All listening to space, waiting for the other one to actually say something.

  • by Chemisor ( 97276 ) on Saturday June 02, 2012 @06:24PM (#40196507)

    Having no detectable radio emissions does not preclude possibility of a civilization. Our civilization's emissions are already mostly in spread-spectrum format, which is by design indistinguishable from noise unless you know the encryption key. The transmitters we do have usually do not radiate omnidirectionally; that would be wasteful. Antennas are designed to cover the intended audience and minimize leakage outside of it, which makes detecting their radiation unlikely at any appreciable distance.

    Futhermore, natural inverse square weakening of the signal makes the signal fade into the background before leaving the solar system anyway. Our TV and radio transmitters are not going to be heard outside the solar system. It is no coincidence that our satellites communicate with highly directional dishes. Directed signals are the only ones that will make it to the next star, so what SETI is really looking for is aliens who are actively broadcasting toward Earth. I don't know why they would be doing such a dumb thing, but who knows, maybe they are a not-too-intelligent life.

  • Re:Validation? (Score:5, Informative)

    by mbone ( 558574 ) on Saturday June 02, 2012 @06:40PM (#40196597)

    This result isn't surprising [...] but it does validate VLBI as a very exciting means

    I'm a little confused as how a negative result validates the excitement-quotient. Or how this could even be validated in a more meaningul sense -- there's no way of checking the data. Maybe it was a false negative and there's oodles of aliens there.

    The biggest technical problem in radio SETI is RFI. A signal from the star in question would have a very specific Doppler shift between the VLBI antennas, different from the relative Doppler shifts from any terrestrial RFI, even spill-over from a satellite. You can still have a saturated receiver if the RFI is too strong, but that is less of problem (it's easy to detect), and VLBI really reduces the chance of a false positive to almost nil. You also don't need an actual signal to show that this technique works.

  • by kasperd ( 592156 ) on Saturday June 02, 2012 @07:08PM (#40196823) Homepage Journal

    we sent a lot of signals into space, none of it is actually intended for alien consumption

    One could argue that the Arecibo message [] was designed for alien consumption.

  • by mbone ( 558574 ) on Saturday June 02, 2012 @07:44PM (#40197133)

    FTA :

    From our results we place an upper limit of 7 MW Hz1 on the power output of any isotropic emitter located in the Gliese 581 system, within this frequency range.


    That means they could detect a 7 megawatt carrier tone with very narrow bandwidth of 1 HZ (or a 70 MW signal spread over 10 HZ, etc.) BUT, that is isotropic power. If the ETI was using something like the DSN antennas, much less something like Arecibo, they might have a gain of 60 dB, which means we could detect a signal down in the 10 Watt range. This search has enough sensitivity that there are lots of broadcasters on Earth (weather radar and airport radars, for example), that could be detected by this survey, if there happened to be a clone of our civilization at Gliese 581.

  • by RubberDogBone ( 851604 ) on Saturday June 02, 2012 @08:19PM (#40197369)

    This is true. Nearly all of what we have ever broadcast has been trashed into junk RF by the time it passes through our own heliopause. Voyager 1 and 2 are helping us learn that it is a fantastic filter aggressively scrubbing and sterilizing radio. Perhaps only a few terrifically strong military radar signals or intentional interplanetary signals (i.e. the powerful Arecibo transmissions) might have made it through. Decades of TV and radio have not. For practical purposes, our Sol system is silent. We are not emitting potent enough repeating signals of the sort we ourselves are seeking.

    It is logical to expect a similar result for other planetary systems where something like a heliopause exists. RF would be trashed and never make it into interplanetary space.

    For even more discouragement, remember that most of life on Earth does not use radio. A planet teeming with life might yet have nobody emitting even weak signals. Radio derives from the human need to communicate, constantly. Especially while driving. It is entirely possible that another similarly advanced species might not have that need to talk talk talk and entertain at a distance.

    Anyway, the universe is a very big place. It's a long way down the road to the pharmacy, but that's nothing compared to the universe. Most of it is empty. On average, we don't actually exist at all. Sigh.

  • by vlm ( 69642 ) on Saturday June 02, 2012 @08:22PM (#40197381)

    Actually we are broadcasting VERY little now, and most of what we are transmitting won't leave our own system. this is why if other life exists its gonna be hell to catch a signal as there is a very small window between finding out how to broadcast and switching to digital, if other life follows a similar pattern.

    "Radio Astronomy" by John Kraus has a section on this topic. Well obviously he predates the digital transition. Not all that surprisingly large scale planetary radar has the best range, but the strongest long term signal used to be the constant (hopefully stable freq) of AM radio transmission carriers. You can integrate the carrier over months I suppose if necessary, detectable long below the data level

  • by Chemisor ( 97276 ) on Saturday June 02, 2012 @10:04PM (#40197929)

    the vast majority of licensed bands are still the age old single peak and often at insane output powers

    I wouldn't call a few kW insane. Only the largest transmitters in the world go into the megawatt range; the vast majority are things like wifi, which are small and weak, at
    They are looking for aliens who have set up multi-Gigawatt transmitters all over their planet, just like we have.

    You obviously don't know much about transmitters. There are no multigigawatt transmitters anywhere in the world. The most powerful transmitter in the world is the Roumoules transmitter in France [], which outputs 1.4MW, three orders of magnitude less than you think. It is notable that only its longwave broadcasts can be heard past 100km, because those reflect off the ionosphere. At night, the medium waves can do that too and so can be heard farther.

    Notice that most of that radiated power will be reflected from the ionosphere and won't even make it as far as Earth's orbit. But for argument's sake, let's assume a full half of the signal makes it through. The antenna is somewhat directional, though the wikipedia article does not specify the beam width. Let's be generous and say it's a cone 30 degrees wide.

    This cone will form a moving beam across the sky as the Earth rotates. A 30 degree beam will illuminate any particular star in its path for 2 hours each day. The study in the article we're discussing listened only for 8 hours, which is too short to always catch it.

    Even if the signal is not reflected from the atmosphere, it will be significantly attenuated. Let's say 1MW makes it through. Gliese 581 is 20 light years away, ~2e17m. The base of the radiated cone is 2e17*tan30=1e17. The area of the base is pi*1e17^2=3.75e34. 1MW/3.75e34m2 = 2.67e-29W/m2.

    Minimum detectable signal [] with a 1kHz bandwidth is -144dBm of the 1mW reference signal. That's 4e-15W. As a dumb estimate, we can calculate that 1.5e14m2 of continuous antenna area would be necessary to receive such a signal. That's approximately equal to the cross section of the Earth.

    Of course, that's if the signal can be received at all. At low levels like that electrons in the antenna are unlikely to absorb anything at all. The ground state energy, for example, is 13eV = 2e-18J, 11 orders of magnitude lower than the signal per square meter. I find it difficult to believe that any excitation can occur here.

  • by Jappus ( 1177563 ) on Sunday June 03, 2012 @04:43AM (#40199627)

    You might want to reread that quote from Adams. He did not express any of the erroneous formulas that you stipulate. Here's the full quote in all its glory:

    It is known that there is an infinite number of worlds, but that not every one is inhabited. Therefore, there must be a finite nuber of inhabited worlds. Any finite number divided by infinity is as near to nothing as makes no odds, so if every planet in the Universe has a populations of zero then the entire population of the Universe must also be zero, and any people you may actually meet from time to time are merely the products of a deranged imagination.

    So what he's actually saying is this:
    X = # of worlds;
    Y = # of inhabited Worlds;
    X = infinite;
    Y = finite;
    infinite number >>> finite number (infinity has a higher magnitude than any finite number)
    It follows: X/Y = 0 - epsilon, where epsilon approaches 0 infinitely close.
    Thus: Average density of life per world so close to zero, that it functionally IS zero (remember, just like: 0.99999... = 1)

    Summary: Any life one sees must be the product of a deranged mind. You could even go so far as calling it imaginary.

    Actual summary: Expressing humorous quotes in terms of maths is exactly what it takes to take the humour out of them.

  • You obviously don't know much about transmitters. There are no multigigawatt transmitters anywhere in the world. The most powerful transmitter in the world is the Roumoules transmitter in France, which outputs 1.4MW

    The HAARP project [] directs a 3.6 MW signal, in the 2.8â"10 MHz region of the HF (high-frequency) band, into the ionosphere. Keep trying, you'll get it eventually.

Vitamin C deficiency is apauling.