Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

ET Phone Home

  1. Jan 20, 2004 #1

    FZ+

    User Avatar

    Much work has been done into detecting potential signals from extra-terrestrials, but I want to discuss the more active approach of sending signals ourselves...

    What attempts at such communications have we attempted ourselves? I remember hearing of attempts to broadcast radio waves into space, which strikes me as too weak to attract an alien's attention, or the gold plaques on the voyager probes, which strikes me as rather... laughable.

    It seems to me that a potential beacon would have to satisfy a number of criteria.

    Speed: It must travel at around c. Else, it simply is not worth bothering with. By the time it is detected, we would be extinct.

    Magnitude: The signal must be powerful, as its strength diminishes at it spreads out by an inverse square law. Dust and so on may also absorb some.

    Coverage: We don't know where the aliens are. Therefore, we need a way of covering the entire sky, and signaling each spot for as long as possible.

    Universality: It needs to be a technique that a similar type of being would have anticipated and be prepared to detect.

    Any others? Any ideas as to how to (eventually) fulfill this? Based on our current standard of radio telescope, what is the maximum range of the signals we are sending out right now?
     
  2. jcsd
  3. Jan 21, 2004 #2

    Phobos

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Offhand, I recall 2 attempts with radio waves...both with limited targets.

    Here's the one from 1974...
    http://www.seti-inst.edu/science/a-message.html

    Then there was this one in 1999...
    http://www.seti-inst.edu/science/enc-2001.html

    hmm...wasn't there one other?

    Perhaps (odds are very...very low that they will be found). But I suppose it's a nice gesture given that the Voyagers may be roaming around for billions of years. Also, it has some good public interest for the public back here on Earth. Hey, maybe future space-faring generations of humans will find it and consider it an archaeological treasure.

    Agree - thus, the 2 radio signals to date.

    That's the catch-22, isn't it. Do you use the power to send the message broadly & weakly or narrowly & powerfully?

    I think the 1974 broadcast was more publicity than an actual attempt at contact (since their target was 21000 light years away - - see link above).

    The 1999 broadcast targeted 4 nearby stars (60 light years).

    The chosen (attempted) language was math/science (on the basis that the laws of physics are the same everywhere). Of course, there is still a lot of human-bias to be found in the messages.

    Interesting to note that SETI is starting up "Optical SETI" which searches for optical (laser) pulses rather than radio signals from ETs. This method has some advantages over radio signals (e.g., can use the power to more narrowly target specific star systems, but can quickly go from star to star & pack in a lot of info in short pulses).
    http://www.seti.org/seti/our_projects/optical_seti/overview/Welcome.html
     
  4. Jan 21, 2004 #3

    Njorl

    User Avatar
    Science Advisor

    Binary representations of a series of prime numbers would be a good universal signal. It says, "We are here and we can do math."

    There is a nice section in Sagan's "Contact" about the multi-level message the aliens sent.


    I have often wondered what the biggest manmade signal is that reaches space. Is it the 60-hertz signal generated by all the households in the US? Is it a single TV broadcast? Is it something designed to be transmitted via satellite?

    We might be able to send some message to another planet's analogue of a SETI program cheaply. If we can find a way to modulate the noise of a very big source, we could encode a message into it. Every electric power plant in the world is essentially an EM noise source at whatever frequency their high-tension wires carry. It might be possible to cheaply modulate that frequency a little bit, and encode a message in the modulation.

    Njorl
     
  5. Jan 21, 2004 #4
    Possibly will be possible also to send signals using the method of neutrino modulation. That is, you modulate a beam of neutrinos instead of a beam of electromagnetic radiation to code information.Neutrinos have very little mass, they practically travel at the velocity of c
     
  6. Jan 21, 2004 #5
    What advantage might this have... and how would they be detected?
    [?]
     
  7. Jan 21, 2004 #6

    russ_watters

    User Avatar

    Staff: Mentor

    And of course it also says, "we can build a radio transmitter of xxx power."
    Pretty soon the power company may be your ISP. They are close to being able to commercialize that.
     
  8. Jan 21, 2004 #7

    FZ+

    User Avatar

    Just a quick additional question...

    Why radio waves? Why this part of the EM spectrum? Is it just a practical issue?
     
  9. Jan 21, 2004 #8
    Well, I'm sure there are ETs out there and there is a slim chance that they (or us) will detect a signal. However, the real question must surely be 'will we ever be able to conduct a dialogue?' The answer to that is, equally surely, 'No!'
     
  10. Jan 21, 2004 #9

    Nereid

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Some quick inputs (details and links later if anyone's interested):
    - most powerful radio? military radars, by many, many OOM. Only exception may be radar astronomy (narrow beams, narrow wavelengths)
    - neutrino modulation would say 'we understand physics' far louder than AM or FM radio primes (spread spectrum primes imply receivers' knowledge of the codes)
    - why radio? very long 'mean-free path', low energy to generate, high S/N, easy to detect, we understand the technology, we can modulate easily (cf gammas, neutrinos), ...
    - sure 'they' can detect us; with our current capabilities we'd be able to watch 'I Love Lucy' as far away as the signal has gotten (small exaggeration); Sagan's Nazi broadcast (in Contact) wasn't too far off what we *could* detect today
    - dialogue? of course we could! small example: mere Earthly mathematicians have been conducting centuries-long mathematical dialogues for ... well, centuries. It's just that none of us would be around to have our questions answered But what's wrong with great, great, great ... n grandchildren?
     
    Last edited: Jan 21, 2004
  11. Jan 21, 2004 #10
    Neutrinos can pass through a solid, for example a planet, practically without interacting with the atoms of the solid. That's not possible with EM radiation
     
  12. Jan 21, 2004 #11
    That represents its own problem. It would be very difficult to modulate a neutrino beam as well as extracting information from one.
     
  13. Jan 23, 2004 #12

    Phobos

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    ...and something to do with the magic wavelength of 21 cm? (the natural resonating frequency of hydrogen...the most comment element in the universe and, over the vast EM spectrum, a wavelength were ET astronomers are more likely to have their attention parked)

    But my memory is foggy on this point...I'd have to go back to Sagan's book Pale Blue Dot to get the details.
     
  14. Jan 23, 2004 #13

    Phobos

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Exactly, our current neutrino telescopes can only capture a few out of the billions and billions and billions that zip through the Earth.
     
  15. Jan 23, 2004 #14

    Nereid

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    The trouble with the 21 cm line is that there's an awful lot of H in the universe, so it's a very noisy wavelength to use.

    However, the sky is pretty quiet in 'the water hole', so why not look there?

    This page, from a SETI site, explains:
    http://setiathome.ssl.berkeley.edu/about_seti/radio_search_2.html

    Note that the short wavelength side is somewhat misleading; the sky noise becomes dominated by the Earth's atmosphere, rise above that and it's all clear again.
     
  16. Jan 23, 2004 #15

    Nereid

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Quite correct.

    A different perspective: neutrinos were first postulated in the 1930s (1931?) and first detected in the 1950s (1957? our very own SelfAdjoint was there!). Call that a century. A century is insignificant in terms of the life of most things in the universe. Perhaps there's a discovery just waiting to be made about neutrinos, in the 2200s, and we'll know why any sensible form of advanced life doesn't bother to communicate with EM? Or maybe it's the selectrons and winos that are better choices, and we'll find that out in 2020?
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: ET Phone Home
  1. ET phone home (Replies: 1)

Loading...