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

Radio transmission question

  1. Dec 8, 2012 #1
    I sometimes hear on astronomy documentaries that our earliest radio transmissions could be received by aliens in our galaxy. Assuming the stations power was 1000kw at 1 mile(wild guess on my part) the power of the signal at 1 light year would 1000kw/7 trillion squared. Is this right? If so wouldnt the power of the signal be so extraordinarily low even at 1 light year(let alone the 50 or so they claim) as to preclude any possibility of being detected and interpreted.
  2. jcsd
  3. Dec 8, 2012 #2


    User Avatar
    Staff Emeritus
    Science Advisor

    Pretty much. The signal strength of all our transmissions is so low that they MIGHT be detectable at alpha centauri if you have a REALLY big dish. Somewhere around the size of the Arecibo dish, which is 300 meters in diameter.
  4. Dec 8, 2012 #3


    User Avatar
    Gold Member


    Radio frequency signals emitted here on earth decrease in intensity as they pass out of our solar system according the square of the distance law, as you know. Drakkith correctly points out that a “really big dish” is necessary to collect and concentrate the EM energy. Then a sensitive receiver is used to detect it. Here in our solar system we have high gain antennas and extremely sensitive receivers that receive signals from the Pioneer satellite, for example.

    You mentioned fictional aliens light-years away who might try to receive and interpret our emitted signals. There is no limit on how large a fictional high gain antenna they might construct and no limit on how sensitive a fictional receiver they could build. Also, there is no limit to their fictional ability to interpret it. It might become a huge problem for us humans, however, if they were hungry and came here to eat us.

  5. Dec 8, 2012 #4


    User Avatar
    Science Advisor
    Gold Member

    Incidental radio signals, like a television broadcast, would require an illogically powerful transmitter to be detectable beyond a few light years. Radio is also a dubious choice for interstellar communication. An advanced alien civilization would surely have much more sophisticated and efficient communication technology, and might easily conclude that cultures still using radio are too primitive to trifle with.
  6. Dec 9, 2012 #5
    I cant confirm this myself now, but I did ask a top SETI guy the same question about 15 years ago, and he said that his transmitter and Arecibo dish are just capable of reaching a distance of 1/4 the diameter of the galaxy if there was another dish with similar size at the end pointing directly at the beam. Probably the information bandwidth is very low eg. millihertz at that point as well as travel time of 100K years!
  7. Dec 9, 2012 #6
    Chronos are there any other known communication methods other than EM waves?
  8. Dec 9, 2012 #7


    User Avatar
    Staff Emeritus
    Science Advisor

    None that can reach across space. All others require a medium such as air or transmission lines.
  9. Dec 9, 2012 #8
    What the SETI guys are looking for, and what they really expect other civilizations (if any) to detect from earth, are "accidentally" directed high power beams. Examples are weather radar or the signals from Arecibo when it is used as a radar. I don't think any of them expect aliens to pick up our TV shows, or us to pick up theirs.
  10. Dec 9, 2012 #9


    User Avatar
    Science Advisor
    Gold Member

    Beamed signals have a much greater range than unbeamed signals, but, also cover a much smaller slice of the sky. So, an arecibo size dish could talk to another arecibo dish over a significant fraction of our galaxy - assuming they were pointed right at each other and operating on compatible frequencies. The odds are not favorable. There may not [yet] be an practical alternative to EM signals, but, radio signals of the type we currently employ are probably a blind alley. See http://www.bidstrup.com/seti.htm for discussion.
  11. Dec 10, 2012 #10
    Chronos thanks for the link. One alternative would be to use beam directional switching so that the same transmitter can cover 100 times or a 1000 time the single beam width that SETI currently has. Of course our civilisation would also need to be around up to 100K years later to hear the reply. I suspect there are network of civilisations that are Billions of years old out there somewhere, and they should be friendly.. If not then development of civilisations in this kind of universe is pretty limited.

    Come to think of it the rotation of the earth and its orbit around the sun guarantees a large number of beam directions, although not simultaneously. So an alien receiver might pick something up and then lose it for a long time..
    Last edited: Dec 10, 2012
  12. Dec 10, 2012 #11
    Even a large fixed antenna like Arecibo can see a pretty big chunk of the sky every 24h, almost half. If we were listening to one star intentionally (or it to us) rotation and orbit don't play a terribly big role. A bigger problem is deciding what star to focus on since until you hear that first signal, there's no reason to spend a lot of time there.

    As the exoplanet surveys discover more and more seemingly habitable planets, some candidates that may be worthy of more "ear time" will likely emerge. One can always err on the side of optimism and assume the other end of the potential conversation might be doing the same.
  13. Dec 16, 2012 #12
    It is extremely unlikely that there will ever be valid communication between two very- distant-from-each-other locations in our galaxy, at least in our civilization's time frame. Perhaps fun to dream about, but taking place, NO. Yes, IMHO.
  14. Dec 16, 2012 #13
    A question about radio waves and waves in general. We are all familiar with the EM spectrum. In principle, we know that we can wavelength and f are inversely proportional and in theory, it infinite but I read on Wikipedia that the expected limits for wavelength are the planck length at smallest and the size of the Universe at largest. Has anyone investigated the nature of waves and its application in communication beyond radio waves? For example, a wave with wavelength much larger than typical radiowaves, what is its nature, or shorted than Gamma rays? it seems like we only names for that much.

    I want also ask this in the context of viewing things. For example, as galaxies recede, they will get infinitely red shifted and therefore fade from view or so its said. Well, theoretically, we should always still be able to seem them as long as we are looking at the appropriate wavelength correct? Of course, I realize that they will eventually fade from view because some photons will never reach us as the Universe expands.

    I was just curious if there are some unique things that happen past the typical EM spectrum that we use and observe.

    For example, anything larger than radiowaves are just radiowaves with very, very long wavelength?
  15. Dec 16, 2012 #14
    Also, we already have a limit for how fast information can travel in the Universe which is the speed of light. How then would there be any space for more advancement in communications. It would not seem plausible that fictional aliens could communicate any faster than us other than maybe being able to send communications through wormholes as a cheat?

    Maybe we might have to accept that the likelihood of us ever being able to communicate with aliens is very low and that is simply the nature of the Universe.

    On the other hand, I think the problem of communicating over large distances is itself flawed. I think that because whenever you communicate, you also have to wait for response which takes as long as it takes you to get the message out. On the other hand, if all advanced civilizations were rapidly colonizing or moving radially in spheres of influence, even though it will be slower than light, in the same unlikely event that we do cross paths, there is chance for immediate and instantaneous interaction.
  16. Dec 16, 2012 #15


    User Avatar
    Staff Emeritus
    Science Advisor

    Light with wavelengths larger than radio waves is simply not feasible to use for communication. Your antennas need to be horribly large and the bandwidth of any signals would be terribly low. At energies above the visible range, the light has so much energy that you start to become unable to focus it and it starts acting more like a bulldozer, with x-ray and gamma rays smashing into your detectors and ionizing them. Plus the energy requirements would be far to high at that range.

    Yes, the Hubble Space Telescope is barely able to see the earliest galaxies by using its near infrared detector. The light has been redshifted all the way past the visible range and into the infrared range.

    Once you get into the sub-kilohertz region things start to act more like static fields and you can't do much communication with them. I guess they would technically be radio waves, but I think this is similar to how all light above the X-ray range is classified as gamma rays.

    Well, just because we believe there to be a maximum speed in the universe doesn't mean there actually is. Perhaps there is some undiscovered force or a weird property of spacetime that we simply don't know about yet that would allow FTL communication.
  17. Dec 17, 2012 #16
    Thanks Drakkith, those were very clear answers!
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Threads - Radio transmission question Date
B Black hole gravitational pull questions Jan 28, 2018
I Question about an equation Jan 2, 2018
B Multiverse theory question Aug 17, 2017
B Earth's radio bubble Dec 18, 2016
WMAP tend to coincide with radio emissions from the Milky Way Nov 23, 2007