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Questions based on contact

  1. Sep 5, 2015 #1
    Hello I am new here and my question is this:

    Spatially, what is the distance EM travels through space before redshift alters the wave to the extent that a radio signal becomes a microwave signal?

    Also when a radio signal containing information redshifts to the microwave band is the information retrievable still? Such as audio?

    These questions are based off the movie contact.

    is it correct that we cannot as yet, boost a signal, we can merely relay or jump a signal?
    And finally what makes a signal strong as opposed to weak? The shorter a wavelength the higher the energy. This I understand but I do not understand when ones says we have a powerful broadcasting antennae.
    If one has a receiver, why would one say the signal is faint when there isnt enough distance in order for redshifts effects to be pronounced

    once these questions are answered I eventually want to determine whether there is A shell threshold distance aroundearth such that we can determine a distance of nem front which can rule out any other EM front conflicting with ours in the local region. What would happen when the noise front of ones planets radio and video signals clashes with another?

    In the movie contact the star Vega received our signal which was 65 light years away or approximately but the signal wasn't redshifted did the movie assume that they were able toreconstruct the signal using technology or was there not enough distance involved in order for the signal to be red shifted??

    Thank you
    Last edited: Sep 5, 2015
  2. jcsd
  3. Sep 5, 2015 #2


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    The redshift depends on the expansion of the universe during the travel of the light. Not only on the light travel time. Regardless, 65 years with the current expansion rate is way (waaaaaaay) to short for cosmological redahift to be noticable. Also note that 65 lightyears does not even get you out of the galaxy, which is a bound gravitational object. Cosmological redshift does not apply on these scales.
  4. Sep 5, 2015 #3
    Thanks unfortunately I previously knewall the information in your reply. I know that 65 light years is relatively short but shift does occur in that distance albeit small. What I am trying to acertain in an understanding of the metrics here. Such as an EM signal will degrade by one bandwith number per x amount of travel time/space.
    Thanks again
  5. Sep 5, 2015 #4


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    Only peculiar motion would affect the redshift of Vega, as already noted. Vega is actually moving towards earth at about 14 km/s which causes a slight blue shift in its spectrum.
  6. Sep 5, 2015 #5
    What im ultimately attempting to establish is if there is a way we can determine that there are no civilizations that have broadcasted in radio within a certain distance from us. I understand that radio begins at 3ghtz and becomes an infrared wave after 300ghts.

    I would like to know the distance and/or time that a radio signal will travel, assuming its emminated at 3ghtz, for it to redshift to 301ghts

    This is the distance from earth which we can safely say no civilizations with similar technology to ours exist.

    If redshift didn't exist, we could already rule out the entire universe.
  7. Sep 5, 2015 #6


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    Red shift isn't the issue, it's the falloff of signal strength. Signal strength falls off by the square of the distance. The distance at which a signal becomes unreadable is where its strength has fallen to the point where it is overwhelmed by the natural back ground radio noise.
  8. Sep 5, 2015 #7
    thats answers my question partiality. So signal strength is main culpit. That makes the job of ruling out eti's more challenging radio communications isnt effective for long range communication. And directional lasers are also less effective at long range unless you know where the paths of least obstruction are in your local galaxy.

    We may still be able to rule out a certain distance though based upon radio signal strength drop off assuming eti's are using high powered sophisticated radio.
    Its my understanding that voyagers signal outside our solar system is almost undetectable. If advanced species are using radio, than they have fixed relay stations and/or far more powerful broadcasters on their probes/ships

    Aif all these assumptions are true we would have heard something by now.
    thanks for your replies
  9. Sep 6, 2015 #8
    Also, you cannot hide your planets initial em front. So there are 5 possibilities for our non detection

    1 their signals haven't reached us yet
    2 they're to far away to detect their radio signals.
    3 they have since stopped broadcasting in radio before humans started listening
    4 they use radio but from the very start or soon after they found a way to mask it.
    5 their radio is totally directional
  10. Sep 6, 2015 #9


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    you seem to misunderstand red shifting
    when an EM signal is red shifted it lowers in frequency, not increases

    you understand incorrectly

    if that were the case, radio, TV and cellphone ( and many other) transmissions, which are all below 3 GHz wouldn't exist

    Also please get used to using the correct scientific terms :smile: Hz, kHz, MHz, GHz etc

    UHF ( Ultra High Frequency) radio signals go from 300 MHz to 3 GHz
    SHF ( Super High Frequency) microwave radio signals go from 3 GHz to 30 GHz
    EHF ( Extra High Frequency) microwave radio signals go from 30 GHz to 300 GHz
    THF ( Tremendously High Frequency) microwave signals go from 300 GHz to 3 THz

    Lower frequency bands below UHF include ELF, LF, MF, HF, VHF and a couple of other low ones

    As Orodruin said, red shifting of EM signals within our galaxy isn't something that needs to be considered in any general calculations
    that you would be dealing with

  11. Sep 6, 2015 #10


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    would depend on the age of that civilisation and its technological development

    a possibility if their signals are just too weak to detect against the background noise

    yes maybe, but that is also a technology thing and their older signals would still be travelling out into the galaxy anyway even is all the transmitters were shut off

    possibly but not likely

    that would be a disadvantage for their own use ... broadcast signals need to be non directional to make then useful for a good area coverage

  12. Sep 7, 2015 #11


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    I do not understand what you mean by the word "boost". You can relay a signal from the edge of reception from one transmitter by receiving it and re-transmitting it. You can either just use an amplifier (which amplifies all the shash and erroneous bits of the signal) or decode it, tidy it up , using error correction and then re-transmit it. That's (very nearly) as good as starting from scratch at your remote relay site.
    It is not a difficult matter to compensate for red shift by merely setting your receiver to a slightly lower frequency. In optical terms, you can see, with a spectrometer, the characteristic lines from elements in a distant source and, by looking at the patterns - just red shifted by a bit, you can determine what elements are there and their relative abundance. That is effectively getting information from a red shifted signal.
    If you are hoping to spot radio signals from extra terrestrials then any red shift would be pretty much ignored because you wouldn't know what the original carrier frequency was and you would just 'scan' your the signals from your radiotelescope to find 'some sense'.
    So far, I am not aware that CETI or others have actually identified any thing.
    If you want a more sensitive receiving system then the ultimate would have to be a massive receiving dish, out in space, which never points at any nearby interfering sources (Sum, Earth etc) and which could be a big and sensitive as you want (or can pay for). Problem is that sensitivity is always at the cost of directivity or bandwidth so you could look 'further' but with an incredibly narrow receive beam so you could miss something important that was just a smidgen away from where you were looking.
    It would depend on where their transmitter is situated. If they used satellites to broadcast to a planet(s), then the 'broadcast' beam(s) would be pretty narrow and mostly blocked by the target planet. Omnidirectional antennas are mainly for terrestrial sites in the centre of a service area. (Even then, they are vertically directive.)
  13. Sep 7, 2015 #12


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    Yeah, I was thinking of ground based radio, TV other commercial and maybe even amateur type transmissions
    or even those purposely aimed into space
  14. Sep 7, 2015 #13


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    Thinking about it, it is highly unlikely that we would pick up transmissions that were actually aimed at the local population of this new civilisation. What would be the point of them putting out enough TeraWatts (GoogleWatts) of power to reach Earth, if all they wanted to do was serve the locals (a few tens of millions of miles perhaps)? That would be really lousy Engineering and these guys would actually need to be pretty advance Engineers. The only reason we would be likely to receive a signal would be if it was deliberately beamed in our direction (by chance) and at the same time as our very sensitive and directional receive antenna happened to be pointing in their direction.
    The other alternative would be if the signals were sent and received in such a ridiculously narrow bandwidth that they could be dug out of the noise. Then, you could imagine a 'sensible' power of transmitter could be picked up by a 'reasonable' receiver. Problem there is, however, that you'd be dealing with μBaud data rates and waiting for years until the signal could be decoded.
  15. Sep 7, 2015 #14
    In the movie contact, "vegans" received our omnidirectional radio signal from hitlers olympic games broadcast. until recently, I never asked more questions about the premise that earth has a 360 degree and 80 light year longem wave front. While this is true I didnt acount for background noise and signal strength.
    and as was mentioned earlier, when distance is long enough for redshift to impact the signal, that theres no way to extrapolate what frequency it was originally broadcast on nor would it be decodable.

    I believe the signals are out there but for any eti's including us to detect them we would need a vast sophisticated star trek like network of radio receivers.

    Of course if they dont want to be detected lazers tranmitters using light to communication would be better since to be detected you would need to be within the beam itself.

    I believe a work around exists to the no communication theorem

  16. Sep 8, 2015 #15


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    AS in the hokie kokie -That's what it's all about.
  17. Sep 8, 2015 #16
    What? Is AS?
  18. Sep 8, 2015 #17
    Whats hokie kokue?
  19. Sep 8, 2015 #18


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    as = a word = eg as in it was in the past

    or hokey cokey

  20. Sep 8, 2015 #19
    Sorry I meant to post this on a physics forum not reddit, I see im in the wrong place
  21. Sep 8, 2015 #20


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    People have pointed out other errors, but that one is wrong too. TV and radio signals are sent at lots of different frequencies and are decoded in much the same way. All it takes is tuning into the right carrier frequency and figuring out how it is modulated. (which is basically how it went in the movie)
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