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Thread: the bandwidth of little green men

  1. #1 the bandwidth of little green men 
    Forum Sophomore Vaedrah's Avatar
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    Given that interstellar journey's based on our current understanding and available technology is likely to represent a long drawn out multiple generational initiative, it may well seem that the need to communicate in familiar earth-like terms would be open to speculation and rebuff.

    It follows therefore that any signals we may intercept, or any signals that we may choose to intersperse the universe with may operate at very slow rates and therefore associated bandwidths.

    For example, why send data at 100 MB/s if any subsequent journey to a recipient's destination was 100 light years away? Surely the need to send information in a few microseconds is idiotic when compared to 100 years or more of any subsequent travel?

    It follows that the realistic bounds for data transfer rate will be government not by common place technologies like 802.11a/b/g, bluetooth, zigbee etc, but by;

    1. The number of bits needed to send a incontrovertibly non naturally conceived (i.e. made by intelligent design) message and

    2. The time available for interception of such a message


    The SETI radio project doesn't seem to have considered either of these two concepts. SETI looks for "sidebands" on a carrier at ~ -6 dB, corresponding to amplitude modulation at some human rate (i.e. on-off pulses). These are certainly not incontrovertibly created by intelligent design as period occlusion of a radio source could occur in nature (e.g. by an orbiting object). Further, carrier energy can also appear as discrete lines as we see in light spectra, so the existence of a narrow band carrier frequency represents very poor evidence for interstellar intelligence.

    It is also an obvious fault to look for such modulations that could be, say 10 kHz in some relation to our physical experience. Perhaps it would be way smarter to look for very low frequency modulations.

    We can expect some similarity between our biological makeup and extraterrestrials, certainly the opposite assumption that we would be completely unique has no supportive evidence. Equally, if seeking life we anticipate vaguely similar environments to give rise as such, then our planet and solar system probably represent as good a model as any for initial searches.

    Our planet turns once every 24 hours and probably has a "good view" of space every 8 hours of such a period. We would be ill advised to send messages ourselves that exceeded 8 hours in total length now wouldn't we? Perhaps some extraterrestrials come from faster rotational worlds or slower? Any sensible human would allow for this and propose a realistic bound say ten:1 to our_rate to 1:ten - why not.

    Certainly the SET approach could be assumed to ignore this simple reasoning all together.

    The next question is how to make an controvertible signal, whether we do this or if we receive one. Simple on-off pulses are brain dead so why look for these?

    An on off sequence is realistic but needs a format.

    The easiest format is a binary 0,1 sequence, but what is a "0" and what is a "1". This is not quite the same as off or on.

    The only sequence that makes credible sense is a prime sequence. For example, 1,2,3,5,7,11 is a minimum prime sequence MPS (as I may have suggested before). Further, "2" is the only even prime. Therefore 1,3,5,7,11 should be "1" pulses based on their quantity and "2" should be "00" based on gaps between.

    so, 00 10 00 101010 00 1010101010 00 10101010101010 00 1010101010101010101010, 00 10 00 101010 00 1010101010 00 .....

    is a minimum prime sequence that nature would have an incredibly difficult time to mimic. If we received this minimum prime sequence we would have reasonable evidence of a non natural generating source. Weak sequences just like 01010101010... are pathetic in comparative evidence terms - so why do people look for these?

    The MPS has 64 bits of data i.e. 5 dibits of 2=>0 (10 bits), 1+3+5+7+11 = 27*2 bits => 64 bits total.

    So, based on our physically relevant time experience we could expect 64 bits per 8 hours as representing an adequate data rate to look for. Perhaps we might like to receive the message several times to compare bit errors and be sure; so 192 bits per 8 hours would be sensible. Further, allowing a 10:1 upper margin, perhaps 1,920 bits per 8 hours would be intelligent?

    1.92kB per 8 hours = 240 bits per hour = 4 bits per minute = 0.067 bits per second.

    Given this, why do we look for unexplainable fast data rates in radio SETI?

    The real bandwidth expectation will be about 100 milli-hertz based on this very simple reasoning. It is hard to defend the use of extreme bandwidth like MHz especially given that weak signals require very low bandwidths for detection.

    If we are to detect extraterrestrial signals, or to expect such folk to receive ours, then it is obvious that power constraints suggest these signals to be weak, and therefore bandwidths should equally be at a minimum.

    __________________________________________________ ______________
    Arecebo/SETI looks for microwave signals around the spectra wavelength associated with hydrogen (water). Regardless of this (equal folly), when you are looking for a "needle in a haystack" why make the needle so small? A coherent detection system requires

    1. A precise knowledge of the actual carrier frequency and
    2. A phase noise (sideband noise power) performance that is adequate for very narrow band demodulation.

    Given that microwave reception based on traditional superhet approaches will be limited by oscillator phase noise, demodulation bandwidths less than a kHz or so are probably unrealistic. Even if ultra pure LO sources were made available, a multiple receiver bank of 1 kHz "channels" or less would be needed to cover any anticipated band segment where doppler shifts etc may result in frequency offsets.

    It equally, as I have reasoned, appears that a coherent detection approach as per SETI is equally interpretable as folly as looking for some primitive on-switch, off switch message profile. Whilst this level of creativity might be attributable to Homer Simpson, my guess is that extraterrestrials might be a bit more enlightened.

    It may be better therefore to consider incoherent scalar detection approaches. This "crystal set" approach converts wide band input signals to a DC output corresponding to some aggregate of received energy. In optical SETI approaches this could be based on a simple PIN photodiode + optical focusing lens/reflector receiver.

    This alternative approach now allows ultra narrow band demodulation best suited to a reasonably expected very low data rate. The ultra low phase noise requirement for microwave (or optical) local oscillator injection is avoided as scalar (square law) detection doesn't use a LO. The output signal can be placed on a sub carrier (to overcome 1/f noise issues) but if these sub carriers were just a few kHz with bandwidths of 100 milli-hertz then any computer sound card could capture them and using software for subsequent number crunching is simple enough.

    Square law detection is less sensitive than coherent detection, but we can make some reference to our own familiar performance of sight. For example, we can easily see a car's headlight at night even 10's of km away. The bulb might only be 60 watts and even if it's efficiency was as high as 10 %, only 6 watts of optical output would be available. Even then, we could even see light from the side, so the portion radiated in our direction could be extremely small. And yet, we will see the car headlight with a simple "biological square law detector" we know as an eye!

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    Many people ask, "if intelligent life is out there, how come SETI hasn't heard from them". Maybe some candidate signals have been intercepted but is a simple on-off sequence credible? Or is such a puerile expectation insulting to the possibly little green or grey aliens out there?

    Certainly if we adopt a fairly dumb search for high bandwidth signals when we have obvious knowledge that little usefulness is extended in high bandwidth formats, then we can expect such failure or doubt. Further, the modulation format would need to be simple - ASK is definitely in this class. Digital modulation formats require a specific format and unless this format is exactly known, cannot be decoded. Any race with intention to self advertise to the universe, most clearly, would not adopt a specialized modulation format. It is far more likely, from simple reasoning, to adopt simple ASK based on an initial MPS of binary bits.

    Any remote aliens would not need intelligence in advance of our own, but perhaps just some motivation not to engage in folly.

    (If such extraterrestrial life is biological (non robotic/cyborg) then its brain will probably have similar energy needs as our own, e.g. 25 watts only. This is about the lower end of a conventional incandescent light bulb. Similar energy, possibly similar neuron size and count.)


    "The sky cannot speak of the ocean, the ocean cannot speak of the land, the land cannot speak of the stars, the stars cannot speak of the sky"
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  3. #2  
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    I partly agree. Even the messages we've sent out into space have usually been things like music or TV signals, that a computer would have a very hard time decoding into something useful.

    A stream of bits would be the simplest way to get someone's attention, and if you want the signal to go really really far, you have to account for the possibility that it will become red shifted a lot before it reaches its target.

    Still, low frequency transmissions have the advantage that they can pass harmlessly through space dust, background plasma, and even solid rock, at least compared to a high frequency signal that will get distorted by anything it hits.

    An 8 hour long signal is kind of un-realistic, though. One consequence of the signal becoming stretched out by the hubble red shift is that it also becomes stretched out in terms of how long it takes to be received. They'd have to keep the signal short in order to make sure the final recipient(s) has(/have) time to receive it. (Not to mention that they might want people on planets with faster orbital periods than ours to hear it).


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  4. #3  
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    Yes kojax. TV signals are about 10 MHz wide, times wider than a 100 mHz signal.

    Also red shifting is a fundamental problem for coherent detection but not for scalar detection.

    Even if the message length is "stretched" it may only be a few percent at worst - easy to accommodate in computer processing approaches. So taking a longer term signal acquisition approach fits within our cosmological expectations. In stark contrast, expecting to receive TV signals and associated massive bandwidths is difficult to defend as being plausible. Low bandwidth equates directly to best signal to noise ratio (SNR) and given long distance traveled with constraints on available power, the bandwidth is only best determined from the minimum message length required to incontrovertibly send a non naturally produced message and the time available for its reception.
    "The sky cannot speak of the ocean, the ocean cannot speak of the land, the land cannot speak of the stars, the stars cannot speak of the sky"
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  5. #4  
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    So, as I understand ELF (Extremely Low Frequency) transmissions, I've been told the US Navy has been experimenting with their use, but it's causes problems for whales because they use ELF's to communicate, and they can mistake submarines that use ELF's for other whales.

    So my point is: You think maybe whales might occasionally pick up broadcasts from space? It just be really funny if extra-terrestrials were in contact with them, but not us.
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  6. #5  
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    I didnt read all of the question but i did realise the first part of communication with other life forms.

    i was just wondering how we would communicate. its not as if they would understand simple hand signals because its humans who made them. im sure that they would have their own way of communication.
    also im pretty certain they wouldnt look like us either, due to the fact thier home planet would be much different and their evoloution would have been much different to ours and would also not eat the same food as us.

    please help
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