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Thread: How far away is the oldest radio signal?

  1. #1 How far away is the oldest radio signal? 
    The Doctor Quantime's Avatar
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    It's near 75 odd years isn't it? How many stars could it have passed by now. Its clear that no intelligent life lives on Gliese 581 c or they would have sent another message back by now. So again, how many stars has it passed?


    "If you wish to make an apple pie from scratch, you must first invent the universe". - Carl Sagan
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  3. #2  
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    Almost 1300 stars. But most of them would be unlikely to harbor life.

    191 of them are of a like spectral class as our sun (G).


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  4. #3  
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    How far it is? The answer is, "nowhere".

    The first radio emissions (experimental ones) are about a century old, yet the point is, how far can travel a radio emission? The answer is, not very far in universal scale.

    I recall that one estimate said that our current level of radio emissions in all frequencies would be undetectable from 20 light years away, as it would be undinnguishable from the Sun's own random radiation.

    First, because most radio waves never leave our planet, rather are absorbed/bounced down by the ionosphere.

    Second, because even if they leave our planet, they will not meet a perfect void. The solar system is relatively more dense in matter than interstellar space, so it will drain a bit of energy more.

    Then there's the old problem with electromagnetic atenuation -signal loses power by the cube of the distance. 2 times farther means 8 times weaker. 3 times farther it's 27 times weaker. 4 times farther it's 64 times weaker... this quickly kills the signal. Most signals lose all coherence and modulation in the first light year or so. Then they become noise, noise in unusual bands, but noise. And this noise, the overall combined noise of Earth, is undistinguishable from background radiation of the Sun at 20 light years. It's been too absorbed, too scattered, to be noticeable against the natural noise of the Sun. Like a candle sitting by a searchlight seen from the moon, it's impossible to notice it's there.

    The only exception are the radio emissions used in SETI programs; they take advantge of a very specific wavelength which is not blocked by hydrogen and so can travel very far across the interstellar space.
    “If the misery of the poor be caused not by the laws of nature, but by our institutions, great is our sin.” -Charles Darwin
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  5. #4  
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    It's a possibility that if there was any other intelligent life out there to pick up our signals that they might decide that we might be more trouble than we are worth and it was best not to respond and keep silent?

    After all humans seem to relish enjoy killing and blowing up each other so why should other life believe they might get a warm welcome?

    If i observed earth and human activity from a distance, i think i would probably prefer it to stay that way
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  6. #5  
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    The early radio transmmisions would have been on low frequency which would have been blocked by the inosophere.

    VHF-FM radio or TV transmissions do escape the inosphere but only travel a few hundred thousand kilometres . a few million if you are lucky before becoming too weak. They only last a matter of seconds before they would be below the noise floor of a distant receiver.

    You would need transmitters of massive power feeding very large aerials to get anywhere near one light year. Even the SETI Institute say our transmitters are too weak.


    ------------From SETI Institute...

    """If an extraterrestrial civilization has a SETI project similar to Project Phoenix, could they hear Earth?

    In general, no. Most earthly transmitters are too weak to be detectable by Phoenix-type equipment at the distance of even the nearest star. To detect "leakage" radiation similar to our own will require instruments that are many times more sensitive than what we now have."""


    http://www.seti.org/about-us/faq.php
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  7. #6  
    Time Lord
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    It would be funny if someone somewhere has picked up our signals, and radioed us back, but the signal is still on the way (or too weak for us to pick up)
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  8. #7  
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    A broadcast FM receiver might work down to 10E-15 Watts (including a bit of aerial gain).. A tiny signal.

    A transmitter might be 100 kW.ERP (Effective power with transmit aerial gain).. 10E5 Watts.

    So the signal path loss maximum is 10E20 times (diff between power transmitted and that received) . A very big number.


    There are on-line calculators to work at the path loss at different frequencies.. At 100 MHz 10E20 (200 dB) occurs at a distance of 2.5 Million Km..


    http://wireless.per.nl/reference/chaptr03/fsl.htm

    Speed of light is 300,000 Km /sec so the signal will travel for about 8 seconds before becoming too weak.

    2.5 Million km is 1/4,000,000 of a light year. So not very far at all.

    Narrow band communications will travel much further but even then I think the record distance for communication with a space craft is only 1/600 of a light year.


    As someone said signals get a bit garbled over such long distances. They have probs communicating with spacecraft.. There is a 'group delay' problem maybe partly due to the ionosphere but over such large distances very slight differences in the speed of propagation at the different frequencies in a wideband signal cause different bits of the signal to arrive before others.
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