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Cellular phones and reverse SETI

  1. Jan 3, 2010 #1
    Many years ago we started broadcasting radio signals for terrestrial use which inadvertently also radiated out into space. Possibly a civilization with appropriate equipment many light years away could have already detected these signals.

    But recently things have changed. By some estimates there are now 4.5 billion “new” small radio transmitters in use: mobile phones. Each phone transmits about 1 watt so the total power would be about 4.5 billions watts (4.5 gW)

    I am not sure of the physics here, that’s why I am asking!

    The basic question is simple: Are mobile phones making it easier for extraterrestrial civilizations to detect us.

    Obviously not all of this 4.5 gW radio signal would be effectively transmitted, something like the Drake equation might apply.

    Not all of the phones are on, say 0.60 are on at any one time

    Even if the phones were uniformly distributed over the Earths surface not all would be visible, say only 0.40 would be in line of sight to a distant point.

    In addition to the phones there are also the transmitter towers, these are not as numerous but possibly do make a positive contribution, use 1.20

    So 0.60 x 0.40 x 1.20 x 4.5 gW = 1.296 gW

    Much less then 4.5 gW but 1.3 gW is still a significant radio signal.

    Is this argument reasonable? Does any of the radio signal from mobile phones even make it out of our atmosphere? How does this compare with the Earth’s existing total artificial radio emission?
  2. jcsd
  3. Jan 3, 2010 #2


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    Phones emit only when you are speaking or when they communicate with the net, that means only minutes per day.
  4. Jan 3, 2010 #3
    Yes, if correct that would cut things down quite a bit. Might not apply to teenagers who seem to be talking on their phones all the time!

    Do you have any idea what is the "stand by" power for a typical phone?
  5. Jan 3, 2010 #4


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    I don't think cell phone signals are cumulative. 2 cell phones, each emitting 1 watt is not the same as a 2 watt signal.
  6. Jan 3, 2010 #5


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    Zero. For most of the time they sit idle, once for a few seconds they switch to "active listening" - that is, they look for a signal that is adressed to them (incoming call). But even then they don't emit anything yet (well - they probably do, as any working electronic device, but that's not the signal we are talking about). Only if there is an incoming call they start to emit signals to communicate with the closest station.
  7. Jan 3, 2010 #6
    Cell phone bands are easily swamped out by TV and radio stations which transmit in Kilo-Watt and Mega-Watt range, whereas a cell phone transmits at 100 milli-Watts when it has poor reception. When close to the tower, its transmitting power is reduced.

    Having millions of these 100 mW pests leaking signal uniformly around the earth doesn't increase the signal power which would be crucial in deciphering it, but rather it increases the average luminosity. Luminosity is also measured in watts, but it is a total energy radiated per unit time which would look brighter in the sky in a very narrow microwave band. But regardless, given the vast interstellar distance, low luminosity is blended with the background noise, and only the strongest signal will survive, or travel the farthest before succumbing to noise floor.
  8. Jan 3, 2010 #7


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    GSM and TV/radio frequencies are about a GHz apart, while it makes some sense to treat them both as combined signal, aren't they easily separated once detected?
  9. Jan 3, 2010 #8
    yes, the frequencies of GSM and TV are close to each other relatively speaking, and assuming that an alien civilization has a dish with a diameter of a moon, and are less than a dozen light years away, the first thing they would see is a spike from the TV/radio stations a few decibels above the noise floor. That's like seeing a gamma ray burst. On the other hand, the combined radio flux density from GSM cell phones would be like a white dwarf analog, radiating low power which would blend in the noise floor quicker than TV/radio. And perhaps with supercomputing and years of integrating time, it might be possible to extract a presence of a signal in the GSM.
    Last edited: Jan 3, 2010
  10. Jan 3, 2010 #9


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    That's exactly opposite of what I thought - correct me if I am wrong, but as far as I know TV and radio operates somewhere around 100 MHz, while GSM - depending on the version - uses approximately 800 MHz or 1.6 GHz frequencies. I feel like that's a huge difference, so it is easy to separate these signals.

    By separate I don't mean being able to pick individual signals, just whole Earth as a source should have different luminosity in different bandwidths - and I always thought spectrum should be relatively easy to analyse and realtively difficult to explain by natural phenomena.

    Large antennas are required for high angular resolution, are they also necessary for spectrum analysis? And we are not talking about very precise analysis, we are talking about differentiating between signals of frequencies different by order of magnitude.
  11. Jan 3, 2010 #10
    yes, that's pretty much where TV operates from 50 - 800 MHz) and radio at 100 MHz. Cell phones 800/900 MHz and 1.8/1.9 GHz. The lowest and highest differ by more than (x10), but their differences (bandwidth) is about 1.5 GHz, and that's a small blip in the vast electromagnetic spectrum. That's why I said "relatively speaking."

    For example, the difference in frequency between a green color and a red color is about 200,000 GHz.

    yes if we are close enough to the source, we could deduce what kind of quartz crystal is inside the phone, and what is the change in ambient temperature from day to day, but in interstellar space the signal becomes very weak and with our current technology it poses certain problems to pick it up.

    yes, large antennas need to collect alot of radio waves for analysis. But the problem that plagues even the best radio receivers lies in the LNA (low noise amplifier) that is sitting at a focal point of every dish.

    Every electronic circuit generates thermal noise from random motion of electrons, and that noise gets amplified by the LNA, and there is power associated with noise - the noise floor.

    Some of the static that you hear in an un-tuned radio comes from such thermal noise being amplified, some from local interference, and some from cosmic microwave background.

    So if you want to pick up a weak signal whose power is less than the power of the thermal noise of the LNA, then it will get buried in the noise as well, and will never be detected (unless using some DSP magic to improve it a little bit, but that's as good as we get)

    Our current LNAs used in most sensitive radio telescopes are very impressive, but still not anywhere close to be able to distinguish another earth a hundred light years away.

    That's why the only signal that has a chance of being picked up must come from the strongest source.
    Last edited: Jan 3, 2010
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