Joined: Jan. 2006
|Quote (Wesley R. Elsberry @ Sep. 11 2007,15:31)|
|IIRC, the SETI FAQ discusses how signal content is lost based on distance and power, and that by only looking for the narrowband carrier, they can expand their search radius for signals to something like 3000 light years. If they limited their search to signals to those where content could be recovered, the search radius would be a fraction of that.|
IOW, the odds are that if SETI is successful, it will be because they picked up evidence of a narrowband carrier from a source too distant to extract content from.
If SETI finds narrowband carriers, then they've discovered a civilization that's not much more advanced than ours.
In a standard AM transmitter, the carrier takes one half of the total power transmitted and it is a pure waste of energy. The receivers beat/mix the sidebands with the carrier to recover the original modulation. If you can generate a carrier locally, in the receiver, you can beat/mix the sidebands with it and the transmitter can stop transmitting the carrier and save the energy.
Single sideband transmitters and recievers do just that. The transmitter just transmits one sideband and the receiver generates a signal to mix with it and the original audio pops out.
Presently, the earth is transmitting scads of high powered narrow band carrier signals. They're called television stations. That are all scheduled to go to digital modulation in a couple of years and that will be the end of their carrier signals because the digital transmitters don't waste energy transmitting them.
That would leave FM radio stations as just about the only source of carriers that make it through the atmosphere and they won't last much longer. HD radio is ready to replace them as soon as enough people are equipped to receive them. The HD signals don't transmit carriers either.
Then there will only be high powered radars searching for incoming ballistic missiles for alien SETI researchers to hunt for. Hopefully we'll get rid of the missiles soon and then the radars won't have anything to look for, so they'll be turned off. What's that - about a 150 year window for a civilization to be easily detectable?