How do scientists working on the SETI program expect to get clear and intelligeble signals from aliens in other solar systemes,when tv and some radio stations sygnals become distoreted and unubtainable after short distances on earth? Wouldn't the vast distances of interstellar space have same effect on tv and radio signals?
There are two main effects that propagation causes on signals:
1. Attenuation: As a signal propagates, its amplitude goes down as most media are lossy. In space, the flux (which determines the amount of signal you can get using your telescope) goes down as the square of the distance traveled. Hence, the receivers that one uses must be sensitive enough to pick up very faint signals. The sensitivity of the receivers used sets a limit as to the distance that one can probe in the search for extraterrestrial intelligence.
2. Dispersion: This occurs to all signals that propagate through interstellar space. What is dispersion? Imagine that the signal is the sum of a large number of signals, each at a specific frequency. In interstellar space, which is not actually perfectly empty (there is very very low density gas and dust), signals at different frequencies travel at different speeds. Hence, the final signal received will be different from the original signal transmitted.
Fortunately, SETI does not suffer from the second problem as one searches for narrow band signals only. Many astrophysical processes cause objects to emit broadband radiation and so it will be difficult to distinguish a broadband signal as from an intelligent source. However, a narrow-band signal (similar to the radio transmissions in Earth) is more likely to be artificial than natural. As a result, most SETI concentrates on finding narrow band signals, and these do not suffer from much dispersion.
Further, the search is done at frequencies that can pass through the Earth's atmosphere. Typical searches are done from 1000–3000 MHz, and these frequencies are not absorbed by the Earth's atmosphere. In contrast, the signal frequencies used for radio and TV transmission are absorbed or reflected by the ionosphere.
The other main problem is to distinguish genuine signals from space from those from Earth itself, and this is called radio frequency interference. There are techniques for doing this, but I won't go into that. You can also check out this website and it will answer a few of the common queries regarding SETI.
This page was last updated June 27, 2015.