Wouldn't the vast distances of space distort SETI signals into unintelligeble forms?
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 you know, 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 ETI.
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, 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.
Get More 'Curious?' with Our New PODCAST:
- Podcast? Subscribe? Tell me about the Ask an Astronomer Podcast
- Subscribe to our Podcast | Listen to our current Episode
- Cool! But I can't now. Send me a quick reminder now for later.
How to ask a question:
If you have a follow-up question concerning the above subject, submit it here. If you have a question about another area of astronomy, find the topic you're interested in from the archive on our site menu, or go here for help.Table 'curious.Referrers' doesn't existTable 'curious.Referrers' doesn't exist
This page has been accessed 18577 times since October 23, 2002.
Last modified: February 2, 2004 5:02:13 PM
Ask an Astronomer is hosted by the Astronomy Department at Cornell University and is produced with PHP and MySQL.
Warning: Your browser is misbehaving! This page might look ugly. (Details)