> If we did receive an intelligent signal from an extraterrestrial source, what form do we think or hope it would take?

For unintentional signals, I think it would take the same form of the early radio broadcasts from here on earth. These are likely some of the weakest signals but they have also traveled further than any other at this point.

These would probably be fairly difficult to detect. The vast majority of the radio messages we have leaked into space over the last 100 years or so weren't focused. By the time they reached a neighbouring star they might be hard to distinguish from background noise. Also let's say someone did manage to detect these signals and they were 80ly's away. They might not have detected them until say 2000, and if they immediately send back a message, we would not receive this until 2080.

I think our best hope is that an Alien civilisation has begun the search for extrasolar planets, like we have. They will be able to analyse the composition of our atmosphere using spectroscopy.

With this information they would either decide that life either exists or is very likely to exist here, and so would begin transmitting a constant and unmissable signal to us in the hope that we would eventually detect it. Providing we manage to monitor the right frequency and look at the right point in the sky..... as life has existed here on earth for Billions of years, perhaps this signal is already been sent to us and we just need to find it....

It's a long shot but perhaps it's the best one we have.

Why is that? We just didn't record it?

1977. The intensity data was the recording according to the standard of the times.

Having read about historical radio astronomy and practiced signals analysis, I think they were a little more sophisticated than that. The recording in this format was made for easy perusal by a human, say, the next morning. I think that was the case with this signal. No doubt they wanted to revisit that spot in the sky looking at intensity, frequency shifting (Doppler), bandwidth, modulation, etc. But they never saw it again.

Historical point of interest: the Big Ear telescope that recorded this was bulldozed to make way for a golf course.

There's still a "memorial" web site:

http://www.bigear.org/about.htm

The Big Ear telescope "was larger than three football fields in size and equivalent in sensitivity to a circular dish 52.5 meters (175 feet) in diameter. The telescope consisted of a flat tiltable reflector measuring 340 feet long by 100 feet high (less when tilted), a fixed standing paraboloidal (curved) reflector which measured 360 feet long by 70 feet high, an aluminum-covered ground plane measuring 360 feet wide by 500 feet long, and two feed horns mounted on a movable assembly."

In 1977 I suspect recording equipment was too expensive for radio telescope work when an alphanumeric graph can suffice.

This is still a problem. I attended a talk a few weeks ago by a member of staff on the Square Kilometer Array Science Data Processor project, and they were talking about data volumes on the order of an exabyte... per day.

Obviously, an awful lot of pre-processing has to be done to get this down to a manageable data volume for analysis (100s of TBs), but if something gets lost in processing in the early stages, it's gone forever. Even managing and archiving the processed datasets will be a challenge.

Oh, and the second stage of the project was going to add 100x to those data volume numbers.

Some day, 30 years from now, people will look back on the term "big data" and laugh at how naive we were.