Fact and Fiction

Thoughts about a funny old world, and what is real, and what is not. Comments are welcome, but please keep them on topic.

Saturday, November 26, 2005

SETI won't work

The search for extra-terrestrial intelligence (SETI) is running a long term programme that searches through radio telescope data for signals that could have been produced by extra-terrestrial intelligent sources (little green men, if you want). You can participate in this search by going to the SETI at home site, and downloading the software there. A few years ago I began to run this SETI software as a screen saver on my PC, but I never found any extra-terrestrial intelligence, and nor has anyone else.

After staring at this screen saver for longer than most sane people would consider healthy, it became obvious that the search strategy that SETI uses is totally naive, and it is bound to fail in its goal. In a nutshell, what SETI looks for are transmissions that resemble carrier signals, which are the signals that are sent out by transmitters which you "tune into" when you twiddle the dial on your radio (or press the auto-search button). Actually, SETI is rather cleverer than that, because a transmitter is likely to be moving on a curved path (e.g. it is on a planet orbiting about a star) which will make the frequency of its carrier signal vary with time, so the search takes account of the variations in the carrier frequency that this causes.

Why is searching for carrier signals a naive strategy for SETI to use?

The answer is that an intelligent transmitter would not use such an inefficient way of transmitting signals. Also, they might not want anyone to eavesdrop on their signals, so they would use a more stealthy means of communication. The most obvious alternative possibility is something that we humans already do; it is called spread spectrum transmission, which was (surprisingly!) invented by the actress Hedi Lamarr. The trick is to not use one carrier frequency, but to use a random sequence of many carrier frequencies, thus spreading the transmitted signal over the frequency spectrum in a way that makes it very difficult to receive if you don't know the random sequence used. If you combine this with appropriate "whitening" of the transmitted signal, then the spread spectrum signal is indistinguishable from background noise, and it is actually impossible to receive if you don't know the random sequence and the whitening method used. Generally, a transmitter is at its most efficient in terms of both bandwidth and stealth when its transmissions look like white noise, and the means used to achieve this could be much more sophisticated than merely being simple variations of the frequency hopping approach described above.

If we are already using spread spectrum techniques ourselves, then it is likely that an extra-terrestrial civilisation would be doing something that is at least as clever, and probably far cleverer. This is why SETI won't work.

In this week's New Scientist there is an article Looking for alien intelligence in the computational universe in which Stephen Wolfram makes the same criticism of the SETI search strategy. He then proposes that we do a search of the universe of all possible algorithms (i.e. the computational universe) for ones that have behaviours that are a cyber-version of an extra-terrestrial civilisation. This is an application of A new kind of science. Wolfram's proposal is that a cyber-version of an extra-terrestrial civilisation is as good as the real thing. This is complete rubbish, and I am surprised that he offers this as a serious proposal. It is like saying that virtual reality is the same as real reality. They might seem to be the same but they are not actually the same.

I think that the computational universe is a very worthwhile place to harvest, because there will be algorithms out there that do very interesting and useful things, and which could be used as the basis for whole new technologies. This is why A new kind of science is a very good thing that more people should pay attention to. However, this has nothing whatsoever to do with SETI.

I will caveat that last remark. There is the possibility that there are algorithms out there in the computational universe that might be used as a sophisticated way of encoding/decoding transmissions, and which extra-terrestrial civilisations might already be using. All we need to do is to find these algorithms, and we can then eavesdrop on the extra-terrestrial conversation, assuming there are no other gotchas standing in the way, such as the very interesting scenario that is described in Piers Anthony's book Macroscope; read the book, especially if your IQ is 150+.

SETI should focus on reality and not create a virtual simulacrum of reality. Remember that science is about connecting with experimental results; computational simulations are very interesting in the same way that pure maths is interesting, but without an anchor in reality they are not actually science. This distinction is the same as the difference between natural philosophy (now discredited) and natural science.

SETI should also extend their search strategy to look beyond communication using mere carrier waves, otherwise they are going to waste an awful lot of computer time hunting through the radio telescope data.

4 Comments:

At 28 November 2005 at 00:23, Blogger Wolfgang said...

Cosma Shalizi wrote a review about Wolfram's book which you may want to read.

http://www.cscs.umich.edu/~crshalizi/weblog/387.html

 
At 28 November 2005 at 00:55, Blogger Steve said...

I had already seen the "A rare blend..." review that I think you are referring to here. I don't pay much attention to such reviews; I prefer to judge books for myself. Don't forget the Parrot Effect.

Yes, I agree that Wolfram has an unconventional way of publishing his output, but I tend to "interpret" rather than "read" his material, and to mine it for useful ideas. I don't know to what extent his output is original, but I have certainly found it to be a very accessible source of inspiration, which has greatly helped in my own research.

 
At 5 May 2008 at 22:38, Anonymous Anonymous said...

Here is another reason why SETI won’t work, at least not with the Allen Array or other current equipment.

SETI (in the words of the SETI Institute) “seeks evidence of life in the universe by looking for some signature of its technology.” Primarily it searches for the radio leakage inadvertently streamed into space by extraterrestrial civilizations.

It has been pointed out (http://www.damninteresting.com/?p=219) that even using a 300 meter antenna (similar to Arecibo’s), Earth’s own radio leakage would diminish to indiscernible static at the relatively short distance represented by Saturn’s orbit, which of course is infinitesimal compared to any interstellar observation. Because of such attenuation, the detection of leakage from a radio-using civilization on a planet circling the closest other possible solar system, at only 4 light years distance, would require an antenna measuring 33,000 kilometers in diameter. Focused beams are more easily detectable, but not by much on the interstellar scale, and one is persuaded that in the vastness of space it is very unlikely that any extraterrestrial beams would casually impinge upon Earth.

It is nice that Mr. Allen has financed the SETI array bearing his name, but neither this (with a 100 meter equivalent diameter) nor anything else on our planet comes remotely close to the minimum basic requirement for the detection of radio leakage from even our nearest potential neighbors. In fact, the current antennas are less than 0.001% of what is needed to get started, aside from the sensible questioning of the nature of any extraterrestrial signals.

One wonders why SETI proponents completely skirt this fundamental detection issue. The SETI Institute does inform its sponsors about its sophisticated instrumentation, but neglects to mention that it is all connected to an antenna array which will be just as effective in receiving extraterrestrial signals as a toaster.

 
At 6 May 2008 at 09:25, Blogger Stephen Luttrell said...

The figures are pulled out of thin air on the web page you mention here, so I have to assume they are correct. Of course, a beamed (rather than omni-directional) transmission would survive the ravages of distance much better, though the inverse square law would eventually win.

The Allen Telescope Array has a large synthetic aperture, so although it may intercept only 100sq.m (your claimed figure) of the signal wavefront, it can be used to form a very narrow synthetic beam, which thus gives a large receiver gain. However, if the signal is truly noise-like then you can't get any extra gain by beam-forming in frequency-space (or whatever), which will limit the types of signal which the Allen Telescope Array can detect.

A truly omnidirectional signal will weaken as the inverse square of the distance from the transmitter, but if it is transmitted as a narrow-band signal (i.e. not spread spectrum, as assumed in my original blog posting) then this will make it much easier to detect against background noise.

If a pseudo-random code is used to create a spread spectrum signal, then you are forced to detect pseudo-noise against background noise, which makes things much more difficult. We are then back to the worst-case scenario (i.e. spread-spectrum) that I described in my blog posting.

The worst case of both spread spectrum and omnidirectional transmission would make things very difficult for the receiver. However, I suspect that the signals would be beamed rather than omnidirectional.

 

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