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The Search for Extraterrestrial Intelligence (SETI) in the

Optical Spectrum


Floor: I think that if we are trying to communicate with other intelligent beings, we have to ask ourselves: Is it the engineers that are going to motivate it, or is it the politicians of the other life forms that are going to motivate it? If it is the politicians of the other life forms that are going to motivate the communications, the assumption is that they are in control of the resources as opposed to the technology, then the question is; "What is their motivation?". If their motivation is to interrogate as opposed to communicate, then it may be that they don't want us to know that we are being interrogated. If their motivation is to communicate, then it's as simple as; if they are visual beings, they would wave or make a noise or do something that would be bringing attention to them. So that's one kind of communication, sort of like at the beginning of a protocol packet, there is a bunch of ones or zeros, or ones and zeros that is sort of calling the meeting to order. But how many thousands of years are they going to do that? There would be no communication in that.

That would be one thing to look for. The second thing to look for, were it to be there, is on-going communications where we're just listeners, and they are already communicating with somebody else. In which case, they may be at a different sub-space band that we are assuming that they are trying to reach us at certain windows of our capability that are far beyond the capability of where we are. The chances of us being able to listen in on technology that's above us - I don't think that they would consider making it easy for us to listen-in on communications that were going beyond us. Maybe the combination of both - maybe we should be listening to these attention-getters which are headers of packets of communication, with things that are behind it that change. There would be some modification - some repetitive signal with some modulation that changes behind it. Those are my thoughts on communications. If somebody's rapping the table, trying to get your attention, then they may follow that by some information.

Charles Townes: Alright, let's proceed; Dr. Kingsley?

Stuart Kingsley: Professor Townes may be the best person here to answer this. To continue on the area I mentioned this morning about interstellar dispersion, I want to get to something more definitive here. What do you believe are the limits in terms of bandwidth transmission in the microwave regime if you have the capability vis-a-vis optics? Assume that there is no limit in signal-to-noise ratio, just a fundamental limitation by interstellar dispersion.

Charles Townes: You would worry about interstellar dispersion in the signal?

Stuart Kingsley: Yes, in the signal, limiting your modulation and effective communication rates.

Charles Townes: I'm afraid that I have never put any numbers into that, but it would be fairly easy to calculate. The dispersion in the radio region would be given primarily by the ionized material. We know its behavior with wavelength. We know roughly its density. It will depend a lot on the particular direction. If we get from relatively clean directions, there will be relative little dispersion.

Stuart Kingsley: What do you call relatively little in terms of nanoseconds, or what have you, over a thousand light years?

Charles Townes: There are directions where the attenuation is almost zero. If the attenuation is zero, you know from the relation between real and imaginary parts that the phase change would also be essentially negligible. I think that there are directions in which it would be very small. In other directions it would differ.

Stuart Kingsley In the galactic plane?

Charles Townes: Right, in the galactic plane. If we try to communicate, let's say across the galactic center or something like that, then we would be in trouble. But there would be quite a few regions where there would be no great trouble like that. It's fun to put numbers into that, but I think the fact that the attenuation is so small means that the phase change would also be small.

Kent Cullers: May I make a comment to that? It seems to me that dispersion is not a fundamental limit. If you want to be clever in your communications link, you can subdivide into whatever small bands you like and put a synchronization signal in them so that the bits get all synched-up right and you can overcome these problems in principle. I don't think that it's really reasonable to say that dispersion sets fundamental limits to communications systems, even at microwave. The obvious thing is that at a gigahertz, you can't have 1014 Hz bandwidth.

Charles Townes: I think there's another aspect to this - the following: If we know the attenuation between us and the place we are communicating with, and we can measure that by measuring the amount of radiation coming in our direction, in that case because we also know the physical mechanism that's producing it namely free-free transitions, we can then calculate the dispersion, allow for it in our signal, and compensate for it. There are some complications involved, but I don't think it's very fundamental - I agree.

Stuart Kingsley: That presupposes a targeted beam.


Copyright , 1993, SPIE

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