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.
Alright, let's proceed; Dr. 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.
You would worry about interstellar dispersion in the signal?
Yes, in the signal, limiting your modulation and effective communication rates.
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.
What do you call relatively little in terms of nanoseconds, or what have you,
over a thousand light years?
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.
In the galactic plane?
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.
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
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.
That presupposes a targeted beam.