Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Ian Scott on 22/07/2008 03:24:46
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Hi
I was interested in SETI while ago but I think it's reliance on searching for an on/off signal is unconvincing - sure -6dB sidebands on a spectrum analyzer would display this by so could a physical phenomenon create AM modulation.
Wouldn't it be better to send a prime number sequence;
1,2,3,5,7, (no 9,)11
The 2 is even so it could be a "space" = 0. so 1, 0 3, 0 5, 0 etc where 2=0 makes a sequence.
(Then lasers are better than radio for communication. Radio gets faint as it spreads out - 700 Tera-watt lasers are better applied to this task).
This is a minimum prime sequence that would be hard to imagine caused by natural effects. An on off sequence is a poor candidate.
SETI people didn't like this suggestion!
Mod edit - formatted the subject as a question. Please try to do this to help keep the forum tidy and easy to navigate - thanks!
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Any number sequence, including your series of primes, would be a sequence of ons & offs if encoded in binary.
Decimal 1 2 3 5 7 11
Binary 1 10 11 101 111 1011
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yes but look at the structure of a message to decode it - binary is just a number base and the structure of a data sequence is independent on the base.
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Maybe this paste will work
http://ian.r.scott.tripod.com/images/conjecturals/SETI%20Receiver%20at%20Home/seti_r6.gif
or
http://ian.r.scott.tripod.com/seti_rx.htm
I hope the pages are fun!
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Why primes?
It may be that any intelligence has different rules.
Can you see this alien sound?
http://www.mediafire.com/?snbxbjjrzdi
Don't get worried, its mine.
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Sorry the link didn't work with Ubuntu - I'll try vista later
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Argh.
I'm on xubuntu...not microsoft so it should work fine. [:-[]
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The link worked for me on Debian
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Did the poison gas capsule fail to go off when you opened the box or was it just kitty objecting when you put her in ?
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?
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I guess some people are a waste of space
But I admire those that think
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Ian Scott
(Then lasers are better than radio for communication. Radio gets faint as it spreads out - 700 Tera-watt lasers are better applied to this task).
Unfortunately, Laser beams do spread out. You can't get past the diffraction limit, whatever you do.
The width of a laser beam when it hits the Moon (even) is very, very finite. Also, your 700 TW laser power will relate to peak power. Its mean power will be much less and that is very relevant.
Lasers are hideously inefficient and it is possible to generate large mean powers of coherent signals at RF for much less supplied energy than for a Laser.
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sophie why do you argue with me? It is disheartening, I think you are better.
Sure lasers spread as we know but much less than radio waves from a yagi or a dish antenna, let's give each other some credit.
The US has big plans for lasers and little interest in radio - 700 TW pulsed as a source is exactly what I suggest for an optical SETI. (prime number sequence at 0.1 % duty cycle say)
But what would I know? Or what should I care?
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Sophie
http://ian.r.scott.tripod.com/seti_rx.htm
please just read and then reply.
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The link worked for me on Debian
Hi DoctorBeaver and thanks for the trouble.
In that small mp3 you can hear my cat fudge meow.
Now put that into visual spectrum analyser and what do you see?
Its a similar principal to Carl Sagans "Contact" with no prime numbers or sequences involved but would be easily recognisable as alien to an alien. Fair enough, you would still be bound by the transmitting power and whether any intelligence could decode it but embedding a simple image rather that a complex formula I think would be better.
Hope you manage to get it Ian Scott.
PS. As you are on Linux the program used to encode the jpg onto the sound is called enscribe and for viewing you could use baudline, both free. [:)]
For those who cant do that, this is the embed picture of my cat.
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fwww2.mediafire.com%2Fimgbnc.php%2F6ed6fa8a39cc1012befd2ac777d5f91b3g.jpg&hash=f2a83ae948fa5e219ab803ad2ababf0d) (http://www.mediafire.com/imageview.php?quickkey=9dendnd12i3&thumb=4)
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I guess I can tell good people from bad
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Sophie
http://ian.r.scott.tripod.com/seti_rx.htm
please just read and then reply.
I have had a look and I must say yo have certainly put a lot of effort into the web page. Well done.
I haven't checked your link budget calculations but I assume you've got them right. I did a lot of those in connection with DBS TV planning.
I have a feeling that a terrestrially based optical system would be more subject to atmospheric attenuation and light pollution than you would like and I still take issue with your 700TW plans. For a reasonably achievable power supply (say a few GW), you would be dealing with a very small duty cycle and, in the end, your narrow band receiver would be integrating over a long time - i.e. matched filtering. Your link budget should really deal with the mean power (power averaged over the response time) rather than the peak power ; that reduces the utility of the method significantly, I think.
The Achilles' heel of many receiver systems tends to be receiver noise and, for best results, you might think in terms of a laser amplifier; these have better noise figures than the devices you discuss - but, for the Amateur, a bit pricey, I guess.
btw, what does your term "transmit benefit" mean? It is not a term I have come across.
As far as the choice of code for transmission is concerned, the basic requirement is to provide a signal which is definitely not noiselike to grab the attention of the listener. You need to expect a lot of dispersion on your transmission path (Years and years of transit time) so you can't expect to have 'pulses' at the receiver end. Anything with an autocorrelation function which is not an impulse would do as your signal. Pulsars satisfy this requirement to, of course, and a signal such as you suggest would do ok, I guess. However, to get the best information transfer rate, the signal should be as noiselike as possible so you would need to alternate a loud and crude "hallooo" with a sophisticated modulation which the remote civilisation (if it was bright enough) could make sense of and get maximum worthwhile info from.
And don't be so touchy about criticism. No one can rely on getting everything right on their own. If you want your Home Brewed system to work best you need to have all the input you can get - negative and positive. There are a number of things which need sorting out before it is worth committing yourself to building anything. During a long career in Engineering Research I was often subjected to (and gave others) a hard time over technical discrepancies. All good fun.
Having read a lot of your posts I notice that you, yourself are not averse to making adverse comments about people. If you dish it out then you should be prepared to take it without getting upset!
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thanks sophie
I am thick skinned and I stand up for myself - still I appreciate the grace of your reply.
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Your comments Sophie on dispersion intrigue me. Is there a frequency selective mechanism in light through space - to the extent that 1 Hz or so notches might appear in the modulation?
The atmosphere of a planet is secondary of course.
Sophie I respect you.
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I am thinking on dispersion now - perhaps gravitational delays by black holes at the center of galaxies might result in a null or a peak in modulation bandwidth. I wonder of the geometry as I like << 1 Hz bandwidths
p.s. I assume an AM optical signal to avoid DC offset and 1/f noise - possibly as low as 20 kHz.
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Sophie
Please don't start me on matched filters - this is a whole new topic but one I seek to publish
Using a resonant filter to extract information is absurd - it is better to do a LMS estimation.
This avoids the need to have integer frequency relationships in digital comms - but then interpolation is needed
Still Sophie if you like to speak LMS I am happy to oblige
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Ian are you familiar with speread spectrum communications and electronic warfare receiver techniques? I did a lot of the things you are talking about in your website under secret military cover back in the seventies and eighties and I doubt if I was the first to plough that field. Look up Instantaneous Frequency Discriminators and Digital Radio Frequency Memories (DRFM). I also had several patents in that area. I say had because the are now well out of their time limit.
A lot of the techniques are now commonplace in advanced digitial communications modulators and receivers
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Thank you Soul Surfer
I am interested in Software Defined Radios and also in 20GB/s optical transmission of wide spectrum for replacement comms (serializers, deserializers). I like the 16 Bit ADC's and DACs from Maxim etc these days and it is a curious place. I also like FPGA's but still study to program them for AI.
I admit I never warmed to spread spectrum but that is an oversight on my part
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I think you need to give much more thought to maximising your SNR (front end stuff) before your demodulation. If it were a trivial task to do what you propose, others would have had some success already.
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I am glad to not think like you Sophie as you believe that other people have already known all the answers. Maybe you need to join a religious cult. I believe it is best to question. We are opposites. Maybe you should not reply to my posts unless you show a mature perspective.
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But, unfortunately, you don't just question. You make assertions with little stated justification and then you get incredibly upset when someone disagrees with you. I cannot understand why you interpret this as 'anger'. I think you would rather take offense than think about the implications of any opposing views; it offloads the responsibility onto the other person. That's emotional laziness, I'm afraid.
If you want to prove the Establishment wrong you have to Prove it by demonstration. Can you? Have you? (No anger here, just curiosity)
My reason for tending to believe the standard view is that, as an Engineer, I have so often seen it to work so well in practical Technology. The equipment you are using to read this is a fair justification for my confidence. Solid state Physics, em propagation, laser optics, information theory, power a/c theory and many others go toward bringing you this information. And, I think you must grant, they do a fairly good job. So where do you begin to part company with what I believe?