Question About SETI & Time Chauvinism

[Kevin the Crackpot]

I get the idea of radio astronomers listening at certian wavelengths (around the quieter area associated with the H+ and OH- "water hole" in the EM spectrum) for signals--perhaps mathematical in nature, like successive prime numbers--but one problem occurs to me: How about the amount of time between each 'beep' in the sequence of prime numbers?

If you can imagine a 78 rpm record played at 33 1/3, you get an idea of what I mean.

So, if I hear a beep with my radio telescope, the next beep may come a month later, followed by another beep in a week. We have the pattern of prime numbers, but so much time between them (because we might be like short-lived, hyperactive mayflies to beings who live thousands of years) that we may not be connecting these signals . . . because of our "time chauvanism" and preconceived expectations of how fast or slow these signals must be strung together.

So, is this point taken into consideration when radio astronomers are poking through all the data from their radio telescopes?

 

[phinds]

So, is this point taken into consideration when radio astronomers are poking through all the data from their radio telescopes?

Hard to see why they would bother since any communication from inside the local group, or even from nearby galactic clusters, will not have this as a significant issue and any radio communication from farther away would be so attenuated as to be undetectable. And at that, I think I'm probably being wildly optimistic in assuming we could detect any artificial radio waves emanating from outside the local group.

EDIT: Hm ... I think I mistakenly thought you were talking about frequency shift due to expansion but you are talking about different time scale process. Guess my comment isn't applicable.

 

[Kevin the Crackpot]

Hard to see why they would bother since any communication from inside the local group, or even from nearby galactic clusters, will not have this as a significant issue and any radio communication from farther away would be so attenuated as to be undetectable. And at that, I think I'm probably being wildly optimistic in assuming we could detect any artificial radio waves emanating from outside the local group.

EDIT: Hm ... I think I mistakenly thought you were talking about frequency shift due to expansion but you are talking about different time scale process. Guess my comment isn't applicable.

Thank you for answering.

 

[DaveC426913]

How about the amount of time between each 'beep' in the sequence of prime numbers?

So, is this point taken into consideration when radio astronomers are poking through all the data from their radio telescopes?

They do a lot of processing on signals, looking for just such patterns.

Frankly, tempo is one of the simplest forms of translations that can be done. When the signal is fed to an algorithm in a computer, it likely doesn't really bother with any absolute scale of signal, only relative. A computer won't even know the difference between a signal that took a minute to record versus one that took a year to record unless you tell it.

Here are some sets of data from radio sources. How far apart are the pulses? One second? One year?

 

[Kevin the Crackpot]

Thank you for clarifying.

 

[sophiecentaur]

It's one thing to detect and identify a signal that is likely to have come from some intelligence. It's an entirely different thing to have a chat with the source. Time scales and time windows act against that. I'd bet there are some (à la Drake) figures about both the maximum likely lifetime of any advanced civilisation and the probable limits to the timescales they would used for their thought processes.

 

[russ_watters]

How about the amount of time between each 'beep' in the sequence of prime numbers?

If you can imagine a 78 rpm record played at 33 1/3, you get an idea of what I mean.

So, if I hear a beep with my radio telescope, the next beep may come a month later, followed by another beep in a week.

So, is this point taken into consideration when radio astronomers are poking through all the data from their radio telescopes?

I would tend to doubt this would be an issue because any advanced civilization is going to want to transmit data quickly. Modulation schemes will vary, but for example a common old modem transmitted 300 bits per second, whereas NASA recently used laser pulses at a rate of 600,000,000 bps. Beings transmitting a bit per month would need to be older than the universe to get the same out of their data as us!

We're all going to be limited by the same laws of physics, but I'd be more worried about missing a signal that was too fast vs too slow.

edit: If you're thinking of the movie "Contact", where they hear the "signal" in real time, that's just a dramatization for the movie. Even in the movie what they heard wasn't even the real signal and the real signal almost certainly would have been found anyway.

 

[phinds]

... because we might be like short-lived, hyperactive mayflies to beings who live thousands of years) that we may not be connecting these signals . . . because of our "time chauvanism" and preconceived expectations of how fast or slow these signals must be strung together.

While I certainly can't say it's impossible for there to exist beings that have a much slower pace of life than us, I seriously doubt it. The problem is that physical events that are likely to be harmful to living beings happen quickly (think predators, storms, etc) and a really slow responding organism would likely not survive.

 

[Klystron]

There's data from caves situated along the Colorado River indicating rapid extinction of giant slow-moving ground sloths following human contact. Beasts nearly impervious to predators apparently succumbed to faster moving cooperative tool users.

 

[Kevin the Crackpot]

While I certainly can't say it's impossible for there to exist beings that have a much slower pace of life than us, I seriously doubt it. The problem is that physical events that are likely to be harmful to living beings happen quickly (think predators, storms, etc) and a really slow responding organism would likely not survive.

That seems reasonable, so thank you.

 

[phinds]

That seems reasonable, so thank you.

Hey, reasonable is my middle name. Unfortunately my first name is un.

 

[sophiecentaur]

any advanced civilization is going to want to transmit data quickly.

"quickly" would have to be appropriate for the channel characterises. Even with an unthinkably powerful transmitter, the planned link budget may limit the data rate to something very modest.

 

[tech99]

It's one thing to detect and identify a signal that is likely to have come from some intelligence. It's an entirely different thing to have a chat with the source. Time scales and time windows act against that. I'd bet there are some (à la Drake) figures about both the maximum likely lifetime of any advanced civilisation and the probable limits to the timescales they would used for their thought processes.

Not only the lifetime of the civilisation. Unless they send signals which are designed to be intercepted, digital communication signals are not likely to be noticed because they are intentionally noise-like. Modern man only used analogue transmissions for 100 years out of his 200,000 total..

 

[DaveC426913]

I think it's a pretty good assumption that the OP was thinking specifically about signals meant for inter-world communication. Hard to imagine a broadcast of prime numbers across interstellar distances serving any other purpose.

Thus, the tick rate would have nothing to do with maximizing throughput, and everything to do with maximizing the chance of being picked up.

 

[sophiecentaur]

If we are suggesting an 'advanced' civilisation then we could expect them to be attempting to second guess the nature of their target civilisation. It's all very much 'message in a bottle' territory (or moreso) or even 'spectral graffiti' unless they are going for targets that are near enough to them for a meaningful conversation. They will have done their own Drake Equation and factored in their own longevity.
The 'A For Andromeda' scenario (BBC TV, years ago) is one of the best SciFi scenarios I have ever come across. Earth receives a set of data which allows them to build an advanced computer which in turn creates a life form (Pretty Blonde Woman aamof) which will take over the World. They manage to destroy her at the last minute - phew!. The idea behind the story is about a form of colonisation and that is the only form of interstellar 'travel' that satisfies my skepticism. An ideal way to spread the species which doesn't need a specific destination; all it needs is for the broadcast beam signal strikes at least one suitable host target.

 

[LURCH]

Modern man only used analogue transmissions for 100 years out of his 200,000 total..

A point that has crossed my mind more than twice, as I’ve thought about the SETI program. The people who run the project have, by necessity, a very narrow definition of “Extra Terrestrial Intelligence “. They define that term as, “aliens with radios” (because that is the only type of ET we are capable of detecting, at the moment).

Many people think of SETI as a long shot because of the extremely great odds involved. This includes the uncertainty that ETI’s even exist anywhere, ever, the chances that they exist at this exact moment in history, and that we will then just happen to be looking at the right place at the right time to see them. I think there is another factor that seldom gets discussed, the odds that they are using radio at the same time as we.

If I think about how recently we have started using radio, and how likely we are to discover something much better in the near future, rendering radio communication obsolete, the notion of looking for such signals from other worlds seems almost useless. Still, one must start somewhere, I suppose.

 

[russ_watters]

I think it's a pretty good assumption that the OP was thinking specifically about signals meant for inter-world communication. Hard to imagine a broadcast of prime numbers across interstellar distances serving any other purpose.

Thus, the tick rate would have nothing to do with maximizing throughput, and everything to do with maximizing the chance of being picked up.

But broadcasting widely spaced pulses of prime numbers is neither how we are trying to call nor listen for ET. It's just a silly movie dramatization.

The Arecibo message was broadcast at the paltry rate of 10bps, or 25 million times faster than the rate suggested by the OP. And not as pulses, but an FM radio signal.

 

[russ_watters]

Not only the lifetime of the civilisation. Unless they send signals which are designed to be intercepted, digital communication signals are not likely to be noticed because they are intentionally noise-like. Modern man only used analogue transmissions for 100 years out of his 200,000 total..

This isn't how it works either. What we're sending and looking for is nothing more than narrow-band transmissions - carrier signals. Just by virtue of being narrow, there is a good probability the signal is artificial. After identifying the signal, then we worry about decoding it.

That's what happened in the movie "Contact" after the silly dramatization of the prime numbers.

 

[sophiecentaur]

The Arecibo message was broadcast at the paltry rate of 10bps, or 25 million times faster than the rate suggested by the OP. And not as pulses, but an FM radio signal.

FM is an efficient choice modulation system, particularly at low mod indices. AM needs to be transmitted at full whack to be efficient and in its basic form it involves a high power mod amplifier. I think that would account for the choice. Detection could possibly be a bit less convenient but not a problem for a bright ETI.

There was a BBC Radio 4 programme this morning in which the Physicist Jocelyn Bell was interviewed. She was the one who first spotted the strings of regular RF pulses from Quasars. When she presented them to her boss, he told her that they must be a man-made source. She had to share the Nobel Prize with him and he always justified that on the grounds that he was 'the boss'. She is not one to bear a grudge and has gone on to earn many more honours since.

 

[russ_watters]

There was a BBC Radio 4 programme this morning in which the Physicist Jocelyn Bell was interviewed. She was the one who first spotted the strings of regular RF pulses from Quasars. When she presented them to her boss, he told her that they must be a man-made source.

This brings up a good point and requires a caveat to what I was saying before. Wikipedia tells me that first pulsar had a period of 1.33 seconds, which is in the range of the movie Contact's "signal". However, while such a signal would be identifiable as man-made vs extra-terrestrial, it wouldn't be likely to be carrying data itself. I suppose you could use it as a beacon, like in Contact, but I doubt it (it strikes me as unnecesary). More likely it would be a non-data carrying signal like a radar sweep.

...though again I suppose a continuous radio broadcast could also have an intermittency of 24 hours if the signal depends on Earth's rotation. Such a signal would still be carrying data, it's just that the pulse isn't the data.

 

[DaveC426913]

But broadcasting widely spaced pulses of prime numbers is neither how we are trying to call nor listen for ET. It's just a silly movie dramatization.

I don't disagree with you, I'm simply drawing attention back to the OP's objective.

The OP did not appear to be after scenarios where an advanced civilization is going to want to transmit data quickly. So, while food for thought, pursuing it is kind of tangent to the OP's question, IMO.

 

[russ_watters]

I don't disagree with you, I'm simply drawing attention back to the OP's objective.

The OP did not appear to be after scenarios where an advanced civilization is going to want to transmit data quickly. So, while food for thought, pursuing it is kind of tangent to the OP's question, IMO.

I guess I wasn't bridging too well the gap between a real-world scenario for purposeful communication with ET and capturing an extraneous signal that wasn't intended for ET communication. An extraneous signal might be difficult to decode. An on-purpose signal would likely not be difficult because it wouldn't be especially fast (as ours wasn't). But in neither case would the signal be widely spaced intermittent pulses.

I'm not an EE, but my expectation is that the 10Hz modulation of a 2,380 MHz signal makes for a really good signal to noise ratio.

 

[Kevin the Crackpot]

Thank you all very much for answering.

 

[LURCH]

But broadcasting widely spaced pulses of prime numbers is neither how we are trying to call nor listen for ET. It's just a silly movie dramatization.

The Arecibo message was broadcast at the paltry rate of 10bps, or 25 million times faster than the rate suggested by the OP. And not as pulses, but an FM radio signal.

However, the Aricebo signal was arranged in 73 lines of 23 characters each. This data configuration was selected specifically because each of these is a prime number, which might help ET recognize and decode the signal. If technological civilizations in other star systems are trying to make their presence known, it is reasonable to speculate that they would send a signal that has some distinctive features which would be recognizable as something other than random noise. After all, they have probably had their own “LGM” experience.

 

[russ_watters]

However, the Aricebo signal was arranged in 73 lines of 23 characters each. This data configuration was selected specifically because each of these is a prime number, which might help ET recognize and decode the signal. If technological civilizations in other star systems are trying to make their presence known, it is reasonable to speculate that they would send a signal that has some distinctive features which would be recognizable as something other than random noise. After all, they have probably had their own “LGM” experience.

Agreed. Identifying a signal is only half (at most) of the battle. If it's a communication you'll want the recipients to be able to decode and interpret it.

 

[sophiecentaur]

I'm not an EE, but my expectation is that the 10Hz modulation of a 2,380 MHz signal makes for a really good signal to noise ratio.

That would depend entirely on the distance the signal needs to travel. Every time you double the distance, the signal power will drop to one quarter. A nice healthy signal at 1parsec away will no longer be so healthy a thousand LY away. SNR can always be improved by reducing bandwidth and a 1Hz modulating signal has a ten times better SNR etc. etc.

 

[Kevin the Crackpot]

That would depend entirely on the distance the signal needs to travel. Every time you double the distance, the signal power will drop to one quarter. A nice healthy signal at 1parsec away will no longer be so healthy a thousand LY away. SNR can always be improved by reducing bandwidth and a 1Hz modulating signal has a ten times better SNR etc. etc.

Doubling the distance drops power to a quarter?

I'm very familiar with the inverse square law of EM propagation, but I thought this rule was more relevant to a point source (or approximate point source) of light (or other EM radiation) like a light bulb.

Was the parabolic dish at Arecibo a point source of radio waves? I seem to remember (from a book by Carl Sagan) that the signal was "aimed" at a globular cluster that is something like 40,000 light years away.

 

[mfb]

For long distances everything is a point source, and even with non-isotropic emission the intensity drops with the inverse distance squared. You can't focus signals over a distance of more than (emitter diameter)²/wavelength (order of magnitude estimate).

 

[Kevin the Crackpot]

Thank you.

 

[sophiecentaur]

Doubling the distance drops power to a quarter?

I'm very familiar with the inverse square law of EM propagation, but I thought this rule was more relevant to a point source (or approximate point source) of light (or other EM radiation) like a light bulb.

Was the parabolic dish at Arecibo a point source of radio waves? I seem to remember (from a book by Carl Sagan) that the signal was "aimed" at a globular cluster that is something like 40,000 light years away.

With any telescope, what counts is its aperture that gathers the incoming power. ISL determines the amount of power per unit area, however big the dish or lens. In Astro terms, the ‘point’ that you can consider as the position of the scope (or transmitter) can be some distance behind the aperture. I’m terms of the actual distance to or from a star, of course, it’s too tiny to count in the ISL calculation. But to get an enhancement of 60dB (say) or whatever, is good value.
ISL works in a funny way, compared with losses in a coax cable. Double the length of cable and you double the total loss in dB. But if you double the distance through space, you only lose 6dB.
Calculations are the same for receive or transmit arrangements.
PS The ISL also applies to lasers, once you are an astronomical distance away.

 

[sophiecentaur]

For long distances everything is a point source, and even with non-isotropic emission the intensity drops with the inverse distance squared. You can't focus signals over a distance of more than (emitter diameter)²/wavelength (order of magnitude estimate).

You can regard what happens as the directive antenna (or light projector) illuminating a portion of a sphere. This would be the equivalent (as far as an on-beam receiver as what you would get from an isotropic radiator with a power of 1/(illuminated area). In antenna spec they use a term Max EIRP or Maximum Effective Isotropic Radiated Power. A receiver wouldn't know the difference between a 1kW isotropic source and a 1W source with a directive antenna with Gain of 30dB.

 

[Kevin the Crackpot]

Thank you.