Radio-Archaeology

[Andrew M]

Radio-waves are the obvious big signal from our planet to the rest of our Galaxy saying we are here. The SETI project was set up to detect similar signals from elsewhere in our Galaxy. The development of FTL travel brought new possibilities. Radio transmissions which were previously out of reach drifting into the cosmic distance, were suddenly returned to the world of the living. If the expense of retreival matched the demand for authentic history, the effect on society would be profound.

Question: How easy is it to detect a radio transmission from space?

Question: Has anyone looked into the implications of this for the Traveller universe?

 

[far-trader]

Radio-waves are the obvious big signal from our planet to the rest of our Galaxy saying we are here. The SETI project was set up to detect similar signals from elsewhere in our Galaxy. The development of FTL travel brought new possibilities. Radio transmissions which were previously out of reach drifting into the cosmic distance, were suddenly returned to the world of the living. If the expense of retreival matched the demand for authentic history, the effect on society would be profound.

Question: How easy is it to detect a radio transmission from space?

Question: Has anyone looked into the implications of this for the Traveller universe?

It all depends:

• Signal strength
• Signal frequency
• Signal type (tight beam v broadcast)
• Background noise
• Detector sensitivity
• Local (to the detector) noise
• etc.

The SETI program is searching for purposefully transmitted messages, not random noise. Said purposefully transmitted messages occupy a narrow range band suited to high power signals capable of interstellar ranges. And it's imagined said signals would be simple and clear, designed to inform of intelligence.

The stuff of interstellar communications consisting of transmission of the Encyclopedia Galactica or plans to build something are the stuff of science fiction. It's not practical. Likewise ideas of two way conversations.

As for the day to day radio transmissions of a society like Earth (and spaceships) istr that the practical limit on detecting them is very very short (on an interstellar scale), possibly not more than a couple light years. They are simple too weak, in the wrong frequency range, and broadcast rather than narrow beam.

So, Interstellar Radio Archeology, while a fun idea, and one I've also toyed with, seems a bit of a stretch for any long range (and long time) archeology. You might be able to do a couple light years, or several with really large detection gear, but nothing on the order of decades or longer.

I do seem to recall that X-Rays make good long range signals though. So while the idea of ET picking up the broadcast of the Berlin Olympic games after decades or more may be unlikely, perhaps they might pick up the signals from the sudden rapid succession of nuclear bomb uses and tests, followed by an equally sudden cessation of same. From which they may debate one of two outcomes:

1. Earth developed nuclear weapons, fought a war or wars with them, and wiped ourselves out.
2. Earth developed nuclear weapons, saw the folly of waging war with them, and instituted a ban.

One of the classic proposed watershed events of "advanced" civilizations.

I do seem to recall the idea of Interstellar Radio Archeology being discussed here on CotI at some time. Maybe someone else recalls the who and where to be able to find it.

 

[far-trader]

I managed to google up a site that explains things in a bit more detail. Seems to (at a quick glance) confirm (or even make my recollection generous) the short ranges possible. A couple quick numbers from the site (presumes an Arecibo size receiver dish, note that none of the starships in Traveller have anything close to that size of receiver):

• An AM radio broadcast could only be detected out to 0.0074 Astronomical Units (AU).
• FM Radio could be detected out to 5.4 AU.
• A 5 Megawatt UHF television picture could be detected out to 2.5 AU, although the carrier wave could be detected much further; out to 0.3 light years

All significantly less than interstellar. The 5MW UHF is probably about what the best starships would have.

Here's the link to the site for further information:

http://www.computing.edu.au/~bvk/astronomy/HET608/essay/

Now of course you could posit your IRA team having access to something like the proposed space based very large array (VLA)* that was talked about (can't find a link to it at the moment) a while back. I don't recall how big it was supposed to be, huge, several kilometers across iirc, which would give you more range. I think the page linked above may have the calculations to figure it out if you wanted to.

* maybe by linking several ships, though that would have its own logistics issues

 

[MR TEK]

If I recall, the very large array was supposed to be the width of the earth. Antennas place 180 degrees from each other around the equator.

As I understand it, basic resolving power at radio frequencies only requires two properly aligned and phased, and the data connection to allow them to be integrated.

It must require more units, but several sites only use an array of antennas and not a full parabola, so it obviously works to only use a limited number of elements.

There was also once a proposal to synchronize units on the moon and the earth and gain an array the equivalent diameter of the distance between the earth and the moon.

It has been a great many years since my skills to calculte the number of units needed myself, and even a number of years since I have seen either of these proposals discussed, so without some googling I can't get teh exact details.

 

[far-trader]

My recollection of basic nuclear bombs was off, no surprise (only my correct recollections surprise me ). The X-Rays are of course rapidly absorbed by the atmosphere and converted into heat. The only chance of them escaping into space would have been from the high altitude tests (of which there were some, and they made pretty impressive pictures). So, not as many X-Ray bursts for ET to detect, and they'd have to be on the right side of the world, and not drowned out by background noise of course. I bet we're still unnoticed unless ET is parked in orbit

 

[Andrew M]

Thats really cool, thanks Dan.

So the lesson is that non-directed communication will turn to undetectable fuzz before its first birthday. That's bad news for our IRA researchers. It puts a severe limit on in-system communications as well. Non-directed radio is going to be for distress signals only, and only then if there is no obvious reciever aim a beam at.

The other possibility was to look for directed radio, which is going to get much more common once the space revolution gets going. The possibilities for jump capable reasearchers are a bit wider than they are for SETI. They can pick where and when to look. How about looking for planet to planet comms? Planetary motions are reasonably predictable aren't they?

I think the controlling factors for the strength of signal will be the sensitivity of the reciever and the distance the beam has to travel. The IRA receiver has to be several factors better than the intended target. How much better depends on how much further the signal has travelled. That means that signals sent to the main world are going to be much more difficult to detect than signals coming from that world.

I think Voyager might be a good case to look at for this, though I'll have to do a little google-fu and sum-fu...

Oh, they have the figures for Pioneer 11 on that site.. Oh, they have the figures for Pioneer 11 on that site.. Signals from Pioneer are detectable at 120 AU range. The strength of the signal sent the other way is not mentioned.

 

[Icosahedron]

The other possibility was to look for directed radio, which is going to get much more common once the space revolution gets going. The possibilities for jump capable reasearchers are a bit wider than they are for SETI. They can pick where and when to look. How about looking for planet to planet comms? Planetary motions are reasonably predictable aren't they?

The problem is that to detect directed comms, you need to be positioned in line-of-sight, and much of the signal will be absorbed by the body of the planet it's aimed at. Even then, a signal designed for interplanetary range will not have enough power to be detected at a range of hundreds or thousands of light years.

I suspect that you need a new premise, unless you're happy with a huge handwave. Radio archaeology may be the stuff of golden age sci fi, but realistically the whole concept doesn't hold water.

 

[far-trader]

Thats really cool, thanks Dan.

De nada It (was) a bit of an interst of mine an age ago.

You might make some handwave for very advanced tech, something on the order of TL13+ maybe. Gravitics, whatever If the goal is fun, science can play second to fiction in my books. As Icosahedron said it's more Golden Age than current, but they were good times. In fact my slightly lubricated memory is saying the idea above of nuclear tests being detected at interstellar range by ETs is from some (or a few) Golden Age sci-fi story.

 

[Andrew M]

The problem is that to detect directed comms, you need to be positioned in line-of-sight, and much of the signal will be absorbed by the body of the planet it's aimed at. Even then, a signal designed for interplanetary range will not have enough power to be detected at a range of hundreds or thousands of light years.

I suspect that you need a new premise, unless you're happy with a huge handwave. Radio archaeology may be the stuff of golden age sci fi, but realistically the whole concept doesn't hold water.

Icosohedron, I won't do the maths. I'll just throw the scenario at you. If you send a probe to Pluto. How tight a beam are you able to send at 40+ AU distance? Would this beam stay clear of Pluto/Charon, Encomass some part of the planetary system or take in the whole planetary system?

Whichever way you envision it the planet does didley squat to the signal.

 

[robject]

Didn't Bruce Alan Mackintosh design a set of thorough sensor rules for T4?

 

[aramis]

IIRC, a 1° beam should be about 1000x the effective source power... or somewhere around 30x the range.

 

[Icosahedron]

Icosohedron, I won't do the maths. I'll just throw the scenario at you. If you send a probe to Pluto. How tight a beam are you able to send at 40+ AU distance? Would this beam stay clear of Pluto/Charon, Encomass some part of the planetary system or take in the whole planetary system?

Whichever way you envision it the planet does didley squat to the signal.

It's not just the mass of the planet, though. Many planets will have magnetic or ionisation fields that may significantly interfere with the signal as it passes.
However, the biggest problem remains the signal strength. Any EM signal designed to travel 6 light-hours will have a quarter of its received power at 12 light hours, a sixteenth of its power at one light-day, a sixty-fourth of its power at two light-days, etc, following the inverse square law. I won't do the maths either, but I reckon this signal will be undetectable before it's first birthday, too.

 

[Andrew Boulton]

Didn't Bruce Alan Mackintosh design a set of thorough sensor rules for T4?

http://home1.gte.net/res04u7k/Traveller/hrules.html

 

[far-trader]

Didn't Bruce Alan Mackintosh design a set of thorough sensor rules for T4?

But it uses the standard T4 ranges right? And so it is flawed from the outset by ignoring the inverse square law. Not saying it doesn't work for a game, just that it doesn't model reality.

 

[aramis]

Bruce accounted for the InvSqLaw in the mods; the scale is log based

 

[Andrew M]

The remote probe "New Horizons" was launched in 2006 and is due to rendezvous with Pluto in 2015. It has a 2.1 meter diameter receiver dish. The distance to Pluto is roughly 40AU. There are 63 324 AU to a light-year, which makes a light year just shy of 1600 times the distance to Pluto. The area required to detect a signal is proportional to the square of the distance, so the diameter is directly proportional. This makes our remote reciever 3.36 km accross. If you wanted to see it 30 years later the reciever would have to be equivalent to a 100km dish.

The largest telescope we have is 305 m and there is a 500 metre telescope (FAST) under construction. With arrays of telescopes it is possible to achieve very large apparent appertures with widely spaced telescopes. The largest net so far has an effective aperture of (lost the reference to this so take it lightly) 22 000 km. Unfortunately this uses the same telescopes several times over the day as the Earth revolves, so you could not use it to read radio transmissions. This still means you should be able to put together some mighty arrays in space, since you can put things so much further apart.

 

[aramis]

Synthetic aperature arrays have issues: IIRC, they have a RESOLUTION almost eqivalent to the full size, but an equivalent diameter of the square root of the sum of the component areas for signal STRENGTH.

For radio astronomy, most of what they are looking at is strong enough for a 10m dish, and the vast majority for a 30m dish. what they don't have, however, is the resolution needed. By using the large arrays, while they do up the gain, they vastly improve the resolution. The more scopes, the better the resolution, and the further afield, the better the resolution... within the limits of the integration technology.

For radio, TV, and commo, you don't need resolution; you need strength.

 

[Andrew M]

Synthetic aperature arrays have issues: IIRC, they have a RESOLUTION almost eqivalent to the full size, but an equivalent diameter of the square root of the sum of the component areas for signal STRENGTH.

For radio astronomy, most of what they are looking at is strong enough for a 10m dish, and the vast majority for a 30m dish. what they don't have, however, is the resolution needed. By using the large arrays, while they do up the gain, they vastly improve the resolution. The more scopes, the better the resolution, and the further afield, the better the resolution... within the limits of the integration technology.

For radio, TV, and commo, you don't need resolution; you need strength.

Thank you Aramis. I was worried about this. I do have to make it clear, everything I am writing comes from broad sweeps of the internet and back of the envelope calculations.

I noticed that virtually all of the SETI program used single very large telesopes and not arrays. What made me think that synthetic aperture arrays might be relevant was that they commissioned the Allen Array http://www.seti.org/ata. They way they are used for the SETI project is substantially different from the way synthetic aperture arrays are used for radio-astronomy. I does not appear that they bring the same scale of benefit, either in resolution or amplification. This looks very much like a prototype. There are plans to build a similar square kilometer array. http://www.skatelescope.org/

Another thought was that improved resolution might help if you knew precisely where the source was.

 

[far-trader]

I wonder now too if you might not find a certain limitation due to absorption by intervening cosmic dust and oort clouds? No matter the signal strength and receiver sensitivity/size, if the signal is blocked/bounced/redirected over the distance you just won't have it. Of course cosmic dust may be so minor that it won't affect anything except the smallest waves. Oort clouds, depending on the actual density and size of material might be a bigger problem.

 

[Andrew M]

I wonder now too if you might not find a certain limitation due to absorption by intervening cosmic dust and oort clouds? No matter the signal strength and receiver sensitivity/size, if the signal is blocked/bounced/redirected over the distance you just won't have it. Of course cosmic dust may be so minor that it won't affect anything except the smallest waves. Oort clouds, depending on the actual density and size of material might be a bigger problem.

I had not considered this but I don't think it would be significant. The Oort clouds would be more noticable if it interfered with light. There was some mention http://setifaq.org/faq.html#1.2.3 of interstellar plasma smearing the signal bandwidth. This was said to put a lower limit on the useful bandwidth of the reciever, but if it had any attenuating effect on the signal this was not mentioned.

My guess is that the space dust between here and Pluto would be every bit as disruptive to the signal as the space dust beyond.