Sometimes I think there are not abundant intelligent life forms wafting about the universe. We would see things in our careful, highly accurate, detailed looking at a sampling of the universe. But, I suppose we've only been scanning with super amazing instruments for a few years, and only scanning a small fraction of the universe. But certainly, in a decade or two we'll be able to say that radio-communicative or emitting intelligent life is either out there somewhere, or not likely to be. Absence of evidence will evolve into evidence for pessimism, at the very least.
Meanwhile we get these little quirks. And, the latest is a burst of radio-info that the experts on this all seem to be saying is not a thing, looks like lots of other things that are also not things, but one guy somewhere put out a press release so now everybody thinks it is a thing.
But it is not. Not a thing. Nothing.
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Let's keep things in perspective. We have a pair of spacecraft that are now more than 100 AU away, which are still communicating with us here on Earth.
Think of these as "alien life forms transmitting an intelligent signal" in our direction. (The Voyager spacecraft are certainly "alien" to most human beings.)
How strong is the signal coming from them? We're still able to pick up their signal, but only at a VERY low bit rate -- much slower than the LEO spacecraft that "we're using every day". Much, much slower.
Putting that aside, the signal strength here on Earth, as we receive it, is about 1x10(-16) Watts. That's "incredibly weak" -- by an standard.
So, 1x10(-16) Watts for "an alien signal" being sourced at 20 Watts from a distance of 100 AU.
The nearest star is 4.24 light years. One LY is about 63200 AU. That's 632x farther away than Voyager. Will the signal be 632x weaker if Voyager were at Proxima Centauri? No...
Signal strength falls off as r-squared. We can estimate, then, that Voyager's signal would be 1/400,000x as strong. Just to maintain the same received signal strength, it's transmitter would need to be 400,000x more powerful: 8 megawatts.
That's a hefty amount of power to put on a spacecraft, but let's say that it's a ground-based transmitter on a planet around Proxima Centauri. Let's further make it easy on ourselves and wave our hands and ignore the huge source of noise just next to it, the star itself...
In order to receive the signal from Voyager at 100 AU, we have to use the largest antenna we have: One of the 70-meter antennas of NASA's DSN. This means, by implication, an antenna with very high gain...
The problem with using antennas that have very high gain is that their beam width, i.e., the area of the sky they're "listening to" is very small. The higher the gain, the smaller the area.
This has another important implication: You had better know, with exquisite detail, exactly where to point your big antenna in the sky, or you'll miss the signal entirely. Entirely. As in "no signal at all".
Worse: Your source is moving... Even if it is a planet and not a spacecraft. Worse yet: Your antenna is moving -- fast (by comparison). You now need to be able to point your antenna with great precision and keep it tracking your source with great precision, too -- or you loose your signal.
The DSN has a tough job -- even for something as close as 100 AU...
But what are the chances of an alien being as close as 4.2 LY? Not much. After all, the Russian signal was from a source 95 LY away. 95/4.2= 22x farther away. But that means a signal that's 500x weaker again.
Better make that alien transmitter 4 GIGAwatts.
Oh, and another confounder... That 4 GW transmitter? It has to also be part of a very high gain antenna, otherwise, more watts... (4 GW will power several good- sized cities on Earth. These aliens had better have some good justification for that kind of electricity bill...)
More troubles: That alien high-gain antenna pouring 4 GW of signal had better being pointing EXACTLY at Earth... the ENTIRE time it's transmitting. It has the same tracking issues that our DSN antennas have: You have to track your s/c in the sky just as accurately while transmitting to it as you do when receiving its signal.
The odds that an alien civilization "out there" would build and power a super-high-powered antenna and decide to track & broadcast to us, specifically? Because if they're not pointing right at us, we're getting nothing.
As Greg says, "But it is not. Not a thing. Nothing." Sorry.
They can avoid this limitation by broadcasting over the full 4π steradians, but as you note, this would require a proportionate boost (several orders of magnitude depending how tightly beamed the signal is) in transmitting power. Traditional broadcast radio and TV signals use this method because they want their signal to be received by many different people in all directions (at least azimuthally) from the broadcast tower. But the most powerful such stations have transmitting powers of the order of 100 kW. Communications and navigation satellites also use this method because they want the signals to be picked up in wide enough angular region that they may as well broadcast to the full 4π steradians. But that's far less than what you can do with ground-based transmitters.
Suffice to say that the alien civilization that can afford transmitters that are that powerful is almost certainly way beyond us in technology.
So, if there were another civilization somewhere out there, and they
a) were sufficiently advanced to be able to design communication systems as powerful as needed to get a meaningful signal here
b) had the resources and time to devote to building those systems
what, exactly, would be their point? If they were advanced enough to consider such an action, isn't it likely they would be advanced enough to be able to explore their local space (to a greater extent than we have ours) and know that travel to systems like this one isn't really feasible? It would seem that mounting a mission from there to here would require so much technology and time that it would be judged impractical.
#3 Dean - in addition they had to have existed at that stage of advancement at a time when their signal would be receivable by us x light years in the future, we have to be looking in the right direction at that time and with equipment capable of receiving it. If as a life-form they are anything like us there is a high chance they will kill themselves off near the time they reach that level of advancement so the window of opportunity here will be very small. I suspect that if the universe is filled with alien civilization many have died out, many are not interested in, or physiologically incapable, of pursuing technology in that direction, many have not reached that level of maturity yet, and lots more "manys".. Maybe they are all out there scanning the skies for someone else to communicate with them first - who will blink first :)
That's touching on another confounder I didn't mention: In order to have a communication (including mere 'detection'), the transmitting group has to transmit "at something", and do so in co-ordination with the receiving group looking directly at the transmitter at the same time (with the expected adjustments for time-of-flight).
For JPL, this means having precision clocks on Earth and the spacecraft, tightly synchronized. And careful navigation predicts so that we know where the s/c will be at time T, and how it will move in the sky over the transmission period. On the spacecraft, it has to similarly know where earth will be during the reception period in order to point its antenna.
And then both have to know when the signal is/is to be airborne, so that the s/c and ground are receiving/transmitting in co-ordination with each other. No point in listening if you tune in after the signal is gone.. or listen too soon (and then it goes "over the horizon").
Not to mention that both sides have to have a carrier frequency agreed upon. And a data modulation scheme that's also pre-determined. (Not done yet...) And they have to agree on the data rate that's being modulated.
Sorry, but it gets worse! Since the sender & receiver are moving with respect to each other, the frequencies of the signals are changing too -- doppler effect. So either the transmitter must pre-compensate its transmission frequency so that a constant frequency is received on the other end, or the receiver must be able to track the change in the received signal -- but it has to know where to start. And when. (Voyager 2 has to use the first technique; its receiver lost most of its tracking bandwidth when its loop capacitor shorted out.)
So you require:
High (enough) gain antennas,
High (enough) transmit power,
Precision pointing of the transmitter antenna,
Precision pointing of the receiver antenna,
Matching carrier frequencies,
Matching data modulation schemes,
Matching data rates,
Pre-agreement on session start time,
Predicts for doppler compensation,
--and for the high gains required--
Predicts for navigation to aid pointing and tracking.
Get any of those wrong and you get NO SIGNAL. Given the distances involved, there is no technology that will allow you to broadcast low gain with a compensating power boost suggested by Eric to reduce the pointing requirements. (The current maximum that NASA can transmit is about a megawatt -- nowhere near interstellar needs. Higher than that and you melt the waveguides...)
No problem. All ET needs to do is build a dyson artifact with a big gap in it. A quick bodge-up of a stellar lighthouse! A solar system's worth of material resources would probably be required, but think of the dividends.
Some tens, hundreds or thousands of years later, the blinkety-blink is detected here, on Earth.
What a payoff!
"It was a quiet night in the galaxy, not a signal to be found..."
We are going to detect intelligent life on other planets using something other than radio signals.
It will probably be an isotopic signature.
Even there, you have to watch out for natural confounders. There are acceleration processes in interplanetary space that are known to modify isotope ratios, particularly 3He/4He and 22Ne/20Ne (presumably others as well, but those are two of the easiest to measure). The 3He/4He ratio in particular can change by several orders of magnitude, so it might be possible to detect these processes with a sufficiently powerful spectroscope (the frequency of certain spectral lines will be shifted slightly due to the different mass of the nucleus--this effect has been observed in the lab for deuterium vs. hydrogen in the lab, but the processes in question don't change the D/H ratio very much).
There are also processes that affect the 13C/12C and 18O/16O ratios in biological systems. These processes are correlated with temperature, so they are sometimes used as proxies for paleoclimate. But these changes are at the parts-per-million level, so they would be hard to detect at stellar distances. And even if they were detected, it would only mean that there was life on some planet in that stellar system, not that it was intelligent.
My hunch, based on the Fermi Paradox and Earth's previous geological and palaeontological history, is that life maybe relatively common in the cosmos but highly "advanced" technological sentiences like ours may be very few and far between. I'd love to be proven wrong though!
Meanwhile in somewhat related news; that new found Proximan planet is an amazing and wonderful discovery indeed - but with a range of planetary temperatures from minus thirty three* to the "high hundreds" in degrees Celsius suggested in an article in the latest New Scientist magazine (pages 8 & 9, Jacob Aron, 27th August 2016** ) calling it earthlike is a bit misleading! It could be frozen solid apart from the odd blast of extreme radiation by the Proximan flare winds or similar to Venus depending on atmospheric details. It could also be a world that's carbon rich - with an ashphelt surface over diamond core like 55 Cancris e , could be all ocean or had most of its atmosphere blasted away resembling a super sized Mercury or be the stripped down core of a gas or ice giant or .. ??? We don't yet know. Sure fun to speculate but have to conclude we've insufficient data as yet - and must therefore learn more.
* That's minus twenty seven point four Fahrenheit according to an online converter thingammyjig.
** * Despite that range and unknowns this article is headlined "The Earth next door" which strikes me as being rather misleading and poor journalism.