Let’s say that, via some method that doesn’t really matter here, we find conclusive proof that there is an advanced and intelligent alien civilization inhabiting This Expolanet of That Star, which is, say, five light years away. Could we communicate with them via existing technology? Or would whatever signal we sent them degrade and be useless across that much space?
I don’t know the answer to your question, although the SETI people certainly think so. But even if it were possible to exchange symbols, can you imagine how long it would take to establish a common language if any exchange took ten years back and forth?
The internet tells me that the two Voyager probes transmit with a power of 23 watts, and we are able to receive their signals with 10^-18 watts on Earth. The probes are 0.0025 light years away, that’s 1/4000 the distance of your ten light years star, so the law of inverse square would tell us that the signal would have to be sent from the other end at 23 * 4000^2 watts. That’s 368 megawatts. That’s an awful lot of power that would have to be pumped just into transmission, about a third of the power of a nuclear plant just in terms of electromagnetic signals. On the other hand, I would guess that we’re not pointing our most sensitive receivers at the Voyager probes.
You got something better than the Deep Space Network?
Though of course, we could make something better, if we had reason to.
For perspective, there are about 12 star-like objects within 10 light years of Earth, including the Sun and including 4 or 5 brown dwarfs (failed stars).
That calculation assumes that the Voyager probes are right at the limit of what we can detect, and we could not detect anything fainter. I doubt that that’s true.
The example that comes to mind is using the Sun as a gravity lens, which has been proposed as a method for detecting and/or communication with extragalactic civilizations. Much less ones within a few light years.
Putting a large transmitter/receiver station out past Uranus is well beyond what we can do now of course, but it’s one of those “only an engineering problem” issues. No new physics would be required, just lots of money, time and effort. Which any attempt at interstellar communication would require anyway.
That’s why you don’t do a straight exchange. Just constantly transmit data until they can do their own translation and send questions back, while they do the same with us.
Like Asimov suggested in “My Son the Physicist”
Ah! That’s actually the story I originally got the concept from (seen it elsewhere since). I just couldn’t recall the name or author. I’ve always privately thought of it as “the gossip method” because of that.
I thought that might be the case!
Voyager benefits from us knowing exactly where it is in the sky and knowing what frequency and data coding is used.
The signal to noise ratio and bandwidth defines a hard limit on the data rate we can achieve. It doesn’t matter if the signal is below the noise if you know it is there. The data rate just gets lower. Knowing it is there is the key.
The Deep Space Network has receivers that are on the bleeding edge of technology. Electrical sensitivity isn’t really something that can be much improved as it is limited by the thermal environment, and thus the intrinsic noise.
A bigger antenna is an answer and possible, especially if we build in space or maybe on the moon. And we get freedom from the background noise of modern technology. That gets you more signal in both directions and better signal to noise. You don’t need a dish if you have enough electronics to run a phased array. That is a big if as the array size increases. But is a matter of scaling not an intrinsic limit.
Eventually you have a core problem. Get the other end to notice you. Then you work out how to exchange information.
Getting noticed may be really hard. We can stack the odds. Assume the other end has enough knowledge to have a good idea of the planets in range. So assume they will take particular notice of us. So they might be listening. Blast a suitable message. It can start much like those Carl Sagan and friends crafted. It will be very low information rate. But it needs to contain a lot more information - enough that a receiving civilisation can work out how to craft a useful reply. Where useful is something that is worth a 10 year round trip. The message can be long, and we just keep repeating it, year in year out. And look for a reply after the first 5 years. If the signal to noise ratio at the other end is really low, which it will almost certainly be, the message modulation needs to draw attention to itself.
The big problem is that maybe the remote civilisation listened for a few decades a couple of million years ago and got bored. The time scale of technology versus time in the universe is against us lining up.
If the question is not about the physics of the transmission but about how to establish mutual understanding in an exchange, the OP might be interested in Lincos. It’s not exactly a language but rather a protocol for communication to assign mutually understood meaning to signals to allow for communication:
Fair point. Every now and then there’s a story about some amateur astronomer or radio operator picking up a Voyager signal, so I suppose the DSN could pick up something fainter than that. OTOH, there’s surely a difference between “picking up a faint signal occasionally” and “establishing stable reception for meaningful communication”.
One issue with accidental communication is that compressing data makes it look more like random noise, and in our history, at least, there was a very short window between when we started using radio and when we started compressing our transmissions (and in another species’ history, that gap might not exist at all: They might have started compressing before they had radio).
Of course, no intentional signal looks completely like random noise. It’s usual, after you wring out all of the redundancy you can from a data stream, to deliberately put a little bit of redundancy back in, in a precisely controlled way, so you can detect and possibly even fix small errors in transmission. And in practice, that’s usually done in a fairly obvious, intuitive way: There have been cases of different software engineers independently implementing such solutions and coming up with the same thing on the first try.
Concerning signal strength: a recent study claimed that an Earth-like civilization would be able to detect the emissions of another Earth-like civilization at a distance of about 12,000 light-years, or about halfway from here to the center of the Galaxy. The key is using “planetary radar” telescopes for the transmission side of things, which are extremely powerful (and beamed, which allows for less wasted power.)
Unfortunately, according to Wikipedia there’s only one currently operating radar telescope, but there are others that have been proposed and/or planned. (The most famous such telescope, at the Arecibo Observatory, collapsed in 2020.) But the important point is that we don’t lack the technology to do this, we just lack the infrastructure.
EDIT: I should also note that the above study was concerned with detection rather than communication. The latter might probably require a higher signal-to-noise ratio to be practicable, and so would be more limited in range.
A “radar telescope” transmits signals to an object in space, that then bounce off of it, and it detects the return signal. There’s only one of those, because there’s very little demand for it, because it’s only practical for very close ranges (within the solar system). But there are plenty of other radio telescopes, which detect signals originating from other objects, and any of them could very easily be converted into a transmitter if there were some reason to do so.
OK, you know how in Sci-Fi, the crew of The Spacecraft “scan for signs of life” when approaching a new planet? Is this a thing that can be done a) at all and b) from Earth? Consistent, predictable and meaningful radio signals would be one way (there’s an advance civilization down there sending radio signals). Are there any others? Can we detect, say, certain compounds that only exist in living organisms?
The obvious answer is to look for significant amounts of oxygen in the atmosphere, since we don’t know of any non-life way of producing that. But the problem is that, first, there might still be some way we don’t know of, and second, not all life produces oxygen.
In practice, even for the next closest planet, and even with actually sending probes there, we’re still not completely certain. For anything more difficult than that? Fuhgettaboutit.
Your calculation is much too pessimistic.
Using the Friis formula and various calculators on the Internet, under ideal conditions a pair of 100 m diameter dishes 10 lightyears apart operating at 1 MW and 10Ghz each should be able to achieve a received power in the 1e-18W range. That’s easy enough to do. Multiply power by 10 to overcome real word inefficiencies and you still only need 10 MW.
Entirely feasible.
You also need to make sure to point at the right bit in the universe. The required precision of a few arcseconds is doable and in fact is being done every day. The required accuracy of knowing where your desired target will be in 10 years time relative to where you are now is also possible in principle. In the context of the OP, it depends on what knowledge you have already gained about the other side.
This actually has a surprising twist: An antenna on Earth listening to what was sent 10 years ago will likely point in a different direction than an antenna on Earth sending something that will be received in 10 years time.
But this also shows the real problem here: Both sides need to know, or at least be actively searching for, where exactly to listen for an incoming transmission.
Not really true; an ocean planet with a deep high-pressure ice mantle could have an oxygen-rich atmosphere simply by photolysis; certain types of water-rich, tidally-locked worlds could have oxygen-rich atmospheres for a similar reason. But if we ever get a good look at nearby terrestrial-type worlds then these abiotic oxygenated worlds should be relatively easy to identify, and would look nothing like Earth.