Anyone who has been in an email conversation where the chain split into different threads, or has been in chat where they receive two diverging comments requiring a response before a reply to the first is complete, know what is referred to here.

Now imagine there are light years separating the communications, not just your speed on the keyboard. There is no chance for a phone call to clarify things, and it can be years before you get back a response that the other party is lost.

If you communicate with someone who left Earth on a journey you may set up rules. If you communicate with a previously unmet entity, you would not have an existing rule.

Do any studies or protocols give best practices to maintain clear communications with significant time lag? If yes, have there been any tests to validate their effectiveness?

  • $\begingroup$ Voyager 2 is 0.67 light days away from Earth. If a 20 kW transmitter of DSN is sufficient for that distance, about 5.9 GW will be necessary for only 1 light year. We should be happy with a communication over a light week distance using a 2 MW transmitter. 43 MW for a light month. $\endgroup$
    – Uwe
    Oct 28, 2017 at 15:31

3 Answers 3


Historically, we haven't had the instantaneous technology that we enjoy today. However, we have never had in place a way to communicate at a distance without setting up some sort of protocol beforehand. Let me discuss how some of this has been handled historically, how it is handled with distance spacecraft today, and a bit of how we could figure it out with an unknown entity in the future.

Historically, when there was a months lag in communication between, say, Europe and the Americas, an ambassador was sent to another country, with some specific guidelines to follow, but given some lee-way to exercise judgement if something fundamentally different happened. This is the same way that explorers worked, and any other number of people making important journeys, they were given some direction, but had to exercise autonomy to fulfill said directions.

Okay, so how do we manage communication with distance spacecraft today? Essentially you have to set up an asynchronous communication protocol. You send a packet, each packet identifying where it is in the stream. Some packets will be lost, but as a whole, most will make it. You listen carefully to get as many of the packets as you can, but you might miss something. Depending on the spacecraft, a re-transmit request might be made. It is actually fairly similar to how the internet works. The further you are, the less likely you will request a retransmit. In most instances, they listen on a different frequency than they transmit, allowing 2 way communication to happen. This is called Full Duplex communication, by the way.

Okay, so what to do about really large distances? Fundamentally, it is the same as we communicate with Voyager. Send a packet, request a response, and be listening the entire time for said response. You should see the response come in eventually, especially for a machine, the time can be predicted when it will arrive.

Okay, so what about a human communication approach at a huge distance? You either need to set up a time scale to know when said communications can be expected, or you can have something listen all the time. Both ends will require asynchronous communications, as well as requiring pointing a large dish at each other, so you have to time it correctly, or else the other party won't be listening. You will have to allow them some time to send a response.

As to how to communicate with an alien entity? The first step is to decode their message pattern, so you can understand it. At first, you are either going to have to do a simple (Return on the same frequency), or a close duplex, as you will not know where to send a response that they can listen to. You should be able to get the right thing figured out in the end. You will have to apply the lessons from above, but as you can't receive when you transmit on the same frequency, you will have to be very careful as to when you transmit. Or better yet, have a station on one part of the earth receive, and another part transmit.


Of particular interest in scenarios such as this where the delay could be extremely large and the communication medium lossy is the combination of repetition and multiple options:

If the craft is populated by humans you can allow a certain amount of autonomy, but you may still want to provide alternative messages. As a simplified example:

If scenario x occurs, follow the instructions in envelope A, otherwise follow the instructions in envelope B.

How this would translate into modern day comms would be to send a series of alternative instructions giving guidance/orders based on possible outcomes as predicted from Earth:

If that asteroid is iron based, take 4 extra days to analyse; or if it is water ice, replenish reaction mass; or if it is populated ask to see their leader

A problem with providing multiple messages is that loss of packets could cause confusion, hence the requirement to repeat. Unlike TCP/IP where the recipient can state "I missed packets 10405 and 11200" the lag would be too long, so a continuous stream of message is likely to be needed. As you can imagine, this can lead to a requirement for multiplexing all the comms that may be needed to the explorer on one channel (or more) and all the return comms on another.


You start with a range of predicted scenarios (back to the original messages in envelopes scenario - or store them in HAL's memory) and leave it up to the explorer.

  • $\begingroup$ I don't see why you'd need to send multiple messages to convey different possibilities. It'd be perfectly possible to convey that information in a single message (just pick a message encoding that allows multiple possibilities and action responses), and appropriate use of FEC and some sort of checksums should be able to take care of most of the rest. $\endgroup$
    – user
    Jun 8, 2014 at 20:58
  • $\begingroup$ Michael - that means the same thing. $\endgroup$
    – Rory Alsop
    Jun 9, 2014 at 15:52
  • $\begingroup$ I guess I found it confusing because of your use of "instructions" and "messages", particularly in combination with the paragraph after the second block quote ("a problem with..."). Maybe this answer can be edited slightly to clarify that part? $\endgroup$
    – user
    Jun 9, 2014 at 15:54
  • $\begingroup$ Mind you, I'm not saying this is a bad answer (and assuming we can transmit at all over such distances, I imagine it'd be done much in terms of multiple-choice commands based on autonomous decision-making); I just figure if I misunderstood that part, it's certainly possible that others might, too. $\endgroup$
    – user
    Jun 9, 2014 at 15:56

This is one of those things that, for the time being, everyone is looking at the possibility of the use of quantum entanglement as means of relaying information error free and over enormous distances at greater than light speeds:

Repeated experiments have verified that this works even when the measurements are performed more quickly than light could travel between the sites of measurement: there is no lightspeed or slower influence that can pass between the entangled particles. Recent experiments have shown that this transfer occurs at least 10,000 times faster than the speed of light; this merely establishes a lower limit to the speed — according to the formalism of quantum theory, the effect of measurement happens instantly.

But short of quantum entanglement for purposes of communication, we currently have no means of relaying information faster than light, and they all come with various drawbacks, mainly the signal strength and with it required power for communications signals to traverse without loss of data over such vast distances. So, of course, for any missions at lightyear distances, you'd want your probe to be as autonomous as possible, possibly have some form of Artificial Intelligence (AI) onboard, at least somewhat controlling the probe to avoid any obstacles it might encounter.

Similar semi-autonomous systems are already in use with several unmanned probes that we sent in space, for example the Rosetta spacecraft has the ability to avoid close proximity obstacles, something that will likely need as it starts closing the distance to the 67P/Churyumov–Gerasimenko comet it is chasing and when it approaches the Sun increases its coma and tails, with possible debris chipping off it. Even being merely around 1 Astronomical Unit (AU) away is still distant enough to cause communications delay that is utterly unmanageable for avoiding collisions from the Earth in real time.

So having more advanced AI in place and have our probes eventually 100% autonomous, not depend on having to communicate with us over distances in the magnitude of light years, and do their work completely independent of our constant control of their functions is, arguably, just a few decades shy from materializing and certainly within the reach of even our current technology. That ought to be a lot faster than we'll be able to develop any interstellar drive technology getting us there by the time our probes are still expected to operate and anyone here still ready to listen to data it transmits back.

I guess what I'm trying to say is the communications delay might not matter so much, if we learn to design around this obstacle with the use of AI, which seems to advance a lot faster than drive technology needed to reach other stars in a reasonable timeframe is. And when quantum entangled communications will be possible, all these drawbacks in communications delay and power requirements needed to produce strong enough signals for us to eventually pick them up will be gone.

As for the off-chance of encountering yet unmet sentient beings out there that would be technologically advanced enough to detect our presence - all of this is a matter of opinion, like with the rest of dealing with the unknown. My opinion (take it as such) is, that luckily for us, chances of encountering another civilisation in their counterpart to our 16th century development and though are extremely remote to say the least, so any proper protocols that should be in place in the eventuality we'd encounter previously unmet entities should be a lot easier to achieve than what popular culture would have us believe in science fiction movies and literature.

If they'd be any less technologically advanced, you'd likely pass by undetected, or our presence dismissed as something that the civilisation you encountered can relate to. And if they're same or more technologically advanced than we currently are, they will know well enough that it's a robotic probe they've detected, as such a proof of intelligent life elsewhere in the vastness of the Universe, and will likely rejoice and have a golden record with greeting from the Earth to place on top of their charts. And if they won't and are combatant in nature, they will most likely be involved with themselves too much to pay attention to some other civilisation, light years away, and by the time they could, their civilisation will be back to stone ages.

I don't think we should preoccupy ourselves with how advanced some theoretical civilisations that aren't ours and are light years away might be, and rather more frequently look back and reflect on ourselves in which of the two directions described we are heading. My 2 cents. ;)

  • 1
    $\begingroup$ Quantum entanglement can't be used for communication at all. en.m.wikipedia.org/wiki/No-communication_theorem $\endgroup$
    – rlbond
    Oct 24, 2014 at 14:35
  • $\begingroup$ @rlbond I mention that everyone is looking at the possibility I don't claim that it's possible. Tho that's a personal view I disagree with your point. We don't know if it's possible, that's why it's called on that wiki page a theorem and not a theory. It might as well prove that communication by using quantum entanglement might be possible but not at the speed greater than light. It would still be useful as such, for example it could enable high bandwidth comms with the surface of Venus or anywhere else where there's physical obstacles or radio spectrum refraction over critical angle. $\endgroup$
    – TildalWave
    Oct 24, 2014 at 14:46

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