EDIT: Apparently the rover has an antenna that can directly communicate with Earth at 10 bits per second! That's obviously not useful for pictures or anything, but it's an amazing capability for emergency comms.
Very low bandwidth, very low power, very long distance comms are really cool. The limiting factor tends to be the clock frequency precision - if you have precise enough clocks at both ends, there isn't anywhere in the universe you couldn't communicate with, as long as you're happy to go slow enough.
> there isn't anywhere in the universe you couldn't communicate with
Right, assuming you are emitting infinite amount of photons, there are no obstructions, black holes and you ignore universe expansion and GR which can make your precise enough clocks go out of sync.
I think that the double slit experiment shows that this isn't an issue... Since we are summing with all other noise in the universe, it doesn't matter exactly which photons came from my transmitter, if any at all, but the total count will still (probabilistically) reflect the the we're after...
On long distances you are talking about receiving single photons. And while we may view it as probabilistic, it will either interact with our equipment or it won't. If it doesn't, communication failed.
But yes, I wondered about that when writing my comment. Certainly photons are more than just "bullets" that are getting more and more sparse. So I got curious and it seems[1] that in context on our detector you can indeed put hard bounds on where the photon may be received. As far as I understand it, based mostly on the abstract, I do not really grasp it.
But ignoring the paper, I think I get your point. If photon has any chance of reaching the target (even lower than virtual particle spawning) and even if you don't know the future of the Universe (other photons hitting detector), you can use some complex encoding so that observing certain (allegedly otherwise unlikely) patterns allows you to say you received some message with some probability (which is what communication always is). If you have synced clocks. Simultaneity is relative though so there's also that.
Noise, by definition, doesn't have a qualifying characteristic. It just means "all signals we're not interested in". That means your ability to filter out noise is strongly dependent on having pre-existing knowledge about the shape of the signal you're interested in.
Self-clocking signals are useful when the ambient noise level is low with respect to the signal, but if your signal is just "these two droplets in an ocean" you will not be able to filter out enough of the signal to detect the edges of the original signal.
Having a very reliable clock frequency means you can apply a very exact filter to lock on to the signal you want, thereby making the transmission less sensitive to interference from other radio signals. Just having a sensitive receiver isn't enough, because a sensitive receiver will also pick up more of the other radio signals.
It is possible to send a signal weaker than the noise floor. The maximum information you can send is defined by shannon's channel capacity formula, which clearly says a signal can be weaker than the noise.
If you use a self-clocking signal, then effectively the information you are sending consists of both the clock timings and the information you're really trying to send. If your channel capacity is very low, you will find there is no channel capacity for even the clocking information, let alone the information you really want to send.
https://old.reddit.com/r/IAmA/comments/lpzbzo/were_scientist...
EDIT: Apparently the rover has an antenna that can directly communicate with Earth at 10 bits per second! That's obviously not useful for pictures or anything, but it's an amazing capability for emergency comms.