There have been numerous means of sending a message from point a to point b over the span of human existence, within the past couple centuries it has become possible to ask someone at point b what the weather is like without actually sending someone to physically deliver your missive. Naturally people have started to take the ability to receive an instantaneous response for granted and most science-fiction (and a few fantasy) authors have naturally incorporated it into their works, even including some form of “interplanetary internet” in some cases. Though sometimes they don’t think things through too much, making mistakes such as interstellar wi-fi, to prevent such errors why don’t we take a quick look at how communications may work across interplanetary and interstellar distances.
First off there’s the single most common medium of transmission since the mid-20th century, radio waves. Transmitters translate text, verbalization, or other forms of data into discrete or continuous pulses of electromagnetic radiation (aka light) with wavelengths ranging from 1 millimeter to 100 kilometers and frequencies of 300 GHz to 3 kHz and a receiver detects and re-translates the information sent. Their low frequency and long wavelengths mean that radio waves have very little energy compared to other forms of EM radiation (and most definitely cannot cause cancer) but can potentially carry information for light-years before losing coherence. However radio waves are limited to the speed of light, so any attempt at calling someone further out than a light-minute or two (for reference, the sun is about eight light-min from earth) is going to experience a considerable amount of lag as the time it takes the waves to travel to their destinations becomes noticeable. In addition signals sent using radio will become incoherent with distance, depending on the frequency, the absolute limit being one or two light-years.
Another common means of communication is concentrated pulses of visible light, usually along glass fiber-optic cables which shield the signals from interference by the atmosphere. This method allows for far superior data quality than radio but atmospheric gases or particles can block them easily, as can physical objects that radio waves can pass through. In the vacuum of outer space there is considerably less matter in any form that can block an optical signal, however, especially if the signal is transmitted in the form of a laser capable of maintaining integrity over great distances. Lasers are also less susceptible to jamming or disruption by solar flares. But there has to be a clear line-of-sight between the transmitter and receiver and even lasers spread out and become incoherent over interstellar distances.
As for how the internet might cope with space travel, e-mail and social networks would still be possible, and probably the primary form of communication between planets, but instant messaging would no longer be “instant” and if you think AOL back in the 1990s took a long time to load webpages, you probably wouldn’t have the patience to try surfing the internet from Mars. In all likelihood deep space colonies would form their own separate internets, with unique web sites inaccessible on earth or any other fairly distant regions. Certain websites that may be determined to be “important” enough might set up localized servers that would receive updates from one another at specified intervals, but you’d have to wait several hours and most likely need a massive transmitter to look up any other sites based outside your local region of space.
Neutrinos, those supposedly massless particles that don’t interact with most normal matter and instead pass right through it, gained some publicity a few months ago when readings by CERN supposedly indicated that they travel slightly faster than the speed of light. Those readings were determined to be an equipment failure (a disconnected wire) but another group of researchers managed to do something not quite as amazing with neutrinos, but still significant. They managed to use neutrinos to send a one-word message through 240 meters of solid rock. Granted, the transmission speed was very slow, only 1 bit/second, and it took a particle accelerator to send the message, but still the neutrinos experienced negligible interference from materials that would block radio or optical signals completely. They could be very useful for communicating for people deep underground or underwater, or on the other side of a planet or star even. Neutrino transmission would need to be very tight beams like lasers to compensate for the low transmission rate, but the advantages of a transmission medium that is near impossible to block are considerable. Of course, if someone managed to place a neutrino detector between the sender and the receiver they could read the message without anyone knowing.
One of the science “buzzwords” of the century is “quantum mechanics”, relating to the behaviors of subatomic particles. One thing that science-fiction authors have extrapolated from the various “weird” properties covered under quantum mechanics is the use of “entanglement” to send messages instantaneously over any distance. The idea is that when two particles are “entangled” at the quantum level they can be separated and whatever happens to one particle happens to the other one instantaneously. Somewhere along the line someone decided that that could allow communication faster than the speed of light. In addition to sending messages instantaneously a quantum entanglement communique would be impossible to intercept as it would be teleported to the receiver. The harsh reality is that the act of observing an entangled particle breaks the connection with the paired particle, attempting to send data with entangled particles would by necessity require observing them.
However, quantum entanglement can be used to encrypt messages sent by conventional (currently only dedicated fiber optic cables) means such that only those who possess one of two “keys” can interpret the data. By encoding a transmission in the form of quantum states of a particle one ensures that the very act of intercepting it would corrupt the data and alert the holders of the keys as to how much of the message was intercepted. And it actually has been done, some governments and companies who consider security worth the expense use quantum cryptography for their most secure data transmissions, the Swiss canton of Geneva used it to send national election ballot results to the capital in 2007 for example. There have also been experiments with sending quantum encrypted messages over radio as well, it seems likely that the technology will become more prevalent over the next few decades. Though of course it only works between two specialized devices that have to be physically transported to their working locations.
The Utterly Fantastic
Of course, even quantum-encrypted FTL neutrinos would take years to travel from one solar system to another, so many authors have turned to the farther fringes of science in order to maintain “instantaneous communication”. For example, tachyons which are highly hypothetical particles that travel faster than light and which most scientists don’t believe exist. Or if their universe allows physical travel through some sort of “hyperspace” they might send radio transmissions through that same dimension where the normal laws of physics don’t apply. Heck, you might even use mentally “bonded” telepaths, worked for Heinlein.