The Culture has access to faster than light travel, including faster than light communication and the ability to communicate during faster than light travel. But it isn't instantaneous faster than light communication. This means that for two Minds with the ability to monitor and process the events of an entire solar system with billions of people on it, and which are not located in the same solar system, communication is incredibly high-throughput but also incredibly high-latency. Imagine shipping hard drives by camel—there is no practical limit on the amount of information one can send, but getting replies from one's correspondent is strictly limited.

For this reason, the communiques between Minds tend to be along the lines of novels and not text messages, and contain in addition to summarization and conclusions a vast amount of data and reasoning to support each summary and conclusion. The communication between Minds almost never requires one to ask the other "why do you believe that". It would be a waste of latency for a Mind to say that it believes something without providing its full reasoning behind it.

Mind-to-human communication is very different. Humans aren't able to process a fully justified derivation of a decision tree from raw data. They want to be able to ask their own questions, not see them predicted and answered in petabytes of analysis. And the information processing speed of a human is low enough that the latency of communication is no longer the bottleneck of the conversation—although if one ever speaks to a Mind over a long distance, it's likely that the distant Mind has simply provided the entire dump to a local Mind which is carrying out the conversation in proxy

GSV I Won't Tell If You Don't is receiving incoming communications from 23 other Minds. And then, all at the same time, it isn't.

A Mind doesn't receive communications like a human programmer working on top of a networking stack. It doesn't manually parse the entire packet structure every time, but nor is it delegated to a hardcoded subroutine which has no flexibility in what it analyzes or how it returns errors. A human programmer working on top of a network stack encountering this anomaly would receive segments up until a short time before the anomaly, at which point the network stack would stop receiving new messages and wait a fixed period of time before reporting the connection as dead.

This is a longer period of time when one is dealing with faster-than-light communications at a galactic scale than when one is dealing with local networks on an orbital or a planet, which can largely assume that a connection with no data received over it in one hundred seconds is dead, and which can be debugged by sending data to nearby devices and seeing what they send back.

The nearest other Culture ship—the nearest other computer of any kind not on I Won't Tell—is 23 minutes away.

A GSV doesn't manually monitor the analog levels on the multiple sensors it uses for communicating with other ships, but it does monitor the rate at which data is arriving on each connection, and it will notice when that rate drops to zero. It will then inspect the most-recently recorded analog levels of the communication sensors—this data is too large to be stored indefinitely, even for a GSV, but it's written to a ring buffer and flushed to long term storage whenever a higher part of the networking stack encounters an error—and determine that at the exact same moment, all of its network sensors stopped recording an analog signal sweeping between peaks of ones and dips of zeroes, but instead only recording low-intensity noise near the zero-point of the sensor. There is some data in the higher levels of the network stack—the data link layer has the first half of a frame, the network layer has part of a packet waiting for the next frame from the data link layer, and all of that is eventually part of a request at the session layer waiting for it's next segment.

I Won't Tell If You Don't has recovered the data sitting in the lower levels of the network stack and recorded it to long term storage—only a few thousand bytes would be lost, but something weird is definitely going on, and those few thousand bytes may be important.

I Won't Tell If You Don't switches into emergency mode and begins the long and arduous process of confirming that everything is working as intended. The amount of data being stored surges to ten times the previous level, and a truly ridiculous number of automatic analysis programs are run over all of the previously stored data since I Won't Tell left the Drixike orbital. Included in this list of emergency programs are:

Programs which are recomputing known physical constants from new observational data, to check if any of them have deviated from their normal values. This makes more sense than it actually sounds—lots of new physics is discovered by finding out that things that look constant actually vary due to local conditions, such as the curvature of spacetime and general relativity, or the speed of light and and four dimensional faster-than-light-travel.

Programs which measure local conditions like curvature and speed of light. They're supposed to be riding pretty close to the energy grid, deep into infraspace, but now they have to recompute that fact from new observational data instead of historical data.

Programs which map the galaxy based on infraspace waves—a largely futile effort for anyone who wants an actual galactic map, but one that should be capable of confirming their position, and failing that, that they aren't about to wander into a black hole.

Programs which validate the integrity of the storage, memory, communication, and compute units across the entire ship. This includes sending messages between all pairs of antennas and validating their error rates are below standard limits, as well as all three entirely-redundant Mind-instances cutting off communications with each other, running internal validity checks, and then checking the validity proofs of the others before fully opening communications again and merging back into I Won't Tell If You Don't.

Programs which validate the number, locations, health, and safety of the inhabitants of I Won't Tell—all 3 billion 102 million 723 thousand 605 of them. 606 if you include the Mind running the ship, 608 if you count each Mind-instance as a separate entity even while merged.

Programs which scan all known methods of communication, including ones that really shouldn't work in space, like pressure waves or chemical dispersion (also known as smell), because it would be really embarrassing to miss one. These scans aren't looking for anything that matches known communication protocols, although of course it will try all of the communication protocols it knows, but rather for any data at all that is statistically distinguishable from noise.

Programs which alert level-minded and trustworthy passengers of I Won't Tell that something weird has happened.

Here's how it plays out in real time.

1.6 milliseconds in, I Won't Tell If You Don't has noticed the connection throughput has dropped to zero, and data is recovered from the network stack and written to long term storage.

12.9 milliseconds in, I Won't Tell has completed initiating the list of programs described above, except for alerting the passengers. The last program it initiates at this time is the one to self-verify.

603.8 milliseconds in, each Mind-instance has completed self-verification, and they send each other their proofs.

748.4 milliseconds in, the Mind-instances have verified each other's proofs, and they remerge.

749.0 milliseconds in, I Won't Tell sends an alert to the humans gathered in one of the many operational centers, including its bridge.

I Won't Tell doesn't need a bridge. No humans are involved in the operation of a GSV. But humans are involved in the decision making processes of a GSV, to different degrees depending on the personality of the Mind running the show, and I Won't Tell is old-fashioned despite being less than a century old. It wants a bridge, and it wants a human leader archaically referred to as "captain", and would dislike making the important decisions for its humans in the same way a parent would prefer to trust their teenaged children instead of making all their decisions for them.

So I Won't Tell has a bridge.

The bridge is not what a member of a level 3 civilization would expect a bridge of a starship to look like. It isn't a sterile white or sleek steel-and-glass room with chairs and monitors and a viewscreen. It also isn't a junky, cramped cockpit with thousands of switches and exposed wires. In fact, a member of a level 3 civilization wouldn't guess they were on a starship at all

A massive cavity three layers tall, five layers below the surface of I Won't Tell, contains a garden. The surface of I Won't Tell is styled a humid subtropical mountain range, but there are interior gardens which provide spaces filled with plants from other ecologies. This garden is a pine forest, tall dark trees ascending into the highest reaches of the room, shading its occupants from the artificial sun. The floor carpeted in springy moss and dull red pine needles. This room isn't always the bridge, but it is today.

Around a fire, sit a collection of elected officials, appointees, favorites of I Won't Tell, and their friends chat and eat. The vibe is a cross between a security operations center during a quiet day and a group of friends. Most, but not all, of the members present are actual biological humans; the rest are drones surrounded by abstract and brightly colored forcefields.

Usually this group would be alerted of something meriting their attention via a floating notification over the fire, but this calls for something more dramatic.

Red neon signs, unsupported by any structures, light the air around them like confetti frozen in the air by a fast shutter.

"What's going on?"

"Total communication loss with the rest of the Culture. Round trip to nearest contact is 46 minutes away. Inquiries have been sent to the nearest ten thousand contacts, including port of departure and port of arrival."

"Any other anomalies?"

Some physical constants take longer to recompute than others. They just require more observational data, and are subject to more noise. Additionally, when you're reconstructing from scratch every constant or local condition from fresh observational data, some of your measurements will come back abnormally high or abnormally low via total chance. Some of the constants have been verified to within standard precision. Some of the constants still have extremely spread out probability distributions. Some of the constants seem to be converging to values outside of their standard values, but not with enough confidence to distinguish them from noisy outliers which will eventually settle down—and of course this means that some of the constants which currently appear within standard precision may still drift outside of that range as more data is gathered.

"Nothing yet, although standard integrity checks are still running."

"What are your top three hypotheses?"

"Signals were lost all at the same time, all with the same profile.

"Most likely is someone has massively increased the scale and distance of displacers, and has displaced the entire ship. No new physics needed for that one, only requires a significant technical breakthrough on a bunch of hard problems. Within that hypothesis, most likely source is unknown aliens, second most likely is another Mind playing a prank. If this is the case we'll resume communication in 46 minutes.

"Second most likely is that someone is riding closer to the energy grid than us, and running perfect jammers on all of our incoming communications. This doesn't require any new physics or even technical breakthroughs, but it'd be a near impossible feat of engineering to coordinate all of the jammers on all of our connections to start at the same time. Especially while we're moving. If we don't get a reply in 46 minutes we can start moving erratically and try to throw off potential jammers.

"Third most likely is that all of the nearby Minds coordinated to prank us. Based on my personality assessments of them, this is highly unlikely, but it's better than anything else I have right now."

"Anything that requires new physics?"

"There are a number of physical theories which would explain sudden loss of signals. In all of the cases I have discovered and simulated, however, they invariably also immediately lead to either your or my death. I have not stopped running simulations of edge cases of possible physical theories, but the fact that we are conversing means that none of the theories I have yet tested are plausible."

"If we are dealing with hypothesis one, how far would we have to have moved for signals to decay to zero?"

"All the communications were over tight-beam infraspace channels, so a small move could have been engineered to carefully place us in a region with no transmissions. Say, a few light days at minimum. Additionally, I think you're about to ask if the ping I sent to nearby contacts could miss them—that ping was sent over a high-powered non-directional protocol reserved for emergencies."

Most humans don't like it when a Mind does that—predicts their questions and answers them in advance—but Reil actually prefers it. It feels more honest to talk to a superintelligence that doesn't avoid giving you information it knows you want just because it you want to ask the question yourself.

"Got it. How long until we have any idea where we are? Do we need to be worried about running into a black hole, if we've been moved maliciously? Could we figure out where we are faster if we dropped into regular three-space?"

I Won't Tell If You Don't knows this fact about Reil, of course, and has modified the way it talks to pre-answer more of Reil's questions, even to the point where it starts to annoy Reil a little, because Reil meta-likes being annoyed by it. I Won't Tell is making sure not to annoy him right now, though. One doesn't want one's captain annoyed during an emergency, even if they meta-like it, even if they are more the captain of the crew than the captain of the ship.

"Our infraspace mapping would have revealed any nearby black holes in the first few seconds of search, if any existed, and none do. We would be able to map our location significantly faster if we dropped out of infraspace, but normal three-space is more dangerous if we are being attacked by something, and it will take longer to reestablish connections if we keep changing our trajectory.

"As for how long—that depends on how far we've moved. If we're less than a light year out of place, we will know where we are in less than 29 minutes. If we're halfway across the galaxy, it could take hours to days, depending on what we're near."

Certain astronomical phenomena give of stronger natural infraspace waves, although all natural infraspace waves are pretty weak. Black holes, pulsars, and quickly orbiting binary star systems give off the strongest ones. If they're near something like that, they should be able to identify it and look it up in their star charts to figure out where they are.