# How would a radar work while inside a FTL dimension?

In my universe FTL drives are easily mass manufactured and most citizens own them and use them.

The FTL drives work by compressing a certain powder which then explodes, ripping open a wormhole which the ship is sucked into. The ship will enter a dimension of infinitely folded spacetime.

The dimension is completely black and is shaped almost like many ant colony tunnels. Each part of the walls in the tunnel is linked to a certain place inside the galaxy, so if the ship drives itself into one of the walls at that point it appears outside of the dimension in that place.

The problem is the dimension is completely black with no light at all. It’s extremely large and there is absolutely no way for the human eye to navigate this dimension safely without bumping into a wall and appearing somewhere random.

When inside the dimension the ship would act like it’s just in space. There is nothing inside the dimension except the occasional debris of ships but other than that it’s very plain and empty.

So engineers invented a machine that can navigate through this dimension and safely bring the ship to its destination. The question is how would it work?

EDIT: for clarification the ship never actually travels at the speed of light in or out of the space time dimension. They are just called FTL drives because if done right you can literally travel thousands of light years in a matter of days, hours, or even minutes. Almost like teleportation.

• Why would just keeping track of how you have moved and where you started not work as navigation? Feb 28, 2022 at 20:00
• As you present it, this dimension seems to be some kind of maze, and you've set out to travel faster than light through the maze.. Now suppose you'd have radar, you still need to circumvent collision in real time. At FTL speeds, that could be quite demanding on the pilots.. Is this space static or dynamic ? If static, did you consider a map of it ? Does/Can the traveler set a route in advance.. Feb 28, 2022 at 20:37
• Maybe radar doesn't work at all, there may be infinite FTL dimensions even if 2 spaceship share the exact same channel they will still never meet but rather be in a state of superposition ;D Mar 1, 2022 at 1:09
• @Goodies that makes no sense at all even before the edit. Since the FTL method is traveling through another dimension and the radar signatures are in the dimension alongside the ship, the speed differences between the ship and radar would be maintained inside the dimension (otherwise walking in the ship would not be possible either). Mar 2, 2022 at 16:30
• A lot of information about your folded space needs to be added before an engineering answer can be attempted. Please put down some folded space physics explaining how things like light and magnetism and radio functions in folded space, that is not like our real space. Mar 2, 2022 at 17:22

Lit buoys.

Your dimension is kind of like boating at night. You can't see anything. It is easy to get lost. It is easy to hit shallow water.

So there are lit buoys. These mark the path.

Your dimension has the well travelled parts marked with buoys. They are colored lights seen from a distance and they also broadcast a radio signal with the coordinates of the light and what it looks like. The buoys use radar to detect nearby ships. The radar operates all the time but the colored light and radio signal only turn on when a ship is in proximity - this to save energy.

A buoys will ask for a battery when its own battery gets weak. It is considered good form for a traveler to stop and change the battery if they find such a buoy. Civic minded travelers carry spares.

Your space is big and it is possible to get out past the buoys. When people do this they often leave little buoys behind them like a trail of breadcrumbs. Out in unexplored areas it is not uncommon to run across depleted old buoys in the dark.

• And this approach offers soooo much flexibility story-wise! Mar 1, 2022 at 10:51
• Babylon 5 did something similar, though their hyperspace is more of a suffusion of reddish light, there's still nothing that can serve as mileposts that is naturally occuring. Mar 1, 2022 at 13:51
• Wouldnt it be smarter to turn it around? The bouys are simply reflective, the ship simply shines its radar signals across the channel and illuminates the bouys, no power in the bouys needed and no degradation/computer components either. Added advantage: since there seems to be no background radiation inside the dimension (and no negative consequences despite that) you can use an ultra-low energy frequency to illuminate the bouys as long as you have high-sensitive sensors to detect the return. Mar 2, 2022 at 16:36
• @Demigan - that is a slick idea. I wonder if one could include the coordinate info in the radar reflective profile. Actually given no atmosphere and no background radiation there is no particular advantage to radar - you could use visible light. Mar 2, 2022 at 18:53
• @Demigan - I really like this idea, but I think they'd be a little more complicated. The presence of a buoy just says "Someone was here." To be navigationally useful, they would have to say "wall near here exits near X." I'd copy the idea of an RFID. When RF at a certain frequency hits the buoy, that powers a circuit inside it, which spits out the data you care about. Mar 2, 2022 at 22:22

The simple option would be to "feel" your way through.

The cheap but dangerous way would be to use some kind of projectile... shoot ahead of you and bounce a radar off the back of the projectile. When the return signal abruptly stops, you know there's a wall and the projectile shot through it. Keep shooting to map out "space" around you. It isn't necessarily very nice if there happens to be someone or something the other side of the "wall" where the projectile is exiting though.

A fractionally less cheap but rather safer technique would be to use tiny drone probes that fly out ahead of you, and you watch for them suddenly vanishing. Depending on the rules of your hyperspace, you could literally feel around with a sufficiently long tethered probe, but that might not be very practical if the tunnels were very large relative to the size of the ship.

• I like the idea but keep in mind this dimension is VERY big and every piece of the wall could be tethered to your desired star system so with this in mind your strategy could take multiple years if not more. Feb 28, 2022 at 20:02
• @HomegrownPotatoes you didn't specify sizes. That said though, the system does scale up relatively well... the blast front from a nuclear shaped charge travels quickly and spreads out nicely and is nice and warm and so observable via infrared. Just keep popping off nukes until you've made a nice map. Feb 28, 2022 at 21:34
• @StarfishPrime I like this Idea, but what about just firing off sending of little retro reflectors. Cheaper then a probe. Mar 1, 2022 at 1:46
• @GaultDrakkor because of the speeds involved and the potential risks, mostly. I already suggested radar-reflective projectiles after all... problem is that once they pass through a "wall" they're largely indistinguishable from a surprise railgun attack to anyone who happens to be on the other side, whereas drones might reasonably be able to slow down or at least avoid things on the other side. Mar 1, 2022 at 8:31
• @StarfishPrime username checks out Mar 1, 2022 at 9:16

# Track the gravity.

Routes are regularly worked out by drones with years of effort exploring places. When a new route is found, they stick a massive object into hyperspace, generally an asteroid, but sometimes something as large as a planet. Some people who are OK living away from people live on these asteroids permanently, offering repairs and refuels to ships.

They have extremely sensitive sensors on ships which can detect this gravity. Now, new tunnels can be found even if the dimension shifts or debris blocks a tunnel. Just track the unique gravitational signal of each asteroid.

# It's just a normal radar

"When inside the dimension the ship would act like it’s just in space"

It's in another dimension. The distance covered in this wormhole may put your ship thousands of light years away in a matter of minutes/days to our normal world, but inside the wormhole, you aren't moving that fast relative to how fast you would be moving in normal space.

## The best example I can give is this

If you know of Minecraft's nether dimension, this is basically what I mean. Every 1 block in the nether is 8 or so blocks in the normal world. You still run the same speed in both dimensions, moving 100 blocks in both dimensions takes the same amount of time. But if you move 100 blocks in the nether, and then return to the overworld, you would have moved 800 blocks. This is basically how that dimension works.

So, you don't need a radar that can move faster than the speed of light, because in the warp dimension you aren't really moving that fast, you just cover a lot of distance. And I am assuming for navigation you would rely on charts and certain landmarks and maps for navigation. Like "there is a bend here in the tunnel, with a branching path that is very distinct on the radar. Travel 800m from that point left and you'll find this point to return to the normal dimension" kind of thing.

• Simple and makes most sense. Mar 1, 2022 at 6:58
• This assumes the walls of the dimensional tunnels are visible on the radar. But since matter simply exits the dimension when it hits a wall, why would radar be reflected? Wouldnt it just exit the dimensional tunnel and never return? Also unless you go into the dimension at an exact location, the rules of the tunnel walls could relate to completely different area's of space (as indicated with exiting at a random place in space if you bump into the wall). That would make maps useless unless you use an exact point the mapmaker used. Mar 2, 2022 at 16:56

Radar propagates as electromagnetic ripples. In space time, a ripple consists of a periodic gravitational deviation (that is: a sinewise gradient variation in space time) which is also known as a gravitational wave.

How to set it up

I'd suggest to build an rotating array of gravitational wave detectors, and a gravitational wave emitter of some kind. You could e.g. generate the gravitational waves as a side effect of the Alcumbierre drive. The drive will produce some kind of exhaust, consisting of miniature space-time gradient variations, which propagate through space-time. When there's something in the way, the reflections will reach your gravitational wave detector.

Its limitations..

The wave would reflect on dimensional boundaries, so you could at least chart your space and circumvent collission with these boundaries.. but look out for black holes ! these are nasty objects that would probably not be seen by this kind of radar.. as are blue giant stars for that matter, you fly into these easily, when you cross a dimensional boundary. There's something your radar cannot anticipate.. the gravitional waves do not pass dimensional folds.

Take a break, look around and plan your jumps..

Gravitational waves travel with the speed of light, so when the ship is moving FTL, your waves won't return in time. This question is not science-based, maybe you could change the strategy of travel, that is take a break and look around before jumping. The rest of the jumps better be good (FTL) of course, else it won't be of any use to attempt travel FTL through space with obstacles. See also my opening comment, consider having a map and a travel plan.

• Your opening paragraph isn't wrong, per se, but seems to imply the term "gravitational wave" is somehow inaccurate. "Wave" is a technical term for a periodic, sinusoidal deviation. So "gravitational wave" isn't just common parlance; it's the correct technical term. Mar 3, 2022 at 0:15
• @MichaelS +1 your comment seems reasonable, but I'm afraid I don't understand what you mean. Would you please edit my question to show what you mean ? Mar 3, 2022 at 7:05
• It's not huge either way, but I think just replacing the word "commonly" with "also" would help. Mar 3, 2022 at 7:12
• @MichaelS I see... thank you! Mar 3, 2022 at 17:26

This answer assumes that "completely black" means that the walls of the dimension do not interact with any kind of electromagnetic radiation. Maybe objects within it are even hidden from each other, though I assume that light works at least within a ship.

How do you navigate something which you cannot detect? Answer: By trial-and-error. Explorers more or less randomly move within this dimension until they eventually emerge somewhere in realspace. They take great care to log the location, direction, and velocity of their entry and exit points, as well as their exact movements within the FTL-dimension. Your people might want to use cheap drones for this. Over time, this will establish a network of known useful routes (and many more completely useless ones), which others can follow.

Hopefully the structure of the FTL-dimension is at least somewhat continuous, such that small errors in navigation still mean that you are likely to end up in roughly the right place (the scale of the offset at the exit point may be orders of magnitude higher and if you are really unlucky, you bump into a wall somewhere along the way).

If the wall structure is not stable over time but drifts somehow, you will need additional effort via drones to continuously maintain and update your established routes. Woe to the colony who finds itself suddenly cut off from the rest of the galaxy because all FTL-routes leading there suddenly fail.

• I'm surprised this isn't the accepted answer. This is essentially what you want, points of entry and exits that are regulated and have all necessary equipment to communicate as much as possible with traveling ships and handle any possible emergencies that could happen. Like airports, you plan the route ahead of time and the airports help for blind navigation if needed. Mar 1, 2022 at 10:41
• Bob Shaw, Nightwalk had a similar setup... Mar 1, 2022 at 11:53

Dust scanner

You mention that the dimension is empty save for the occasional debris of other ships. The debris will provide you a path of travel, as due to the ships having exploding once upon a time or whatever ill fate has befallen them they have scattered an untold number of dust and gas particles into the void of this dimension, to float endlessly or until they bump into the dimension walls and appear somewhere in the universe.

The particles will bump into each other as well, slowing themselves down as they lose the energy that was imparted to them in the initial explosion and settling down into a 'cloud' or 'atmosphere' of sorts that permeates the dimension tunnels and then it will be possible to, with the right sensitive-enough equipment, map out a path of travel by way of particle density, or even by the presence of particles at all since beyond the dimension walls the scanners would not receive any bounce-back at all and it will then be a dark zone on the particle radar. I can imagine the presence of particles, and thus flyable space within the dimension, to be represented by smears upon a screen in the shape of the tunnels while anything beyond the dimension walls on the screen will simply be dark.

Now obviously you won't be able to see beyond a turn as in the image due to the dimension wall eating up the scanner's rays or pulses or whatever you use but it does allow you to visualize the effect of a scanner being so sensitive that it picks up dust and gas and not merely large objects. The scanner will also double as a great wreckage and potential impactor detector, allowing the crew to slow down and nudge things out of the way or go around them deal with them in whatever way that is appropriate.

You'll also need a rather sizable amount of collisions or fights to have occurred within this dimension for this to be viable, as without which you'll be hard-pressed to pick up much of anything. You could manually pump the dimension with gas or dust as well but such an operation will costs time and resources, possibly being like the road pavers of today.

• You can also just see the black-body radiation coming off the dust. It's being blue-shifted as you travel through the tunnel, which might or might not be useful, depending on how fast you're traveling. Active scanners would be more sensitive, providing a plot device requiring ships to slow way down if such scanners are broken. Mar 3, 2022 at 0:20
• Such ancient dust particles would have drifted into walls and get transported back to normal space long ago. They won't build up in the tunnels. Mar 3, 2022 at 13:03

So, thermodynamically the walls are extremely cold within hyperspace.

As ships use reaction drives, they'll spew out hot particles. Those particles will raise the average temperature of the tunnels.

So you just have to "thrust" both forwards and backwards, and ensure that the wave of hot particles in front of you goes out far enough. Then you measure how far the hot particles in front of you reach, and that tells you if you are approaching a wall.

The "forward thrust" is going to be a different kind of rocket, as you'll want it to be reasonably "wide" stream of particles (so you can see more), and (as you don't actually want to stop) you'll compensate with (narrower) thrust behind.

This is going to be a lot like feeling your way around in the ocean by using bottom-sensing radar. You'll probably prefer to have maps and buoys for faster travel. The buoys can be made out of radar-reflective materials and have lights and even emit radio waves. Something like RFID, where they broadcast (or reflect) their identity when hit with sensors could also be good.

Your maps would then have the buoys and known "walls" on them. You'd triangulate your position (maybe even in a GPS-like way) to figure out where you are, and only have to rely on "hot gas" radar in limited situations.

Military buoys would exist that would not be reflective, and would only respond to specific cryptographic signals with cryptographic (and possibly tight-beam) responses.

Moving buoys would be a way to engage in piracy; make someone not realize where a wall is, and have them drift into it. Then jump them in normal space before they can get out.

As well as the buoys, you'll want to have relay stations for communication purposes. These stations would communicate using tight-beam to the next station.

Post offices, stations with ships in them in hyperspace, would exist that would allow you to send messages back to normal space.

As sending a ship back and forth through hyperspace transition requires certain limitations on ship design, there are going to be hyperspace-only ships that are more efficient. Similarly, you'll have specialized ships that exist only to ferry supplies over the hyperspace barrier. Some post offices could develop into railheads, with specialized hyperspace cargo carriers moving goods to the destination along well navigated routes.

Hyperspace fleet carriers which again never leave hyperspace (except in extreme situations) that deploy spaceship attack craft would be one kind of military solution. There is no stealth in normal space, and there is no practical defence; in hyperspace, the walls are cover, so you can keep your capital assets "over the horizon" (behind a twist in a hypertunnel).

I will assume hyperspace is hyperbolic, and the tunnels are local 3 space 1 time but are embedded in a 4+ space dimensional brane with positive curvature. This means that instead of n^3 volume at n distance, you get more than that; space is a saddle. This also means that attempting to form a ring of warning stations gets crazy exoensive.

Imagine if a typical tunnel goes 1 km before forking into two. Then from any one point within 10 km there are 1000 tunnels; within 20 km 1 million, within 30 km 1 billion, within 40 km 1 trillion, and it keeps going. Building early warning along all paths becomes insanely expensive; and even exploring them all is impractical.

Simply scale the branching rate to whatever level you want to make the radius of practical defence be however far you like. The average curvature of the hyoer tunnels puts a limit on hyperspace travel speed (together with engine tech limits) and how far you can see. The branching frequency as a function of travel speed and time between systems lets you know if multiple systems can form a single defensive ring, which puts geographic pressure on government sizes.

• Re your last paragraph: The way I'm reading the question, normal space would detect hyperspace fleets, but hyperspace wouldn't see normal space fleets, since stuff in hyperspace pops out when it hits the boundary, but only traverses the other direction under specific circumstances. So not only could your planet sense the fleet indirectly because of weird signals leaking through, you could also just put radar beacons in hyperspace that send data directly to your planet, sensing hyperspace fleets long before they got to your system. Mar 3, 2022 at 0:26
• @michaels If the fleet had a wall between itself and the target, no; so your fleet carrier would hide behind a "corner" if tge tunnels. Then launch short-range attack space ships; realspace stations otger than the target one are lightspeed-range limited for warning. So you'll want beacons and light houses and warning towers along hyperspace approaches, to notice incoming hyoerfleets. But if hyperspace is hyperbolic, thick warning layers is exceedingly expensive...
– Yakk
Mar 3, 2022 at 0:41
• I think you're presuming there might be many, many tunnels that lead to essentially the same realspace position, where I was presuming there would generally only be one or two tunnels that might or might not lead anywhere near your destination. A worldbuilder could then go anywhere in between those scenarios to give realspace or hyperspace the advantage. Mar 3, 2022 at 0:45
• @michaels I would think the middle is most fun; backdoor tunnels are rare/expensive but exist. Also, the dark tunnels nobody knows what is down them are a danger; one nations would neglect at long term peril.
– Yakk
Mar 3, 2022 at 0:53

## Intertial navigation systems and a map.

Given your description of the dimension it would appear that by default humans have mapped its 'geography' because the crews obviously have a series established departure/destination points on record. (Otherwise their jumps would literally be random.) So you have a map of the dimension and probably one that is presumably being expanded as explorers try 'new departure points' and in so doing get a fix on local stellar cartography every time they emerge at a new point in 'normal' space. (Well the ones that survive do anyway.)

That being the case you have the first key element: **A map of the dimension ** showing currently know exit points and their corresponding positions in the real universe.

The second element; a very accurate (atomic clock) (at least one possibly multiple ones aboard each ship.

The third element; very sensitive and accurate sets of accelerometers and gyroscopes. These are 'based on laser on a chip designs (which already exist and are in use). The accelerometers measure liner acceleration along the XY&Z axis on the ship. Basically (with the aid of clock ) how fast and how long you have traveled. The gyroscopes measure orientation and angular velocity (how fast a body rotates around its own center of rotation). Again with the aid of the clock the gyros let you measure the orientation of a vessel along any part of the journey from start to finish.

With all three systems aboard your ship and a computer the vessel can basically track its own path through void by comparing its motion in any direction and velocity against the on-board map. No radar is required although one assumes it would be useful for takeoffs, landings and threat evasion.

For extra efficiency you can also add active homing beacons at known safe exit points that act like GPS satellites and let you get a fix of your location relative to other exit points on the map. Then all your ship needs are directional radio receivers to detect their broadcasts (and maybe a beacon saying 'here I am' to other ships).

• This was going to be my suggestion. It is based on one assumption; that the "hyperspace" dimension is linear and consistent - that is, every time it was created, the same mappings between points in the normal universe were kept and that if you created 2 of these dimensions starting from the same point, if you travelled the same distance in the same direction, you would pop out at the same place into the normal universe. The only reason you might require radar would be to help you avoid running into other travellers in the dimension. Mar 2, 2022 at 13:48
• @MikeWorsley: It doesn't have to be linear. As long as you can track a path with sufficient accuracy, you can repeat the same path. It just has to have reasonable amounts of continuity along known routes, so tiny deviations along the way (mostly) don't have a large changes in your destination. But you can certainly have fractured volumes in hyperspace, and non-Euclidean space/hyperspace mappings, such that there are boundaries where millimeter differences in route result in mega-lightyear differences in destination (or unplanned disassembly of the spaceship). Mar 3, 2022 at 0:32
• I do wonder at the accuracy of such systems though. How far can you realistically go through space before you're way off course? Accelerometer-based dead reckoning is notoriously bad in real life. Perhaps that's a plot point: better navigation systems can maintain accuracy for longer, and you have to pop out of hyperspace every so often to check out the local stars and reset your origin coordinates. Mar 3, 2022 at 0:49
• The latest versions of 'on-chip' internal navigation systems are being put into drones & guided weapons on a test basis as we speak. Some reports indicate that error rates of less than 5 meters an hour are achievable. So for example a passenger jet flying from Melbourne to London would be say 100 to 125 meters or so off course by the time it arrived. Which isn't bad.
– Mon
Mar 3, 2022 at 3:28

## When it's dark, just turn on the lights

The OP says the FTL dimension is completely dark, but he doesn't say anything about how the walls react to artificial light shining upon it. If they react in any way (e.g. reflect some of it), just turn on your ship's outboard lights and you can see the walls (or measure whatever reaction they give). Or if they don't react to light but to some other type of radiation (e.g. radar, but could be any type, electromagnetic or not), use that to illuminate them.

If the walls are completely black, in the sense of absorbing anything that touches them (both matter and energy) without any sort of reaction, then the other answers can be used. I'll add a few more for that case here:

## Release a bottle of compressed gas and illuminate it with a laser

A ship can release a cloud of gas and then use a laser beam to illuminate the expanding gas cloud to see where the gas particles disappear. If gas expanding in vacuum moves too slowly for the distances needed, you'll want to shoot particles out of some kind of particle accelerator and detect where they disappear with a laser in the same way.

## Magnetic fields

In free space, magnetic field lines behave in a certain way. If magnetic fields can penetrate the walls (reaching the normal space beyond those walls) the change in the shape of space should affect the magnetic field, and be measurable by the ship generating it. If magnetic field lines do not penetrate the walls, that is also measurable and the walls would effectively repel magnets.

## Beams of electrons and positrons

The ship emits two parallel (or close to parallel) beams, one of electrons and one of positrons. The electron and positrons attract each other, and when they make contact they annihilate and emit gamma rays. How far the electrons/positrons travel before contacting each other depends on their speed and the angle between the two beams, so that can be controlled by the emitting ship. By measuring the annihilation gamma rays you know that there is no wall up to that distance out. In this way you can 'feel' your way around, and it doesn't require gas that you can run out of, only energy.