The "moving walkways" pass close to local solar systems fairly far from their suns. They are very small, and thus hard to detect. Each "walkway" consists of two columns: dust and debris moving at a significant portion of the speed of light, about 400m in diameter passing by our solar system on a useful trajectory above and parallel the ecliptic.

They are like outbound and inbound subway tunnels. Though, to step in these tunnels would be deadly. Simply going near them is very dangerous. But, with time, we learn how to harvest energy by dipping a "catcher's mitt" into the stream to catch the impact of a few of the particles. This causes eddies and collisions which threaten to knock relativistic particles out of the tunnel so it must be done very carefully.

We don't have the technology to tell where these long, curves begin or end. Is there a source for the fast moving dust? A destination?

The first task is simply to exploit it to move the unmanned probe out of the solar system to explore the nearest star. The extra speed is hard to pass up. If that works the probe will keep going to see where the dust stream leads.

But, I'm not certain if relativistic dust could even be exploited for speed in this way. So, can we use those "walkways" to send a probe to where they lead?

  • $\begingroup$ Sounds similar ish (though i dont really understand you precise concept) to the Bussard ramjet; en.wikipedia.org/wiki/Bussard_ramjetBussard%20ramjet. Or perhaps a Fusion pallet highway. The idea being you ram into fusion pallets which power a drive. Both of these concepts dont actually work though. $\endgroup$
    – ErikHall
    Commented Jul 3, 2023 at 23:05
  • 3
    $\begingroup$ Please remember to articulate an actual question. Yours is by no means the worst offender I've seen, but the only question marks in the post are related to the backstory. Never assume people can see what seems obvious to you. Further, while I'm going to take a crack at an answer, it would be helpful to know what tech level we're working with. What can your civ do and not do? $\endgroup$
    – JBH
    Commented Jul 3, 2023 at 23:46
  • $\begingroup$ Hmm such walkways have to intersect somewhere, in case they are gravity-bound to curve around the galaxy. And they should carry no charge, otherwise ionized interstellar gas would shine them out (and even then, there should be areas that have increased ionic activity near those "walkways". Also the walkways have to be confined with external force, otherwise the dust stream would naturally expand due to internal and external collisions with matter, and colliding those two oppositely directed steams of matter would cause quite some energy-based effects. $\endgroup$
    – Vesper
    Commented Jul 5, 2023 at 9:11
  • $\begingroup$ So, it'll be better if the walkways are one stream instead of two, otherwise your hypothetic civ that made them would just achieve a galaxy-sized collider of sorts, with those streams quickly inflating the area of collision with incoming matter. Since it didn't yet happen, there is stuff to control the stream(s) which you don't describe, as physically they should not be stable uncontrolled. $\endgroup$
    – Vesper
    Commented Jul 5, 2023 at 9:14

2 Answers 2


Assumption #1: The civilization does not have the ability to accelerate to the same velocity as the stream with any practicality. They may have the mechanical tech to do it, but something limits them. Not enough fuel. Not enough time. Something. They can't do what the stream does.

Assumption #2: They do have better material science than Earth of 2023 such that what I'm about to suggest is believable, even if not plausible or possible.

There are conditions for any object of any positive mass regardless its initial velocity such that its impact with another object is entirely kinetic. — Professor Emeritus Hubére Dinglehopper to the Intraplanetary Association for Interstellar Studies (IAIS, expatriated); April 1, 2147.

Phase I

Your probe has electric ion thrusters. That's important, because the source of energy is a paddle with a magnetic loop at the end. The loop's purpose is to create the conduction of electrons via the high-velocity particles running through the stream. I can't speak to whether or not relativistic speeds create magnetic fields (they should...) in which case you could claim electrical conduction just like an alternator. But I suspect I can claim velocity interference with the magnetic field creating static electricity. In other words, your thrusters are now on steroids, accelerating the probe quickly without threatening the structural integrity of the probe. You're not taking advantage of the kinetic energy at all, just the disruption of magnetism to induce electricity.

Phase II

In the beginning (and for a while after that) the relationship between the probe and the stream can be modeled as V(probe)=0 and V(stream)=c/Y. However, somewhere in the middle the relationship changes as the probe's velocity begins to catch up with the stream. At this point a second paddle is deployed that's a mesh of cool metals (that I'm not totally making up) like titaniopalladiarosenburg23. Yup, unobtainium. What we're looking for is something more along the lines of a particle accelerator. The high-V particles slam into the mesh, releasing all kinds of energy, which is sucked into the system for use with... Bussard-style ramjets. In fact, the entire ramject could be set into the stream at this point — so long as it's not so bulky that the physical impact of still-honking-fast particles don't tear it to pieces. If you want to get really funky, use the paddle in the design of a waveguide (since stuff close to light speed is kinda like a wave and a particle. Actually everything is like a wave and a particle, but the velocity-vs-mass is usually heavily on the particle side). Anyway, you could use that to route the high-V particles into the ramjet. Now you're taking advantage of some of the kinetic energy of the particles.

Phase III

This is the fully kinetic part. Once your probe is materially up to the same velocity of the stream, it simply extends a series of paddles about its circumference. The pitch, yaw, and rotation of the paddles can be controlled to simulate thrusters. Call (or make) them sails, for lack of a better word. Except, rather than thrusting, they're receiving impacts. Should the probe slow, impacts move it back up to velocity. These paddles could be made of something like titanium because, unless your stream particles are irradiated (that would be bad), at this phase they're impact isn't causing tremendous damage.

BTW, I really like this idea. It's akin to humanity discovering the great oceanic currents that first let them travel the globe. I can imagine the distortions of gravity causing the path: the individual gravities of planets having almost no affect but the aggregate gravities of solar systems and solar clusters, etc. creating the 3D curves for the flow. I'll leave the source up to you... but I could imagine bookin' around the central black hole.... I'm just sayin'

  • $\begingroup$ I'm a bit surprised nobody else took a crack at answering this. I thought it was an interesting question. $\endgroup$
    – JBH
    Commented Jul 5, 2023 at 3:59
  • $\begingroup$ I might've. Only seeing this Q now though. Once you're up to velocity, you should take advantage of the high-energy reference frame of the interstellar medium and move out of the stream to further accelerate. In an ideal case, the power needed to run a rocket is 0.5×Thrust×Exhaust Velocity, while the power generated by a windmill is Drag×Windspeed. The missing factor of 0.5 is key for net acceleration upstream (the concept is called Q-Drive, "Q" here meaning "dynamic pressure"). $\endgroup$
    – BMF
    Commented Jul 5, 2023 at 7:33
  • $\begingroup$ The construction is like that Veritasium video about accelerating upwind. As long as you have sufficient reaction mass and efficient enough conversion of plasma windmill power to electric thrusters, you can hypothetically accelerate upwind. $\endgroup$
    – BMF
    Commented Jul 5, 2023 at 7:34
  • 1
    $\begingroup$ Hmm, if the particles that move with relativistic speeds carry no charge, there will be no magnetic field at Stage 1, and your whole system would fail to take off. I am considering whether a stupid mass absorber could allow flight with the stream, provided that there is low to zero difference in speed over the stream's individual parrticles, and whether their density is low enough to not annihilate anything that enters. Yet, if an object would enter one of these, I expect that the debris would influence the other stream, creating an area of dense matter that would absorb both streams - BOOM! $\endgroup$
    – Vesper
    Commented Jul 5, 2023 at 9:18
  • $\begingroup$ If your probe is actually encompassing the entire stream at Stage 1, then yes, inducing a current around the stream would generate momentum on the probe, Yet, any miscalibration on the loop, and one of its parts hits the stream - FOOM there goes your probe. And there is the other one flowing dangerously close, the first stream spreads even in the slightest to the other (a single particle could do if they are dense enough - double FOOM and we have a positive feedback loop with colliding matter somewhere along the walkway. More matter comes in, more energy is released, more collisions happen. $\endgroup$
    – Vesper
    Commented Jul 5, 2023 at 9:27

Frame challenge: streams' interaction with interstellar matter

You claim that there are streams of particles relatively close to each other, moving opposite directions, each having a speed of no less than 0.01*c with a fixed volume of stream. Skipping the focusing part and acceleration dispersion part (let's say they both consist of atomic hydrogen with speed vectors of exactly zero difference, sent from somewhere to we want to know where - and using neutrons fails due to them being unstable), the streams still face the interstellar matter. Solar wind, comet traces, mere "floating" protons that come into the volume these streams occupy. No matter the density, eventually there will be a collision of one of the streams' particles with a "stationary" object, be it a proton or bigger thing, this doesn't matter. This process would happen with calculatable probability with each particle that is a stream's part over distance travelled, and these interactions result in either stream getting less focused over distance travelled, so their width or diameter would increase. Worse, even if the interstellar space would be devoid of particles, it's not devoid of photons, and these carry impulse as well, AND those impulse vectors are pretty random, as stars are everywhere and the universe background radiation is also omnipresent and will interact with either stream.

So, since the initial trajectories are declared antiparallel, and the widths of both streams continually increase over distance, there will be place where those streams would intersect - provided the streams themselves are long enough. Yet, a speed difference of a mere 1 m/s in a stream, omnidirected, at a base speed of 0.5c (the lower the worse) would cause the stream to grow in diameter over the solar system's diameter by 12*3600*c*1/(0.5*c) = 86400 meters, way more than your 400 meters asserted diameter, and the energy required for a H1 atom to gain 1 m/s sideways velocity is a mere 0.8e-27 J (1.6e-27*1*1/2), or one quantum of frequency about 1.2 MHz. The visible light carries orders more energy per quantum, thus it's only a matter of time spent with the stream until its part would reach the opposite one.

Now, the opposite streams collide, and what would happen? This largely depends on the amount of matter transferred by either, down to almost nothing should the streams' density at collision area lower to at least the density of a nebula (10^6/m^3). Of course, some collisions at double speed would still happen, providing both "stationary" matter and extra photons of high energy in the local area to influence the streams, with them both gaining width the process would cause local density increase together with local temperature increase, but unconfined, it won't cause much harm unless the streams' density would cross some threshold that depends on their contents, speed and initial trajectory distance. The best case would be two largely non-interfering streams of gas flowing opposite directions, with occasional bursts of energy when they would get crossed by something solid, like a comet kernel in an Oort cloud.

The worst case is that they would create an area of superheated gas that would become increasingly opaque to both streams, yet not dispersing fast enough so that the amount of matter in the cross-section of both streams would increase over time. This in turn would make the emerging gas cloud to enlarge with speed about equal to initial of the streams' particles, yet a great part of their kinetic energy would be transformed into local gas's internal energy and outgoing photons, creating a visible effect of an emerging star without actual star being formed. Still anything entering the "walkway" from either side would end up annihilated in that gas cloud, defeating the purpose of them being a media assistant route for interstellar travel. So there would be no reason of those that created the accelerators to ever start creating the streams, since all they would achieve would be an acceleration of star formation somewhere in the distance.

  • $\begingroup$ If the streams are charged you might be able to handwave some induced Z-pinch effect to counter the scattering forces of the medium, keep them narrow. $\endgroup$
    – BMF
    Commented Jul 5, 2023 at 14:07
  • $\begingroup$ Charged stream particles would start reacting on the stars' magnetic fields and their local anomalies, I really doubt this effect would be intended. $\endgroup$
    – Vesper
    Commented Jul 5, 2023 at 14:10

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