# Why wasn't speed of particles in engine exhaust limited in the "Slow zone"?

In the Expanse universe, both books and series, ships came to the space where their velocity is forcefully limited. This is allegedly because of alien station justly considered fast-moving objects as dangerous. But Epstein drives are so good exactly because of extremely high speed of cast reactive mass. That is, their exhaust is extremely dangerous too.

Why wasn't speed of particles in engine exhaust limited in the "Slow zone"? Or it was, but I missed it? I think not, since limited exhaust couldn't push huge mass of ship.

UPDATE: Thanks to Null for exact pointing to original text. Now it is obvious that it's a plot leak. Epstein drive had exhaust speed near 11 000 000 m/s, and the whole idea is simple, to accelerate particles with set of magnetic fields. If you cast a kilo of reactive mass with 11 000 kilometers per second, your 1000-ton ship get 11 meters per second. And if you could accelerate particles only for 600 m/s, ummmm... I have bad news for your 1000 ton ship.

• I ... uh, I guess it depends on how exactly the velocity is being limited, but I don't think it would necessarily change the propulsion efficiency. Certainly if the exhaust was slowed down after leaving the ship, it wouldn't make any difference. Nov 5, 2019 at 9:00
• @Harry Johnston, Think about small system, like handgun. There is problem, if expanded gases have normal speed, and bullet speed will be limited in the barrel, we will get out-break of these gases through all constructional gaps. Now imagine state-of-the-art propulsion. Nov 5, 2019 at 9:14
• If this mystery "slow zone" field were capable of stopping every particle within a ship simultaneously, then the people and objects within a given ship would not have been thrown about so violently when the ship itself was first affected. Everything would have stopped moving simultaneously. We don't know enough about the field to explain why it works the way it does, but either it can't target things that small, or the ship itself somehow blocks or shields the field from affecting things within (so these particles aren't affected until they've left the ship - imparting their momentum en route) Nov 5, 2019 at 14:27
• @Steve-O, good observation about people who were smashed when braking. nevertheless, hull or structure of any ship was not deformed. so unlikely that braking was collision-like, ship was stopped as a whole. Nov 5, 2019 at 14:48
• It's not a plot leak/hole. While the "magic" of the Ring Builders' inertial dampening field is not explained (and you wouldn't expect it to be) the story is internally consistent and therefore does not have a plot hole. The particles can be accelerated >600m/s when they are in the interior of the ship. They are decelerated as soon as they exit the ship but their momentum is absorbed by the slow zone not by the ship.
– Null
Nov 6, 2019 at 14:45

The slow zone's properties are described fairly well in the book:

But the most intriguing factor of the slow zone, and the one that gives it its name, is the absolute speed limit of six hundred meters per second. Any object above the quantum level traveling faster than that is locked down by what seems to be an inertial dampening field, and then dragged off to join the garbage circling the central station. At a guess, this is some sort of defensive system that protects the Ring Station and the gates themselves. Light and radar still work normally, but radiation made up of larger particles like alpha and beta radiation does not exist inside the slow zone. At least outside the ship, that is. Whatever controls the speeds here only seems concerned by the exterior of the objects, not the interior. We’ve done radiation and object speed tests inside the ship, and so far everything works as normal. But the last probe we fired was immediately grabbed by the field and is now making its way down to the garbage ring. The lack of alpha and beta radiation leads me to believe that there’s a thin cloud of loose electrons and helium nuclei orbiting that station as part of the garbage ring.

Even electrons are big enough to be affected by the slow zone so presumably the exhaust from the Epstein drive would be affected as well. However, the description notes that only the exterior of an object is affected. I think the likely explanation is that the particles ejected from the ship as exhaust are initially propelled while "inside" the ship and are therefore not affected by the slow zone until those particles fly "outside" the ship (at which point they are presumably suddenly slowed down to the speed limit). This initial high speed propulsion within what the alien station considers the "interior" of the ship is presumably enough to propel the entire ship.

• It's been a while, but I think the internal velocity of exhaust doesn't matter much; only the velocity at the point at which it leaves the ship. Nov 5, 2019 at 15:14
• @DavidW There's just enough ambiguity in how the slow zone works (what is considered "interior" vs. "exterior" by the alien station) that I think we just have to assume the velocity of the exhaust particles is high for just long enough to propel the ship before the particles are slowed down.
– Null
Nov 5, 2019 at 15:18
• Well, if the exhaust is actually slowed down over some non-zero distance, as opposed to being instantaneously braked to the speed limit, then you would get at least a significant fraction of nominal thrust. I was somehow interpreting the effect as an instantaneous one. Nov 5, 2019 at 15:37
• @DavidW Whether the exhaust is slowed down over some non-zero distance or instantaneously slowed to the speed limit, the ship will be propelled so long as that happens when the exhaust particles are sufficiently outside the ship. Exactly what happens to all the momentum of the exhaust particles when that happens is a mystery of the Ring Builders, but that momentum is somehow absorbed by the inertial dampening field in the space around the ship instead of the ship itself. Therefore the ship is propelled.
– Null
Nov 5, 2019 at 18:17
• @HarryJohnston The only issue if they are slowed down inside the ship is that one would wonder if the momentum was transferred back to the ship itself, in which case it wouldn't be propelled.
– Null
Nov 5, 2019 at 18:48

1) Propulsion is based on the conservation of momentum, which in turn is a consequence of Newton's 3d law. That means that the force of propulsion is the force that drives the particles away from the ship. If particles are accelerated, then ejected from the ship's exhaust, and then the force that makes them decelerate to the zone's speed limit is not exerted from the ship, there is no reason for it to decelerate.

Thus, the engine could accelerate particles to very high speeds, which would then be decelerated almost immediately by the zone. What matters, is where (and by what) the forces are applied. Even if the particles were decelerated inside the ship, as long as the ship itself is not exerting the decelerating force, propulsion would not be affected.

2) In fact, even if the particles were not accelerated beyond 600m/s, as long as the ships drive exerted the same force on them, the ship would still be propulsed. In that scenario, when the particles reach 600m/s, their mass becomes effectively infinite; the force of the drive cannot accelerate them any more, or it imparts on them negligible, undetectable acceleration.

3) The whole "slowing field" idea is of course a paradox, there is no known physical mechanism that could produce such an effect. Also, momentum conservation would be violated unless somehow momentum was transferred to the station that created the strange field which caused the speed limit effect. The station should experience acceleration according to momentum conservation, if of course the station was actually the source of this weird force. Now, if the station was tethered to a planet, a planet has sufficiently large mass to "absorb" this momentum with minimal, perhaps not even detectable acceleration. Still, I can't imagine any way to produce such an effect.