But the system can work. First, crucially, the cycler must broadcast its status continuously; it will never be traveling as fast as this broadcast, and for any cycler going less than 50% lightspeed, the announcement of what its current capacity is will reach the next world on its ring many months in advance of the cycler itself. So world A will know beforehand what the carrying capacity of a given incoming cycler is.

How, though, does world A coordinate with world B? This is where it gets interesting. Negotiation isn't possible because of the time-lag in communications. World A must be able to send a cargo with prior knowledge that world B will receive it. For this to work, world A must have some idea of world B's cargo capacity (i.e. how many incoming cargos it can decelerate at any one time), and, more crucially, it must be mandatory for world B to decelerate any incoming cargo (if possible). Regardless of expense.

Other worlds of the Compact can't know what the current economic state of world B will be when a given cargo arrives there; they can only know what it was a few years ago. In other words you have to trust that world B will be able to receive you when you leave world A--despite economic and technical difficulties, political changes, coups, piracy etc. The beam systems that decelerate incoming cargos will likely be expensive and power-hungry. A budget-conscious government might be tempted to refuse to receive some incoming cargos, especially if those cargos contained, oh, let's say, supporters of the previous regime...

So the rules of transport in the Cycler Compact need to be enforced. Yet there is no way to police distant planets. The Compact has no navy; you need the cooperation of a world to visit it, unless you want to spend years decelerating using magsail braking alone. And a navy that crawls into your planetary system over the space of three or four years is an easy target.

Yet the Compact has one highly effective threat to hang over any world that flouts the rules: isolation. If world B starts refusing to receive cargos, or ships up cargos that max out passing cyclers' life support or supply allotments, the Compact can ostracize world B. In fact, it'll happen naturally: a recent article in Nature showed that human groups tend to punish freeloaders--in fact, altruistic cooperation seems to be based on such a mechanism (check out the New Scientist article on the same subject). If world A sends a delegation of diplomats to world B and they don't arrive, world A is unlikely to send another. Once word gets around (which admittedly will take a number of years) nobody will send cargos to world B anymore. Freeloading might have some short term advantages, but it's disastrous in the long term.

So your world maintains its membership in the Compact by continuously broadcasting the number of cyclers you expect to receive, plus your capacity for decelerating cargos; and when a cargo comes in, you decelerate it, even if doing so stretches your resources a bit. If you do this, you earn the right to be able to ship cargos out yourself. Thus interstellar cyclers bring about a far-flung, cooperative civilization, without the need for navies or complex constitutions.

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