Call it teleportation, or a portal device.
It's a classic tale where men on earth create a device to teleport one thing from one end of the room to the other. Only it didn't work. So, instead the corporations use the portal to make things disappear as the ultimate get-rid-of-it garbage hole. Over the next two years several people committed suicide using the device, including a murderer.
Thing is, surprise, two years later the other machine which was the receiver starts working, and all the stuff starts to come back. Teleportation, instantly as far as the person using it is concerned, but two years later. Knowing when the people are coming back the police stand by to receive them and the trial for the murderer is all set to go as soon as he falls through.
There are various uses and social upheavals from this. Sure, you have to wait two years, but after that you have a perfect pipeline for oil that can't be blown up by terrorists or interfered with by mother nature. The same for water and gas. Long-term planning for such a thing changes the world too as people have to re-think how to do their logistics. People who are sick or moments away from death can walk through portals to a future two years later where the doctors have planned two years in advance to treat them, or however long it takes to get a cure.
Best of all, a starship can be made with unlimited fuel supplied two years in advance. Even food and water and parts. And people can come back whenever they want on a schedule.
There's a few plot points. One is where people are seeing social changes over several decades. Another where the ship is encountering difficulties avoiding objects, and there's relativity involved, and won't have the fuel two years down the line to avoid it, because there's a four year delay.
I think there was a battle on the starship too, with a drug-overdosing captain, but I'm not sure.
What is this book?