A Krasnikov Tube is a proposed method of faster-than-light travel that is essentially an extremely limited one-way time machine. The tube is a distortion of spacetime that can be intentionally created (using hypothetical technology) in the wake of travel near the speed of light. The Krasnikov Tube allows for a return trip that takes you back to the time right after you left.
This several light-year long man-shaped "tube" might arguably constitute a megastructure, but unlike most megastructures it's not constructed of physical matter such as titanium or plastic, but instead is just a warpage of spacetime.
Krasnikov Tubes in action:
So, you leave earth in the year 2200 AD, and go to some planet 10 light years away, creating a Krasnikov Tube behind you as you go. Let's say that using your normal sublight space drive it takes just over 10 years to get to where you are going, so you arrive in 2211 AD. You conduct your business on the other planet, and then return by charting a course through your Krasnikov Tube. You arrive back on earth in 2200, just minutes, hours, or days after you left. For your own personal timeline, the encounter in 2211 has already happened, but for the rest of the world it hasn't. Various equations show that this doesn't violate relativity of simultaneity, so it's not "officially" time travel. Just the same, you know what 2211 will be like on that other planet, while living in 2200 back on earth.
The Krasnikov Tube can't reduce your initial trip's duration, just the duration of the return trip. You can arrive back shortly after you left, but not before it, and you can't "meet yourself" or prevent your initial visit. If the tube could be kept stable via some technology, you'd have a long-term reusable one-way wormhole.
Note that it is a time machine in the sense that a person born in 2210 could be loaded into a ship in 2211, and head towards earth. They'd arrive on earth in 2200, nearly 10 years before they were born (according to the records on the world of the birth, anyway). As odd as that sounds, it's no more bizarre than the Twin Paradox, a principle of general relativity that has been experimentally proven.
Note also that the two ends of the tube are each anchored in a specific frame of reference, which is part of why you can't travel back to before the departure of the ship that built the tube. One end of the tube opens onto 2200 at earth, the other at 2210 on the other planet. It would appear that this is a scientifically valid system that would incorporate the otherwise dubious concept of San Dimas Time. If not, then the tube could be used similar to a Roman Ring.
(On the wikipedia page, it's argued that any two parallel-but-opposite Krasnikov tubes are effectively a roman ring or break down causality, but that doesn't seem to jive with Krasnikov's original concept. Could be the source that argues for causality breakage doesn't understand San Dimas Time.)
Unlike some other models of traversible wormhole, to use a Krasnikov Tube you need a very fast space ship. You can't just drift into or across the tube and get the effect, you have to race along its length.
Lastly, it's worth mentioning that my numbers / dates were pretty much chosen randomly and arbitrarily, and probably don't match up to the real math. My stats aren't terribly accurate, but they convey the concept easily and may even be close enough for gaming purposes. Both of the sources listed below have serious numbers that give proper respect to relativistic math. Please, trust them before you trust the random examples of this Arcanist.
Solving the Fermi Paradox
The Fermi Paradox basically says "the math tells us there should be other intelligent life in the universe, yet despite lots of searching we have yet to find it!" How can that be?
Well, if the aliens have Krasnikov Tube technology, it would make perfect sense. They could connect the worlds in their sector of space with such tubes, and use them for transit and communication between themselves.
Why haven't the aliens visited us yet? They may not have noticed us, because our radio waves haven't reached them yet. When they do, they may come for a visit, but the initial journey will have to be at sublight speeds and it may take hundreds of years to reach us.
Why haven't we noticed their radio waves yet? Because they no longer use such broadcasts for communication - instead, they transmit information through the Krasnikov Tubes via directional beams. This would be a functional version of FTL communication that can't be eavesdropped from other directions. Or, they may use standard broadcast technologies like our own - but the wave forms become terribly weak over distances, as recent SETI research suggests, and is lost amidst the background radiation of the cosmos due to the Inverse-Square Law.
Game and Story Use
- Many of the galactic shortcut and galactic exploration ideas presented on the wormhole page would be equally applicable to the Krasnikov Tube. A galactic civilization could be connected by hundreds of such shortcuts.
- Thanks to the Twin Paradox, if the PCs were the crew of a ship that built these things, they'd only experience very minimal aging. Again with my admittedly flawed math that gets across the concept while being inaccurate in the specifics: I leave 2200, and head across 10 light years at speed a little less than light in what seems like a couple days to me, but appears like years to observers using telescopes from earth. Then I return via the Tunnel in what seems like minutes to me, to arrive while those observers are still setting up their telescope. Because of the crazy physics of the tube, they might not even be able to observe any return flight.
- Since the tubes can explain the Fermi Paradox, there could already be thousands of them criss-crossing the universe. Twenty Minutes Into The Future, the tube builders establish First Contact with the earth.
- If the SETI research is right (and radiowaves get lost past a range of a light year or two, because of the Inverse-Square Law), there's two possible takes on how radiowaves interact with the Krasnikov Tube:
- The waves transit the tube as if it were a shorter distance, and broadcast through it with minimal break-up.
- The waves are lost in the tube or in the background noise caused by moving faster than light. In this case, the tube builders have to rely upon some sort of trans-tubal postal service.
- It should be noted that physics may have a definitive answer to that conundrum that I just haven't encountered. But if it ain't readily discoverable via an internet search, the GM is probably free to define it as they see fit for the campaign. Have fun on the intergalactic Pony Express.