SciFiFilter FTL-ish Plausibility Check
June 21, 2014 6:53 PM   Subscribe

In a discussion elsewhere on the internets a twist on a common science fictional transportation technology was proposed. A spaceship leaps from one point in space to another, but while it is instantaneous for the passengers, the transit actually takes some small amount of time longer than light would take to cover the distance (let's say the Planck time). Would the time delay prevent the violation of causality? It appears that everyone is staying in their light cones, what am I missing?

As an example, a spaceship bound for Alpha Centauri would engage the drive and disappear from our solar system. Without any intervening travel, it would reappear near that group of stars just over 4.37 years later.
I studied physics many many years ago and what little I mastered of Lorentz factors has long since dissipated.
Thanks for indulging my nerdly inquiry. If this were ever used in a story I wrote, MeFi would receive the acknowledgement it merits.
posted by Octaviuz to Science & Nature (13 answers total) 1 user marked this as a favorite
not a physicist, but that sounds reasonable to me. how do you think it might violate causality?
posted by russm at 6:58 PM on June 21, 2014

Presumably during the leaping it travels at the speed of light (no experience of time), but in a hidden path slightly longer than the distance (so it takes a little longer than light traveling normally.)
posted by michaelh at 7:07 PM on June 21, 2014

Yeah, that's exactly what would happen if you could somehow impart a staggeringly huge acceleration and deceleration to your spaceship without pulverizing the occupants. As the speed approaches c, the proper time observed along the spaceship's world line approaches zero.

So if causality isn't violated with that kind of conventional travel, then adding a magical "does-not-actually-occupy-space-while-in-transit" device shouldn't make any difference.
posted by teraflop at 7:13 PM on June 21, 2014 [4 favorites]

Ken MacLeod's "Engines of Light" Trilogy contained an FTL engine which worked like this, although the transit time was precisely lightspeed. The characters weren't really aware of that when they hit the go button (it was alien tech) and so after learning their new distance from home, they quickly understood that there was nothing familiar to go back to. (This wasn't the focus of the story, it was writer handwaving to eliminate any thoughts in that direction by the reader and keep on the actual focus.)
posted by Sunburnt at 10:03 PM on June 21, 2014

In some SF discussions this kind of device is called NAFAL — nearly as fast as light — as opposed to FTL travel.
posted by hattifattener at 1:47 AM on June 22, 2014

Humbug, I had tried to look around for prior examples and didn't manage to find any, thanks MeFi :-)
posted by Octaviuz at 9:08 AM on June 22, 2014

If you don't want to squish your passengers, say by keeping your acceleration to around 1G, then it will take some time to get close to c...the closer you get, the longer it takes (to the outside observer, anyway)...~.75c in a year, ~.99999c in like 10 years. From inside the ship things go a bit faster ~.75c in like 6mos-1yr, ~.99999c a few weeks later, though IIRC the amount of fuel you burn has to keep increasing to stay at 1G.
posted by sexyrobot at 12:26 PM on June 22, 2014

(Oh, and if you're looking for specific writers, Arthur C. Clarke tends to keep things in the 'don't violate c' camp...)
posted by sexyrobot at 12:29 PM on June 22, 2014

Humbug, I had tried to look around for prior examples and didn't manage to find any, thanks MeFi :-)

Ursula Le Guin uses this concept (NAFAL) extensively in her Hanish universe.
posted by His thoughts were red thoughts at 5:13 PM on June 22, 2014

Some more search terms for you if you're still interested, NAFAL also goes under the names "time dilatation" and "relativistic (speed)".
posted by anaelith at 6:41 PM on June 22, 2014

Another data point - this concept is a major plot point in Orson Scott Card's Ender's Game - he uses 'relativistic speed', but also trades on Le Guin's work a bit.
posted by His thoughts were red thoughts at 6:49 PM on June 22, 2014

I feel like there is, at least, a genre distinction between NAFAL settings (a magic-ish drive like the *Engines of Light* setting, where transit is quick but you arrive inside your leaving lightcone) and ultrarelativistic settings (usually an "accelerate conventionally until close to c" style drive, with magic-ish ramjets or reaction rockets to make that possible).
posted by hattifattener at 11:10 PM on June 22, 2014

Not to threadsit but yes, the two are different MacGuffins. The reaction engine with unlimited power is at least a conceivable endpoint of current technology ("all" you need is a Saturn V that maintains its first stage thrust for a few months without having >99.999% of the ship be reaction mass and there you have it), the idea I was asking about (like Engines of Light) contains a bit more unobtanium than that.
posted by Octaviuz at 9:17 AM on June 23, 2014

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