faster than light speed travel, but not too much faster

Question

A friend of mine is writing a hard sci-fi novel about interstellar colonisation. He wants to use the real Earth and the surrounding stellar neighborhood. The plot requires that the initial voyages take 50-60 years to get to a habitable planet (let's say up to 100ly from Earth) but that, over time, and in little increments, the travel time is reduced to around five years.

He does not want to resort to things like hyperspace, warp speed, or "jumps" (like in the Foundation series), he feels that these methods are too fantastic and the travel time is too short for his plot. He is also worried about trying to explain how relativistic effects were overcome.

My advice to him is to not explain the technology behind his story and rely on the readers' suspension of disbelief. But does anyone have some technical explanation that would fit these details? Thanks, much obliged.

Update: in the plot the amont of time passed on the planets is not important. It is the amont of time on the ships travelling that has to fit the above criteria.

Answer 1

Go sublight, and let time dilation take care of the subjective travel time.

Your friend has painted himself into a corner with conflicting requirements. But since he's "worried about trying to explain how relativistic effects were overcome", one course of action is not to try.

His spaceships can accelerate to a significant fraction of $c$ for half the trip and decelerate the other half. If it's a high-enough fraction, time dilation will reduce the subjective time on the ship to the time frame he's interested in.

As the story progresses, improvements in drive technology will allow ships to reach a higher fraction of $c$ and cut down the subjective travel time.

Of course, a lot more time will pass on the places the ships are travelling between than on the ships themselves, but is that really important?

Answer 2

Here's an idea:

Wormhole random walk

Outside of the immediate vicinity of the solar system, natural wormholes open and close rapidly, and ships can enter into them if their hulls are appropriately polarised to pass through the event horizon, allowing FTL travel with almost no energy cost. However, these natural wormholes take ships only a small distance in a seemingly random direction. This means that to arrive at a location, a ship needs to take a massive number of jumps, and indeed a ship might not arrive at all.

Later technological improvements allow the possibility of measuring the approximate direction of a wormhole jump before it is taken, and mapping out directions where wormholes are more probable (and thus quicker), improving journey times. But FTL still takes ages.

Answer 3

He does not want to resort to things like hyperspace, warp speed, or "jumps" (like in the Foundation series), he feels that these methods are too fantastic (...)

You need to look up the definition of "fantastic". There is a warp drive that is the stuff of scientific papers, called Alcubierre Warp Drive.

The Alcubierre drive, Alcubierre warp drive, or Alcubierre metric (referring to metric tensor) is a speculative idea based on a solution of Einstein's field equations in general relativity as proposed by Mexican theoretical physicist Miguel Alcubierre, by which a spacecraft could achieve apparent faster-than-light travel if a configurable energy-density field lower than that of vacuum (...)

In 2012, a NASA laboratory announced that they had constructed an interferometer that they claim will detect the spatial distortions produced by the expanding and contracting spacetime of the Alcubierre metric.

You can call this warp drive hypothetical, speculative, and even probably invalid once a theory of quantum gravity is further developed, but fantastic? Nope. Fantastic is that which belongs to the realm of fantasy. And scientists don't write fantasy in their papers.

Anyway, with an Alcubierre drive, you can say that each technological development allows for finer control of the Alcubierre bubble, allowing for faster travel with less destruction (this thing theoretically obliterates everything on its destination when going back to subliminal speeds).

Answer 4

Wormholes, but unlike another question here, the short ones are cheap and the long ones are expensive.

Adjust the numbers in the following as required.

So the energy required to open a wormhole grows extremely rapidly as the length of the hole grows. To go, say, 0.1 light year costs \$1 per cubic meter transported. But to go 1 light year costs \$100,000 per cubic meter. And it requires about 10 hours to open each wormhole. So to go 1 light year you need to make 10 short hops, each one requiring 10 hours.

So you can get a rate of something like 100 hours per light year. And assuming each person can fit in 10 cubic meters, including luggage, air, food, facilities, and service equipment, it means \$100 per light year at that 100 hours per light year pace. But to go 10 times as fast would cost 100,000 times as much, or about \$10 million per light year. Maybe there are emergencies when a government would do it, or maybe there are massively rich guys who would do it. But ordinary people would not be able to.