# faster than light speed travel, but not too much faster

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.

• Outside of something fantastic exceeding the speed of light is not possible. Instead you can choose closure stars. There are 4 exoplanets within 11ly of earth that are potential inhabitable (en.wikipedia.org/wiki/List_of_nearest_exoplanets). – goryh Jan 10 at 19:17
• I'd agree with you that a better option is to not explain and focus on the story. – Wiggo the Wookie Jan 11 at 11:01

## 4 Answers

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?

• "Painted himself into a corner"... yes that's about right. And the the time passed on the planets is not important in this case. – ssimm Jan 11 at 18:21
• You can use the equations on the relativistic rocket page. The equations are only for continuous acceleration, but if the ship accelerates at 1G for the first half of the journey and decelerates at 1G for the second half (so everyone on board feels like they're in Earth gravity), then the onboard time will just be twice what the equations tell you it would be for half the distance. For example, if we want to go 100 light years, we first find what the equations say is the onboard time for 50 light years, then double it. – Hypnosifl Jan 11 at 20:36
• As pointed out on the page it's easier if you use units like years and light-years for time and distance so that c = 1 in these units, and in these units 1G acceleration is said to be a = 1.03 light-years/year^2. So, onboard time for 50 light years would be T = (1/1.03)*acosh((1.03*50) + 1) and if you plug that into the calculator here you get a time of about 4.52 years, so the onboard time to go 100 light years would be 9.04 years. If you switch it to 2G acceleration or 2.06 ly/y^2, time to go 50 ly is 2.59 years, so 100 ly is 5.18 years. – Hypnosifl Jan 11 at 20:42
• Orson Scott Card’s Ender books use this approach, though he breaks it with (spoiler) FTL communications. – Joe Bloggs Jan 12 at 9:19
• This is perfectly in keeping with the plot. At 1G, a trip takes about 1 year + the distance in light years. So going to Proxima would take about 5 years at 1 G. So at the start, limit acceleration to some smaller value, and then as the story proceeds, increase the acceleration. That seems perfectly natural to me. – Maury Markowitz Jan 16 at 21:18

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.

• That is a cool mind twisting idea... I bet someone could write a book just around that idea! – ssimm Jan 13 at 13:03

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).

• It's difficult to have an Alcubierre drive without it being easy to convert into a machine for backwards time travel though, see here. In addition this section of the Alcubierre drive article mentions a result by Krasnikov showing there isn't any way to get the bubble to move FTL without a bunch of devices placed along the path manipulating spacetime in the needed way. – Hypnosifl Jan 13 at 21:25

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.