# If a spaceship can travel 1 light year in 3 months, how fast is the ship going?

Greetings. I am but a humble fledgling writer that wants to do a sci fi story.

In my story, the science of space travel revolved around a dangerous technology of endless acceleration. It is possible to travel 1 light year in a straight line instantly, but the ship and its crew will not survive it.

Why?

I'm still working on it.

There are other means of FTL travel but it is not always available.

The point is this: Maintaining a constant speed is much more important in the story than reaching top speed.

Anyways, the 1 light year in 3 months is the 'safe' top speed for conventional space travel. What I want to know is what is that speed.

I am not proud of it but I suck balls at math and science.

Please lend some wisdom to this not so smart author.

### 16 Answers

- ignoramusLv 74 weeks agoFavourite answer
What units do you want it in ?

3 months, say 90 days = 90 x 24 = 2160 hours

1 ly = 6 x 10^12 miles = 6000 x 10^9 miles

(6000 x 10^9) / 2160 = 2.777 x 10^9 mph, or 2777 million mph. (without taking any relativistic effects into account.)

- TomLv 74 weeks ago
You have to consider "Ship time" and "outside time" for the people left behind on EARTH. On the ship, time slows down----And gets slower the closer one gets to Light speed (which cant be exceeded)------In ship traveling close to light speed, A 4 light year trip may seem like only a few weeks, but it would be 4 years later ON EARTH. At 99% light speed, for instance, one might make a 100 light year trip in what seems like a month, but over 100 years would have passed on EARTH.

There is a mathematical formula that reveals how long ship time to distance is according to the percentage of speed of light your ship is traveling---and how much time passes back on Earth-----I don't recall it right now. But BOTH Ship Time and Earth time must be considered in your story.

The only way to avoid the Earth time vs Ship time discrepancy is to have some "Physics Shortcut" like "Warp Drive", or something involving "Quantum Entanglement" to both exceed light speed and not have a time differential----I use a " Particle Wave attribute" position changing technique, inspired by HUT (Holographic Universe Theory) Physics in MY stories.--resulting in instant "Space jumps" that shorten most of the journey.

- JimLv 74 weeks ago
I assume you mean relative to another craft. In that case you use Einsteins Special Theory of Relativity gives t' = t / √(1 - (v²/c²))

- alan PLv 74 weeks ago
Don't get too hung up on the technical details. You need to keep your eye on the human story. However you explain faster than light travel it has to be only a part of the story. In the physical universe that we live in faster than light travel is not possible. If you could accelerate at 1g it would take the best part of a year to approach the speed of light but you could never reach it. You will reach a point where time on the ship is passing 4 x less quickly than on Earth due to the time dilation effect. At about 97% of the speed of light the travelers will, from their point of view, be traveling at 4 x the speed of light. But you need a fabulous amount of energy to do this. It might be easier to say that the spaceship has been diverted into a parallel universe with different laws of physics where you can find the situation that you want for your story.

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- ZirpLv 74 weeks ago
Well, it would have an AVERAGE speed of 4C

"It is possible to travel 1 light year in a straight line instantly, but the ship and its crew will not survive it. Why?"

Because the acceleration would crush them? We are used to only 10 m/s².....

- Enough TrollsLv 74 weeks ago
If you are writing within current theory no thing can be accelerated to light speed that has mass. At absolute best it can move one light year in a fraction more than a year.

- AmyLv 74 weeks ago
1 light-year in 3 months = 1 light-year in 1/4 year = 4 times the speed of light.

In more familiar units, that's over 4 billion kilometers per hour. But nobody uses numbers that big. Just call it 4 times the speed of light.

You might want to read up on Star Trek style "warp" FTL. Warping space instead of moving fast lets you avoid having to break relativity. As I recall there was an episode that set a 'safe' top speed based not on damage to the ship but damage to the surrounding space.

- billrussell42Lv 74 weeks ago
1 light year in 3 months, 1/4 year means 4 light years in one year.

that is 4 times the speed of light, 4c

or 4x3x10⁸ m/s or 12x10⁸ m/s

that's neglecting time compression due to travel.

It's possible to traverse 1 light year with the occupants experiencing 3 months of elapsed time, if they travel at a high percentage of c.

math is:

time dilation

t' = t/γ

γ = 1/√(1–(V²/c²))

3 = 12/γ

γ = 4 = 1/√(1–(V²/c²))

1–(V²/c²) = (1/4)² = 1/16

V²/c² = 15/16

V/c = √(15/16) = 0.97

so at 97% of c, travelers experience 3 months while external observers experience 1 year (actually slightly over 1 year), while they traveled 1 LY