View Full Version : % of light speed

jallenecs

04-18-2012, 06:35 AM

I'm writing about a spaceship traveling at sublight speeds. I know that covering one light-second of distance will take four hours, seventeen minutes.

How would I name that speed in terms of mph, kph, or a percentage of light speed. I thought I knew what calculations to do to figure it out for myself, but I confused myself, and now I don't trust my judgement on the matter.

I'm sure there's a webpage out there to tell me the answer. Anybody care to help out a math-challenged old broad?

Drachen Jager

04-18-2012, 08:17 AM

19441 metres per second.

Unless my math is way off.

299792458÷((4×60+17)×60) was my calculation.

Or 12 miles per second.

186282÷((4×60+17)×60)

benbradley

04-18-2012, 09:20 AM

I'm writing about a spaceship traveling at sublight speeds. I know that covering one light-second of distance will take four hours, seventeen minutes.

How would I name that speed in terms of mph, kph, or a percentage of light speed. I thought I knew what calculations to do to figure it out for myself, but I confused myself, and now I don't trust my judgement on the matter.

I'm sure there's a webpage out there to tell me the answer. Anybody care to help out a math-challenged old broad?

Yes, there's an online calculator that can do this, it's called Google. Let me see ... I always do this a little at a time to see if I'm getting what I want. First, for the percentage of lightspeed, divide the one second by the longer time, just as Drachen Jager did:

http://www.google.com/search?q=1+second+divided+by+4+hours+17+minutes

Nah, let's go ahead and make that times 100 to get percent:

http://www.google.com/search?q=1+second+divided+by+4+hours+17+minutes+ti mes+100

That's 0.0064 percent of lightspeed. To get an absolute speed, go back to the first and multiply by the speed of light in whatever units you desire:

http://www.google.com/search?q=1+second+divided+by+4+hours+17+minutes+ti mes+speed+of+light+in+miles+per+second

And this gives 12.08 miles per second.

Or:

http://www.google.com/search?q=1+second+divided+by+4+hours+17+minutes+ti mes+speed+of+light+in+miles+per+hour

43,490 miles per hour.

Mac H.

04-18-2012, 12:31 PM

As others have mentioned, you can get google to do the work for you.

If you type in: " 1 light second per 4 hours 17 minutes in km/hr " it will do the math for you.

I'd just round it off to being 70 megs. (Megs = Mega-meters/hr. ie: 1000km/hr)

Most

areteus

04-18-2012, 01:31 PM

Or avoid the issue altogether and use your own arbitrary scale as they did in Star Trek...

You can even name it after a famous scientist in your continuum who came up with it.

i.e. 'We're running at 14.4 Einstiens, Captain, in 20 minutes we will have accelerated to 90% light speed...'

You can decide in your own head what the new scale equates to but you don't have to tell anyone else what it means so you avoid geeks getting all mathematical at you :)

ironmikezero

04-18-2012, 08:34 PM

Why not use generic naval speed references (relative to the vessel) for sub-light velocities...

"All ahead full"

"Course 270*, ahead one-third"

"All stop - hold position"

...etc... you get the idea...

There's always some merit to keeping it simple.

BRDurkin

04-19-2012, 12:36 AM

I've seen a lot of science fiction writing use the Astronomical Unit (AU) in reference to distances and speeds. An AU is the approximate distance between Earth and the Sun (149,597,870.7 kilometers). However, in this case, it seems your ship is moving at a very very small fraction of an AU per hour, so I'm not sure it would a good unit to use.

Plus, if your ship is in a universe in which Earth doesn't exist, the AU would be irrelevant anyway. Just thought I would throw that out there.

RichardGarfinkle

04-19-2012, 12:33 PM

The speed of light in a vacuum is around 300,000 kilometers per second (kps) or 186,000 miles per second (mps) none of which you need since you have the time your ship needs to travel 1light second. Just divide 1 by the number of seconds which should be 15420 to get the fraction of light speed.

In physics the speed of light is simply written as the letter c. So you could write this as c/15420 or 0.0000648c which is a little clumsy. Measuring speeds in space realtive to c is a good method since it is a universal constant.

The SFF forum is a good place to come if you want more on this kind of thing.

http://absolutewrite.com/forums/forumdisplay.php?f=39

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