# Alpha centuri .4 light year relative to who?

1. Jul 18, 2009

### matttan

Alpha centuri.....4 light year relative to who?

Hi,

Here it goes.

Suppose it takes 4 light years to get to alpha centuri.

So if I could travel at 99.99999% the speed of light and the people on earth remains stationary(as in moving very slowly which is << C).

So does my clock shows 4 years when I get to alpha centuri or the clock on the earth shows 4 years.

I am a high school student so if possible avoid any technical explanation

Thanks

Last edited: Jul 18, 2009
2. Jul 18, 2009

### Staff: Mentor

Re: Alpha centuri.....4 light year relative to who?

A light year is a distance, not a time. A light year is the distance that light travels in a year.

Could you please rephrase your question, so that times are in years, and distances are in light years? Thanks.

3. Jul 18, 2009

### Nabeshin

Re: Alpha centuri.....4 light year relative to who?

Uhh, assuming (which I think you mean to) years instead of light years, the trip takes about sixteen hours for an observer moving at 99.99999.

4. Jul 18, 2009

### matttan

Re: Alpha centuri.....4 light year relative to who?

Yeah thats what I mean. So the time on earth measured is 4 years?

5. Jul 18, 2009

### JesseM

Re: Alpha centuri.....4 light year relative to who?

Yes, if we assume the distance from Earth to Alpha Centauri is exactly 4 light years in Earth's frame, and the ship is moving at 0.9999999c in Earth's frame, the time in Earth's frame is 4/0.9999999 years which is just a smidge over 4 years (around 4 years and 13 seconds!)

6. Jul 18, 2009

### qraal

Re: Alpha centuri.....4 light year relative to who?

The time onboard ship would be almost 16 hours.

A problem arises because accelerating to that speed takes a lot of distance & time or very high acceleration. If you did it in 1 hour of ship-time the acceleration would be over 71,000 gees - very unhealthy for all on board. Even accelerating for a year the acceleration required would be over 8 gees.

7. Jul 18, 2009

### DaveC426913

Re: Alpha centuri.....4 light year relative to who?

Well, 99.99999% of c might be tough but I'd calced a way back that getting to 99.9% c only takes a month or two at 1G. I'll have to check my numbers again.

Ah right. I was looking at http://www.davesbrain.ca/science/gliese/index.html" [Broken], which is 20 light years away and 6 years subjective. So a month or two of acceleration is small change.

Last edited by a moderator: May 4, 2017
8. Jul 18, 2009

### HallsofIvy

Re: Alpha centuri.....4 light year relative to who?

Does that include decelerating to a stop at alpha centauri or do you just "blow on by"?

9. Jul 18, 2009

### qraal

Re: Alpha centuri.....4 light year relative to who?

You figured it out, Dave?

Last edited by a moderator: May 4, 2017
10. Jul 18, 2009

### DaveC426913

Re: Alpha centuri.....4 light year relative to who?

Declerating.

11. Jul 19, 2009

### qraal

Re: Alpha centuri.....4 light year relative to who?

Dave, just how did you compute that? Reaching a rapidity equal to c at 1 gee takes a year of subjective time, and that's only a speed of 0.76 c. Getting to 0.999c - a rapidity of 3.8 c - takes somewhat higher gees to cram into a few months.

Last edited by a moderator: May 4, 2017
12. Jul 19, 2009

### DaveC426913

Re: Alpha centuri.....4 light year relative to who?

You know what? Now I'm not sure.