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Beards
2006-Aug-29, 01:38 PM
Imagine 2 space rockets leaving earth and heading for 2 distant stars. Star 1 is 100 yl away and star 2 is 200 ly away.
Imagine both rockets accellerate and reach light speed the moment they leave the earth.
Now as I understand it, for anything travelling at the speed of light, time = 0. Therefore both rockets would reach their stars instantaneously even though there is a difference of 100 ly. Is this correct?

If it is correct then how fast would the rockets appear to travel from a FOR for an outside observer? And how long would each journey appear to take?

:question:

Any help appreciated as I'm trying to wrap my head around this "relativity" thing !! LOL

CuddlySkyGazer
2006-Aug-29, 02:00 PM
I take it these are rockets that suddenly become massless, because otherwise they cannot travel at c (speed of light in a vacuum)!

The answer to your questions are: c, 100 and 200 years respectively (as the speed of light is the same to all observers).

Beards
2006-Aug-29, 02:08 PM
I take it these are rockets that suddenly become massless, because otherwise they cannot travel at c (speed of light in a vacuum)!
Yep - magical massless machines. I did put "Thought experiment" at the top of the post in case you thought I was a total imbecile!!

So an obvious answer then. Thanks for that.

Anyone out there reccomend some "light" reading (yes I know - hilarious pun!) about relativity? I'm not the cleverest of people but I am really interested in finding out more, so anything written in Laymans terms with as little maths and physics as poss would be ideal.

Then I'll stop asking such obvious questions!!!

Cheers.

Ken G
2006-Aug-29, 02:45 PM
Don't worry, your question does get to the bottom of something very strange-- although each ship thought it took the same time (zero) to reach their destination, in another frame, there was a 100 year difference. This is not unusual in relativity-- in fact, you can sometimes find reference frames where the order of two events are reversed! But you can never reverse a cause and an effect, there are some limits. There are a lot of good online places to look into relativity-- have you tried Ned Wright's cosmology tutorial? That can be a nice resource (Google it).

trinitree88
2006-Aug-29, 10:27 PM
Yep - magical massless machines. I did put "Thought experiment" at the top of the post in case you thought I was a total imbecile!!

So an obvious answer then. Thanks for that.

Anyone out there reccomend some "light" reading (yes I know - hilarious pun!) about relativity? I'm not the cleverest of people but I am really interested in finding out more, so anything written in Laymans terms with as little maths and physics as poss would be ideal.

Then I'll stop asking such obvious questions!!!

Cheers.

Beards...try "Thinking Physics" by Epstein in your bookstore. cartoons...good stuff simplified. The old stuff..Mr. Tompkins...etc by George Gamow. Ask your librarian to help. Good luck. Pete.

Ufonaut99
2006-Aug-30, 12:52 AM
Hi Beards, and welcome to the board,

The rockets would reach their destination instantaneously for the crews on board ship - ie. they would take off and, before they knew it, immediately be at the stars.

For the folks left back on earth, they would see the rockets take 100 years and 200 years (although, of course, they would only *see* the rockets arriving after 200 and 400 years, since the light of their arrival has to make the trip back).

Beards
2006-Aug-30, 07:12 AM
Thanks for the replies.
It was a bit of an easy question for you lot!
I shall do some reading (Nice one trinitree88 - a few for me to start with) then I shall return with more obvious questions. LOL

Thanks for all not laughing at my simple question.