Talk:Time dilation/Archive 4

message: In the section entitled 'Time dialation and space flight' the sentence, 'Indeed, a constant 1g acceleration would permit humans to circumnavigate the known universe (with a radius of some 15 billion light years) in one human lifetime. ', is interesting.  Plugging 82 billion light years into the t* equation gives an experienced time of a little more than 25 years.  But if you replace g with a slightly lower value, say 1.5 m/s2 then you get an experienced time of a little more than 150 years.  It seems a strange coincidence that the universe fits us so well; If we lived on a planet with less gravity and were therefore incapable of sustained exposure to anything like 1 g, or if our life spans were similar to that of a dog, or if the universe were only a few times bigger then we could never hope to circumnavigate it.  Yet our life spans, the size of the universe and the gravitation of Earth are all unrelated variables.
Are these equations accurate?  I had heard than even with a high constant acceleration and time dialation it was impossible to reach the centre of the galaxy in a lifetime.
198.103.34.97 17:58, 9 June 2006 (UTC)
replies: text: I will take your calculations on faith, as I know that a constant acceleration really does get up to near light speed (with respect to the starting frame of reference) within a few years.  At that point, your clock is all but stopped.  For the galaxy:  Going to the center within a human lifetime now does seem viable (and back too).  However, where are you going to get the energy from for the accelerations and decelerations necessary for pulling off that trick?  As for circumnavigating the universe, there is a problem:  the Hubble expansion.  As a practical matter, you will never get even halfway around it. --EMS | Talk 05:26, 10 June 2006 (UTC)
replies: text: The phrasing of the question was careful about avoiding the expansion problem, if I remember it correctly (specified a sphere with the diameter of the presently-observable universe, as opposed to our universe, or even a spherical volume within our expanding universe). Regarding the equations, I was considerig doublechecking it yesterday, but so far have been too lazy. I'll do this within the next couple of days here on the talk page, showing my work. There are at least three ways of setting up the problem, though: Applying a thrust of constant magnitude from a stationary observer's reference frame, applying a thrust of magnitude scaling with the relativistic mass of the ship from the stationary observer's reference frame, and considering the ship to apply constant thrust in its own reference frame. These will give wildly different answers for travel time (in fact, the second option might make you approach C in a finite time, with thrust approaching infinity, now that I think about it). --Christopher Thomas 17:34, 10 June 2006 (UTC)
thread_title:  Time dialation and experienced time to circle the universe 

message: :If there is one thing I think this article could do with it is a Graph, showing Time Dilation effects in relation to speed.
:I saw a graph like this years ago in an old book and was always interested in it, it showed how much time would appear to slow down if you were in a spaceship travelling at various percentages of the speed of light. I assume the graph was made using the various formulas on this page, it would be really great if someone could create a graph like this for the article.
:In case I'm not explaining this well, what I mean specifically is a graph with one axis as speed (in percentages of C), and the other axis as the lengthening of time (i.e. at zero speed 1 second = 1 second, at the Speed of light 1 second = Infinitely long time, at-least I think that's how it goes).
:A graph like this would be very useful for people to see at what speeds Time Dilation really starts to affect things, like if I wanted to know say, what would be the dilation at 25% C, then I could just go look (instead of trying to calculate it with those formulas).
:--Hibernian 05:53, 19 July 2006 (UTC)
thread_title:  One thing this article needs