Physics Help Forum Special Relativity Question

 Special and General Relativity Special and General Relativity Physics Help Forum

 Aug 29th 2009, 07:26 PM #1 Junior Member   Join Date: Aug 2009 Posts: 4 Special Relativity Question I just started special relativity in school and, being somewhat confused, I found myself asking this question. An observer A is travelling at a substantial fraction of the speed of light relative to observer B. Since they are both moving at that speed relative to one another, how do we know which one experiences time dilation/length contraction? I know it must have something to do with one of the observers having had to accelerate (I think), but I don't understand exactly why. Thanks in advance
 Aug 29th 2009, 07:43 PM #2 Physics Team     Join Date: Jul 2009 Posts: 310 Yes, the observer that travels near the speed of light relative to a stationary observer experiences special realitivistic effects. __________________ "Dissent is the highest form of patriotism." - Thomas Jefferson. "Give me control of a nation's money and I care not who makes her laws." -Mayer Amschel Rothschild I study Mathematical Physics at the University of Waterloo. -DC
 Aug 29th 2009, 07:55 PM #3 Junior Member   Join Date: Aug 2009 Posts: 4 Right, but my question involves the fact that A is moving at some speed relative to B, but B can be considered to be moving at that same speed relative to A. What determines which one we choose to regard as stationary? I'll rephrase it in this idealized form: 2 observers, A and B, in empty space are travelling relative to one another at a substantial velocity v and with no reference points except themselves and one another. Each observer considers the other to be moving while he himself is stationary. Which observer experiences the relativistic effects and why? And if it has to do with whichever was initially accelerated, what is it specifically about his acceleration that causes the effects to apply to him alone?
 Aug 29th 2009, 11:28 PM #4 Physics Team   Join Date: Feb 2009 Posts: 1,425 You are right we can't decide who is moving as long as they travel with uniform velocity with respect to each other beacuse uniform motion is relative not absolute. So what happens is this. A in his own ship eperiences no relativistic effects, but when he looks at B, he finds that B experiences them,since from A's stand point, it is B who is moving with relativistic velocity. And vice-versa for B. Special relativity states that the speed of light is constant in all inertial frames of reference and that the laws of physics also have the same form in all inertial frames of reference. The price that has to be paid for this is that things like mass, time and length which were always taken to be absolute are no longer so in relativity. However if any of them accelerates, he will feel a force. So accelerated motion can be detected and is not absolute. However for this problem, we need not bring in acceleration . The postulates of special relativity are not valid for accelerated frames of reference.

 Tags question, relativity, special

 Thread Tools Display Modes Linear Mode

 Similar Physics Forum Discussions Thread Thread Starter Forum Replies Last Post luinthoron Special and General Relativity 2 Apr 30th 2014 10:59 AM brentwoodbc Special and General Relativity 0 Jan 19th 2009 05:02 PM ah-bee Special and General Relativity 0 Nov 10th 2008 02:43 AM evabern Special and General Relativity 2 Oct 6th 2008 04:26 AM ah-bee Special and General Relativity 3 Aug 31st 2008 04:49 PM