Physics Help Forum Travel at speed of light?

 Special and General Relativity Special and General Relativity Physics Help Forum

 Aug 4th 2014, 04:25 AM #1 Member   Join Date: Mar 2013 Posts: 37 Travel at speed of light? Speed of light is 299792458 m / s, Speed is related to 2 objects for comparison, can traveling at speed of light be achieved by comparing 2 spaceships moving in different directions at 149896229 m / s? If one observes another spaceship to each other, they are traveling at speed of light related to other spaceship, would it be correct? Does anyone have any suggestions? Thanks in advance for any suggestions
 Aug 4th 2014, 04:33 AM #2 Physics Team     Join Date: Jun 2010 Location: Naperville, IL USA Posts: 2,171 If spaceship A is moving away from you to the right at, say, 0.9c, and ship B is moving away from you to the left at -0.9c (negative because it's moving left), then to you the difference in velocity between the two ships is 1.8c. But to the pilot on ship A the relative velocity is different - the formula for adding relative velocities at relativistic speeds is: w= (u+v)/(1+uv/c^2) So in this case he measures you traveling left at 0.9c and ship B moving left at 2(-0.9c)/(1+(0.9)^2) = -0.994c.
 Jan 6th 2017, 03:01 PM #3 Junior Member   Join Date: Jan 2017 Posts: 4 Yes I believe so. But, if the spaceships are going towards each other at c, the observer will still measure the spaceship the speed of light. Because, remember, you still have to take time dilation and length contraction into account. You will eventually see the spaceship going at the half of the length it should travel, and half of the time you will measure(you are going towards it at the same speed), and the final result gives you c, the speed of light. So basically, no matter what position you are at, or what speed you are going, and what direction you are going towards, the speed of light you measure will always be c.
Jan 7th 2017, 11:09 AM   #4
Senior Member

Join Date: Aug 2010
Posts: 150
 Originally Posted by JustKiddingImNotAPhD Yes I believe so. But, if the spaceships are going towards each other at c, the observer will still measure the spaceship the speed of light. Because, remember, you still have to take time dilation and length contraction into account.
And then the two spaceships will turn into pink flamingos! My point is that if you start with false hypotheses you can come to any conclusion. A space ship cannot travel at the speed of light to begin with.

 You will eventually see the spaceship going at the half of the length it should travel, and half of the time you will measure(you are going towards it at the same speed), and the final result gives you c, the speed of light.
No, it won't.

 So basically, no matter what position you are at, or what speed you are going, and what direction you are going towards, the speed of light you measure will always be c.
Yes, but that has nothing to do your previous assertions.

 Tags light, speed, travel

 Thread Tools Display Modes Linear Mode

 Similar Physics Forum Discussions Thread Thread Starter Forum Replies Last Post timemachine2 Light and Optics 0 Mar 4th 2016 04:27 PM kiwiheretic Light and Optics 4 Apr 2nd 2014 10:52 AM urooj177 Special and General Relativity 5 Jan 5th 2013 06:30 PM danielcarey General Physics 3 Nov 13th 2012 04:58 AM hapflir Kinematics and Dynamics 1 Sep 13th 2011 02:45 PM