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-   -   Travel at speed of light? (http://physicshelpforum.com/special-general-relativity/10303-travel-speed-light.html)

 oem7110 Aug 4th 2014 05:25 AM

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

 ChipB Aug 4th 2014 05:33 AM

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.

 JustKiddingImNotAPhD Jan 6th 2017 04:01 PM

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.

 HallsofIvy Jan 7th 2017 12:09 PM

Quote:
 Originally Posted by JustKiddingImNotAPhD (Post 33794) 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.

Quote:
 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.

Quote:
 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.

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