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How to deduce the theorem of addition of velocities?

Let's say that you are in a rocket speeding at 90% the speed of light away from Earth. Now fire a bullet inside the rocket that is also going at 90% the speed of light. According to Newtonian physics, we add both velocities. Thus, the bullet should be going at 180% the speed of light.

But we now know that according to Einstein the sum of these velocities is actually close to 99% the speed of light, because length contracts and time slows down.

But how can you determine mathematically, that bullet is going at 99% the speed of light?
Additionally, can you also calculate, how much time is slowed down, if you know the velocity? Is there any formula?


marked as duplicate by Sklivvz, Waffle's Crazy Peanut, Qmechanic Dec 26 '12 at 13:55

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

  • $\begingroup$ This question doesn't really evince much research effort. Have you familiarised yourself with the Lorentz factor? $\endgroup$ – Richard Terrett Dec 26 '12 at 12:40
  • $\begingroup$ You could also search this site for the transformation equations (if that's what you are looking for). They're called Lorentz transforms. There have been plenty of questions asked about similar topics before. $\endgroup$ – Kitchi Dec 26 '12 at 12:48
  • $\begingroup$ No, I am not familiarised with Lorenz factor. But thank you very much for this information. I found some articles. It may help me to understand. $\endgroup$ – kk-dev11 Dec 26 '12 at 12:56
  • $\begingroup$ This doesn't really make sense: Einstein's formulas are supported by experiments. You can't prove anything physically just by using maths. Not everything that is valid mathematically, is also a valid physical law. Both Newton and Einstein velocity composition laws are solid, mathematically speaking. Only one is right though. VTC. $\endgroup$ – Sklivvz Dec 26 '12 at 13:01
  • $\begingroup$ Ok. @Sklivvz I didn't say proving physically. Just to solve the problem mathematically and get a basic idea since I can't do that experiment on my own. $\endgroup$ – kk-dev11 Dec 26 '12 at 13:05

Velocities are strange beasts. Relativity theory tells us rapidities (the sum of the accelerations experienced by an object) are more intuitive quantities. Rapidities are directly observable quantities (external observers relate these to blue/redshifts). And unlike velocities, rapidities do sum up. Velocities depend non-linearly on rapidities, and therefore velocities follow a more complex addition rule.

Imagine three observers $A$, $B$ and $C$ all moving along a railway track. Observer $A$ measures $B$ to have velocity $v_{AB}$. From $B$'s perspective $C$ has a velocity $v_{BC}$. And to close the circle, from $C$'s perspective $A$ has velocity $v_{CA}$.

Common experience (encoded in so-called Galilean relativity) tells us these velocities simply add up to zero:

$v_{AB} + v_{BC} + v_{CA} = 0$

This is wrong. It ignores a non-linear term that becomes important at speeds approaching the speed of light $c$. Lorentzian relativity tells us the correct equation is:

$v_{AB} + v_{BC} + v_{CA} + v_{AB}.v_{BC}.v_{CA} / c^2= 0$

This is all the math you need. Give it a try. Enter $v_{AB}=0.9 c$ and $v_{BC}=0.9 c$ and see what value you get for $v_{AC} = -v_{CA}$.

  • $\begingroup$ If I do the math.. $v_{CA} = -1.8$. From C's perspective A is moving at -180% speed of light? is it correct? $\endgroup$ – kk-dev11 Dec 26 '12 at 13:46
  • $\begingroup$ 0.9 + 0.9 + x + 0.9 0.9 x = 0. Determine x. $\endgroup$ – Johannes Dec 26 '12 at 13:49
  • $\begingroup$ great!! x= -0.9944.. That's really awesome. And that's what I wanted. Thank you very much. :) $\endgroup$ – kk-dev11 Dec 26 '12 at 13:52

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