Speed of Light Limit: Earth & Spaceships

lnsanity
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If Earth is here 0 and I go at 90% the speed of light in that direction <--in my spaceship and my friend is going a 90% the speed of light in that direction --> in is spaceship so it look like this. 90% <-- 0 --> 90% relative to people on Earth we each go at 90% the speed of light in oposite direction, how come relative to my friend I would not be going at 180% the speed of light ? and if so what would be my speed relative to my friend if not 180% ?

From the point of view people on Earth it certainly seem to me that they would see us distance each other at faster than light speed... !?
 
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From the Earth's point of view, the distance between you two is increasing at 1.8c. From your point of view, and from your friend's point of view, the distance between you two is increasing at only .9945c.

See: http://en.wikipedia.org/wiki/Velocity_addition_formula
 
Thread 'Can this experiment break Lorentz symmetry?'

Thread 'Can this experiment break Lorentz symmetry?'

1. The Big Idea: According to Einstein’s relativity, all motion is relative. You can’t tell if you’re moving at a constant velocity without looking outside. But what if there is a universal “rest frame” (like the old idea of the “ether”)? This experiment tries to find out by looking for tiny, directional differences in how objects move inside a sealed box. 2. How It Works: The Two-Stage Process Imagine a perfectly isolated spacecraft (our lab) moving through space at some unknown speed V...
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Thread 'Relativator (Circular Slide-Rule): Simulated with Desmos - Insight'

Insights auto threads is broken atm, so I'm manually creating these for new Insight articles. The Relativator was sold by (as printed) Atomic Laboratories, Inc. 3086 Claremont Ave, Berkeley 5, California , which seems to be a division of Cenco Instruments (Central Scientific Company)... Source: https://www.physicsforums.com/insights/relativator-circular-slide-rule-simulated-with-desmos/ by @robphy
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Thread 'A sufficient condition for integrability of equation ##\nabla g=0##'

In Philippe G. Ciarlet's book 'An introduction to differential geometry', He gives the integrability conditions of the differential equations like this: $$ \partial_{i} F_{lj}=L^p_{ij} F_{lp},\,\,\,F_{ij}(x_0)=F^0_{ij}. $$ The integrability conditions for the existence of a global solution ##F_{lj}## is: $$ R^i_{jkl}\equiv\partial_k L^i_{jl}-\partial_l L^i_{jk}+L^h_{jl} L^i_{hk}-L^h_{jk} L^i_{hl}=0 $$ Then from the equation: $$\nabla_b e_a= \Gamma^c_{ab} e_c$$ Using cartesian basis ## e_I...
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