Two Spaceships & Light: Measuring Speed from Earth

  • Thread starter Thread starter keepitmoving
  • Start date Start date
keepitmoving
Messages
97
Reaction score
0
two spaceships send a light beam toward Earth at the moment that they pass each other, one sends light backward toward us and one sends light forward toward us.
On Earth we measure the light speed as equal but let's back up to when the two lights were emitted. What speed would we measure for the two lights?
 
Physics news on Phys.org
c

They always measure c.
 

Thread 'Describing the Big Bang'

I started reading a National Geographic article related to the Big Bang. It starts these statements: Gazing up at the stars at night, it’s easy to imagine that space goes on forever. But cosmologists know that the universe actually has limits. First, their best models indicate that space and time had a beginning, a subatomic point called a singularity. This point of intense heat and density rapidly ballooned outward. My first reaction was that this is a layman's approximation to...
View full post »
Thread 'Dirac's integral for the energy-momentum of the gravitational field'

Thread 'Dirac's integral for the energy-momentum of the gravitational field'

See Dirac's brief treatment of the energy-momentum pseudo-tensor in the attached picture. Dirac is presumably integrating eq. (31.2) over the 4D "hypercylinder" defined by ##T_1 \le x^0 \le T_2## and ##\mathbf{|x|} \le R##, where ##R## is sufficiently large to include all the matter-energy fields in the system. Then \begin{align} 0 &= \int_V \left[ ({t_\mu}^\nu + T_\mu^\nu)\sqrt{-g}\, \right]_{,\nu} d^4 x = \int_{\partial V} ({t_\mu}^\nu + T_\mu^\nu)\sqrt{-g} \, dS_\nu \nonumber\\ &= \left(...
View full post »

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...
View full post »

Similar threads

I Why is Rømer's light speed measurement not one-way?
Replies
42
Views
3K
A 1-Way Speed of Light
Replies
42
Views
680
B Meaningfulness of the speed of light
Replies
25
Views
2K
I For a relativistic spaceship, what is the most recent news the ship gets from Earth?
Replies
6
Views
1K
I Effects of time dilation for near-speed-of-light travel
2
Replies
65
Views
10K
I Conventionality of the One-Way Speed of Light
Replies
5
Views
355
I Bell's Spaceship Paradox, Born Rigidity, and the 1-Way Speed of Light
Replies
47
Views
4K
B Is this an Unavoidable Collision between 2 spaceships traveling at 0.6c each?
Replies
34
Views
2K
B One-Way Speed of Light
2
Replies
93
Views
5K
B Why do you need a light clock?
Replies
7
Views
2K

Hot Threads

Recent Insights

Back
Top