Light speed independent of source

  • #1
Martyn Arthur
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TL;DR Summary
Please bear with me I am trying to get a grip with underlying principles.
Starting to try and understand Einstein’s second postulate and distinguish that the speed of light is independent of the speed of the source – v - of objects, other than light with travel initiated independent of the speed of the source.
Please bear with me I am trying to get a grip with underlying principles.

Starting to try and understand Einstein’s second postulate and distinguish that the speed of light is independent of the speed of the source – v - of objects, other than light with travel initiated independent of the speed of the source.

Much is 'made' that the speed of light is independent of the speed of the source.

But will not a ball “A”, hit by a bat, in any inertial frame, or indeed in any circumstance devoid of interference such as gravity, also travel independently of the speed of the source.

Why thus does the independence of light on source velocity figure so significantly
 

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  • #2
Ibix
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But will not a ball “A”, hit by a bat, in any inertial frame, or indeed in any circumstance devoid of interference such as gravity, also travel independently of the speed of the source.
No. Throw a ball out of a moving car and its velocity as seen by someone on the ground will be your throwing velocity plus the velocity of the car. But light is always doing ##c## as measured by any inertial observer.
 
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  • #3
PeroK
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But will not a ball “A”, hit by a bat, in any inertial frame, or indeed in any circumstance devoid of interference such as gravity, also travel independently of the speed of the source.
 
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  • #4
kuruman
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Same idea as in post #3 but implemented by amateurs.

 
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  • #5
PeroK
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Same idea as in post #3 but implemented by amateurs.


Which proves that neglecting air resistance does not necessarily lead to significant inaccuracies!
 
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  • #6
Ibix
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Which proves that neglecting air resistance does not necessarily lead to significant inaccuracies!
Neglecting the rigidity of your ramp or the error in your release direction, however...
 
  • #7
cianfa72
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No. Throw a ball out of a moving car and its velocity as seen by someone on the ground will be your throwing velocity plus the velocity of the car. But light is always doing ##c## as measured by any inertial observer.
Just to be clear, @Ibix means your throwing ball velocity w.r.t. the car plus the velocity of the car w.r.t. an inertial observer on the ground.
 
  • #8
kuruman
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Which proves that neglecting air resistance does not necessarily lead to significant inaccuracies!
I am not so sure. Note that the brake lights are continuously on when the car comes into view. This slowdown presumably compensated for losses due to air resistance. Like I said, amateurs.
 
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  • #9
PeroK
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I am not so sure. Note that the brake lights are continuously on when the car comes into view. This slow down presumably compensated for losses due to air resistance. Like I said, amateurs.
Hoist by their own petard, in any case!
 
  • #10
cianfa72
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But light is always doing ##c## as measured by any inertial observer.
When you say the speed of light is always ##c## as measured by any inertial observer, are you actually talking about the one-way or the two-way speed ?
 
  • #11
Vanadium 50
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There is no such thing as a one-way speed of light.
 
  • #12
Ibix
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When you say the speed of light is always ##c## as measured by any inertial observer, are you actually talking about the one-way or the two-way speed ?
I think this is straying somewhat from the OP when he hasn't yet had a chance to read the on-topic responses.
 
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  • #13
Baluncore
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Why thus does the independence of light on source velocity figure so significantly
When you throw a stone into a pond,
the stone has a horizontal velocity,
but the ripples in the pond form circles,
because the ripples propagate through the water, not the stone.
 
  • #14
Martyn Arthur
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Just to be clear, @Ibix means your throwing ball velocity w.r.t. the car plus the velocity of the car w.r.t. an inertial observer on the ground.
Of course; sorry, not the brightest of questions by a long shot!
 
  • #15
Ibix
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When you throw a stone into a pond,
the stone has a horizontal velocity,
but the ripples in the pond form circles,
because the ripples propagate through the water, not the stone.
While this is true, it is analagous to an ether model of light propagation. Don't take this too literally, OP.
 
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  • #16
PeroK
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When you throw a stone into a pond,
the stone has a horizontal velocity,
but the ripples in the pond form circles,
because the ripples propagate through the water, not the stone.
Not the greatest poem I've ever read, I'm sorry to say.
 
  • #17
Baluncore
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Not the greatest poem I've ever read, I'm sorry to say.
Too long for a haiku.
Bonus points to anyone who can express it in three lines of haiku math.
 
  • #18
PeroK
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Too long for a haiku.
Bonus points to anyone who can express it in three lines of haiku math.
Water is perturbed,
By vertical displacement:
Horizontal wave.
 
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  • #19
cianfa72
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If the OP has not questions anymore, I would like to go in detail on this point:
There is no such thing as a one-way speed of light.

I believe if we want to avoid any definition of simultaneity/synchronization, we must actually limit ourselves to talk about constancy/invariance of the two-way speed of light alone.
 
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  • #20
Meir Achuz
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Einstein (1905): "light is always propagated in empty space with a definite velocity c which is independent of the state of motion of the emitting body."
But, "sound is always propagated in still air with a velocity which is independent of the state of motion of the emitting body."
I think Einstein could have said it better.
 
  • #21
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I think Einstein could have said it better.
No doubt, but considering language effects (English was not Einstein's native language) and that he was often not writing for a lay audience this is not surprising.

Stuff like this is why we generally recommend that people learn the physics from more modern treatments. The authors' understanding may be no deeper, but the exposition will be much easier to foillow.
 
  • #22
cianfa72
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Einstein (1905): "light is always propagated in empty space with a definite velocity c which is independent of the state of motion of the emitting body."
If the above is meant as the one-way speed of light in empty space, then the above is actually a statement about a property of the synchronization procedure for spatially separated clocks according that the one-way speed of light is the invariant ##c##.
 
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  • #23
Meir Achuz
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It's what Albert wrote. Check with him about what is meant.
The quote was translated from his German 1905 paper.
This thread started with: "Einstein’s second postulate ... that the speed of light is independent of the speed of ... the source." We seem to agree that "Einstein could have said it better."
 
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