# Light speed independent of source

• I
• Martyn Arthur
In summary: 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...
Martyn Arthur
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

Martyn Arthur said:
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.

cianfa72, FactChecker and vanhees71
Martyn Arthur said:
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.

DrClaude, vanhees71, Dale and 1 other person
Same idea as in post #3 but implemented by amateurs.

vanhees71 and Ibix
kuruman said:
Same idea as in post #3 but implemented by amateurs.

Which proves that neglecting air resistance does not necessarily lead to significant inaccuracies!

vanhees71, kuruman and Ibix
PeroK said:
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...

vanhees71
Ibix said:
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.

PeroK said:
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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kuruman said:
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!

Ibix said:
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 ?

PeroK
There is no such thing as a one-way speed of light.

vanhees71
cianfa72 said:
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.

vanhees71 and Motore
Martyn Arthur said:
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.

vanhees71
cianfa72 said:
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!

Baluncore said:
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.

vanhees71 and cianfa72
Baluncore said:
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.

vanhees71
PeroK said:
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.

Baluncore said:
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.

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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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.

Meir Achuz said:
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.

Meir Achuz said:
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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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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cianfa72

## What is "light speed independent of source"?

"Light speed independent of source" refers to the scientific principle that the speed of light in a vacuum is constant and does not depend on the source of the light.

## Why is the speed of light considered to be independent of its source?

This principle is based on the theory of relativity, which states that the laws of physics are the same for all observers in uniform motion. This means that no matter where the light is coming from or how fast the source is moving, the speed of light will always be the same.

## What evidence supports the idea of light speed being independent of its source?

One of the key pieces of evidence is the Michelson-Morley experiment, which showed that the speed of light is the same in all directions, regardless of the motion of the Earth. Additionally, the phenomenon of time dilation in objects moving at high speeds also supports the concept of light speed being constant.

## Is the speed of light truly constant in all situations?

While the speed of light in a vacuum is considered to be constant, it can be affected by the medium through which it is traveling. For example, light will travel slower in water or air compared to a vacuum. However, the speed of light in a vacuum is considered to be the maximum speed at which anything can travel.

## What are the implications of light speed being independent of its source?

This principle has major implications for our understanding of the universe and how it works. It allows for the development of theories such as relativity and quantum mechanics, and has led to advancements in technology such as GPS systems and particle accelerators.

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