# Speed of light measured the same by all inertial observers?

• metrictensor
In summary, the conversation discusses the concept of the speed of light and its measurement by inertial and non-inertial observers. The question is raised whether the speed of light is always measured as c by any observer, and the limitations of this concept are explored. Ultimately, it is determined that the speed of light may only be measured as c at the origin of an accelerating frame, with variations appearing above and below the origin.
metrictensor
What is the speed of light measured the same by all inertial observers?

$$3*10^8 \frac{m}{s}$$
or
186,262 miles per second

A Google search would have found this faster.

(Or did you mean to ask why??)

Is this reasoning correct : suppose we have a non-inertial observer (towards another one)...then considering the local (in time) observer fitted at every time to the non-inertial one...then the speed of light is c for it, so it has to be c for the non-inertial observer at any time...so the speed of light is c for any observer ?

That sounds like it's awfully close to Zeno! If I understand what you are saying: "A non-inertial observer is accelerating but at each instant he is going at exactly the same speed as some inertial observer. For that inertial observer, the speed of light is c, therefore for the non-inertial observer, the speed of light is always c also!"

I'm not at all sure that is valid. The fact that, at each instant, the non-inertial observer is going at some speed doesn't mean that he observes what an inertial at the same speed would observe.

kleinwolf said:
Is this reasoning correct : suppose we have a non-inertial observer (towards another one)...then considering the local (in time) observer fitted at every time to the non-inertial one...then the speed of light is c for it, so it has to be c for the non-inertial observer at any time...so the speed of light is c for any observer ?

No, beacsue the coordinates tha he measures the speed of light to be c at any instant, whilst being stationary to him at thta isnatnt change from instant to instant as they arel accelarting relative to our non-inertial obsrebre at thta instant.

kleinwolf said:
Is this reasoning correct : suppose we have a non-inertial observer (towards another one)...then considering the local (in time) observer fitted at every time to the non-inertial one...then the speed of light is c for it, so it has to be c for the non-inertial observer at any time...so the speed of light is c for any observer ?

This is true for the specific case of an observer accelerating in a constant direction at a constant rate (a constant proper acceleration), with some important limitations.

The main limitation is that the speed of light in such an accelerating frame is equal to 'c' only at the origin of the frame. "Above" and "below" the origin of the frame clocks appear to run at different rates- one can use the usual formula for gravitational time dilation to calculate the apparent rates at which clocks run, one can also derive the same result from the Lorentz transform without using the gratatioanl time dilation formulas.

DaveC426913 said:
$$3*10^8 \frac{m}{s}$$
or
186,262 miles per second

A Google search would have found this faster.

(Or did you mean to ask why??)

A Google search would have found

$$c= 299,792,458 \ \frac{\mbox{m}}{\mbox{s}}$$

Daniel.

## 1. What is the speed of light?

The speed of light is a fundamental constant in physics and is defined as the speed at which electromagnetic radiation travels in a vacuum. It is approximately 299,792,458 meters per second.

## 2. Why is the speed of light considered constant?

The speed of light is considered constant because it is independent of the motion of the observer. This means that no matter how fast an observer is moving, they will always measure the speed of light to be the same value.

## 3. What does it mean for the speed of light to be measured the same by all inertial observers?

This means that no matter how fast an observer is moving, they will measure the speed of light to be the same value. This is a fundamental principle in the theory of relativity and has been extensively tested and confirmed by experiments.

## 4. Are there any exceptions to the speed of light being constant?

So far, the speed of light has been observed to be constant in all experiments. However, there are some theories that suggest the possibility of variations in the speed of light in certain extreme conditions, such as in the early universe or near black holes. These theories are still being studied and have not been confirmed.

## 5. How is the speed of light measured by inertial observers?

The speed of light is measured by observing the time it takes for light to travel a certain distance. In inertial frames of reference, this measurement is independent of the observer's motion and will always be the same value. This is known as the principle of relativity.

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