# Speed of Light: Is it An Assumption or Empirically Demonstrated?

• revo74
In summary, the entire theory of relativity is based on two postulates: the principle of relativity and the principle of invariant light speed. The latter states that the speed of light in vacuum is constant regardless of the motion of the light source. This is supported by empirical evidence and is a fundamental concept in our models of physics. However, there are also tacit assumptions involved, such as the isotropy and homogeneity of space and the independence of measuring rods and clocks from their past history. The question of who is correct remains open to interpretation and further research.

#### revo74

Someone said that entire theory of relativity hinges on the assumption that the speed of light is constant in a one way direction between any two points and that this cannot be proven scientifically; it must be assumed.

A response to this was: constancy of the speed of light through a given medium is not an assumption. We assume this is constant in all reference frames because we have no evidence otherwise. Thus, it is empirically demonstrated in a reliable fashion and our models based on this data work.

Who is correct? Please elaborate. Thank you.

The second paragraph seems to contain a contradiction.

http://en.wikipedia.org/wiki/Special_relativity#Postulates"

The Principle of Relativity – The laws by which the states of physical systems undergo change are not affected, whether these changes of state be referred to the one or the other of two systems in uniform translatory motion relative to each other.

The Principle of Invariant Light Speed – "... light is always propagated in empty space with a definite velocity [speed] c which is independent of the state of motion of the emitting body." (from the preface). That is, light in vacuum propagates with the speed c (a fixed constant, independent of direction) in at least one system of inertial coordinates (the "stationary system"), regardless of the state of motion of the light source.

The derivation of special relativity depends not only on these two explicit postulates, but also on several tacit assumptions (made in almost all theories of physics), including the isotropy and homogeneity of space and the independence of measuring rods and clocks from their past history.​

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Phrak said:
The second paragraph seems to contain a contradiction.

http://en.wikipedia.org/wiki/Special_relativity#Postulates"

The Principle of Relativity – The laws by which the states of physical systems undergo change are not affected, whether these changes of state be referred to the one or the other of two systems in uniform translatory motion relative to each other.

The Principle of Invariant Light Speed – "... light is always propagated in empty space with a definite velocity [speed] c which is independent of the state of motion of the emitting body." (from the preface). That is, light in vacuum propagates with the speed c (a fixed constant, independent of direction) in at least one system of inertial coordinates (the "stationary system"), regardless of the state of motion of the light source.

The derivation of special relativity depends not only on these two explicit postulates, but also on several tacit assumptions (made in almost all theories of physics), including the isotropy and homogeneity of space and the independence of measuring rods and clocks from their past history.​

How exactly does this show there is a contradiction?

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revo74 said:
Someone said that entire theory of relativity hinges on the assumption that the speed of light is constant in a one way direction between any two points and that this cannot be proven scientifically; it must be assumed.

A response to this was: constancy of the speed of light through a given medium is not an assumption. We assume this is constant in all reference frames because we have no evidence otherwise. Thus, it is empirically demonstrated in a reliable fashion and our models based on this data work.

Who is correct? Please elaborate. Thank you.

Claude.

revo74 said:
Someone said that entire theory of relativity hinges on the assumption that the speed of light is constant in a one way direction between any two points and that this cannot be proven scientifically; it must be assumed.
To measure the "one-way" speed of light from A to B you need two clocks, one at A and one at B. To measure the "two-way" speed of light from A to B and reflected back to A again, you need only one clock at A.

We have lots of experimental evidence to show that the two-way speed of light in vacuum is constant.

The one-way speed of light depends on how we synchronise the two clocks at A and B. The "assumption" that the one-way speed of light in vacuum is constant, is really a definition of how to synchronise clocks.

## 1. What is the speed of light?

The speed of light is a fundamental physical constant that represents the speed at which electromagnetic radiation (such as light) travels through a vacuum. Its value is approximately 299,792,458 meters per second.

## 2. How was the speed of light first measured?

The speed of light was first measured in 1676 by Danish astronomer Ole Rømer. He observed the varying times of eclipses of Jupiter's moons and deduced that light takes approximately 22 minutes to travel across the diameter of Earth's orbit.

## 3. Is the speed of light an assumption or has it been empirically demonstrated?

The speed of light has been empirically demonstrated through various experiments, including the Michelson-Morley experiment in 1887 and the Fizeau experiment in 1851. These experiments have consistently shown that the speed of light is a constant value.

## 4. Can the speed of light be exceeded?

According to the theory of relativity, the speed of light is the maximum speed at which all energy, matter, and information in the universe can travel. So far, no experiment has been able to exceed the speed of light.

## 5. How does the speed of light affect our daily lives?

The speed of light plays a crucial role in many aspects of our daily lives. It allows us to communicate through technologies such as radio, television, and the internet. It also helps us understand the behavior of light and other electromagnetic radiation, which has various practical applications in fields such as medicine, telecommunications, and astronomy.