# Speed if light

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.

## Answers and Replies

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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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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Claude Bile
Science Advisor
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.

Google the Michelson-Morley experiment.

Claude.

DrGreg
Science Advisor
Gold Member
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.