# The Naked Scientists Forum

### Author Topic: Is the velocity of light constant??  (Read 1093 times)

#### ijaz

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##### Is the velocity of light constant??
« on: 09/12/2015 13:21:32 »
is the velocity of light constant??

#### Colin2B

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##### Re: Is the velocity of light constant??
« Reply #1 on: 09/12/2015 13:56:31 »
No.
But I assume you know that and the circumstances under which it is not?

#### Phractality

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##### Re: Is the velocity of light constant??
« Reply #2 on: 09/12/2015 16:00:27 »
The speed of light in a vacuum is constant by definition. Passing thru a denser transparent medium slows the light.

The definition of the speed of light in a vacuum establishes the ratio of the meter to the second. This ratio was first measured when the meter and second were defined in terms of the size and rotational speed of Earth. Earth's polar circumference is 40,000,000 meters and the second is 1/84,600 of a mean solar day. We got so good at measuring the Earth that we discovered it's size and rotational speed vary slightly; so we needed a more stable standard. The new standard is the frequency and wavelength of a certain spectral line of the Cesium atom. That wavelength and frequency now define the meter, second and speed of light, so it is now pointless to try and measure them; they can not change as long as they are constant by definition.

Because light is affected by gravity, and light defines the space-time grid, the grid is warped by gravity. If you defined a space-time continuum to be flat (not warped), you would then need to define space-like and time-like units in which the speed of light is variable. Those units could be identical to meters and seconds in the absence of gravity, but light would go faster in a gravity well, when measured in those space-like and time-like units.

Einstein's general relativity ensures that the laws of physics operate the same everywhere. In a flat-by-definition space-time continuum, those laws would need to be different in different places.

#### Colin2B

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##### Re: Is the velocity of light constant??
« Reply #3 on: 10/12/2015 08:53:48 »
Defining a calibration standard does not change the laws of physics.
An instrument calibrated in as stationary in one frame of reference or gravitational field will show time changes if moved to another.
It is however an interesting question as to whether we consider time constant and the speed of light changing or vice versa, but hey, that's all relative!

#### The Naked Scientists Forum

##### Re: Is the velocity of light constant??
« Reply #3 on: 10/12/2015 08:53:48 »