Constancy of the speed of light.

[Endervhar]

I have just come across the following quote by Nigel Calder:- "Light travels faster...toward the centre of gravity than away from it."

How does one square this with the constancy of the speed of light?

 

[JaredJames]

As far as I'm aware, gravity only alters the direction of light, not it's speed.

 

[omerusta]

I have just come across the following quote by Nigel Calder:- "Light travels faster...toward the centre of gravity than away from it."

How does one square this with the constancy of the speed of light?

Speed of light is not constant. It changes, sometimes dramaticly, according to the medium it travels.
Actually not the speed but spectrum of light changes near big objects.

 

[JaredJames]

Speed of light is not constant. It changes, sometimes dramaticly, according to the medium it travels.
Actually not the speed but spectrum of light changes near big objects.

No, the speed of light is constant. It is it's interaction with the medium that makes it appear to travel slower.

 

[omerusta]

No, the speed of light is constant. It is it's interaction with the medium that makes it appear to travel slower.

Interesting!
It fools us then.

I don't think so...

 

[HallsofIvy]

Speed of light is not constant. It changes, sometimes dramaticly, according to the medium it travels.
Actually not the speed but spectrum of light changes near big objects.

No, the speed of light is constant. It is it's interaction with the medium that makes it appear to travel slower.

Interesting!
It fools us then.

I don't think so...

It depends upon how you look at it. Light, traveling a given distance through water will take a longer time than the same distance through vacuum- it travels slower.

But if you look "closely" you will see light traveling at the same speed as in vacuum between molecules of water, until it it is absorbed by an atom of hydrogen or oxygen. Then, after a brief delay, it is re-emited. It is the "brief delay" that causes the macro "slowness".

 

[omerusta]

Now we agree. Good explanation.
By the way, please note Cherenkow radiation.

http://en.wikipedia.org/wiki/Cherenkov_radiation

 

[Endervhar]

Thanks, that was more or less my understanding, but it still leaves me wondering what Calder meant. I shall have to identify the source and find out if he was mis-quoted.

 

[OnlyMe]

But if you look "closely" you will see light traveling at the same speed as in vacuum between molecules of water, until it it is absorbed by an atom of hydrogen or oxygen. Then, after a brief delay, it is re-emited. It is the "brief delay" that causes the macro "slowness".

Can you provide a reference for this? (other than a purely theoretical construct)

The emission spectrums of oxygen and hydrogen are well defined. They result in well defined absorption lines in the light from more distant sources. And well defined emission lines when excited sufficiently to emit photons.

If the variation of the velocity of light through transparent mediums were the result of absorption and re-emission, as photons traveled through intervening mediums, information unique to a distant source would be lost.

What we "see" is that some of the photons from a more distant source do interact with atoms in an intervening medium resulting in absorption lines and a portion reach us without direct interaction. If this were not the case even the lenses in our optical telescopes would change the nature of the light from distant galaxies.

 

[Rap]

It depends upon how you look at it. Light, traveling a given distance through water will take a longer time than the same distance through vacuum- it travels slower.

But if you look "closely" you will see light traveling at the same speed as in vacuum between molecules of water, until it it is absorbed by an atom of hydrogen or oxygen. Then, after a brief delay, it is re-emited. It is the "brief delay" that causes the macro "slowness".

This was my belief too, until I read the general physics FAQ: https://www.physicsforums.com/showthread.php?t=104715. See post #4.

 

[cosmik debris]

The speed of light is constant locally. If you attempt a measurement remotely over large distances in spacetime you will not get the usual value. If for example you measure the speed of light as it passes a large mass you will get a slower answer for the total trip, This is called Shapiro time delay.

http://www.astro.ucla.edu/~wright/deflection-delay.html

 

[Drakkith]

Let's not confuse terms and stuff here.

The speed of light IN A VACUUM is a constant. The speed at which light will propagate through a medium changes depending on the medium, but is constant in each separate medium as long as it is uniform. Sudden changes in density and other physical effects can alter the direction and speed of light in that medium.

Note that things can and do travel faster than light in a medium. That is the reason for Cherenkov radiation. The speed c is the "speed limit" that nothing can reach other than light, and that is only in a vacuum when it is not interacting with matter.

Gravity does not change the speed of light, but it can change its direction and wavelength depending on the observer.