## Speed of Light in Vacuum

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MuggsMcGinnis
Asternaut
Posts: 2
Joined: Sat Mar 03, 2012 7:39 pm

### Speed of Light in Vacuum

I have been in a discussion on another site and would like some expert advice: either I need to be straightened out or I would like to be able to defend the statements I've made. I have been insisting that, for all observers, the speed of light in a vacuum is a constant. Here's the statement I've made:
"Relativity is based on the fact that, for all observers, the speed of light in a vacuum is the same. It doesn't matter whether the light was emitted from a "stationary" or "moving" source. It doesn't matter if the measurements are done under zero, constant non-zero, or variable acceleration. The path of light will curve but the speed will not change."
Is that correct or am I confused?

Chris Peterson
Abominable Snowman
Posts: 17013
Joined: Wed Jan 31, 2007 11:13 pm

### Re: Speed of Light in Vacuum

MuggsMcGinnis wrote:I have been in a discussion on another site and would like some expert advice: either I need to be straightened out or I would like to be able to defend the statements I've made. I have been insisting that, for all observers, the speed of light in a vacuum is a constant. Here's the statement I've made:
"Relativity is based on the fact that, for all observers, the speed of light in a vacuum is the same. It doesn't matter whether the light was emitted from a "stationary" or "moving" source. It doesn't matter if the measurements are done under zero, constant non-zero, or variable acceleration. The path of light will curve but the speed will not change."
Is that correct or am I confused?
That is mostly correct, although I'd say that Special relativity is based partly on the assumption that the speed of light in a vacuum is the same for all observers (an assumption that is well supported by observation, as well as other theoretical concerns). Because the assertion is restricted to Special Relativity, it applies only to observers in inertial (non-accelerating) frames. An observation of a photon by an accelerating observer is analyzed by General Relativity, and is much more complex.
Chris

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Chris L Peterson
Cloudbait Observatory
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MuggsMcGinnis
Asternaut
Posts: 2
Joined: Sat Mar 03, 2012 7:39 pm

### Re: Speed of Light in Vacuum

So, in what sort of reference frame would light in vacuum have a different speed? The statement I made isn't restricted to Special Relativity. I refer to measurements done under non-zero and variable accelerations. Is the speed of light in vacuum different measured in a lab on Earth?

Light will follow a curved path, geodesic, with respect to an accelerating reference frame but isn't it true that its speed along that path is still c?

neufer
Posts: 18805
Joined: Mon Jan 21, 2008 1:57 pm
Location: Alexandria, Virginia

### Re: Speed of Light in Vacuum

MuggsMcGinnis wrote:
So, in what sort of reference frame would light in vacuum have a different speed? The statement I made isn't restricted to Special Relativity. I refer to measurements done under non-zero and variable accelerations. Is the speed of light in vacuum different measured in a lab on Earth?

Light will follow a curved path, geodesic, with respect to an accelerating reference frame but isn't it true that its speed along that path is still c?
Time stands still on the event horizon of a black hole.

Hence everything (including photons) will come to a halt at the event horizon from the point of view of an outside observer.
Art Neuendorffer

Chris Peterson
Abominable Snowman
Posts: 17013
Joined: Wed Jan 31, 2007 11:13 pm

### Re: Speed of Light in Vacuum

MuggsMcGinnis wrote:So, in what sort of reference frame would light in vacuum have a different speed?
In any non-inertial (accelerated) frame.
The statement I made isn't restricted to Special Relativity.
I think it needs to be.
I refer to measurements done under non-zero and variable accelerations.
I don't know if any experiments have been conducted under conditions where the accelerations were large enough to produce measurable deviations from what would be observed in an inertial frame. If so, I'd expect they involved particles in a circular accelerator.
Light will follow a curved path, geodesic, with respect to an accelerating reference frame but isn't it true that its speed along that path is still c?
Yes, but you need GR to understand how to measure that. From an accelerated frame, I think light can appear to be traveling at something other than c. That is never the case in an unaccelerated frame.
Chris

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Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com