## Is the speed of light actually constant or just always measured to be the same?

 Quote by DaveC426913 Your thoughts are becoming disjointed. No part of the above addresses anything in the current thread. Take some time to compose your responses so as to contribute to the topic at-hand. (Also, maybe calm down a bit. If you have facts on your side, use them instead of exclamation marks to make your case.)

 Quote by harrylin Please focus on answering questions instead of criticizing the contributions and opinions of others, OK?
Seriously dude, are you going to just copycat me?

Can you explain what
 "Time" isn't a "building block", it even has no substance!
has to do with the topic at-hand?

 Quote by Ken Natton Perhaps this also contributes nothing of significance, but what I am struggling with here is why anyone thinks that the constancy of the speed of light is open to question under relativity. It isn’t some side effect of relativity, it is one of the two postulates. Take it away and you are not discussing relativity. Or so it seems to me. Some one else on these forums pointed out that if you increase the value of c (hypothetically of course) you get closer and closer to Newtonian mechanics until, when c is infinity, hey presto! Newtonian mechanics! That is to say, the constancy of the speed of light is the difference between Newtonian mechanics and special relativity.
When c is infinity and c does not relate to the speed of light, then you obtain Newtonian mechanics. However, "Relativity" commonly means General relativity, in which the second postulate is only locally valid.

- http://www.bartleby.com/173/22.html

Cheers,
Harald
 Well okay, perhaps these are just the ramblings of someone who doesn’t understand very well, but it has always seemed to me to be a problem that we call the phenomenon ‘the speed of light’. It has a tendency to make people think that light is the issue. It isn’t. It isn’t about the speed of the propagation of electromagnetic waves. Light travels at that speed but it isn’t the driver of the phenomenon. Here’s another point that I believed to be a significant insight, perhaps this is also wrong but maybe discussing it will help to clear up the misalignments of thought that are dogging the discussion. Something I read suggested that in point of fact the speed of everything is always constant through space time. The only variable is what proportion of your travel through space time is travel through space and what proportion is travel through time. When you are stationary in space, all of your travel through space time is taken up with travel through time. When you are travelling through space at c, time is stopped all together. The only variable is where you lie between these two extremes.

 Quote by DaveC426913 Seriously dude, are you going to just copycat me? Can you explain what has to do with the topic at-hand?
Also seriously, that would be appropriate as your behavioral advice was really good. This was merely to illustrate that nit-picking on other contributors is not constructive - so I can but won't elaborate. Peace dude (yes that was a copycat).

 Quote by Ken Natton Well okay, perhaps these are just the ramblings of someone who doesn’t understand very well, but it has always seemed to me to be a problem that we call the phenomenon ‘the speed of light’. It has a tendency to make people think that light is the issue. It isn’t. It isn’t about the speed of the propagation of electromagnetic waves. Light travels at that speed but it isn’t the driver of the phenomenon. [..]
Yes that's an essential insight. The speed of light is a boundary condition from which the Lorentz transformations followed; light itself isn't an issue.

Harald

 Quote by harrylin It's more than semantics! Such questions relate to philosophy and physical models. Relativity was the natural outcome of the older physical models of mechanics and optics; however, the result was a theory that is based on principles (a "principle theory") which does not directly relate to physical models.
Philosophy is just semantics. Scientific method requires that we focus on the observable and testable, not something that "actually" happens according to your unsubstantiated belief system or philosophical system and that somehow cannot be measured in any way whatsoever or have any effect on the observable nature.

 Quote by harrylin The speed of light is a boundary condition from which the Lorentz transformations followed; light itself isn't an issue.
So then is it not important to be clear about what is being discussed? Are we discussing the speed of the propagation of electromagnetic waves, the possible variablility of which does not necessarily undermine the basic principles of relativity, or are we discussing the boundary condition, the speed that is the fastest it is possible to travel through space and at which time stops, whose invariabliity is central to the whole concept of relativity? Is that perhaps the source of the misunderstandings here?

 Quote by DaveC426913 Well, OK, you've linked to your own posts. Do you have any references that aren't you?
Let's get this out of the way by putting up the math, this is not the first time you are making innuendos I do know what I am talking about.

Let's take a pair of stationary test observers R2 and R3 in a Schwarzschild solution with a Schwarzschild radius of R.

Do you agree that the ruler distance between them is:

$$\rho = R \left( \sqrt {3}\sqrt {2}-\sqrt {2}+\ln \left( -\sqrt {3}+\sqrt {3} \sqrt {2}-\sqrt {2}+2 \right) \right)$$

If so do you agree that the radar distance T in coordinate time between them is:

$$T = R+R\ln \left( 2 \right)$$

And that the radar distance in proper time for R2 is:

$$\tau_{R2} = 1/2\, \left( R+R\ln \left( 2 \right) \right) \sqrt {2}$$

And for R3:

$$\tau_{R3} = 1/3\, \left( R+R\ln \left( 2 \right) \right) \sqrt {6}$$

With me so far or anything wrong with the math?

From this you can calculate the (average) speed of light, if you do this you will find that both the coordinate speed and the speed from r2 to r3 (r2 < r3) in proper time is always < c. Only the speed from r3 to r2 in proper time is > c.

Agreed? No? Where do I make a mistake?

For you reference if we value the R2 and R3 values we can chart it, here is a 2D plot.

Below is a plot of light speeds between pairs of static observers (o1, o2) separated a fixed ruler distance of 1 with the radar distance as measured by a clock at observer o1. In the plot you can see the ruler distance (which is 1 for each pair) divided by the radar distance, this ratio is larger for pairs closer to the EH.

And here is a 3D plot:

Any mistakes?

If not, could you please stop making innuendos I am wrong?

 Quote by Passionflower ...this is not the first time you are making innuendos I do know what I am talking about.... If not, could you please stop making innuendos I am wrong?
I am not making innuendos, I am simply asking for a reference to something other than your own work for the claim you are making. I get to do that.

 Quote by DaveC426913 I am not making innuendos, I am simply asking for a reference to something other than your own work for the claim you are making. I get to do that.
The 'claim' that I am making is obvious if you follow the math. It is very simple: it is true or false, either my math is right or it is wrong.

So you think titles and reputations are more authoritative than the mathematics? If so I pity you.

Recognitions:
Gold Member
Quote by DaveC426913
 Quote by Passionflower When we measure the speed of light between two points in curved spacetime then the speed will generally not be c.
Yah, I'm not really sure what leads you to say this.
This is a well-known fact of non-inertial frames (and there are no such things as inertial frames in curved spacetime, only local approximations to them).

See for example Physics FAQ: Is The Speed of Light Constant? - General Relativity subsection

To clarify, in curved spacetime (or even in flat spacetime in a non-inertial frame) when you measure the speed of some light passing right next to you, you always get c (using proper distance and proper time) but if you measure light that is some distance away from you, you will almost always get a different answer.

OK:
 Quote by Passionflower The 'claim' that I am making is obvious if you follow the math. It is very simple: it is true or false, either my math is right or it is wrong.
I do not have the math background to follow it. Why do you think I'm asking for a reference so I can read up on it?

 Quote by Passionflower So you think titles and reputations are more authoritative than the mathematics?
Where on Earth did that come from? Not from me.

No, I think that I like to verify something that goes against my understanding, by checking with multiple sources.

 Quote by Passionflower If so I pity you.
You don't have a middle zone do you? You're either normal or in Full Attack. Try being a little less defensive. And maybe a little less vicious.

I'm asking for references to your claim so I can understand it. I have not made the slightest suggestion that you are wrong. Go back and check.

 Quote by netheril96 Philosophy is just semantics. Scientific method requires that we focus on the observable and testable, not something that "actually" happens according to your unsubstantiated belief system or philosophical system and that somehow cannot be measured in any way whatsoever or have any effect on the observable nature.
I agree with the focus of modern science - thanks for enhancing my point. However, I doubt that any philosopher will agree with your claim about their profession.

Harald

 Quote by Ken Natton So then is it not important to be clear about what is being discussed? Are we discussing the speed of the propagation of electromagnetic waves, the possible variablility of which does not necessarily undermine the basic principles of relativity, or are we discussing the boundary condition, the speed that is the fastest it is possible to travel through space and at which time stops, whose invariabliity is central to the whole concept of relativity? Is that perhaps the source of the misunderstandings here?
I do think that the OP is talking about the speed of light rays. However, I see no consequence for this thread of understanding this thread to be about either light rays or the limit speed, as they are supposed to be equal and the validity of relativity is not questioned here. What answer do you think would change with the interpretation of the question?

 Quote by DrGreg [..] See for example Physics FAQ: Is The Speed of Light Constant? - General Relativity subsection To clarify, in curved spacetime (or even in flat spacetime in a non-inertial frame) when you measure the speed of some light passing right next to you, you always get c (using proper distance and proper time) but if you measure light that is some distance away from you, you will almost always get a different answer.
Yes it's very basic, it even was one of the first things that Einstein explained to the public about GR. Here's the reference once more: http://www.bartleby.com/173/22.html

Harald

PS: I now see that the writers of the FAQ are unfamiliar with the use of "velocity" in older English literature (and still some modern literature of other disciplines). It usually stands for "Geschwindigkeit" which commonly means speed; and here it is a translation of "Ausbreitungsgeschwindigkeit" which means propagation speed (that is, non-vectorial).
- http://www.ideayayinevi.com/metinler...rie/oggk03.htm
- http://de.wikipedia.org/wiki/Ausbrei...eschwindigkeit

And if you check his 1916 scientific paper on GR you will understand that his explanation only makes sense if it is understood to mean just that, from his use of the Huygens principle.

 Quote by harrylin I do think that the OP is talking about the speed of light rays. However, I see no consequence for this thread of understanding this thread to be about either light rays or the limit speed, as they are supposed to be equal and the validity of relativity is not questioned here. What answer do you think would change with the interpretation of the question?

Well okay, and perhaps my intervention has added nothing, I apologise if so. Clearly I was not successful in defusing the argument which is what I presumed to be doing. My perspective was just this – for someone who has a view of the reality in which they live that might be described as Newtonian mechanical – even though they themselves might not even know what that term means – it is a big struggle to understand how it can be possible for two different observers, one of whom is stationary and the other of whom is moving at some significant proportion of the speed of light, to both observe the same beam of light and measure its velocity to be the same. For such a person, grasping how it can be that all reference frames are relative and yet the speed of light is constant for all observers requires a fundamental shift in their understanding of the reality in which they live. Falling out over minute details about the speed of propagation of electromagnetic waves seems to me to be getting bogged down in a detail that is less than entirely essential.

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