# Is the speed of light always 299 792 458 m / s

by Bjarne
Tags: light, speed
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P: 17,330
 Quote by Bjarne Corrrect I was just trying to explain that seen from any overall perspective - "the distance" - "it self" - (between the Earth and the distant star) is not changing, - (regardless which overall observer that would measure it),
This is not correct. There is no such thing as the distance "itself". There is only the distance according to a specified reference frame/coordinate system. And that does change wrt different systems.

 Quote by Bjarne But still the point is what happens to 1 meter, - I guess the same thing. ... In the end of the day it comes down to that B would say to A, - I don't trust your ruler is exactly 1 meter. Prove it to me.
Again, the answer depends on the method of comparison. What experiment would B accept as a method of comparison?

I don't know why this concept is so hard for you to understand. I thought you understood it after our previous conversation on the same topic.
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P: 17,330
 Quote by nitsuj Does this point have any purpose? It seems to me to be of an opinion of interpretation, for example with russ' one measures distance as 80 million miles an other measures the same points as 90 million miles. That is two separate physical realities. I can even do the math 90-80 = 10 see they're not the same length. I appreciate the perspective that the interval is the same, and stands as a "true" measure of spacetime, in turn one physical reality. But i don't think that is your point with calling different measures of length time of the same physical reality. To reword my point of view, the Twins are different ages, at one point they experienced different physical realities...comparably.
No, they are not different realities, they are merely different perspectives on the same reality. The point is that there is no such thing as distance by itself. Only distance relative to some coordinate system. So the fact that distance relative to A is not equal to distance according to B does not imply multiple realities.

Even in non relativistic physics this happens with some quantities. Would you say Newtons universe implies a different reality for each Newtonian reference frame?
P: 1,098
 Quote by DaleSpam The point is that there is no such thing as distance by itself. Only distance relative to some coordinate system. Even in non relativistic physics this happens with some quantities. Would you say Newtons universe implies a different reality for each Newtonian reference frame?
I wouldn't, and that's a good point Dalespam regarding lengths. Now I can't say two separate physical realities
P: 344
 Quote by DaleSpam No, they are not different realities, they are merely different perspectives on the same reality. The point is that there is no such thing as distance by itself. Only distance relative to some coordinate system. So the fact that distance relative to A is not equal to distance according to B does not imply multiple realities. Even in non relativistic physics this happens with some quantities. Would you say Newtons universe implies a different reality for each Newtonian reference frame?
I both agree and disagree.
On the one hand we can easy end up to have unlimited "space-time realities", that could be a confused problem to deal with.
On the other hand, - if you say, “they are merely different perspectives on the same reality” – which “space-time reality” is the basic one then ? - that too could be confused to answer , right ?

 Quote by DaleSpam This is not correct. There is no such thing as the distance "itself".
Right, this is why I wrote "itself" and not itself. - implying there are several (overall) perspectives possible.

 There is only the distance according to a specified reference frame/coordinate system. And that does change wrt different systems.
Right.

 Again, the answer depends on the method of comparison. What experiment would B accept as a method of comparison?
I don't think any direct method exists. If so, - I would be happy to hear about it. Left are only one (or several) logical conclusion(s).

 I don't know why this concept is so hard for you to understand. I thought you understood it after our previous conversation on the same topic.
The discussion, whether there was comparable differences between a ruler in different space-time, - ended unsettle, - because how would it finally be possible to prove what is right or wrong.
Well I agree to that, but that shouldn’t prevent us from asking; - what is the most logical conclusion?.
So long we haven’t fully understood such important consequence of the theory of relativity, - we cannot say that anyone fully have understood the full range of that theory.
 P: 362 Light moves on geodesics with null magnitudes, that is all that needs to be said
P: 1,098
 Quote by Bjarne I both agree and disagree. On the one hand we can easy end up to have unlimited "space-time realities", that could be a confused problem to deal with. On the other hand, - if you say, “they are merely different perspectives on the same reality” – which “space-time reality” is the basic one then ? - that too could be confused to answer , right ?
I see where you coming from,

The interval is the same, that's invariant and is a "measure" of the two "different realities".

we can have different perspectives of that same interval, such as length. A different perspective doesn't necessitate a different reality.

such as orientation of a pencil in 3D, is it shorter if I look at it from a different angle than you? Is that then a separate reality.
P: 344
 Quote by nitsuj I see where you coming from, The interval is the same, that's invariant and is a "measure" of the two "different realities". we can have different perspectives of that same interval, such as length. A different perspective doesn't necessitate a different reality. such as orientation of a pencil in 3D, is it shorter if I look at it from a different angle than you? Is that then a separate reality.
We are suspended in language in such a way that we cannot say what is up and what is down. The word "reality" is also a word, a word which we must learn to use correctly.
Quote, - Niels Bohr
P: 1,098
 Quote by Bjarne We are suspended in language in such a way that we cannot say what is up and what is down. The word "reality" is also a word, a word which we must learn to use correctly. Quote, - Niels Bohr
.....
 Emeritus Sci Advisor P: 7,634 That's far from my favorite Bohr quote. My favorite would be "Never express yourself more clearly than you can think." Going back to the original question, the modern definition of the meter is, in fact, 1/299,792,458 of a second. With this modern defintion, the speed of light is no longer measured, it's defined to be a constant. Anyone who is using the modern defintion of the meter rigorously would talk about measuring the length of the prototype meter bar, rather than measuring the speed of light. As far as being "relative to local time dilation", there's nothing in the standard about that, and it seems slightly "off" to me as written. If instead one says "the meter is measured by local clocks" I would completely agree, and not have lingering reservations. Time dilation is something that enters only when you define a coordinate system. One can (and should) think of distance as something that's independent of coordinates, therefore it's in my opijnion not good to include a coordinate correction factor into one's notion of distance when the notion of distance is defined independently of coordinates.
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P: 17,330
 Quote by Bjarne I both agree and disagree. On the one hand we can easy end up to have unlimited "space-time realities", that could be a confused problem to deal with. On the other hand, - if you say, “they are merely different perspectives on the same reality” – which “space-time reality” is the basic one then ? - that too could be confused to answer , right ?
It's a good question, and there is no confusion at all to the answer. According to relativity they are all equally valid and none is singled out as more basic than the others.

 Quote by Bjarne I don't think any direct method exists. If so, - I would be happy to hear about it. Left are only one (or several) logical conclusion(s).
Several methods exist, but I dont know which you might characterize as direct.

 Quote by Bjarne The discussion, whether there was comparable differences between a ruler in different space-time, - ended unsettle, - because how would it finally be possible to prove what is right or wrong.
Easy, pitck a method for comparison and do the comparison.

 Quote by Bjarne Well I agree to that, but that shouldn’t prevent us from asking; - what is the most logical conclusion?. So long we haven’t fully understood such important consequence of the theory of relativity, - we cannot say that anyone fully have understood the full range of that theory.
At this point it isn't a flaw in the theory, just a failure by one individual, yourself, to clarify which of many length comparison methods he will use.
 P: 79 Have enjoyed reading this thread so far. I've mulled over similar considerations as Bjarne's. Especially liked pervect's quoting of Bohr: "Never express yourself more clearly than you can think." Hadn't heard that one before. I consider Bohr to be sort of the Yogi Berra of physics.
P: 344
 Quote by DaleSpam It's a good question, and there is no confusion at all to the answer. According to relativity they are all equally valid and none is singled out as more basic than the others.
Yes this is true…
But on the other hand, our reality begins (or end, - if you prefer) where the photons reality begin. From the perspective of the photon there can (mathematically) not exist distances, time, and I guess even not space, - it must be nothing.
To be extreme “reality” as a whole includes nothing and everything between nothing and everything, depending on space-time 'perspective'
It is at least attempting to talk about “different space-time realities” – because "they" can be so diffrent, - but is as you point out also not perfect to do so, - because "these" are not clear separated but rather “one” reality - ("seperated" by time, - at least photons, neutrinos etc. seems to be) and as you see now I contradict what I Just wrote. .
"Never express yourself more clearly than you can think."
Hmmmm.... Maybe Bohr had Einstein in his mind.

"We are suspended in language in such a way that we cannot say what is up and what is down. The word "reality" is also a word, a word which we must learn to use correctly. "
Hmmmmm

We can measure a photon existence, and the speed of it, - but seen from the photons own perspective, it doesn’t exist (in space and time) because by the speed c, space-time doesn’t exist.

What is the ultimate relality ?

Who is not confused ?

 At this point it isn't a flaw in the theory, just a failure by one individual, yourself, to clarify which of many length comparison methods he will use.
As I see it, I cannot see any direct experiment is possible, to reveal that.
As I wrote; only logical indirect conclusion.
How can A prove to B that his definition of one meter is "correct" (meaning universal).
The problem is A is still in top of the skyscraper and B in the cellar.
Where must the comparison take place?
I simply have no idea, and don’t think such experiments are done already.
Should I be wrong I would like to hear about it.
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P: 17,330
 Quote by Bjarne Yes this is true… But on the other hand, our reality begins (or end, - if you prefer) where the photons reality begin. From the perspective of the photon there can (mathematically) not exist distances, time, and I guess even not space, - it must be nothing. To be extreme “reality” as a whole includes nothing and everything between nothing and everything, depending on space-time 'perspective' It is at least attempting to talk about “different space-time realities” – because "they" can be so diffrent, - but is as you point out also not perfect to do so, - because "these" are not clear separated but rather “one” reality - ("seperated" by time, - at least photons, neutrinos etc. seems to be) and as you see now I contradict what I Just wrote. .
This is so self contradictory that it doesn't even rise to the level of philosophy, let alone science.

 Quote by Bjarne We can measure a photon existence, and the speed of it, - but seen from the photons own perspective, it doesn’t exist (in space and time) because by the speed c, space-time doesn’t exist.

 Quote by Bjarne As I see it, I cannot see any direct experiment is possible, to reveal that. As I wrote; only logical indirect conclusion. How can A prove to B that his definition of one meter is "correct" (meaning universal). The problem is A is still in top of the skyscraper and B in the cellar. Where must the comparison take place? I simply have no idea, and don’t think such experiments are done already. Should I be wrong I would like to hear about it.
Here I post, for reference, a relevant answer for comparing clocks from our previous conversation. With a small amount of thought you should be able to turn those into methods for comparing distances instead.

http://74.86.200.109/showpost.php?p=...&postcount=151

The result you get, as I have said many times, depends on the method of comparison, and GR can correctly predict the result for each method.
P: 344
 Quote by DaleSpam This is so self contradictory that it doesn't even rise to the level of philosophy, let alone science.
It was intended. There was no conclusion, - it should not make sense.

Which mean a mathematical "reality" contradict logical rational thinking, - and doesn’t make logical sense (?). I can only agree to that.

 Here I post, for reference, a relevant answer for comparing clocks from our previous conversation. With a small amount of thought you should be able to turn those into methods for comparing distances instead. http://www.physicsforums.com/showthread.php?t=511170......... (copy past)...>> Sure, I can see at least 4 ways to compare two distant clocks: A) Broadcast a reference signal, measure the frequency of the signal locally at each clock B) Take a reference clock, physically transport it from one clock to the other and measure the rate of the reference locally at each clock C) Agree on a standard physics experiment as a reference, perform it locally at each clock and measure the time for the experiment D) Agree on an astronomical reference and measure the time for the astronomical reference locally The beauty of GR is that it is a single law of physics which explains A, B, C, and D all together
I have read it long ago.
These experiments are already (more or less) done, and with reference to the context of this thread to my opinion only “indirect scientific evidence”.
I don’t think this is enough to finally determinate, whether the definition of 1 meter is universal or not? – What do you think?
 The result you get, as I have said many times, depends on the method of comparison, and GR can correctly predict the result for each method.
Right
But what about the question; is the definition of 1 meter (based on the current definition of 1 second) universal or not? – Or must the definition be “floating” and only true when it is be based on a relative local stretching or contracting “second” ? – What exactly do you think.
 P: 362 This thread is very lacking in any quantitative discussion, and most of it is talk of meta-physical ideas such as "reality" and what not.
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 Quote by Bjarne But what about the question; is the definition of 1 meter (based on the current definition of 1 second) universal or not? – Or must the definition be “floating” and only true when it is be based on a relative local stretching or contracting “second” ? – What exactly do you think.
I think every time you measure a meter it will be the same length. An objects physical dimensions may change from the point of view of your frame of reference due to relativistic factors, but you will never measure a meter as being different.
P: 344
 Quote by HomogenousCow This thread is very lacking in any quantitative discussion, and most of it is talk of meta-physical ideas such as "reality" and what not.
The right brain sees the forest, but not the (quantitative) trees (it is not important), - the left brain see the trees very detailed, but not the forest. The confused brain that try to see the forest cannot because there are too many (quantitative) trees.
P: 344
 Quote by Drakkith I think every time you measure a meter it will be the same length. An objects physical dimensions may change from the point of view of your frame of reference due to relativistic factors, but you will never measure a meter as being different.
Yes this is what seems to be at least very logical, (based on holistic thinking).

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