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Do we know why is the speed of light 3*10^8 m/s? What determines this value? (I'm not asking about system of units)

S.

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Do we know why is the speed of light 3*10^8 m/s? What determines this value? (I'm not asking about system of units)

S.

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Same goes for your second question. Some have theorized that the speed of light might have changed over the life of the universe, but there's been no evidence of this.

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The value of c can be derived by postulating Maxwell's equations. The derivation shows that c = 1/sqrt(eu) where e = permitivity of free space and u = permiability of free space.

Do we know why is the speed of light 3*10^8 m/s? What determines this value? (I'm not asking about system of units)

S.

That is a theorem (something derived) and not a postulate.Anything that has momentum without mass is seen to travel at 'c' by any observer. This was a postulate of special relativity, and has many times been experimentally verified.

Pete

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My interpretation of the OP's original question is that he was asking about why c is invariant and also how the value of c is obtained. c is obtained using Maxwell's equations. The invariance of c is a postulate and as such is not derived. However we can postulate the Principle of Relativity and Maxwell's equations and then derive the invariance of c. So it really depends on what you're starting with.

Please provide the derivation of which you speak.peter0302 said:The fact that v=c when m=0 can be derived from E=pc does not make it a theorem because it already assumes that which it is trying to prove.

A theorem is, by definition, that which is derived from other theorems, from postulates/laws/axioms or from both. As far as E = pc then this too is something derived and not postulated. This relation can be derived from Maxwell's equatons. As such it too is a theorem. If you hold that v = 0 given m = 0 and E = pc then please proof a proof since I'm a bit uncertain as to what you're assuming as given (i.e. what are you assuming is true in that derivation?). How exactly are you defining mass m anyway?

Definiton of

If you disagree then please define the term "theorem" as you understand it and, if possible, please provide a source where you learned such a definition. Thank you.

Pete

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here is my 2 cents :

1. constancy of speed of light wrt all observer is a postulate of relativity.

2. Maxwells eq gives a theoritical proof of what the value should be, but doesn't imply that it is contant for all frames.

3. even if the value of C is changing, it is same for everybody/frame at a given pint in time.

4. to understand why it is same for all frames, if you have to deal with higher dimension. Light is actually travelling perpendicular to 4D, it is travelling in 5th dimension, but trapped by the 4D universe so that it just grazes the surface of 4D. Anything travelling in 5th dimension will not be seen as travelling with varying speed to any observer in 4D. Don't compare with X/Y axis here. XY axis is straight, where as 5th dimension is curved. Efeectively what is happening is this

you tarvell at a speed in Y axis.

observer is in X axis, now assume Y axis is curved although always pernedicualr to X. the component of its velocity projected on X axis will remain identical irrerespective of the movement along X axis. in eucledian X-Y axis this component is zero. in our higher dimension, where things to bend, the component is = C.

Any object without mass, can't exist in a gravitational field, it must escape the field = escape the universe but since the higher dimensions are al curved, they graze the surface of 4D universe.

Read kaluza-Kein theory of 5 dimension you will get the hint. If you want to know further you have to get into string theory or some such ideas.

In fact the equations of SR can be derived from this - light is travelling perpednicular to 4D. I can't draw images here. poor at editing.

1. constancy of speed of light wrt all observer is a postulate of relativity.

2. Maxwells eq gives a theoritical proof of what the value should be, but doesn't imply that it is contant for all frames.

3. even if the value of C is changing, it is same for everybody/frame at a given pint in time.

4. to understand why it is same for all frames, if you have to deal with higher dimension. Light is actually travelling perpendicular to 4D, it is travelling in 5th dimension, but trapped by the 4D universe so that it just grazes the surface of 4D. Anything travelling in 5th dimension will not be seen as travelling with varying speed to any observer in 4D. Don't compare with X/Y axis here. XY axis is straight, where as 5th dimension is curved. Efeectively what is happening is this

you tarvell at a speed in Y axis.

observer is in X axis, now assume Y axis is curved although always pernedicualr to X. the component of its velocity projected on X axis will remain identical irrerespective of the movement along X axis. in eucledian X-Y axis this component is zero. in our higher dimension, where things to bend, the component is = C.

Any object without mass, can't exist in a gravitational field, it must escape the field = escape the universe but since the higher dimensions are al curved, they graze the surface of 4D universe.

Read kaluza-Kein theory of 5 dimension you will get the hint. If you want to know further you have to get into string theory or some such ideas.

In fact the equations of SR can be derived from this - light is travelling perpednicular to 4D. I can't draw images here. poor at editing.

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I don't see where you get that idea from. Photons certainly exist in gravitational fields. In fact if they didn't then I'd be unalbe to read your posts. As far as remaining in a gravitational field its a well accepted notion that a photon can orbit a massive spherically symmetric body. However ther orbit is unstable.Any object without mass, can't exist in a gravitational field, it must escape the field = escape the universe but since the higher dimensions are al curved, they graze the surface of 4D universe.

Nah. Unless you learn it at a layman's level then learing string theory requires an education in quantum field theory. And its a a minority of physicists know QFT. A great deal of people who post here, such as myself, have never learned string theory at a detailed level. Namely because QFT is very hard to learn. Its said to be extremely hard, if not impossible, to learn through self teaching.Read kaluza-Kein theory of 5 dimension you will get the hint. If you want to know further you have to get into string theory or some such ideas.

I'd have to see such a derivation before I'd believe it. Its meangless in SR to speak of light traveling perpendicular to 4d spacetime because relativity only uses 4 dimensions and having something perpendicular to spacetime requires another dimension. One then has to specify the meaning of that dimension before one can discuss it too.In fact the equations of SR can be derived from this - light is travelling perpednicular to 4D. I can't draw images here. poor at editing.

Pete

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I quess your first question has 'independent of the relative speed and direction of the observer' added on the end.Why is the speed of light the same to any observer

And the 2nd question:

Do we know why is the speed of light 3*10^8 m/s? What determines this value? (I'm not asking about system of units)

S.

I think that it is to do with light behaving like a wave. The frequency and/or the wavelength changes (red shift/blue shift) but the relationship between these two components stays the same, hence c stays the same.

As for the second question, I would imagine that the answer would be similar to why sound has a specific speed through water or any other medium. The only difference is that we are dealing with the natural frequency of something which has resonant inertia but is not matter.

Nick

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HallsofIvy

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Yes, the constancy of the speed of light is a "postulate" of relativity- and it was chosen as one because that was what experimental evidence showed.

As for

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I don't follow. Why do you say that we can postulate whatever we want to in mathematics? This is certainly news to me. In math the basic postulates are things like Peno's postulates, the distributive laws, the associative laws etc. But we don't make up postulates. This follows from the definition ofI am a bit uncomforable talking about "postulates" for physics as opposed to mathematics. In mathematics we can "make up" whatever systems we want with whatever postulates we want- in physics we are constrained by reality!

postulate: to assume or claim as true, existent, or necessary : depend upon or start from the postulate of to assume as a postulate or axiom (as in logic or mathematics)

Pete

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This is a very good question. Turns out that light speed is

Do we know why is the speed of light 3*10^8 m/s? What determines this value? (I'm not asking about system of units)

S.

The

Now, as to your thread title:

The answer has been given by others already: it is a

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Those are not the basic posulates of math. Math has no basic postulates. Those postulates, or axioms are a set of axioms for the natural numbers. That's all they cover. There are many problems domains that go beyond natural numbers such as quantum mechanics and string theory, where the concept of addition or commutativity are not applicable.I don't follow. Why do you say that we can postulate whatever we want to in mathematics? This is certainly news to me. In math the basic postulates are things like Peno's postulates, the distributive laws, the associative laws etc. But we don't make up postulates. This follows from the definition ofpostulate(as used in this context)

postulate: to assume or claim as true, existent, or necessary : depend upon or start from the postulate of to assume as a postulate or axiom (as in logic or mathematics)

Pete

So using math you can develop axioms for all sorts of things, it doesn't matter, and maybe some applied mathematicians such as physicists will use them and the derived theorems to help solve their problems. But math itself certainly shouldn't concern it's self with our own experiences as HallsofIvy correctly stated.

So in math make up whatever axiom you want and perhaps the theorems derived will cover some part of reality we have yet to know. Betrand Russel has a famous quote:

"Thus mathematics may be defined as the subject in which we never

know what we are talking about, nor whether what we are saying is

true." -- Bertrand Russell

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it appears that youDo we know why is the speed of light 3*10^8 m/s? What determines this value? (I'm not asking about system of units)

the only fundamental physical fact imposed upon us by reality is that

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a consequence of an even more fundamental postulate that the laws of nature are the same for all inertial observers.here is my 2 cents :

1. constancy of speed of light wrt all observer is a postulate of relativity.

[tex] c = \sqrt{\frac{1}{\epsilon_0 \mu_0}} [/tex]2. Maxwells eq gives a theoritical proof of what the value should be, but doesn't imply that it is contant for all frames.

if3. even if the value of C is changing, it is same for everybody/frame at a given pint in time.

of course, one would ask, if we defined the speed of light to be however fast my Toyota goes flat out on the highway, would that make it so? it's actually theThis is a very good question. Turns out that light speed isset by defintion.

and that was when the meter was defined as the distance between two little scratch marks on a bar of platinum-iridium (the "prototype meter" ) in the BIPM in France.Thevalueit is set to (299,792,458m/s) was obtained averaging the results of the most recent and most precise experiments.

this postulate is a little bit explainable. it really is dependent on the more fundamental postulate that the llaws of nature are the same, both qualitativelyNow, as to your thread title:"Why is the speed of light the same to any observer?"

The answer has been given by others already: it is apostulatederived from multiple experimental observations. Postulates are not explainable.

i tried to beat this horse to death in this thread.

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:-)of course, one would ask, if we defined the speed of light to be however fast my Toyota goes flat out on the highway,

Yes, back in 1983.and that was when the meter was defined as the distance between two little scratch marks on a bar of platinum-iridium (the "prototype meter" ) in the BIPM in France.

This is an interesting one, I have seen occasional claims of :this postulate is a little bit explainable. it really is dependent on the more fundamental postulate that the llaws of nature are the same, both qualitativelyandquantitatively, for every inertial observer, even those that are moving (at constant speeds) relative to each other. it is a consequence of the very reasonable postulate that all observers that are not accelerated haveequalclaim to being stationary. if two inertial observers, moving relative to each other, have equal claim to being stationary (and "it's the other guy who is moving, not me") then the laws of physics, including the quantitative expression of them (two such quantities are [itex]\epsilon_0[/itex] and [itex]\mu_0[/itex] which determinec),mustbe identical to both observers.

-dependency of the second postulate on the first one

(re)constructing SR based only one the first postulate

but I could never find the respective papers/books. Do you know if the above claims are provable?

OK, I am now going to look your horse in the mouth :-)i tried to beat this horse to death in this thread.

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If these inertial observers don't have equal claim to being stationary does Maxwell's formula become untenable?two inertial observers, moving relative to each other, have equal claim to being stationary

The reason I question it is that there is speculation about reference frames and the distortion of these near areas of high gravity. The existence of a reference frame at all may suggest that there is a theoretical 'at rest'

Nick

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no, i don't think so. but if these inertial observers have unequal degrees of absolute velocity, and their absolute velocity is determined relative to some absolutely stationary frame of reference (that really doesn't exist, but we would call itIf these inertial observers don't have equal claim to being stationary does Maxwell's formula become untenable?

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actually, earlier than that. 1959, more like.Yes, back in 1983.rbj said:and that was when the meter was defined as the distance between two little scratch marks on a bar of platinum-iridium (the "prototype meter" ) in the BIPM in France.

depends on what you mean by "the laws of physics". if parameters that appear in the laws of physics are, themselves, part of the laws of physics, then it is an obvious logical construction to conclude that the parameters of the laws of physics (namely [itex]\epsilon_0[/itex], [itex]\mu_0[/itex], andThis is an interesting one, I have seen occasional claims of :

-dependency of the second postulate on the first one

(re)constructing SR based only one the first postulate

but I could never find the respective papers/books. Do you know if the above claims are provable?

some people might mean that the structure of the laws of physics remaining constant do not mean that the parameters inside them must remain constant, but that is not what i mean when i say "the laws of physics remain constant" for various inertial observers. is that what Einstein meant? i think so, but someone else might disagree. but it doesn't matter because Einstein closed the door on this but explicitly stating that the laws of physics and, at least the parameter we call

one reason i think that it is semantically silly (and logically silly) to say that the parameters inside the laws of physics aren't part and parcel to the laws of physics and do not share the same degree of invariancy as the laws of physics is that, for any particular law, let's say Newton's 2

[tex] F = k \frac{dp}{dt} [/tex]

if

it is no different of an issue regarding the appearance of

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The answer is in 'velocity measurement' http://wizdum.awardspace.info" [Broken]

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DaveC426913

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No, I think he really is asking why the speed of light is the speed that it is, independent of whatever unit you measure it in.it appears that youareasking about units.

the only fundamental physical fact imposed upon us by reality is thatcis finite. it doesn't matter what the finite value is, and we may as well call it 1.

Yes, you can measure the speed of light in light-seconds per second, in which case its speed is 1ls/s, or in furlongs per heartbeat. But I do think he's asking why

I would speculate wildly that the speed was soon after the creation of the universe and is somehow related to the vacuum energy or the mass of the Higgs Boson. Or the amount of plasma vented from the warp nacelles.

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but Dave, respectfully, then either he is asking what ultimately is a meaningless question (why is lightspeed 1 light-second per second?) or heNo, I think he really is asking why the speed of light is the speed that it is, independent of whatever unit you measure it in.

Yes, you can measure the speed of light in light-seconds per second, in which case its speed is 1ls/s, or in furlongs per heartbeat. But I do think he's asking whythatspeed.

i'm with Michael Duff, John Barrow, Gabriele Veniziano, and John Baez about this. the speed of propagation of the fundamental interactions of nature is finite, real, and positive (it is necessary physics to come to that conlusion) but the actual value of that finite value is not operationally meaningful. it just defines the scaling, the tick marks, that the rest of reality conforms to. if the speed of light somehow suddenly doubled (from the POV of some God or "Q" or some omnipotent being who does not submit to the laws of nature), we mortals wouldn't know the difference. not unless some dimensionless parameter (like the fine-structure constant) changed, andI would speculate wildly that the speed was soon after the creation of the universe and is somehow related to the vacuum energy or the mass of the Higgs Boson. Or the amount of plasma vented from the warp nacelles.

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DaveC426913

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Why is this a meaningless question?but Dave, respectfully, then either he is asking what ultimately is a meaningless question (why is lightspeed 1 light-second per second

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NateTG

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It is generally agreed (and experimentally verified) that the speed of light is the same in all inertial frames of reference. The prediction that the speed of light is constant comes out of Maxwell's equations as others have mentioned. This struck people as odd, and a huge number of experiments were run allmost all of which support that the speed of light is constant. The most famous of which is the Michelson-Morely experiment.And the 2nd question:

Do we know why is the speed of light 3*10^8 m/s? What determines this value? (I'm not asking about system of units)

http://en.wikipedia.org/wiki/Michelson-Morley_experiment

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Any ideas?

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Any value attached to c is purely due to our choice of units and by a certain choice of units it could be 1 or 99 or whatever. But if we ignore units, to ask why it is whatever it is is a valid question.

As far as i have been taught Maxwell's equations require it to be a ratio of certain physical properties of space.

Matheinste.

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