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Regards,

Nenad

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Tide

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for sound, this is wave propagation within air by minute compressions and rarefractions of the gas and we can tell if the medium, air, is moving past us or not. and if it

but how do we tell the difference between a moving vacuum and a stationary vacuum? if we can't, if there

how is it different? whether you are holding the flashlight or moving past it at high velocity, Maxwell's Eqs. say the same thing regarding the nature of E&M in the vacuum.

so then, there is no reason to expect a different observed speed of light for the different observers (the one on the rocket and the other who is "stationary").

does that get to your question or did i misunderstand it?

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Nenad, i think, at least pedagogically, that you have cause and effect mixed up.Nenad said:Its because when you begin to travel close to the speed of light, things begin to change: length contracts, mass increases and time dilates. These all contibute to you (the observer in the inertial reference frame) seeing the speed of light being constant no matter how fast you go.

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quasar987

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True, if you want to get technical.rbj said:Nenad, i think, at least pedagogically, that you have cause and effect mixed up.Becausethe speed of light (the very same beam of light) is the same for both observers that are moving relative to each other, this has the effect that both observers observe the other's clock to be ticking more slowly, both observe length contraction of the other, etc.

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Nenad

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quasar987 said:

I am very interested to know if one can derive the speed of light being constant from the statement that the physical laws take the same form in every inertial frame. So far, I have learned when formulating special theory of relativity, we need the both postulates. By the way, I was wondering if someone can elaborate what we mean by 'physical laws take the same form'??

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quasar987

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Nonsense was here.

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learningphysics

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'Physical laws take the same form' mean that if we perform an experiment in one frame... and perform exactly the same experiment in another frame (with the same equipment... everything is identical except which frame we are in) ... the results observed will be identical in both cases.HungryChemist said:I am very interested to know if one can derive the speed of light being constant from the statement that the physical laws take the same form in every inertial frame. So far, I have learned when formulating special theory of relativity, we need the both postulates. By the way, I was wondering if someone can elaborate what we mean by 'physical laws take the same form'??

I'd just say the physical laws are the

I like the way Rindler derives the constancy of the speed of light in "Introduction to Special Relativity"... he takes the relativity principle:

and this axiom:

And from these two axioms, he deduces:

(since we have one frame where the speed of light is constant at c... the principle of relativity dictates that in every frame the speed of light is constant at c... since all frames require equivalent outcomes of the same experiment)

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The simple, primary, natural law responsible for this characteristic of spacetime is the homogeneity of time.LedZep_Kamal said:I am sooo intrigued. The speed of light is independent to the speed of the observer. Why??????

http://www.everythingimportant.org/relativity/

http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=AJPIAS000043000005000434000001 [Broken]

http://arxiv.org/PS_cache/physics/pdf/0302/0302045.pdf [Broken]

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These approaches essentially discuss a "maximum signal speed" of a theory of relativity, which is infinite for the Galilean case but finite for the Einsteinian case. So, along these lines, the natural question to ask is why is the maximum signal speed finite, and why is that speed that of light? Certainly, one can appeal to experimental observation. However, I'm not sure if the original poster would be satisfied with that.Perspicacious said:The simple, primary, natural law responsible for this characteristic of spacetime is the homogeneity of time.

http://www.everythingimportant.org/relativity/

http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=AJPIAS000043000005000434000001 [Broken]

http://arxiv.org/PS_cache/physics/pdf/0302/0302045.pdf [Broken]

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None of those papers presuppose a "maximum signal speed." Relativity is a consequence of the homogeneity of time.robphy said:These approaches essentially discuss a "maximum signal speed" of a theory of relativity

You can ask the question but no one knows the answer.robphy said:the natural question to ask is why is the maximum signal speed finite

It's an irrelevant assumption. There are theories where light is thought of as not moving at the maximum possible speed.robphy said:and why is that speed that of light?

If the original poster is dissatisfied with the limited knowledge of physicists, then he might want to entertain a short theological response: God selected the spacetime structure constant to be what it is for no unavoidable special reason. The assigned present value of c is just as workable as an infinite number of other possibilities.robphy said:I'm not sure if the original poster would be satisfied with that.

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Noether's theorem is not required. A maximum possible speed is an easy consequence of the relativity postulate.quasar987 said:

There is no need for both postulates. The first postulate is sufficient and we can get by with an even weaker axiom. The answers to your questions are all contained in the first, second or third link that I offered.HungryChemist said:I am very interested to know if one can derive the speed of light being constant from the statement that the physical laws take the same form in every inertial frame. So far, I have learned when formulating special theory of relativity, we need the both postulates. By the way, I was wondering if someone can elaborate what we mean by 'physical laws take the same form'??

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I never said that such a thing was presupposed.Perspicacious said:None of those papers presuppose a "maximum signal speed." Relativity is a consequence of the homogeneity of time.robphy said:These approaches essentially discuss a "maximum signal speed" of a theory of relativity

As you say in your response to quasar987, "A maximum possible speed is an easy consequence of the relativity postulate."

As you are probably aware, as one attempts to boost an observer's 4-velocity, one finds an upper bound, which is infinite for the Galilean case and finite for the Einsteinian case. These corresponding speeds coincide with the eigenvectors of the Galilean and Lorentz boosts, respectively.

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If we are moving away from a star at .5c and are at a distance of 1lyr when it goes nova, what will the distance between us and the nova when the light reaches us?

.5c = 539,626,424.4kph

time to reach us = t = 1lyr

1 year = 8,765.81277075hr

.5c * 8,765.81277075 = 4,730,264,202,440km = distance added since light left source

4,730,264,202,440km + 9,460,528,404,880km = 14,190,792,607,300km = 1.5lyr

1.5lyr = .000000459892177752mpc

1.5lyr * hubble’s constant = .0000326523446204km/s

1yr = 31556925.9747s

.0000326523446204km/s * 21556925.9747s = 1,030.40762209km

1.5lyr + 1,030.40762209km = 14,190,792,608,330km

14,190,792,608,330km = 1.50000000011lyr

The nova would be 14,190,792,608,330km from us when the light reached us.

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I don't think that Hubble's constant belongs in the answer. Before the question can be answered, we need to know - is the ship 1 light year away from the star in the star's frame, or is the star 1 light year away from the ship in the ship's frame?dalamar96 said:Along these lines for both the question posted before and the fact that this is the first place I saw the speed of light discussed. I would like to post the following. Could anyone tell me if I am off on my thoughts here? Bear in mind that I did not keep the significant digits throughout (obviously)

If we are moving away from a star at .5c and are at a distance of 1lyr when it goes nova, what will the distance between us and the nova when the light reaches us?

A similar remark applies to the distance when the nova light reaches the ship, it wil depend on the frame used. Here, though, there is a reasonably good reason to prefer the star frame - because the explosion will appear to be symmetrical only in that frame. Thus if we want to calculate how much radiation the ship will receive, that's the logical frame to use.

I will now proceed to make the assumption that all distances are being measured in the star frame, because that's the frame in which the explosion will appear to be symmetrical. Then we can write the following equations

x_ship = 1 + .5*c*t

This gives the position of the ship at time 't' where time 't' is defined in the star's frame of reference.

x_light = c*t

This gives the position of the nova light at time 't'.

Setting them equal, we solve for the time t when x_ship = x_light and we get

1 + .5*c*t = c*t

Solving this, we find that t=2, and that x_ship = x_light = 2. Therfore the ship will be 2 light years away from the star when the nova light reaches it

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