# Is the speed of light constant or fixed

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#### Saiph

##### Guest
How about this siarad, In a vacuum we measure the speed of light to be a constant 3x10^8 m/s.<br /><br />No matter what we do, how we move, or how the source moves, etc...etc....etc...<br /><br /><br />Now, the only way to explain this is: The speed of light is <i>always</i> 3x10^8 m/s in a vaccum. <br /><br />As such, everyone will measure this (and does, demonstratably so). <br /><br />The laws of physics must account for this.<br /><br />The only way to measure a different speed for light in a vaccuum is 1) To be innaccurate or 2) The speed of light isn't constant, it varies...which violates observation. <div class="Discussion_UserSignature"> <p align="center"><font color="#c0c0c0"><br /></font></p><p align="center"><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">----</font></em></font><font color="#666699">SaiphMOD@gmail.com </font><font color="#999999"><em><font size="1">-------------------</font></em></font></p><p><font color="#999999"><em><font size="1">"This is my Timey Wimey Detector.  Goes "bing" when there's stuff.  It also fries eggs at 30 paces, wether you want it to or not actually.  I've learned to stay away from hens: It's not pretty when they blow" -- </font></em></font><font size="1" color="#999999">The Tenth Doctor, "Blink"</font></p> </div>

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##### Guest
<blockquote><font class="small">In reply to:</font><hr /><p>How about this siarad, In a vacuum we measure the speed of light to be a constant 3x10^8 m/s. <br /><br />No matter what we do, how we move, or how the source moves, etc...etc....etc... <p><hr /></p></p></blockquote><br />Sorry been away for 10 days.<br /><br />So to be both fixed & constant it means our measuring device is not constant.<br /><blockquote><font class="small">In reply to:</font><hr /><p>The only way to measure a different speed for light in a vaccuum is 1) To be innaccurate or 2) The speed of light isn't constant, it varies...which violates observation. <p><hr /></p></p></blockquote><br />Observation based upon measurement with a varying device, how is that accurate.<br />I've asked several times has an external time source been use to make the measurement, such as a pulsar.<br />

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##### Guest
That's why I've asked about using a pulsar, as time is <i>defined</i> & varies with speed, it's not absolute.

S

##### Guest
No, I see a confusion of words. I don't mean the fixed speed as defined by International units (amazing that) but as in my topic.<br />Topics always run away after about six replies.<br />I thought my two topics, this & coloured photons would sort a problem out but I see failure. <br />Incidentally the Caesium time clock varies with acceleration & distance travelled & wow, time stops at absolute zero!

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#### rpmath

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Tigerbiten: <font color="yellow"><br />Since 1983 it has been "The meter is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second."<br /></font>#8230; <font color="yellow"><br />The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom. <br /></font><br />After this, light speed is fixed (defined to be 299 792 458), and second and meter are measured.<br /><br />If you find some place where light takes more time to cross it than it takes to surround it, you wont get a slower value of c, you will get a bigger value for traveled distance, and say “the space is expanded”, instead of “light is slowed”, I think this is the case in gravity fields.<br /><br />The problem is how to define a good set of clock and measuring rod to measure c.<br />Everything we use is affected by light speed, even the school ruler or the Michelson-Morley interferometer: the size of visible things is set by the interaction between the atoms electrons and the photons of the Electro Magnetic field. The shape of things is affected by their speed in the same way light is affected so we cannot see speed effects.<br /><br />We can base a model in the axiom that space is Euclidean instead of light speed been constant. In this model, gravity reduces light speed, and clocks, rods and light speed are affected by gravity and speed, but there is still no way to know which clock and rods are right, so you will get measures of space, time and light speed relatives to which set of clock and rod you choose…

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##### Guest
You jumped on the easy bit now try the hard bit

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#### Saiph

##### Guest
Siarad, Sorry I've been away for weeks.<br /><br /><br />Now, the time device used is in the frame of reference you're measuring it in. That cannot be helped. If you use an "outside" source, like a pulsar you pass by, it will appear to be different as well.<br /><br />Measurements have been made from a multitude of reference frames, and simultaneously. In the moving frame, in a "stationary" lab frame, in a seperate moving frame. you name it.<br /><br />They all get the same answer. No matter where the clock is, who's got it, who's reading it, etc, etc, etc. <br /><br /><br />Also, the mere fact that the time sources are changing, are different, and no longer agree, I.e. that time varies, is a major discovery.<br /><br /><br />Throw in the facts that many many secondary effects can be explained, calculated, and predicted using relativity (and thus the constant speed of light) also supports the idea of a constant speed of light.<br /><br />If we only said it was constant, but it wasn't , we'd eventually run into inaccuracies. GR and SR are as accurate as we can measure them to date (which is pretty good!) <div class="Discussion_UserSignature"> <p align="center"><font color="#c0c0c0"><br /></font></p><p align="center"><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">----</font></em></font><font color="#666699">SaiphMOD@gmail.com </font><font color="#999999"><em><font size="1">-------------------</font></em></font></p><p><font color="#999999"><em><font size="1">"This is my Timey Wimey Detector.  Goes "bing" when there's stuff.  It also fries eggs at 30 paces, wether you want it to or not actually.  I've learned to stay away from hens: It's not pretty when they blow" -- </font></em></font><font size="1" color="#999999">The Tenth Doctor, "Blink"</font></p> </div>

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#### rpmath

##### Guest
The point with light speed measuring is:<br /><br />To calculate speed you need to measure space and time (v = d / t), to do so, you need a calibrated clock and a calibrated measuring rod.<br /><br />The problem is that our reference units include a fixed value of c in their definition. Really you only can measure time and use light speed to get distance (d = c t). You will always get light speed to be c: <br />v = d / t = /> v = (c t) / t => v = c.<br />How can you compare such measures with whatever else?<br /><br />Think about an indirect measurement:<br />Put 3 observers A, B, C forming an equilateral triangle.<br />You can know they form such a triangle if the time light needs to go from each point to a neighbor one and back is the same (from A-B-A, B-C-B, C-A-C). <br /><br />Now put an observer D in the middle of the segment AB.<br /><br />If light speed is constant, the time light takes to go D-C-D must be exactly (square root 3) / 2 times what it takes to go between other pair like A-B-A.<br /><br />There are such cases…<br />imagine a black hole near the center of the triangle…<br />the light between C and D will take more time that the expected.<br /><br />The standard answer to why the test fails some times is: "Space is expanded"<br />And you will always be able to use that just because that’s what your measuring rod will say.<br /><br />Is space really expanded or is light slowed?<br />There is no way to know which is the real answer.<br />This depends on the way you use to define space and time measurement.

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#### Saiph

##### Guest
Our definition of a meter is defined using the speed of light, however that does not mean the length of a rod is dependant upon that speed.<br /><br />If I take a rod, of whatever length, say the length of my forearm, and use that as a distance measurement, we have removed this depedance on the speed of light.<br /><br />We can then measure the speed of light across a distance, without refering to itself again (like you said).<br /><br />And we can tell if space is warping or light is slowing. If space is warping our physical rod will no longer resemble it's initial state. It will appear as a different length to outside observers (who do indirect measurement of length via angle observations and some simple geometry).<br /><br /> <div class="Discussion_UserSignature"> <p align="center"><font color="#c0c0c0"><br /></font></p><p align="center"><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">----</font></em></font><font color="#666699">SaiphMOD@gmail.com </font><font color="#999999"><em><font size="1">-------------------</font></em></font></p><p><font color="#999999"><em><font size="1">"This is my Timey Wimey Detector.  Goes "bing" when there's stuff.  It also fries eggs at 30 paces, wether you want it to or not actually.  I've learned to stay away from hens: It's not pretty when they blow" -- </font></em></font><font size="1" color="#999999">The Tenth Doctor, "Blink"</font></p> </div>

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#### newtonian

##### Guest
Saiph - Hi!<br /><br />Three quick questions:<br /><br />1. The speed of light is per second. What if time slows down - relativity- from our reference point. Would light then slow down proportionatley since the seconds in c are running more slowly?<br /><br />2. The speed of light is constant in a vacuum. What of the more dense state of our early universe? Did that higher density medium slow light measurably?<br /><br />3. In inflation theory is true, does this mean some of our universe has escaped the light of most or all of our universe. Clearly it could not escape its own light.

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#### Saiph

##### Guest
1) yeah, it would....but relativity also says distances change, thus keeping C, well, C.<br /><br />2) Sure, light moves slower when it's in a medium. <br /><br />3) Inflation says so, yes. Inflation may be caused by the speed of light being different then (that's a new speculative theory...needs confirmation and criticism) <div class="Discussion_UserSignature"> <p align="center"><font color="#c0c0c0"><br /></font></p><p align="center"><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">----</font></em></font><font color="#666699">SaiphMOD@gmail.com </font><font color="#999999"><em><font size="1">-------------------</font></em></font></p><p><font color="#999999"><em><font size="1">"This is my Timey Wimey Detector.  Goes "bing" when there's stuff.  It also fries eggs at 30 paces, wether you want it to or not actually.  I've learned to stay away from hens: It's not pretty when they blow" -- </font></em></font><font size="1" color="#999999">The Tenth Doctor, "Blink"</font></p> </div>

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#### newtonian

##### Guest
Saiph - Thank you.<br /><br />Yes, I remember various hypothesis indicating light speed may have been different.

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#### rpmath

##### Guest
<font color="yellow"><br />2. The speed of light is constant in a vacuum. What of the more dense state of our early universe? Did that higher density medium slow light measurably?<br /></font><br />This question looks interesting…<br />If the light was moving slower at the start of the universe, it would take more time than what is expected due to current light speed. Early objects would look far than they really are, and expansion may look accelerating when it really isn’t … <br />May be we don’t need dark energy after all…

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#### Saiph

##### Guest
:::Shakes head:::<br /><br />The speed of light may have been slower due to interfering media, however the speed of light in a vacuum could still be our familiar constant (even if the early universe didn't have a vacuum).<br /><br />Now, if the speed of light in a vacuum was far higher then, than it is today, it may be the reason for the inflationary period in cosmology. And things would need to be looked at differently. It may have been slower, in which case things would need to be looked at differently. But just cause light in a medium is slower, doesn't meen much.<br /><br />Things can exceed the speed of light in a medium. I.e. a particle can pass through a medium (like water) faster than a photon can. The result is cherenkov radiation. <div class="Discussion_UserSignature"> <p align="center"><font color="#c0c0c0"><br /></font></p><p align="center"><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">----</font></em></font><font color="#666699">SaiphMOD@gmail.com </font><font color="#999999"><em><font size="1">-------------------</font></em></font></p><p><font color="#999999"><em><font size="1">"This is my Timey Wimey Detector.  Goes "bing" when there's stuff.  It also fries eggs at 30 paces, wether you want it to or not actually.  I've learned to stay away from hens: It's not pretty when they blow" -- </font></em></font><font size="1" color="#999999">The Tenth Doctor, "Blink"</font></p> </div>

S

##### Guest
<blockquote><font class="small">In reply to:</font><hr /><p>Can you tell us how a second, as measured by an atomic clock, has light speed, cee, built into it? <p><hr /></p></p></blockquote><br />The BBC sent an atomic clock to Hong Kong & back which lost time in transit. I assumed light speed was built in as e/m = C^2. By what mechanism did it lose time?<br /><blockquote><font class="small">In reply to:</font><hr /><p>Scientific logic (verbal logic) grounds the measures in real, existing event, rather independent of such assumptions. That's the point and why a 'fixed speed of light' has no real meaning. It's not independent of distance and time.<p><hr /></p></p></blockquote><br />This is why I started the topic. Astronomers seem to assume c is 'fixed' in measuring distances. Surely if it was 'constant' these distances would be immeasurable.

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##### Guest
Thanks, you too show C involved.<br />Anyone know how to write decent formula here?

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#### Saiph

##### Guest
an additional note on the michelson moreley experiment, that determined the speed of light to be constant.<br /><br />It doesn't require any known distances. The setup of the device uses interferance. Without any special calibration, you get an interference pattern where the two split beams are recombined. What pattern you get depends upon the relative differences in the distance each seperate beam takes.<br /><br />Now, what that pattern is, doesn't matter. Because we're interested in any changes in C.<br /><br />So as the device is moved in different directions we look for a shift in the interference pattern.<br /><br />1) The device isn't altered. So any change in the pattern will be due to the devices' motion.<br /><br />2) If we see a shift, we know the speed of light isn't constant, that it depends upon the relative velocities between the source, and the detector.<br /><br />3) If there is no shift, the speed of light is independent of the relative velocities, and thus is constant.<br /><br /><br />We see #3. <div class="Discussion_UserSignature"> <p align="center"><font color="#c0c0c0"><br /></font></p><p align="center"><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">----</font></em></font><font color="#666699">SaiphMOD@gmail.com </font><font color="#999999"><em><font size="1">-------------------</font></em></font></p><p><font color="#999999"><em><font size="1">"This is my Timey Wimey Detector.  Goes "bing" when there's stuff.  It also fries eggs at 30 paces, wether you want it to or not actually.  I've learned to stay away from hens: It's not pretty when they blow" -- </font></em></font><font size="1" color="#999999">The Tenth Doctor, "Blink"</font></p> </div>

S

##### Guest
<blockquote><font class="small">In reply to:</font><hr /><p>Uh, that's the equation for the quantitative relationship between energy and mass. It has NO relevancy to the speed of light measurement<p><hr /></p></p></blockquote><br />e/m = C^2 has no relevancy to the speed of light? it seems it is on one side of the equation. Isn't the time change due to increased inertial mass? <br />I thought I'd carefully & deliberately described my meaning of fixed in the topic.<br />

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#### rpmath

##### Guest
Light speed in vacuum was measured up to 1983...<br />but since that time it is defined to be 299 792 458 m/s.<br /><br />If you get other value your meter rod is wrong calibrated.

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#### rpmath

##### Guest
<font color="yellow">Can you tell us how a second, as measured by an atomic clock, has light speed, cee, built into it?<br /></font><br />Well… the second hasn’t it embedded, but the meter has.<br /><br />From other point of view, the exchange of photons among electrons and atom nuclei is affected by c.<br /><br />If you are moving you atomic clock, the photon exchange will be slowed.<br />If it needs at rest a time t<sub>0</sub> = r / c<br />When you are moving at a speed v, the photon must travel a distance<br />d = squareroot(r <sup>2</sup> + (v t) <sup>2</sup> )<br />then:<br />c t = squareroot(r <sup>2</sup> + (v t) <sup>2</sup> ) = squareroot( (c t<sub>0</sub>)<sup>2</sup> + (v t) <sup>2</sup> )<br /><br />where you get:<br />t = t<sub>0</sub> / squareroot(1 – (v/c) <sup>2</sup>)<br /><br />This is the same equation used in relativity to define time dilation…<br /><br />There is a little flaw with this way of see it:<br />How it works the same way in the other direction?<br />... but it can be explained including the space measurement equations.

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#### rpmath

##### Guest
Saiph<font color="yellow"><br />1) The device isn't altered. So any change in the pattern will be due to the devices' motion. <br /></font><br />There is an alternate explanation:<br />The device is affected by motion in the same way light is, so it cannot show any effect and we don’t see any change in the pattern.<br /><br />May be this is because the shape of know objects is due to the electromagnetic field between their nuclei and electrons, and this field is carried by photons like light is…

S

##### Guest
Aha! this is the very reason I started the topic.<br />I didn't state it such as it would have come to a standstill & I needed extra information to progress on another problem. You've shown the problem between fixed & constant, the only measurable, value of C. <br />Replies have caused me to start another topic on Relativity & Reality. <br />I sneakily posted above that time stops at absolute zero & was called over the coals but in fact I'm right as the electronic counting used in such time clocks can't work at Absolute zero. I was trying to point out measurement doesn't necessarily prove reality & lack of a measurable result doesn't prove non-existence.

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#### rpmath

##### Guest
<font color="yellow">I sneakily posted above that time stops at absolute zero & was called over the coals but in fact I'm right as the electronic counting used in such time clocks can't work at Absolute zero.<br /></font><br />The electron movement around the nucleus doesn’t stop at absolute zero. The jump between two quantum states don’t stops at absolute zero. The atomic clock still works… just the thermal movement is stopped.

S

##### Guest
Try putting your laptop in a freezer it may change your mind <img src="/images/icons/laugh.gif" />

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