Question about the speed of light

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L

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Hey guys,<br /><br />Current tought is that the speed of light is a constant relative to the motion of any observer in any frame of reference. <br /><br />i am confused by how this theory has come to be. logically the speed of light should be dependant on the speed of its source. <br /><br />how can we say that c is always 299 792 458 m/s or thereabouts when we have no absolute frame of reference. <br /><br />I'm confused as to how this figure was calculated absolutely when any measuring frame of reference is moving.<br /><br />Any help to clear this up in my head would be much appreciated.<br /><br />cheers

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kmarinas86

S

Guest
C is the square root of the reciprocal of permittivity & permeability of free space, thus is fixed. Can't post formulae here.<br />The <i><b>measurement</b></i> of C depends upon the frame & is always the same.<br />Therefore it's not possible to <i>determine</i> any travel w.r.t. light.<br />I'm no scientist but it's how I view it

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newtonian

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lopadotem - Some have questioned whether the speed of light has been constant throughout the entire history of the universe.<br /><br />For this post I will assume c has been constant [albeit I actually assume nothing].<br /><br />C would not change in different reference frames. However, light cones are dependent on reference frames. And therefore so are visibility horizons.<br /><br />C is also dependent on time.<br /><br />Variant movemenet of reference frames which are in each other's light cone will not effect the speed of light but will effect the wavelength: blue shifted if motion is approaching; red shifted if motion is receeding.<br /><br />Note that faster than light (FTL) can exist - notably inflation theories and projected FTL future (or even present) expansion; and tachyons.<br /><br />Can dark energy propagate FTL?

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Saiph

Guest
logically, using the framework for physics newton layed out...yes, the speed of light should be dependent on the speed of the source in the case of a particle, or of the medium in the case of a wave.<br /><br />However...that is <i>not</i> what observation tells us. Look up the "Michelson & Morely" experiment, you may need to use keywordes "ether/aether" and "interferometer" to get more specific results.<br /><br />So, logic goes out the window...sorta. Rather we've got to construct a new system that incorporates this observation.<br /><br />The figure is calculated absolutely using Maxwell's equations for electromagnetism. Electric fields induce magnetic fields, and vice versa, which causes them to push and pull on eachother, creating the propagating wave of electromagnetism we call light.<br /><br />This pushing and pulling however, can only occur at a certain rate. When you work the equations out to determine how fast these things can push and pull, you get an answer that is only two fundamental constants of nature (permitivity and permeability of free space, epsilon and mu). There is <i>no</i> term for the speed of a medium, or the source, giving us the first indication that something is amiss. <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

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ranur - Yes, how our universe was 'informed' to have its specific laws and properties is one of the ultimate questions in astronomy.<br /><br />I believe these properties, notably e=mc^2, were set by God at creation - fine tuned to allow for stars and life to exist.<br /><br />How the speed of light was set though is, to my knowledge, unknown.<br /><br />Obviously, as you accurately note, it involves the specific properties of time in our universe.<br /><br />The aether does not have intelligence to tell matter and time how to behave.<br /><br />God does have that intelligence - and it has already been told!

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unclefred

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Your statement that "logically the speed of light should be dependant on the speed of its source" is correct if light is a particle. It is totally wrong if light is a wave. Thus, as Spock would say, the statement is ill ilogical<br /><br /> Consider this situation: There is a train engine sitting stationary at the station. A second engine zooms through the station and the two engineers toot their horns and flash their lights at the exact second they are side by side. You are stationed down the track some distance. Which horn do you hear first and which light do you see turn on first?<br /><br /> Answer: there is no difference. Both sound and lights are simultaneous.<br /><br /> Conclusion: there is no reason to expect that the speed of light should be dependant on the speed of its source<br />

S

Guest
But we can travel w.r.t. either, it's just we can't measure it but just notice a Doppler shift.<br />The <i>scientific trick</i> is to say time & distance changes <img src="/images/icons/rolleyes.gif" />

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Saiph

Guest
the thing is this:<br /><br />Light can be observed to act as a particle (photo-electric effect, specifically the minimal cut-off energy).<br /><br />Light can be observed to act as a wave (interference, diffraction, etc).<br /><br />As a particle, the speed of light should be dependent upon the velocity of the source (relative to the detector). This is not the case, and it isn't a matter of accuracy (we can measure the speed of light very accurately).<br /><br />As a wave the speed is dependent upon the medium transmitting the wave. Sound travels at ~700 mph w.r.t. the air it's traveling through. If the air is traveling at say, 30 mph w.r.t. the detector, the speed of sound will be measured anywhere from 730 to 670 mph (depending on the orientation of the sound in the moving air stream).<br /><br />So, physicists look for the medium in which light travels. We create a vacuum, we create electrically and magneticly shielded chambers (akin to making and E&M vacuum), all sorts of things, and light can still travel through it. So whatever medium it travels through, has to be super-permeable (meaning it gets everywhere, evenly, it's ubiquitous and we can't do much to mess with it).<br /><br />This medium, is the aether.<br /><br /> lets assum the medium isn't fixed relative to the earth's surface (i.e. it doesn't rotate with us, or travel with us about the sun)...which is reasonable, since it has to be even between the stars, why should it be locked into position by the presence of a random (albeit life bearing) planet?<br /><br />If so, that means that since the earth moves in a circle around the sun, and it rotates about it's axis, we move with respect to the aether. A sensitive enough device should be able to detect the shift in the speed of light as we rotate (first going with, then against the aether) and as we orbit (first one way, than another). The speed of light should oscillate in many ways, but specifically should have daily, and yearly components.<br /><br />We m <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 - sorry, but I disagree on one point: namely: that space and time are interchangeable.<br /><br />Remember, our universe specific space-time was caused or created, and cause and effect cannot proceed without time.<br /><br />Also, the speed limit of light does not apply to expansion of the fabric of space - apparently. I.e. past FTL expansion (inflation theories) and present acceleration of expansion to FTL speeds is possible.<br /><br />I.e. motion on the fabric of space is limited to light speed (at least for ordinary matter - tachyons ignored for now), but the actual expansion of the fabric of space is not limited to the speed of light.<br /><br />Perhaps the latter is limited to the speed of dark energy???<br /><br />Now, if you call the space-time fabric aether (semantics, btw), then aether is supported by these theories involving FTL expansion.<br /><br />I would simply call the fabric of space a fine gauze, as in Isaiah 40:22, whose threads and filaments are generally expanding.

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Saiph

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I probably should have used the term maleable instead of interchangable. <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
Thanks that's a very long & reasoned reply.<br />Too rushed to reply now <img src="/images/icons/frown.gif" />

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L

Guest
Thanks for the replys guys. <br /><br />I still can't understand how the speed of light is the same for all observers regardless of their speed. I have heard this principle everywhere but has it ever been experimented on and tested and if so how?<br /><br />i understand that the speed of light has been measured on earth with the source and the measuring device being in the same reference frame i.e earth. but how do we know that the velocity of light isn't increased by a faster moving source if measured from a different inertial frame? <br /><br />example if you are sitting on a train that is travelling at .5 c (hypothetically) and you turn on a torch and face it forward and at the same moment and place on earth someone stationary on the ground outside turns on a torch in the same direction what would someone far in front of them detect each ones speed to be? <br /><br />wouldn't the light from the torch on the train travel faster than the light from the torch on the ground? i don't see how they can travel the same speed.<br /><br />Surely the light should be emitted out from each torch at around C with respect to their respective torches. <br /><br />Any thoughts to help me understand would be much appreciated.<br />

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Saiph

Guest
In order to understand how light can have a constant speed, one must abandon some fundamental assumptions of classical physics, a constant time and space for every oberver.<br /><br />Has it been tested: Yes. First, you should know that the speed of light can be measured very accurately, down to differences in speeds of miles per hour (probably less actually).<br /><br />The michelson morely experiment was the first decisive confirmation of the constant speed of light (and they were hoping they <i>wouldn't</i> find that result!).<br /><br />It measures the speed of light going in two directions, at right angles to eachtother. As the earth moves around the sun, it first goes ~30 km/s one way, then 30 km/s the other (6 months later). It also rotates in place, at some thousand miles per hour, which should also cause a shift in the speed of light on a 24 hour period.<br /><br />The way the light speed is measured is using "interferometry". When light beams intersect, they create a pattern of dark and light fringes. If the distance the light travels changes, the patterns shift. OR, if the distance doesn't change (as is the case for the Michelson Moreley experiment) but the speed does, the patterns will shift. This can be purposefully caused by putting glass or some other material in the way, that has a different speed of light inside the material.<br /><br />However, in the M&M experiment...they didn't do that. They just watched it, over the days, weeks, and months, having isolated everything they could. They found no change in the speed of light, even due to the huge changes in speed caused by earths rotation, and revolution. <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>

L

Guest
Thanks saiph...<br /><br />but in the case of the michelson morely experiment the source of the light they tested and the measuring device were in the same reference frame so of course they would find no change in speed no matter which direction the light faced. <br /><br />this would be the same as if someone walked on a moving train at 3km/h forwards then backwards then left or right. if measured by a device on the train his speed will always be 3km/h. but if measured from the ground his speed would alter depending on which direction he travelled.<br /><br />why isn't this the same case for light. how is light different?<br /><br />the michelson morely experiment never explained how light speed is the same for all observers no matter what speed the observer travels. all it did was verify that light doesn't need a medium to travel through i.e. aether. <br /><br />what if light doesn't need a medium like sound. what if light particals such as photos are sprayed out from the source like water from a hose.<br /><br />can someone explain to me how it is believed light speed is the same for all observers no matter what their speed relative to the light source and if this has been tested and verified. <br /><br />cheers.

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Saiph

Guest
I made a slight mistake:<br /><br />The M&M experiment was designed to see if light moved at a constant speed with respect to a medium the earth was in (i.e. say, with respect to the solar system, or somesuch) and so the earth's motion through this medium should produce a difference. It didn't produce a positive result, light did not move with respect to an external medium (the aether) that the earth moved through.<br /><br />This left two possibilities: No aether, or the aether moves with the earth entirely...which is ruled out due to it's requirement that the earth is special, or in part (an "aether" atmosphere)...which is ruled out due to lack of sufficient stellar aberations.<br /><br /><br />Now, the constancy of the speed of light can still be determined using interferometry, you just put a detector on a moving cart, and measure the speed of light from a stationary source, then flip (moving source, "stationary" detector).<br /><br />You get the same speed in both cases. <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>

L

Guest
Saiph.... <br /><br />has the cart experiment you mentioned above or something similar actually been done and if so by who and how. where do you get your assumpton that the speed measured will be the measured exactly the same. <br /><br />the only way i beleive we can reliably conclude that speed of light is always the same is if the moving observer is travelling a substantial speed relative to te source, enough for the measurement t be accurate.<br /><br />you still haven't given me any proof/reason to beleive that the speed of light is the same in all reference frames.

S

Guest
As far as I know the speed of light hasn't been measured in free space only within the solar system gravity.<br />Does anyone know of its measurement in free space ?

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kmarinas86

Guest
<font color="yellow">As far as I know the speed of light hasn't been measured in free space only within the solar system gravity. <br />Does anyone know of its measurement in free space ?</font><br /><br />Same as here, however, that is only according to the proper time of the observer.<br /><br />Proper time is not the same as coordinate time. An earth second at sea level is different than a second a 32,000 feet or a second at sun, the moon, other stars, or between them. So locally the speed of light is the same, but if the observer and light are undergoing different gravitational time dilation (i.e. if they do not have the same gravitational potential) then of course, the light will not be observed by this observer, but the second varying with position means that, for instance, the speed of light per sea-level earth second depends on where this light is. If the light is at sea level, then it is 299,792,458 meters per sea level second (that is, if one were to place a vacuum at sea level and measure the speed of light there). For someone time dialated in a reference frame that is more accelerated, such as the surface of the sun, the floor of a rocket with more than 1 g of acceleration, or of a black hole, since their seconds are longer with respect to sea-level earth seconds, the action that occurs within one of their seconds is equivalent to the action that happens in more than one earth-second.<br /><br />Regardless of the velocity (0 or not), time is relative in a way that is not as recognized as it should be.<br /><br />"Rod distance" should be proportional to the proper time <i>of the location</i> in the gravitational field. However it is not interpreted that way by mainstream. The change of distance of light (dx) divided by the change in proper time should always equal 299,792,458 m/s (meters per proper second is invariant). The change of distance (dx) divided by the change in coordinate time should be variable (meters per coordinate second is varian

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Saiph

Guest
the "moving cart" and laser experiment has been done, and can even be done in a standard undergraduate lab exercise in today's universities.<br /><br />As for doing this at "relativistic speeds"...well, that's a bit harder.<br /><br />One thing to point out is indirect proof. Relativity (both theories) use the constancy of the speed of light as a fundamental basis from which all other predictions are derived. I.e. time dilation, length contraction, and all the other oddness of relativity are derived from (i.e. consequences of) the assumption of a constant speed of light.<br /><br />That, doesn't prove the assumption true. What supports the validity of the assumption, however, is how extremely successful and accurate the relativity predictions are when dealing with relativistic speeds. I.e. relativity assumes a constant speed of light, and relativity is very, very accurate. As such, it's a really good support for (but I agree not proof of) a constant speed of light.<br /><br />Also, quantum electrodynamics (a step above and beyond just regular quantum mechanics) is, arguably, more successful than relativity (which it uses as well) and also incorporates the S.O.L. limit (and that the photon has no mass). <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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unclefred

Guest
Some interesting questions from lopadotem. I don't have any references to actual measurements of the speed of light from different moving sources, but there have been lots of measurement, so it would not surprise me if it had been done. It would be easy enough to set up in interferometer and feed it with light from various stars or other sources. Today's equipment is very sensitive so near C speeds may not be necessary and a moving lab source may be sufficient. It would be interesting experiment but everyone expects the speed to be measure the same for all sources. All tests show that the speed is always constant for an given medium. Note that even though the speed is the same, other properties will change (like the frequency).<br /><br /> Has anyone ever measured the speed in free space? To measure speed one must measure the time required to cover a measured distance. Both of those (delta time and distance) are difficult. Probably the best one can do is a spacecraft orbiting a planet or maybe the planet itself. We know exactly where the planet is, thus the distance is known. Bounce a signal off the planet or have the spacecraft relay it back and measure the time. Bouncing radar off objects is routinely done. I don't know the details but I have never heard of anything unusual so I assume their calculations show the speed is "C".<br /><br /> Measuring speeds outside the solar system is almost impossible because one must know the distance and times by some means that does not depend on light. Many distance estimates use light to get them and thus one may have circular logic where one ends up only proving the initial assumptions.

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raghara2

Guest
Observable speed of light. If light is keyed, it would be somewhat hard to find real speed.<br /><br />Muller experiment was more interesting.

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