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Thread: Speed of light ???

  1. #1 Speed of light ??? 
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    Science states that light travels at a speed of 3x10.^8 m/s in vacuum....
    But all objects in the universe move at completely different speeds with respect to each other, so obviously many frames of reference can be generated after considering all matter in the universe. Then how is it possible for light to have a constant velocity with respect to all these bodies (reference frames). Please don't give answers like regionalized time dilation because to me time is only a concept developed by humans to identify correlate the different states in which matter arranges itself in the universe and does not exist as such.
    Could this mean that light accelerates and decelerates to different velocities depending on density and motion of the surrounding matter,but this would mean light's velocity is not a constant...


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  3. #2  
    Genius Duck Moderator Dywyddyr's Avatar
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    Quote Originally Posted by ashwinvinoo View Post
    because to me time is only a concept developed by humans to identify correlate the different states in which matter arranges itself in the universe and does not exist as such.
    Well there's part of your problem.
    If you're going essentially ignore or dismiss time - a fundamental dimension in physics - the rest of physics can't be expected to make much sense to you.

    Could this mean that light accelerates and decelerates to different velocities depending on density and motion of the surrounding matter,but this would mean light's velocity is not a constant.
    No.
    The speed of light is a constant regardless of who measures it, or how fast they're travelling relative to any other observer.


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    Moderator Moderator Markus Hanke's Avatar
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    Science states that light travels at a speed of 3x10.^8 m/s in vacuum....
    Science does not "state" that; it is an empirical, experimental observation which is testable. It is a fact of our universe.

    Then how is it possible for light to have a constant velocity with respect to all these bodies (reference frames).
    That's due to the geometry of Minkowski space-time; at every point of an observer's worldline you can define a unit tangent vector with respect to that observer's proper time. If you calculate the magnitude of this 4-vector you will find that it is always exactly c, for all observers and at all times. Space and time change with your state of relative motion, but the relation between them never changes; it is an invariant property of the geometry of space-time. To be more specific, light ( as all particles with vanishing rest mass ) trace out null geodesics in space-time.

    Please don't give answers like regionalized time dilation because to me time is only a concept developed by humans to identify correlate the different states in which matter arranges itself in the universe and does not exist as such.
    In that case you should not have come to a science forum to ask this, but rather a group dealing with philosophy or metaphysics. In the natural sciences, time is a geometric dimension, an aspect of space-time, and as such it is modelled by the field equations of General Relativity. Needless to say these are well verified experimentally. You shouldn't expect any answers that aren't in accord with scientific consensus when coming onto a science forum.

    I should also note that if you refuse to acknowledge time as physically real, it is no surprise that you end up with problems explaining the constancy of the speed of light. You are creating your own issue here.

    Could this mean that light accelerates and decelerates
    No, light never accelerates or decelerates, as both Maxwell's equations and QED ( quantum electrodynamics ) tell us, not to mention relativity. All basic models of physics, from the macroscopic to the microscopic, agree on this.
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    Moderator Moderator Markus Hanke's Avatar
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    MODERATOR NOTE : Tentatively moved to "New Hypothesis", though the idea of inhomogeneous media in vacuum isn't exactly new.
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  6. #5  
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    But then when light enters materials like glass its velocity drops, but matter is just a form of energy . All vaccum in space has differnent concentrations of radiation energy flowng through them. So why does'nt lights velocity drop there.
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  7. #6  
    Brassica oleracea Strange's Avatar
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    Quote Originally Posted by ashwinvinoo View Post
    But then when light enters materials like glass its velocity drops, but matter is just a form of energy .
    Its velocity drops because it interacts with the electrons (actually is repeatedly absorbed and re-emitted, always moving at c). This obviously makes matter different than energy.

    Both energy and mass will cause gravitational lensing, though.
    ei incumbit probatio qui dicit, non qui negat
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  8. #7  
    Moderator Moderator Janus's Avatar
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    Quote Originally Posted by ashwinvinoo View Post
    But then when light enters materials like glass its velocity drops, but matter is just a form of energy . All vaccum in space has differnent concentrations of radiation energy flowng through them. So why does'nt lights velocity drop there.
    Strange has already given one answer to this question, The way light interact with matter and energy are different. (Water and ice are just different forms of the same thing also, but have different properties).

    But even putting that aside, you would have to consider the relative energy concentrations.

    For example, a 1cm cube of glass masses ~0.0024kg. The energy equivalent of that is 2.16x10^14 joules. So how much is that compared to the "different concentrations of energy in space"? Let's pick an example. the Sun puts out some 3.86x10^26 Joules/sec. But that is spread out over its whole surface, if we divide that by the surface area of the sun in cm comes out to ~6360 joules/sec per square cm. Now this energy streams away at the speed of light, so we have to divide this by 3x10^10( the speed of light in cm/sec.) to get the energy concentration for one cubic cm right above the surface of the Sun. This works out to 0.00000021 joules per cubic cm. Compare this to the 2.16x10^14 joules per cubic cm for glass.

    IOW, the energy equivalence concentration of glass is some 1,000,000,000,000,000,000,000 times greater than that of sunlight at the surface of the Sun. Now considering that the vast majority of space does not have energy concentrations even closely approaching even that at the surface of the Sun, you would expect that any slowing of light due to these energies even if you assume that they could affect the speed of light, would be extremely small.

    But we don't even have to resort to the above to show that there is no significant slowing of light when it travels the vast distances of space. One aspect of light slowing when it passes through a material like glass is that different frequencies are slowed by different degrees. Thus it takes different amounts of time for different colors of light to pass through the same thickness of glass. (As a side note, this also effects how much the different colors of light refract when passing from one medium to another, which is how a prism breaks white light up into a spectrum pattern).

    Now we are able to view events like supernovae that occur in galaxies billions of light years away. If the speed of light where subject to the type of slowing we see in glass in even the slightest amount, the different frequencies of light from a supernova would arrive at different times. We would see the supernova brighten one part of the spectrum first followed by the next part, etc. We don't see this. The Supernova brightens equally across the whole spectrum at once, indicating that all frequencies of light traveled at the the same speed.
    "Men are apt to mistake the strength of their feelings for the strength of their argument.
    The heated mind resents the chill touch & relentless scrutiny of logic"-W.E. Gladstone


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  9. #8  
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    What are the actual proof's of time dilation and length contraction that have been observed? (just asking out of curiousity)
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    Moderator Moderator Markus Hanke's Avatar
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    Quote Originally Posted by ashwinvinoo View Post
    What are the actual proof's of time dilation and length contraction that have been observed? (just asking out of curiousity)
    Have a look here :

    Modern Tests of Relativity
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  11. #10  
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    Thank you all...
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  12. #11  
    Moderator Moderator Markus Hanke's Avatar
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    Quote Originally Posted by ashwinvinoo View Post
    Thank you all...
    No problem - helping someone and receiving a genuine thanks is always a pleasure, and makes this worthwhile
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