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Thread: Speed of light question

  1. #1 Speed of light question 
    Forum Professor pyoko's Avatar
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    Something has been confusing me. There was an experiment that "slowed light down" to 45 mph in a supercooled medium, so relative to an outsider outside of the medium the light was "slow". I realise there is a "catch" with this and it's not actually slowing down that extremely. Or at all.

    And that hypothetically if that medium was at 0K the light would stop completely.

    The second thing I heard was that photons actually travel at C through any medium, and that the slowdown is almost mechanical - in that the photons get absorbed by a particle in the way inside a medium and then re-emitted, which takes time. But that the photons actually still always travel at C between the particles in that medium.

    Can someone just confirm that this is right? And if not, why not?


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    Quote Originally Posted by pyoko View Post
    Something has been confusing me. There was an experiment that "slowed light down" to 45 mph in a supercooled medium, so relative to an outsider outside of the medium the light was "slow". I realise there is a "catch" with this and it's not actually slowing down that extremely. Or at all.

    And that hypothetically if that medium was at 0K the light would stop completely.

    The second thing I heard was that photons actually travel at C through any medium, and that the slowdown is almost mechanical - in that the photons get absorbed by a particle in the way inside a medium and then re-emitted, which takes time. But that the photons actually still always travel at C between the particles in that medium.

    Can someone just confirm that this is right? And if not, why not?
    That sounds quite right.


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    Reviving a thread that could really use more discussion.
    One thing that I'm curious about is that C in a medium depends solely on RI. If cooling makesc(medium)<<c(vacuum), is RI dependent on temp?
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    Quote Originally Posted by Chronum View Post
    Reviving a thread that could really use more discussion.
    One thing that I'm curious about is that C in a medium depends solely on RI. If cooling makesc(medium)<<c(vacuum), is RI dependent on temp?
    There is a small dependence on temperature for the Index of refraction, but is is pretty weak.

    With the experiment mentioned, it is the special state the matter enters when it cooled to such low temps that accounts for the High IOR, and not the temperature directly.
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    Quote Originally Posted by pyoko View Post
    Something has been confusing me. There was an experiment that "slowed light down" to 45 mph in a supercooled medium, so relative to an outsider outside of the medium the light was "slow". I realise there is a "catch" with this and it's not actually slowing down that extremely. Or at all.

    And that hypothetically if that medium was at 0K the light would stop completely.

    The second thing I heard was that photons actually travel at C through any medium, and that the slowdown is almost mechanical - in that the photons get absorbed by a particle in the way inside a medium and then re-emitted, which takes time. But that the photons actually still always travel at C between the particles in that medium.

    Can someone just confirm that this is right? And if not, why not?
    Your explanations seem pretty standard and correct. Besides the absorption of re-admission of light in many mediums, there is also the index of refraction whereby the light refracts at many angles before it ends up at its final destination. This can be seen in a fiber optic cable with an index of refraction of .5 for instance. Light within the fiber optic cable from its souse to its destination. takes twice as long as it would otherwise take if it were not continuously refracted during its travels.

    As you probably know light cannot be projected in just a linear manner such a laser light without it eventually spreading out in a conical like form, from its source. Light can also travel backwards via reflection or refraction, providing there is an available path in that direction
    Last edited by forrest noble; August 10th, 2012 at 01:42 PM.
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    And that can be summarised as an effect of the dual nature of light. Particle like nature is the absorption and re-emission of photons and refraction is the wavelike property of light.
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    Hm, let me break this down hardcore solid-state physics style.

    The speed of light through a medium is determined by its magnetic and electric susceptibility.
    c_material = 1/sqrt(permittivity*susceptibility)

    in other words, for the speed to be zero, either or both the permittivity or susceptibility need to be infiite. This is however physically impossible, as no material can can cary a infinite magnetic or electric field.

    c will never be zero in a material.
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    Quote Originally Posted by Kerling View Post
    c will never be zero in a material.
    But that will never get in the way of a good headline: Light completely stopped for a record-breaking minute - physics-math - 25 July 2013 - New Scientist
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    Quote Originally Posted by Kerling View Post

    The speed of light through a medium is determined by its magnetic and electric susceptibility.
    c_material = 1/sqrt(permittivity*susceptibility)
    Doubt it will be valid at such low temperatures, in the reign of quantum mechanics. That is a relation whose proof is based on many stringent hypoteses (uniform polarization and many others I can't remember at the moment).
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    Quote Originally Posted by Strange View Post
    Quote Originally Posted by Kerling View Post
    c will never be zero in a material.
    But that will never get in the way of a good headline: Light completely stopped for a record-breaking minute - physics-math - 25 July 2013 - New Scientist
    Storeing light is not the same as stopping it.
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  12. #11  
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    Quote Originally Posted by fred91 View Post
    Quote Originally Posted by Kerling View Post

    The speed of light through a medium is determined by its magnetic and electric susceptibility.
    c_material = 1/sqrt(permittivity*susceptibility)
    Doubt it will be valid at such low temperatures, in the reign of quantum mechanics. That is a relation whose proof is based on many stringent hypoteses (uniform polarization and many others I can't remember at the moment).
    If you want, and I have the time I can show you the quantum-mechanical derivation of this? temperature is something which is only usefully defined for ensembles. Whereas the permitivitty and susceptibilty are calculatable for even single atomical set-ups. Or I might mis-interpret your remark. Care to elaborate?
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    Probably misunderstood your comment as a naive one, I'm sorry Yes, I'd love to see the derivation for single atomical set-ups, just this isn't the right place, I fear.
    Davis was questioning the increasing length of John Coltrane solos, and Trane answered "I don't know how to stop."

    Try taking the fucking horn out of your mouth.
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    When light travels through a medium such as glass, its phase velocity is reduced by a factor which is equal to the reciprocal of the refractive index of the medium. However, large reductions in the speed of light are not due to correspondingly large increases in the refractive index. The reduction is due to a change in group velocity, not a change in phase velocity.

    An article on Slow Light can be found here:

    https://en.wikipedia.org/wiki/Slow_light
    Last edited by JonG; July 25th, 2013 at 03:16 PM.
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    Quote Originally Posted by fred91 View Post
    Probably misunderstood your comment as a naive one, I'm sorry Yes, I'd love to see the derivation for single atomical set-ups, just this isn't the right place, I fear.
    well, I spend months finding one for one of my theses. I've only found one in Landau en Lifshitz statistical physics.in Short you summate over the possible transitions and their weighting. and how much change that involves concerning the electric and magnetic strengths.
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    But that the photons actually still always travel at C between the particles in that medium.
    then why is it dark at the bottom of the ocean? if the photon always travels at c between the particles, shouldn''t it reach the ground?
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    Quote Originally Posted by curious mind View Post
    then why is it dark at the bottom of the ocean? if the photon always travels at c between the particles, shouldn''t it reach the ground?
    Because some proportion of photons get absorbed or scattered by the water (or whatever medium they pass through). It isn't perfectly transparent.
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    Quote Originally Posted by Strange View Post
    Quote Originally Posted by curious mind View Post
    then why is it dark at the bottom of the ocean? if the photon always travels at c between the particles, shouldn''t it reach the ground?
    Because some proportion of photons get absorbed or scattered by the water (or whatever medium they pass through). It isn't perfectly transparent.
    then why can we see light from stars billions of ly's away, it has to travel through, i.e. clouds or gases also?
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    Quote Originally Posted by curious mind View Post
    then why can we see light from stars billions of ly's away, it has to travel through, i.e. clouds or gases also?
    Because there is almost nothing in space.

    Interstellar space can be as low as 10-4 atoms (ions) per cm3, whereas water is about 1022 molecules per cm3 (*). In other words, water is 100000000000000000000000000 times as dense. The intergalactic medium is even less dense. Of course, there are areas we cannot see because the gas/dust between is too dense but in general, the Earth's atmosphere is the biggest barrier.

    (*) And different things absorb visible light to different extents.
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    Quote Originally Posted by curious mind View Post
    Quote Originally Posted by Strange View Post
    Quote Originally Posted by curious mind View Post
    then why is it dark at the bottom of the ocean? if the photon always travels at c between the particles, shouldn''t it reach the ground?
    Because some proportion of photons get absorbed or scattered by the water (or whatever medium they pass through). It isn't perfectly transparent.
    then why can we see light from stars billions of ly's away, it has to travel through, i.e. clouds or gases also?
    We see them due to the effect of gravitational lensing.

    However the speed of light in water is about c/1.33=225407863m/s

    It is not that light does not reach the bottom of the sea,but it must be reflected to us before we can see it.and this is were the problem lies.

    The sea water is not that clear and it contains colliods and the we know that the solutes of colliods exhibit tyndall effect(the scattering of light rays).and Ƒor тнιѕ reason the rays don't strike our eyes(the reflected rays are scattered)hence we cannot see it.
    Last edited by merumario; July 26th, 2013 at 12:48 PM.
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    Quote Originally Posted by merumario View Post
    Quote Originally Posted by curious mind View Post
    Quote Originally Posted by Strange View Post
    Quote Originally Posted by curious mind View Post
    then why is it dark at the bottom of the ocean? if the photon always travels at c between the particles, shouldn''t it reach the ground?
    Because some proportion of photons get absorbed or scattered by the water (or whatever medium they pass through). It isn't perfectly transparent.
    then why can we see light from stars billions of ly's away, it has to travel through, i.e. clouds or gases also?
    We see them due to the effect of gravitational lensing.

    However the speed of light in water is about c/1.5=199861639m/s.

    It is not that light does not reach the bottom of the sea,but it must be reflected to us before we can see it.and this is were the problem lies.

    The sea water is not that clear and it contains colliods and the we know that the solutes of colliods exhibit tyndall effect(the scattering of light rays).and Ƒor тнιѕ reason the rays don't strike our eyes(the reflected rays are scattered)hence we cannot see it.
    funny (or sad) part is, i know about both, but couldn't connect them. thanks for this brainstorm.
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    It skips us sometime man! Cheers(curious mind)
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    I have my own speed of light question. As a lever rotates around a motor, the outer atoms travel at a much faster linear speed (distance/time) in order to have the same rotational speed (degrees/time). If you had a big enought lever and motor, you could make the outer tip of the lever go extreamly fast. What if It went so fast that the point 2/3 up the lever had a linear speed of c? Would the tip of the lever loss rotational speed so it stayed below c or would I never be able to make the point 2/3 up the lever going that fast? Thank you in advance.
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    Quote Originally Posted by Jewish-Scientist View Post
    I have my own speed of light question. As a lever rotates around a motor, the outer atoms travel at a much faster linear speed (distance/time) in order to have the same rotational speed (degrees/time). If you had a big enought lever and motor, you could make the outer tip of the lever go extreamly fast. What if It went so fast that the point 2/3 up the lever had a linear speed of c? Would the tip of the lever loss rotational speed so it stayed below c or would I never be able to make the point 2/3 up the lever going that fast? Thank you in advance.
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    Thank you. I like paradoxes.
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