1. Subscribersonhouse
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    24 Mar '16 12:05
    http://phys.org/news/2016-03-optical-slower.html
  2. Standard memberapathist
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    25 Mar '16 17:441 edit
    "Although light does slow down when traveling through clear dense materials such glass or water,..."

    I thought no it doesn't, light seems to slow down only because it is continually being absorbed and then emitted by the dense material. I expect c is still c even in twisted light - the light just travels farther. Braids appear shorter than the hair they are made of, but the individual hairs are still the original length.
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    25 Mar '16 18:446 edits
    Originally posted by apathist
    [b]"Although light does slow down when traveling through clear dense materials such glass or water,..."

    I thought no it doesn't,....[/b]
    https://en.wikipedia.org/wiki/Refractive_index

    "... light travels 1.333 times faster in a vacuum than it does in water...."

    implying that light does slow down when traveling through water.

    Your misunderstanding might come from:

    http://www.rpi.edu/dept/phys/Dept2/APPhys1/optics/optics/node4.html
    .... "Light in a vacuum always travels at the same speed." Those additional three words in a vacuum are very important. . ..."

    In other words, relativity doesn't imply that light cannot travel slower through material than through a vacuum (although it does imply it cannot move faster ) .
  4. Standard memberapathist
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    25 Mar '16 19:541 edit
    Originally posted by humy...
    "... light travels 1.333 times faster in a vacuum than it does in water...."
    implying that light does slow down when traveling through water. ...
    http://www.askamathematician.com/2011/08/q-if-light-slows-down-in-different-materials-then-how-can-it-be-a-universal-speed/

    "Here’s the idea: a medium, whatever it is, is made up of molecules. When a photon (light particle) hits a molecule it is sometimes absorbed. Its energy is turned into raised electron-energy-levels, or vibrations and flexing, or movement. In short order (very short order) the photon is spit out of the other side, none the worse for wear."

    When a photon exists, it always moves at c. When it is absorbed by a molecule, the energy state of the electrons in that molecule is increased and the photon doesn't exist anymore. After a while, the energy level of the electrons in that molecule drop, and photon is emitted. That delay is why light propagates slower through a medium than in a vacuum. In effect, the light is continuously disappearing for a moment and then reappearing, as it works its way through the material. The photons, when they exist, ALWAYS move at c.

    According to the op article, twisted light moves slower than regular light even though they are in a vacuum. I bet c is still c, and the photons in the twisted light are merely traveling a farther distance, so of course they take longer. I think it said 0.1% longer.
  5. Subscribersonhouse
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    25 Mar '16 21:02
    Originally posted by apathist
    http://www.askamathematician.com/2011/08/q-if-light-slows-down-in-different-materials-then-how-can-it-be-a-universal-speed/

    "Here’s the idea: a medium, whatever it is, is made up of molecules. When a photon (light particle) hits a molecule it is sometimes absorbed. Its energy is turned into raised electron-energy-levels, or vibrations and flexing, or m ...[text shortened]... merely traveling a farther distance, so of course they take longer. I think it said 0.1% longer.
    That's what I read into it. The twisting is forcing the photons to take a path length longer than a straight path, sort of a drunkard's walk.
  6. Standard memberlemon lime
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    25 Mar '16 21:02
    Originally posted by apathist
    http://www.askamathematician.com/2011/08/q-if-light-slows-down-in-different-materials-then-how-can-it-be-a-universal-speed/

    "Here’s the idea: a medium, whatever it is, is made up of molecules. When a photon (light particle) hits a molecule it is sometimes absorbed. Its energy is turned into raised electron-energy-levels, or vibrations and flexing, or m ...[text shortened]... merely traveling a farther distance, so of course they take longer. I think it said 0.1% longer.
    That's right. Light is continually stopping and starting when traveling through material, and this is what causes the appearance of traveling slower. But whenever light is traveling it always travels through a vacuum, and it always travels at c.
    I wonder though... when a photon is absorbed and then emitted is it the same photon? The same thing happens when light is reflected, and since the photon emitted is identical to the one absorbed it's essentially the same photon, even if it isn't.
  7. Cape Town
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    25 Mar '16 21:101 edit
    Originally posted by apathist
    When a photon exists, it always moves at c. When it is absorbed by a molecule, the energy state of the electrons in that molecule is increased [b]and the photon doesn't exist anymore. After a while, the energy level of the electrons in that molecule drop, and photon is emitted. That delay is why light propagates slower through a medium than in a ...[text shortened]... merely traveling a farther distance, so of course they take longer. I think it said 0.1% longer.[/b]
    Actually this is quantum mechanics so its all a little bit more complicated than that.

    The photon isn't simply absorbed and re-emitted as it passes through a medium. The photon can be thought of as taking every possible path including being absorbed and remitted in every possible direction by every atom within range and then all those paths interact with each other to give the probability that it will eventually be detected at a given place.

    Similarly the effect in the OP will involve quantum mechanics.
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    25 Mar '16 21:565 edits
    Originally posted by lemon lime
    That's right. Light is continually stopping and starting when traveling through material, and this is what causes the appearance of traveling slower. ...
    Actually, if I understand this correctly, that isn't right and what that link says contradicts modern quantum physics.
    -see my next post;
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    25 Mar '16 22:0611 edits
    Originally posted by apathist
    http://www.askamathematician.com/2011/08/q-if-light-slows-down-in-different-materials-then-how-can-it-be-a-universal-speed/

    "Here’s the idea: a medium, whatever it is, is made up of molecules. When a photon (light particle) hits a molecule it is sometimes absorbed. Its energy is turned into raised electron-energy-levels, or vibrations and flexing, or m ...[text shortened]... merely traveling a farther distance, so of course they take longer. I think it said 0.1% longer.
    The link says

    1) The speed of light is an absolute.

    2) Light slows down when it passes through a medium (like water, glass, air, &hellip😉.

    and falsely makes out 1) and 2) contradict, which they don't because they have been misrepresented. It fails to mention that when we say "The speed of light is an absolute" as in 1) , we mean "The speed of light is an absolute in a vacuum " and that, because of quantum physics, any photon passing through a median with particles with the distances between adjacent particles being much less than half the wavelength of that light is treated by the light photon as if it was continuous as if the particles in that medium where smeared more or less evenly through space throughout.
    Yes it interacts with the particles in the medium, but you cannot normally measure any individual 'start-stop' periods of motion of the photon while it is going through the medium and, in physics in general, if you cannot measure it, it isn't so. In fact, according to quantum physics, there is no such 'start-stop' motion of the photon through such a medium unless you can, at least in theory, actually detect it. The picture that link (and several others I have now just seen ) gives about what is going on is made too simplistic and contradicts modern understanding of quantum physics.
  10. Subscribersonhouse
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    26 Mar '16 02:18
    Originally posted by humy
    The link says

    1) The speed of light is an absolute.

    2) Light slows down when it passes through a medium (like water, glass, air, &hellip😉.

    and falsely makes out 1) and 2) contradict, which they don't because they have been misrepresented. It fails to mention that when we say "The speed of light is an absolute" as in 1) , we mean "The speed of light is an ab ...[text shortened]... what is going on is made too simplistic and contradicts modern understanding of quantum physics.
    If there were 'stop and start' kind of motions, wouldn't that lead to Cherenkov radiation due to deceleration and acceleration effects? If so, the lack of such radiation would indicate there was no such start and stop motion.
  11. Standard memberlemon lime
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    26 Mar '16 03:262 edits
    Originally posted by sonhouse
    If there were 'stop and start' kind of motions, wouldn't that lead to Cherenkov radiation due to deceleration and acceleration effects? If so, the lack of such radiation would indicate there was no such start and stop motion.
    Light doesn't decelerate or accelerate, whether there is start/stop activity or not.

    It just now occurred to me that LED lights basically work on the same principle as photons bumping electrons up into a higher energy state, then spitting out a photon when the electron pushes itself back into its normal state. LED light is created by reuniting electrons with electron holes... this causes the electron to move into a lower energy state, and then it spits out that little bit of extra energy in the form of a photon.
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    26 Mar '16 08:122 edits
    Originally posted by humy
    The link says

    1) The speed of light is an absolute.

    2) Light slows down when it passes through a medium (like water, glass, air, &hellip😉.

    and falsely makes out 1) and 2) contradict, which they don't because they have been misrepresented. It fails to mention that when we say "The speed of light is an absolute" as in 1) , we mean "The speed of light is an ab ...[text shortened]... what is going on is made too simplistic and contradicts modern understanding of quantum physics.
    misedit:
    I made it sound like I meant that the photon itself can actually stop and then start moving again, which is not what I meant. Not sure exactly how to word that so it doesn't sound that way but hopefully you get the gist of what I actually mean.
  13. Standard memberDeepThought
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    26 Mar '16 17:20
    Bear in mind that there's a difference between group velocity and phase velocity. A plane wave - which is a mathematical idealization, no physical photon is ever anything more than an approximation to a plane wave - or a perfectly spherical wave will always travel at the speed of light. Using a mathematical technique called Fourier Analysis one can decompose any wave packet into plane waves. Suppose one takes a wave packet whose momentum is along the z-axis, then some components will be pointing off the z-axis, but the average is along it. Using basic trigonometry you can see that the packet as a whole will not have travelled as far along the z-axis as it would have were it a pure plane wave, even in a vacuum. This is the basis of this type of effect.

    What I don't get is that they seem to be saying that circularly polarized waves will go slower, and I'm not sure if this is a different effect from what I've discussed in the above paragraph or the phys.org writer has the wrong end of the stick and has left something out.
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