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From The Suns Core to The Corona?

  1. Apr 13, 2008 #1
    Last night I watch a show about the birth of our sun on the Science channel. At one point they talked about how it takes thousands of years for the light in the suns core to travel from the center to the Corona "outer shell", and then only 11 minutes for the light to travel from the Corona to earth.
    There explanation why it took so long for it to travel from the center to the Corona did not set right with me. It was explained that the photons do not slowdown; it took so long because they travel in a zigzag pattern. Witch brought up a question, what causes the photons to zigzag?
    Then I thought about how gravity affects time, what if the photons still travel in a straight line but the massive gravity of the sun slows time? To us the observers it would take thousands of years for the photons to travel from the Core to the Corona. But to the photon "if it could sense time" would take merely minutes.
    Now as I was writing this I remember that the speed of light is a constant, never changing!
    Can some one explain this? What causes the photons to zigzag? And how wrong is my slowed time thought? Because I know it is!
  2. jcsd
  3. Apr 13, 2008 #2
    You are right that gravity affects time and a photon sent down to near the event horizon of a black hole and reflected back would take longer than the distance divided by the speed of light. The mass and density of the sun is too insignificant to produce the sort of time dilation mentioned in that program. The gas in the sun is a plasma and photons coming from the centre are probably absorbed and re-emitted in random directions thousands of times. You also have to take account of refractive index. A light signal sent down a long fibre optic would take longer than a signal traveling the same distance through a vacuum. Before anyone asks if the speed of light is actually slower in a medium like glass, look up phonons. ;)
  4. Apr 13, 2008 #3
    This problem deals with the mean free path for photons inside the sun. The photons are constantly being absorbed and reemitted. Check here for a simple derivation:

  5. Apr 13, 2008 #4
    Thanks for explaining, so black holes can slowdown a photon? I thought that light was a constant and never changed speeds.
  6. Apr 13, 2008 #5

    D H

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    Eric, you are thinking too Euclidean here. Spacetime in the vicinity of a black hole is stretched. That said, this has very little to do with the amount of time it takes light to go from the center of the Sun to the surface of the Sun.

    It appears you are envisioning a single photon traveling from the center of the Sun to the surface. That is not what happens. The fusion reactions at the center of the Sun create high energy gamma rays. Matter is quite dense near the core. Think of it as shining a flashlight in a very thick fog.

    Imagine a gamma photon created by a fusion reaction. This gamma photon will not travel very far; some other ion will absorb it. The original photon travels at the speed of light, but only for a short distance before being absorbed. The absorbing ion will re-radiate the absorbed energy, but in a random direction, at a reduced frequency, and this does not happen immediately. The same thing happens with the new photons released by the ion that absorbed the original gamma. The photons bounce around and around. This is called a diffusion process. Another name is a drunkard's walk. Eventually, after bouncing around, the original energy will leave the sun, but it takes a lot of bounces.
  7. Apr 13, 2008 #6
    mean free path

    I found a note I made that

    mean free path = 1 / [tex]\kappa[/tex] [tex]\rho[/tex]

    where [tex]\kappa[/tex] is opacity
    [tex]\rho[/tex] is density

    Can someone confirm this?
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