Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

40,000 years taken for light to reach here?

Tags:
  1. Jun 12, 2014 #1

    adjacent

    User Avatar
    Gold Member

    http://svs.gsfc.nasa.gov/vis/a010000/a011500/a011537/
    This is so confusing. If light travels at the speed of light, c, then how can it take 40,000 year to get out from the sun?
    Photons undergo fusion? hahaha. I never knew that
     
    Last edited: Jun 12, 2014
  2. jcsd
  3. Jun 12, 2014 #2
    Because the photons do not travel in a straight line to the Earth, it is estimated that on average it takes 40K years. Because of extreme density and high energy, the photons "take the extreme scenic route" bouncing about "3 steps forward and 2 steps back" a truly ridiculous number of times and in seemingly random directions. Incompetent slackers, the lot.

    *Whew ! I really need coffee!
     
  4. Jun 12, 2014 #3

    adjacent

    User Avatar
    Gold Member

    What about nuclear fusion of photons?
     
  5. Jun 12, 2014 #4

    Bill_K

    User Avatar
    Science Advisor

    I believe it means "atoms undergoing nuclear fusion". Also, it's not really correct to say it's all one photon involved in the 40,000 year trip. In fact, from https://en.wikipedia.org/wiki/Solar_core:

     
    Last edited: Jun 12, 2014
  6. Jun 12, 2014 #5

    D H

    User Avatar
    Insights Author

    Staff: Mentor

    That's at best misleading. Here's a much better way to express this concept: It takes energy 40,000 years to escape from the Sun. (Only 40,000 years? I've read much longer time spans.) A single photon doesn't move all the way from the core to the Sun's photosphere. At the center of the Sun, a single photon moves but a tiny, tiny distance before it encounters something such as a proton that absorbs that photon. The proton will soon emit another photon, but in a random direction. Or it might emit two photons, or more. The incoming and outgoing photons are different. Different directions, and sometimes different frequencies. This process is repeated over and over and over. The energy created in the center of the Sun eventually does escape the Sun, put the path of that energy is a random walk.

    The fusion in the core of the Sun creates gammas, extremely energetic photons. The energy is mostly in the form of visible and infrared photons by the time that energy reaches the Sun's photosphere. The photon flux is much greater at the Sun's surface than in the Sun's core because a large number of those infrared and visible photons are needed to equal the energy of one gamma.


    You misread. The article you cited says "Particles of light form from atoms undergoing nuclear fusion in the sun’s innermost layer known as the core." (Emphasis mine.) It's the atoms that undergo fusion, not the photons. The photons are created as part of the fusion process.
     
  7. Jun 12, 2014 #6

    adjacent

    User Avatar
    Gold Member

    Oh, I understand now, thank you :smile:
    lol, I misread it I thought the word "form" was a typo.
    I read it like this: Particles of light from atoms, undergo nuclear fusion. :wink:
     
  8. Jun 12, 2014 #7

    D H

    User Avatar
    Insights Author

    Staff: Mentor

    [rant]

    This is exemplary of what I don't like about popularizations of science. There would have been a lot less confusion had the authors of that site had used "energy" rather than "light". Moreover, "energy" is a better word. There are other forms of energy transport inside of stars besides radiation. Using "energy" covers both the convective and radiative regions of a star. Finally, the word "energy" is much less likely to result in the misinterpretation that photons somehow magically move at much less than the speed of light inside the Sun.

    I know the problems with this popularization because I know the subject to some extent. But what about areas of science in which I don't know so much?

    That's my key gripe with regard to these "quantum woo" popularizations. I know the popularizers intentionally mislead, at least in the areas in which I'm somewhat knowledgable. It leads me to think that this misbehavior is universal across the sciences.

    [/rant]
     
  9. Jun 12, 2014 #8

    CWatters

    User Avatar
    Science Advisor
    Homework Helper

    Light doesn't always travel at 3*10^8 m/s...

    http://news.bbc.co.uk/1/hi/sci/tech/655518.stm

    but this isn't relevant to the question why it takes 40,000 years for light to escape the sun.
     
  10. Jun 12, 2014 #9

    adjacent

    User Avatar
    Gold Member

  11. Jun 12, 2014 #10

    D H

    User Avatar
    Insights Author

    Staff: Mentor

    That's another one of those misleading "quantum woo" articles.

    The collective behavior of photons and pseudo-particles such as phonons make "light" move slower in a medium, but it is not because photons move slower. Individual photons always travel at c. Always. However, individual photons lose their a bit of their individuality in a medium. We have an FAQ on this: Do Photons Move Slower in a Solid Medium?.
     
  12. Jun 12, 2014 #11

    Bill_K

    User Avatar
    Science Advisor

    I see nothing woo-ish about the article. IMHO it's both accurate and interesting. It never mentions photons, you did.
     
  13. Jun 12, 2014 #12

    D H

    User Avatar
    Insights Author

    Staff: Mentor

    Bill, you know the subtleties that that article is addressing. Does a lay person?
     
  14. Jun 12, 2014 #13

    Chronos

    User Avatar
    Science Advisor
    Gold Member
    2015 Award

    Science popularizations are not going away. Ordinary people are interested in this stuff and want intuitive explanations - a 'news bite', if you will. Analogies are a means to this end. Unfortunately, no analogy is perfect, and sometimes they are even ambiguous. Reality is always more complicated than any abbreviated description can convey, which is why mathematics is the language of choice in science. Mathematical descriptions are immune to the slipperiness inherent to words.
     
  15. Jun 16, 2014 #14
    well I'm definitely a lay person, but I now understand that it takes energy in some form starting at the core of the sun 40 thousand years more or less, Depending on the rout the energy takes through different kinds of soup to reach the Sun's photosphere where it might escape as light. Something I never knew. but cool.
     
  16. Jun 17, 2014 #15

    Drakkith

    User Avatar

    Staff: Mentor

    Well, unless that energy is in the form of a neutrino. It only takes about 2 seconds for neutrinos to get out of the sun. (Reference the link in the next paragraph below)

    Interestingly, I've read in the following link that it takes 170,000 years for photons to escape from the core, not 40,000. (http://physics.weber.edu/palen/Phsx1040/Lectures/Lsun.html) Wiki's article on the Sun says estimates range between 10,000 and 170,000 years.

    Also, I believe we may be a bit mistaken in saying "energy" instead of "photons". Apparently there is a "photon diffusion time scale" along with a "Kelvin–Helmholtz time scale", the latter of which governs energy transport as a whole and takes a significantly longer time of 30 million years. (http://link.springer.com/article/10.1023/A:1022952621810)

    Per wiki: http://en.wikipedia.org/wiki/Sun

    Since energy transport in the Sun is a process which involves photons in thermodynamic equilibrium with matter, the time scale of energy transport in the Sun is longer, on the order of 30,000,000 years. This is the time it would take the Sun to return to a stable state if the rate of energy generation in its core were suddenly to be changed.[59]
     
  17. Jun 17, 2014 #16

    Matterwave

    User Avatar
    Science Advisor
    Gold Member

    The actual photon diffusion time is pretty model dependent, so various estimates may give different results.

    The Kelvin-Helmholtz time would be the time it took the Sun to lose all of its currently stored thermal energy given its current luminosity.

    The large difference between the Kelvin-Helmholtz time and the photon diffusion time shows you that most of the thermal energy of the Sun is stored in the atoms themselves and not in the photon field.
     
  18. Jun 17, 2014 #17

    Drakkith

    User Avatar

    Staff: Mentor

    Ah, that makes sense. Thanks, Matterwave.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted
Similar Discussions: 40,000 years taken for light to reach here?
  1. Light years (Replies: 26)

  2. Light that reaches us (Replies: 6)

Loading...