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40,000 years taken for light to reach here?

by adjacent
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adjacent
#1
Jun12-14, 10:49 AM
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http://svs.gsfc.nasa.gov/vis/a010000/a011500/a011537/
Quote Quote by NASA's Goddard Space Flight Center
The light that illuminates our planet is made deep inside the sun and takes some 40,000 years to travel through the sun’s layers. Particles of light form from atoms undergoing nuclear fusion in the sun’s innermost layer known as the core. The light then flows through the sun’s interior for millennia, slowly bubbling up like water in a boiling pot. It eventually bursts past the sun’s surface, called the photosphere, and rises into the solar atmosphere. Once in the atmosphere—made up of the chromosphere and corona—the light streams out through the solar system.
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
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enorbet
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Jun12-14, 10:58 AM
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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!
adjacent
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Jun12-14, 11:03 AM
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What about nuclear fusion of photons?

Bill_K
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Jun12-14, 11:14 AM
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40,000 years taken for light to reach here?

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:

The high-energy photons (gamma rays) released in fusion reactions take indirect paths to the Sun's surface. According to current models, random scattering from free electrons in the solar radiative zone (the zone within 75% of the solar radius, where heat transfer is by radiation) sets the photon diffusion time scale (or "photon travel time") from the core to the outer edge of the radiative zone at about 170,000 years. From there they cross into the convective zone (the remaining 25% of distance from the Sun's center), where the dominant transfer process changes to convection, and the speed at which heat moves outward becomes considerably faster. In the process of heat transfer from core to photosphere, each gamma ray in the Sun's core is converted during scattering into several million visible light photons before escaping into space.
D H
#5
Jun12-14, 11:36 AM
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Quote Quote by adjacent View Post
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?
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.


Photons undergo fusion? hahaha. I never knew that
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.
adjacent
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Jun12-14, 11:52 AM
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Oh, I understand now, thank you
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.
D H
#7
Jun12-14, 11:59 AM
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[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]
CWatters
#8
Jun12-14, 12:36 PM
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If light travels at the speed of light, c...
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.
adjacent
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Jun12-14, 01:13 PM
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Quote Quote by CWatters View Post
Light doesn't always travel at 3*10^8 m/s...

http://news.bbc.co.uk/1/hi/sci/tech/655518.stm
That's so interesting.
[off topic]I saw that in my dream. I slowed down light and made a machine which can view past,live.
D H
#10
Jun12-14, 01:22 PM
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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?.
Bill_K
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Jun12-14, 01:34 PM
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Quote Quote by D H View Post
That's another one of those misleading "quantum woo" articles.
I see nothing woo-ish about the article. IMHO it's both accurate and interesting. It never mentions photons, you did.
D H
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Jun12-14, 02:01 PM
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Bill, you know the subtleties that that article is addressing. Does a lay person?
Chronos
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Jun12-14, 03:22 PM
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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.
mongoak
#14
Jun16-14, 07:26 PM
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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.
Drakkith
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Jun17-14, 12:49 AM
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Quote Quote by mongoak View Post
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.
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/Phsx1...ures/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....A1022952621810)

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]
Matterwave
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Jun17-14, 01:00 AM
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Quote Quote by Drakkith View Post
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/Phsx1...ures/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....A1022952621810)

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]
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.
Drakkith
#17
Jun17-14, 01:48 AM
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Ah, that makes sense. Thanks, Matterwave.


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