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  • You may know that it takes light a zippy eight minutes

  • to reach us from the surface of the Sun,

  • so how long do you think it takes light

  • to travel from the Sun's core to its surface?

  • A few seconds or a minute at most?

  • Well, oddly enough, the answer is many thousands of years.

  • Here's why.

  • Photons are produced by the nuclear reactions deep in the core of our Sun.

  • As the photons flow out of the core, they interact with matter and lose energy,

  • becoming longer wavelength forms of light.

  • They start out as gamma rays in the core,

  • but end up as x-rays, ultraviolet or visible light as they near the surface.

  • However, that journey is neither simple nor direct.

  • Upon being born, each photon travels at a speed of 300,000 kilometers per second

  • until it collides with a proton and is diverted in another direction,

  • acting like a bullet ricocheting off of every charged particle it strikes.

  • The question of how far this photon gets from the center of the Sun

  • after each collision

  • is known as the random walk problem.

  • The answer is given by this formula:

  • distance equals step size times the square root of the number of steps.

  • So if you were taking a random walk from your front door

  • with a one meter stride each second,

  • it would take you a million steps and eleven days

  • just to travel one kilometer.

  • So then how long does it take for a photon generated in the center of the sun

  • to reach you?

  • We know the mass of the Sun

  • and can use that to calculate the number of protons within it.

  • Let's assume for a second that all the Sun's protons are evenly spread out,

  • making the average distance between them about 1.0 x 10^-10 meters.

  • To random walk the 690,000 kilometers from the core to the solar surface

  • would then require 3.9 x 10^37 steps,

  • giving a total travel time of 400 billion years.

  • Hmm, that can't be right.

  • The Sun is only 4.6 billion years old, so what went wrong?

  • Two things:

  • The Sun isn't actually of uniform density

  • and photons will miss quite a few protons between every collision.

  • In actuality, a photon's energy,

  • which changes over the course of its journey,

  • determines how likely it is to interact with a proton.

  • On the density question,

  • our models show that the Sun has a hot core,

  • where the fusion reactions occur.

  • Surrounding that is the radiative zone,

  • followed by the convective zone, which extends all the way to the surface.

  • The material in the core is much denser than lead,

  • while the hot plasma near the surface is a million times less dense

  • with a continuum of densities in between.

  • And here's the photon-energy relationship.

  • For a photon that carries a small amount of energy,

  • a proton is effectively huge,

  • and it's much more likely to cause the photon to ricochet.

  • And for a high-energy photon, the opposite is true.

  • Protons are effectively tiny.

  • Photons start off at very high energies

  • compared to when they're finally radiated from the Sun's surface.

  • Now when we use a computer and a sophisticated solar interior model

  • to calculate the random walk equation with these changing quantities,

  • it spits out the following number: 170,000 years.

  • Future discoveries about the Sun may refine this number further,

  • but for now, to the best of our understanding,

  • the light that's hitting your eyes today

  • spent 170,000 years pinballing its way towards the Sun's surface,

  • plus eight miniscule minutes in space.

  • In other words, that photon began its journey two ice ages ago,

  • around the same time when humans first started wearing clothes.

You may know that it takes light a zippy eight minutes

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B2 TED-Ed photon sun surface core proton

【TED-Ed】Sunlight is way older than you think - Sten Odenwald

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    稲葉白兎 posted on 2015/05/22
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