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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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# 【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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