Light is way older than you think

From plants to humans, we all are 'star stuff', as Carl Sagan famously remarked; embedded inside us are the products of stellar fusion, the necessities of sustaining life itself.


The Sun is undoubtedly the elephant in the room. The 1.4 octillion cubic metres gas giant hails its supremacy from the center of the Solar System expulsing its luminosity into the void. Many secrets lie beneath that yellow blanket of light but the interesting thing is that light itself has a secret of its own. It is common knowledge that light takes 8 minutes to reach the Earth from the Sun, so if the Sun extinguishes after some billion years it would take us 8 minutes to notice its absence. The actual answer is 170,000 years plus the miniscule 8 minutes and that giant figure accounts for the time a single particle of light, the photon, takes to travel from the Sun’s core to its surface.


Surprisingly the time light takes to travel 1 AU or 1.496*108 kilometers (the distance between Earth and the Sun), is exponentially lesser than it takes to cover a mere 0.9% of that distance. The reason to this is that when, under the 225 billion atmospheres of pressure, the Hydrogen atoms of the Sun fuse together to form Helium atoms and energy in the form of gamma radiation is given off. The particle of gamma radiation, also a photon, is initially moving at the speed of 300,000,000 m/s. As the better part of the Sun’s mass is made up of plasma, protons are separated from their electrons and form a soup of charged particles. The photons collide with these particles when they move outwards ricocheting off like bullets and losing some energy to the photons after every collision. That is why the true speed of light is 299,792,458 m/s because the original light loses some of its kinetic energy to these protons.


These photons don’t simply bump into a proton and be on their way. When they encounter one, they rebound in random directions missing almost half of the photons in the way. So they walk quite an entangled pathway before being pushed into the vacuum of space. The distance it travels on this path is calculated by the ‘Random Walk Problem’ that is constituted by the formula:


Distance = ‘Step size x Square root of N’

where N is the step number until which the distance is to be found


Preliminary results showed that the time taken for the photon to reach the Sun’s surface from the core was 480 billion years! Naturally this solution was questioned because the age of the Sun itself is only 4.6 billion years.


A solar prominence, one of the Sun’s countless grandeurs, erupts in August 2012, as captured by the Solar Dynamic Observatory, SDO

The problem with this calculation was that the density of the Sun was left unaccounted for. The Sun is denser around the inner 25% of its radius where the core lies and its density further decreases as you get closer to the surface, much like Earth’s atmosphere owing to the pull of gravity. Many other attempts followed that suggested edits to the number all either being very large or having left some factors undiscussed.


Romas Mitalas at the University of Western Ontario, in 1992, used a detailed model of the Sun’s interior and through computed calculations refined the time to be 170,000 years. As of now, this is the most accurate time a single photon takes to reach the Sun’s surface, scientists have calculated. Maybe with a better understanding of the Sun’s structure and development in more accurate computer problem-solving we will further refine that number, but for now, it is astounding to contemplate that the light we see was born two Ice Ages ago, when our ancestors first started wearing clothes.


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