It’s always sunny above the clouds

lyme bay sunset
The Sun setting over Lyme Bay – photons arriving at the end of a journey which has taken 170,000 years

The amazing journey of sunlight to the Dorset coast

Science feature by Professor Glenn Patrick

BLUE Monday – the most depressing day of the year – supposedly falls on the third Monday of January. This claim made by a psychologist in 2004 is often quoted in newspapers, but does not appear to be based on any real science.

What is true, though, is that mid-way through the winter months, when the Christmas and New Year celebrations have faded, we start to yearn for the warm spring and summer sunshine.

Charles Dickens neatly captured the essence of our relationship with sunshine in his novel Oliver Twist when he said, “The Sun, the bright Sun, that brings back not light alone, but new life, and hope, and freshness to man”.

sun
Deep inside the core of the Sun lies the nuclear furnace which is the source of our photons of sunlight [Credit: SOHO (ESA & NASA)]
Without the Sun, life on Earth would, of course, be impossible. We are incredibly lucky to live on a planet that orbits within the habitable zone of our local star receiving just enough stellar radiation – or sunlight – to maintain a temperature which enables liquid water to exist on the surface and to support life.

If we were located beyond the outer limits of this zone, the Earth’s temperature would be too low, and the surface water would be frozen. On the other hand, if we were much nearer the Sun, the temperature would be too high, and the water would have vapourised.

One of those Big Questions we all ask is… are we alone in the Universe? Over 4,000 planets orbiting other stars – so called exoplanets – have been discovered in recent years. Astrophysicists pore over the data to detect signs that they also support life, but so far, we remain unique.

It has taken us many centuries to understand the nature of light. Up until the time of Isaac Newton, it was thought that light consisted of streams of particles called corpuscles.

In the mid-17th century, the wave properties of light began to emerge, especially through the work of the Dutch scientist Christiaan Huygens.

In the 19th century, the Scottish physicist James Clerk Maxwell came up with his mathematical equations for electromagnetic radiation, which describe light as oscillations of electric and magnetic fields travelling through space.

These dual personalities of light – particle and wave – became even more interesting when in 1900 the German physicist Max Planck found that he could only explain the spectrum of the Sun by assuming that the light was emitted in discrete packets – or quanta – of energy.

Albert Einstein went on to explain the photoelectric effect in 1905 through the concept that it is light itself that is quantised – kickstarting the revolutionary ideas of quantum physics. These packets of energy – or quanta – were eventually labelled “photons” by the American chemist G.N. Lewis in 1926.

Today, depending on the situation, we use both the wave and particle properties of light to explain optical effects like reflection, diffraction and interference, but the photon remains the fundamental building block – the basic globule – of all light.

Our world would not exist without the photon! Not only do photons enable us to see, but they also provide the energy for photosynthesis, which fuels all plant life on Earth.

The photon is also the particle responsible for one of the four fundamental forces of nature – electromagnetism. Without photons, our atoms would fall apart, and our electronic devices would not work.

In the natural world, the main source of photons is the Sun. Travelling at the speed of light, photons take on average eight minutes and 20 seconds to travel the 150 million kilometres from the Sun to reach us here on Earth.

What is even more astonishing though is that before they undertook this final journey these photons were created many thousands of years earlier by nuclear reactions inside the Sun!

The Sun’s gravity – 28 times stronger than the Earth’s – traps hydrogen deep inside the solar core. At a temperature of 15 million degrees Celsius, the hydrogen gas is stripped of electrons, so it becomes a plasma of protons.

In a series of reactions, pairs of protons fuse together to produce helium along with a burst of energy released in the form of photons. These photons must then traverse through the layers of Sun to reach its surface before they can begin their journey to Earth.

Inside the Sun, the photons do not travel in a straight line because they suffer collisions with other particles and their chaotic journeys are estimated to take 170,000 years!

Once the photons reach the cooler, less dense outer layers of the Sun, they no longer collide and can escape!

As we sit in the Dorset sunshine or gaze at those amazing sunsets over Lyme Bay, we should pause to wonder at the extraordinary journey that each photon takes over thousands of years before entering our eyes and sending an electrical signal to the brain to give us vision and sight.

We also have photons to thank for keeping the Earth at a tolerable temperature. Most of the Sun’s energy is radiated as visible light and during the day this warms the Earth’s surface because this incoming radiation is not absorbed by the atmosphere.

The warm Earth then radiates heat back into the atmosphere in the form of infrared radiation, which has a longer wavelength than visible light.

Nitrogen (78 per cent) and oxygen (21 per cent) in the atmosphere cannot absorb the infrared light, but it is avidly absorbed by traces of water vapour and carbon dioxide. The trapped heat in these gases is what keeps the Earth at a cosy 14 degrees Celsius on average.

This process – the greenhouse effect – often gets a bad press due to global warming and the extra heating caused by human emissions. Without the natural version though, the Earth’s temperature would drop to around minus 18 degrees Celsius – not so comfy!

So, forget all about Blue Monday. Instead, celebrate our good fortune that photons make this incredible journey from the centre of the Sun across the Solar System to this special place that we call Earth.

Remember the words of the author Caitlin Moran who said: “It is always sunny above the clouds. Always. Every day on Earth – every day I ever had – was secretly sunny after all. If you fly high enough, if you get above the clouds, it’s never-ending summer.”

Glenn Patrick is a particle physicist and science communicator who explores the quantum world of sub-atomic particles (including at the Large Hadron Collider) and now lives in Lyme Regis.

Woodmead Halls

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