The Incredible Journey of a Million Year Old Messenger

Amy Fok

For centuries, sitting on this carousel, how many times have poets, scientists and philosophers of the past been inspired by the stars of the night sky? Searching for the secrets of life and nature, wondering what was out there, our imagination has been stretched to the utmost. And yet every time we look and feel the vastness of the heavens, our lives cross with the same friend who has traveled millions and billions of years. This friend is the messenger of the Universe- photons, particles of light.

When I said light, I didn't just mean the light that we see, from red to blue. In fact, visible light is just a part of a long spectrum. There is light that we can't see, like UV light but it can affect photographic plates. The scale of light is described by numbers called frequency. As the number gets higher, we go from radio waves to visible to UV, to X-rays and so on. They are all light! For those who were told at school that light behaved like waves, I want to emphasize the way light does behave- like particles. This is a difficult concept and requires a lot of imagination. We will not deal with this here; instead we will simply concentrate on the journey of one of the many that we meet on a clear night.

Hubble realized that the Universe is expanding; the consequence is that light from the early universe (13 billion years ago) has mostly been shifted to low frequencies that our eyes can't detect. So most of the light that we see today are those that started their lives millions of years ago at the core of a star. Imagine a star just like our sun in a nearby galaxy- the Andromeda Galaxy; we know that nuclear reactions are continuously going on inside a star in order to make them shine. It is precisely this chain of reactions that gave birth to our highly energetic little messenger!

After it's birth, however, our photon had to take a tortuous path to escape the star. Inside the million degree furnace, it constantly bounced off nuclei, being absorbed and re-emitted countless times. After about a million year of this serpentine flight, it would work its way out of the star. And the journey has now begun!

Our traveler rushed through the interstellar space of the Andromeda Galaxy, passing trillions of stars, billions of bizarre planetary systems, or even more exotic monsters. Our photon was lucky not to be absorbed by nebular gases and become the ingredients of new stars and planets. After some hundreds to thousands of years, it would finally come to the edge of its galaxy, hurdling through the globular clusters, it would leave the Andromeda galaxy behind and begin a new stage of its journey- a journey of solitude into the intergalactic space.

The distance from the Andromeda Galaxy to us, the Milky Way, is immense. During this lonesome flight, many things happened in our small world. Ice ages came and went, some species flourished and some became extinct! And yet our photon has not reached us!

After 2.2 million years of solitude, our photon was finally within reach of a most magnificent spiral disc, with bands of bright young stars lighting up intricate swirls in the disc- the Milky Way. Falling into this starry spiral, it would be pulled by an invisible super massive giant lying at the heart of our galaxy. Our friend was fortunate enough not to get too close and swallowed by fiery inhabitants; instead it found its way towards a jewel- Planet Earth.

At last, the million-year journey had almost come to an end, as it entered the earth's atmosphere, its path bent through a process called refraction due to variations of the air's density. Most of its companions ended their journeys by hitting the ground and trees. Our photon would however end its journey by crossing its life with us.

Now I have been calling our friend a “messenger”, why is that? Through understanding the information contained in light, it was possible for humanity to describe events from the first microsecond after the Big Bang to the formation of galaxies, and explore the mysteries of dark matter and black holes. For instance, during its flight, our photon would follow the curvature of space. Physicists have, for decades, been trying to figure out the shape of the Universe. This photon knows the answer. Our future of understanding the Universe lies at the heart of detecting and understanding these tiny particles.

On the next clear night when you gaze upon the sky, in the infinite Universe of all possibilities, you are crossing lives with friends from another world who have traveled millions of years to meet us!