Sunday, August 16, 2009
Have you ever looked up into the night sky on a particularly crisp, clear evening to witness the glittering symphony taking place above, from so very far away, from so very long ago? And just when the scene acts to take your breath away, do you contemplate what photons go through to travel unimaginable distances from their source to our retinas? It is a fun thought experiment. Let’s play along together.
Consider all the stars, nebulae, supernovae, and galaxies that emit visible light (photons), and consider that many of these photonic sources are found at great distances (high redshifts) and in some cases appear to us as they were some 11 billion years ago, or 80% of the Universe’s lifetime. Now think about the path these photons have taken to Earth. They pass through vast intergalactic voids, through expansive galaxies, through molecular clouds, and untold numbers of other obstacles. They avoid falling into the event horizons of supermassive black holes dwelling in the center of large spiral galaxies. They bend their paths as result of warped spacetime near strong gravitational fields surrounding stars. Eventually, these tired, travel-weary yet faithful photons arrive at the lens of our telescope to provide us exhilaration about learning something new about the cosmos. And now consider that these specific photons are unique in the sense that they have never been seen by another human being before.
Looking through a telescope can be a memorable experience, especially if the person peering into the eyepiece is properly instructed on how to see all that the telescope can reveal and is prompted to realize that the photons striking his or her retina have been traveling through space for seconds, or centuries, or a great deal much longer. And all those photons, now collected by the telescope and concentrated into your eye are in fact yours and yours alone to savor for as long as the memory lives.
This lasting effect is something to think about carefully. The photons enter your retina after sailing their long intergalactic voyage and stimulate your neurons in such a way as to produce a lasting memory, an experience to be remembered and treasured. Yes, powerful are these massless particles, photons that unlock the secrets of a very big universe.