by Alan Eggleston
Imagine a single photon and its journey from the center of a star.
It doesn’t have to be anything exotic, like from a distant galaxy – choose one from the Sagittarius arm of the Milky Way: Still thousands of light years away, it’s made a long journey.
However, to leave the center of the star, it had to fight its way through the dense core and conflagration of the outer layers. Even an average size star like Sol is multiple hundreds of thousands of miles deep. If the photon was birthed in a blue giant or a red giant, they would be even deeper. A star like Sol, a photon can take tens of thousands to hundreds of thousands (some even estimate a million) years to make it to the surface of the star.
Then the photon is free to travel. It may have to penetrate gas clouds and dust lanes, where it may interact with molecules, perhaps get absorbed and re-emitted before continuing its journey. If it’s fortunate, it just zips through interstellar vacuum at 299,792.458 kilometers (186,000 miles) per second. More likely it will be stretched by gravity, bent by atmospheres, and caromed by encounters with other objects. Still, it journeys onward.
And then there you are standing in your back yard, gazing up at the wonders of the Milky Way, taking in everything amazing the sky has to offer – including that photon. It took thousands of years to escape its sun, thousands of years to travel through space, and you captured it in the blink of an eye.
When a photon enters the eye, the lens focuses it onto the retina. It may land on the wall of the retina, or being more hopeful, it hits a photoreceptor (a rod) and the photon’s energy is absorbed and translated into a signal. The more photons you capture, the more signals the brain receives and gathers into a visual perception – “Oh, I’m seeing the Milky Way!”
Actually, it’s more complex and amazing than that. If you’re curious, read about it here.
It is said that the human eye is capable of seeing a single photon under the right conditions. However, experiments first conducted in 1942 showed that about 90 photons had to enter the eye for 60 percent of respondents to say they were seeing light. Only about 10 percent of the photons were estimated to actually reach the retina, so 9 photons actually reached the receptors. Those photons would have been spread over 350 rods, suggesting statistically that the rods were responding to single photons. So says an 1996 article by Philip Gibbs, “Can a Human See a Single Photon?”
The miracle is, you can look up anywhere in the night sky, at any patch of light that’s emitted from tens to hundreds to thousands to millions … to billions … of light years away and capture a photon and stop it in its tracks. Some of it requires a telescope to see the really distant ones, and then to really “see” them needs some time, some patience. Remember, the eye might capture single photons, but it makes sense out of many. Even the most accomplished telescopes require long time exposures to capture enough photons to create a picture of an astronomical object.
A photon doesn’t have a physical dimension. It isn’t a ball you can physically measure. It is particle like, but it is also wavelike. Estimates of a photon’s wavelength are in the order of about 1/1000th the width of a human hair.
After about 30 minutes in the dark, your eyes become accustomed to the low light of night. With good eyesight, you can look up into the majestic sky and see a smudge, a patch of faint light that’s actually 2.54 million light years away – the Andromeda galaxy. No telescope needed.
Isn’t it amazing what you can do in the blink of an eye?