It’s been said that most of what we know about the universe comes to us in the form of light. But what is light?
Well, that’s a darned good question. Scientists have been struggling with that one for ages, and the answer seems equally puzzling. You see, we know that light is a vibration of an electromagnetic field, but it appears to behave as both a wave and a stream of particles called photons. Pretty bizarre!
Back in the 16th century, scientists didn’t know this, nor did they know if light actually traveled through space or just existed “there” and “here” at the same time.
It was the Danish scientist Ole Rømer who, while observing eclipses of Jupiter’s moon Io, measured the speed of light at a stunning 136,702 miles per second. We’ve since learned that its speed is even greater: 186,272 miles per second!
This means that a beam of light covers a distance of 186,272 miles every second. Hard to grasp, isn’t it? You might try a demonstration that I frequently use with my stargazing guests.
Aim a flashlight toward the heavens, and turn it on for just one second. By the time you flip the switch off, the photons of light you’ve created are already 186,272 miles away, and still streaming outward at the speed of light. (This is only in principle, of course, since your flashlight beam is not powerful enough to penetrate Earth’s atmosphere. But stick with me on the concept anyway.)
Pretty remarkable, isn’t it? Now aim your flashlight toward the bright star Sirius in the southwestern sky shortly after dark. The photons (again, in principle only) will shoot out of your flashlight and speed in the direction of Sirius at 186,282 miles per second.
So when will it arrive at Sirius? Well, since this star lies some 52 trillion miles from us, the light will take 8.6 years to reach it, so we say that Sirius lies 8.6 light-years away. In other words, it will arrive at Sirius around mid-November in 2025.
The same is true in the opposite direction. Sirius itself is also emitting photons of light — much more powerful than your flashlight, of course — and those that stimulate your eyes tonight have taken 8.6 years to reach your eyes. In other words, the photons from Sirius you see tonight left that star sometime during the first week of September 2008.
So what does the light that Sirius is emitting right now look like? Well, to find out, you’ll have to be looking when it arrives, in mid-November 2025!
As amazing as this sounds, Sirius is one of the nearest stars we in the Northern Hemisphere can see tonight. The reddish-orange star Betelgeuse, for example, lies some 500 light-years away. Rigel, on the other corner of Orion, lies some 900 light-years distant.
Just imagine all that has taken place on our planet while these photons were journeying toward your eyes!
If this whole concept intrigues you as it does me, I hope you’ll get outdoors on the next clear dark night and begin your time travel among the stars.
— Dennis Mammana is an astronomy writer, author, lecturer and photographer working from under the clear dark skies of the Anza-Borrego Desert in the San Diego County backcountry. Contact him at mammana@skyscapes.com and follow him on Twitter: @dennismammana. Click here to read previous columns. The opinions expressed are his own.