Ever travelled back in time? In the absence of slippery science fiction, telescopes are the oldest and best portals to our cosmic past. And for the recent past, our eyes do just fine.
The phenomenon results from the finite speed of light. Though it travels as fast as Universal laws allow, a blistering 300,000 km/second, it is, alas, limited. That’s why we perceive the sights and sounds encoded therein as they happened prior to our ‘present time’. Nothing we see for as long as we live happens at the same time we see it. That’s simply the way things work, according to a really keen guy called Einstein.
And so, wherever our eyes might wander, they are looking into the past, even if it’s less than an instant ago. The age of our gaze backward depends on the distance to what we see. The greater the seperation, the older the cosmic histories we may observe. Even very small telescopes reveal objects many magnitudes fainter than the naked eye, and that’s how we capture astronomical objects as they were tens or millions of years ago. The information, the light, has taken that long to reach us, and we need a slightly larger “eye” to see it. In a most powerful way, light speaks to us, as it reveals the history of the observable Universe.
Now, as far as my eyes were concerned, this image appeared instantly in the telescope’s mirror. But in reality, a miniscule, but absolutely quantifiable, fraction of a second elapsed before the light left my face to bounce off the telescope’s mirror, and reach finally my camera’s CMOS chip. Not that it matters, but you’re looking at me when I was 1/100,000,000 of a second younger. Ahhh, the good old days!
Extending outward to the Moon’s average distance of 384,000 km, our powers of perception recognize the time delay rather easily. The most famous examples occurred between Apollo astronauts and Mission Control. Each verbal exchange spanned a round trip of no less than 2.6 seconds, which is the time it takes “radio waves” (longer wavelength light) to travel to the Moon and back. Opening conversations with their comrades on Earth, astronauts paused for a few seconds, careful not to speak until they received a full reply. If either side spoke too soon, the conversation made all the nonsense of Abbot & Costello’s “Who’s on first?” performances.
Even if you could bend physical laws to travel at light’s maximum speed, you’d be too slow to accompany an entire conversation to and from any two points in space. As the first syllable of your first word splits your lips and enters your transmitter radio, it begins a light speed journey across space, leaving the remainder of your words behind.
So in reality, the Universe prohibits us from seeing or hearing original sights and messages more than once. All the more reason to appreciate them them. And that’s probably a good thing. Madness would no doubt get the best of us if we observed everything twice!
Who wants to live in a Universe where deja vu has no novelty?
Not me. I’m happy to see everything for the first time only once.