It has always amazed me that what we see in the night sky is from the past and not the present. And, I have always wanted to talk about it.
When I see a star in the sky, it makes me wonder what would have been happening on Earth when this star was actually like what it seems. As you all must be aware, the celestial objects are so far away from us that it actually takes light from them to take quite some time to reach us, which means that what we see of them at any given moment is in fact already historical. Some objects like planets could take minutes while deep sky objects could take millions of years to send light back at us.
You know how extraordinarily fast light is. If you remotely turn on a light bulb on the 104th floor of the WTC building while standing at the ground level, you can notice how instantly you see it glow (given the verity of all other obvious factors). Light travels at a speed of roughly 186225 miles/s (or 299700 km/s) in air. Imagine, these celestial bodies are so far from us that the scientists decided that it’s best to define their distances from other heavenly bodies in terms of the distance light travels in a year (in vacuum, slightly more than that in air) which is a lot (nearly 6 trillion miles), and called the unit “light-year”. Isn’t that fascinating in itself? So, from a star that is 60 trillion miles away from the Earth, light takes around 10 years to reach us, and we call the star to be at a distance of 10 light-years from the Earth. This also means that what you see of this star today is actually 10 years old. Already wondering what could have been happening in your life 10 years ago, when the light from this star started towards you?
A word of caution here though. The statement “what we see of an object X light-years away is X years old” is not completely accurate. It is actually slightly more legitimate in some regards than others. The theory of relativity comes into play. But let’s not get into all the jazzy details and instead try to remain in the confines of the thought and be amazed!
Now, back to the thought!
If you are wondering how old the Sun‘s image is, that we see at any moment, don’t get too excited! It’s only about 499 seconds (8.3 minutes) old. The Sun is approximately 93 million miles away from the Earth (when Earth is closest in orbit). Just for your information, this distance of the Sun from the Earth is also called 1AU (Astronomical Unit) and this unit is also used to sometimes define the distance of other celestial bodies. The image of Jupiter is roughly 33 minutes (2000 seconds) old at any given moment for us (when it is the closest to us, it will be older when it’s farther), the planet being approximately 365 million miles away from us. Moon is 1.3 light-seconds away from us (roughly 240,000 miles). Alpha Centauri, the star second nearest to the Earth, is around 4.3 light-years away from us. The farthest known galaxy, MACS0647-JD, is a staggering 13.3 billion light-years away from us. Wow! What must have been really happening on Earth when the light from this galaxy started out for us? Well, nothing really! The Earth did not even exist back then. Our planet is only 4.5 billion years old. In fact, the universe itself is considered to be 13.7 billion years old. Ah, even as I speak, I am experiencing goose bumps!
Ever felt like turning back time? I know I did (feel not turn)! Or felt like turning your time traveling dream into reality? Well, who hasn’t? On countless occasions of reminiscing something about my past, I have wished that I had a newspaper from that period so that I could read about what transpired that day. The sky can probably narrate a similar story. Don’t you think so? Well, maybe from the stars whose distance in light years is more than my age in years, I might not be able to dig out much, But, nevertheless, they speak of some story in the past, if not from my life, maybe my parents’ or my ancestors’.
Now. let’s take a step further and look at an even fancier perspective! Consider the time since the Earth came into existence. Although the Earth might never have been bright enough to send light to these stars, but let’s just say magically, without the complete annihilation of the light emitted or reflected by Earth, this light did reach some of these stars. According to the principle of reflection, at least a tiny bit part of this light might have been reflected by these stars. And after further decimation of this light, let’s say the remnant actually made it back to the Earth. So, the light leaving Earth around 9 years ago would have reached Alpha Centauri in around 4.5 years (rounding off the distance of 4.3 light years, for simplicity). This light would then have reflected back from the surface of Alpha Centauri around that period and now it would reach back to us. Woohoo! Does that mean that there might be an infinitesimal bit of possibility that we would actually be seeing something from our own past on Earth? Indeed! I would be seeing a college-going me. Want to go back in time and see the dinosaurs before they were wiped off the face of Earth? Dinosaurs were roaming around on Earth until roughly 65 million years ago. Doing the math, in order for their images to reach us now, any light from them should have reflected off a star about 32 million light-years from us. You might want to check out the Sombrero Galaxy (also known as Messier Object 104) which is roughly that far from us. Or how about reliving a glorious moment that just happened in your life about an hour ago? Go see Saturn!
If, hypothetically, looking at our past was doable just like that, imagine how our lives would be! With sky as our time machine, we could time travel through our eyes and relive our endless past. Fascinating! Wouldn’t that be?