This World and Me

We are a speck, a sliver, a grain in this massive universe. No, "massive" would be a huge understatement. The universe is indescribably large. First of all, there are two kinds of perspectives you can take for sizing the universe. There is the universe, and then there is the "observable universe".

• The universe is the "totality of existence". It's the sum of all matter, energy, and events that exist.
• The observable universe only includes the part of the universe which Earth can detect and observe through the use of tools such as telescopes.

Wait. We can only see a part of the universe? That's right. Everyone knows that light travels at a constant speed in space: the speed of light. Everyone also knows that nothing can travel faster than the speed of light. Light itself cannot travel faster than the cosmic speed limit. From the point of the Big Bang to now, the universe has expanded quite a lot. It's even created us and the Earth. But light emitted by all the celestial objects takes time--a lot of time--to reach Earth. The observable universe comprises only of the objects which light from them has had time (from the point of the Big Bang to now) to reach us.

Visual rendition of the observable universe.

The speed of light is approximately 186,000 miles per second. That's pretty fast. If you could go that fast, you could whip around the equator of the Earth 7.5 times in one second. But the universe is really really big. Light from the Sun requires 8 minutes to reach Earth. As one can imagine, that's a pretty big distance. So technically, when you look up at the Sun, the light you see was actually emitted by the Sun 8 minutes ago. By looking at the sky, you are staring at the past.

It gets even wackier. Imagine you in a spaceship traveling at the speed of light in the direction of the closest star. Even at the highest speed possible, you'd have to travel 4 years until you reach it. There are unfathomably large expanses of space between stars. We use a unit called the lightyear to measure distances in space. Simply put, a lightyear is the distance light travels in one year. The closest star is 4 lightyears away. So technically, when you look up at the night sky and see a star, the light which you are seeing from that star was actually created several years ago, but only just now reaching its ultimate destination: your eyes.

The Andromeda Galaxy, the nearest spiral galaxy to the Milky Way.
It is expected to collide with the Milky Way in 3.75 billion years.

But the universe gets bigger than just the distance between two stars. With regards to the size of the universe, the distance between two stars is very very tiny. A cluster of hundreds of billions of stars is called a galaxy. The distance between our galaxy and the nearest galaxy is 25,000 lightyears. You think that's a big distance? Now imagine that distance between every galaxy of the universe (of course, the distances between different galaxies would be different). Scientists estimate there are 100 to 200 billion galaxies in the universe.

The next time you look up at the night sky, think about the vastness of the universe and realize that you are so very tiny compared to it. It makes you respect this planet which has hosted us for many thousands of years. It also makes you respect all the things you see in the sky, for you are looking at light which was created perhaps long before humans ever existed. And that packet of light had one destiny: to reach your eyes, billions of years later.

Image Credit: 8 Observable Universe (blank).png, Andrew Z. Colvin, CC-BY-SA; Andromeda Galaxy (with h-alpha).jpg, Adam Evans, CC-BY