“Night, when words fade and things come alive. When the destructive analysis of day is done, and all that is truly important becomes whole and sound again.” -Antoine de Saint-Exupery
When you look out into the Universe, what is it that you typically think of? Do you think of reliable, fixed stars and constellations? The vast expanse of the Milky Way, with its memorable dust lanes and amorphous shapes?
The unchanging nature of the points of light in the sky?
Maybe you think deeper and farther than that. Maybe you think about the distant galaxies and clusters, and the deepest deep-sky objects we know of. How the light took millions or even billions of years to reach us, and yet how every time we look at them, we see them exactly the same way.
I couldn’t fault you for thinking like this; from mankind’s point of view, the Universe — for all intents and purposes — doesn’t change at all as we view it from one night to the next.
But does that really mean the Universe isn’t changing?
Let me flip this around on you: how much does anything here on Earth — you, your surroundings, even an entire, vibrant city — change in half-a-millisecond?
Not a whole lot, that’s for certain. You only change with the passage of time, and half-a-millisecond is just 0.00000000002% of a typical human lifetime. It’s too short of a timespan to notice any but the most catastrophic changes, and even then you have to look very closely.
So why should you expect the Universe to change substantially over just 0.00000000002% of its lifetime? That’s how much of the Universe’s lifetime passes between one night on Earth and the next. And yet, if you looked at the right objects, you would be able to see meaningful changes from one night to the next.
The objects within our Solar System, for example, are close enough that we can see them moving from night-to-night. Objects closer to us — like Mars, in the foreground — appear to move more substantially than more distant objects like Uranus, visible in the background.
The great cause of all this motion, of course, is our largest nearby clump of matter: the Sun. Objects like planets move at tens of kilometers-per-second relative to the Sun thanks to its gravity, while Sun-grazing comets can be accelerated up to speeds in the hundreds of kilometers-per-second. If you’re in the southern hemisphere, you may be able to get a good view of one now: Comet Lemmon.
Green because of the carbon and nitrogen interacting with sunlight, this photo does an excellent job of tracking the stars from the Earth along with the Earth’s rotation. What you probably can’t tell is that the comet — with a photo exposure time of over an hour — is blurred.
If instead of tracking the stars perfectly, we tracked the comet perfectly, know what we’d see?
That comet is moving relative to the stars behind it, and our ultra-close proximity to the comet makes it abundantly clear.
But what you may not realize is that these “fixed” stars are also moving at tens-to-hundreds of kilometers-per-second relative to us, and relative to one another! It’s only the vast distances between us — measured in many light-years — that make it impossible to detect these changes from night-to-night.
But we can’t really detect changes in ourselves from millisecond-to-millisecond; you simply need to look on longer timescales!
The stars in our night sky shift positions by many kilometers each second. From night-to-night we might not be able to tell the difference, but just as you or I look different when we go weeks without cutting our hair, we can see just how the Universe changes over long enough timescales.
There are gas clouds and stellar remnants tearing through the interstellar medium at these same speeds, including some that move at thousands of kilometers-per-second, even approaching 1% the speed of light!
There are new stars being born — where nuclear fusion ignites for the first time — and stars that run out of fuel, dying in either a planetary nebula or a supernova explosion, depending on the properties of the star.
And on the largest scales, galaxies merge together, triggering star formation and some fabulous cosmic mashups, in processes taking upwards of hundreds-of-millions of years.
And in some of the fastest and most spectacular changes, catastrophic stellar events — like supernovae — can literally appear from nothing over the timescale of just a few nights!
When you look up at the Universe, it may appear static and unchanging, but that’s only because these objects are so far away and our human experiences are so short in comparison with the age of the Universe.
But stick around for a while, and even the most mundane of objects will change for you. Fuel burns, elements fuse, gravity pulls, and physics happens. Just give it time, and you’ll see it for yourself.
We may only be around for a snapshot of it, but make no mistake, it’s never the same from moment-to-moment. From the way I look at it, there isn’t any doubt about it: the Universe is alive.