Movement in space

Look around you.

Where are you? Where is this place

You are occupying? somewhere in a room, maybe in a city on a continent on a planet orbiting a star in a galaxy among billions.

But… where is all of that? While this may feel like a daft question, it turns out that the concept of an absolute position is something humans made up. In a nutshell, the universe is a big bag of space that has things in it. If someone removed all these things, the stars and planets and black holes and dust, there would just be empty space left. In empty space, the concept of having a position loses all meaning. Empty space is uniform, the same everywhere. The space we occupy is not like a stage under our feet. We can't mark specific points, or staple something to spacetime as an anchor. Without things there is no position. We are really ever only at a position in relation to something else.

Which also means that things we take for granted, like up and down are actually relative too. Ok, let us fill the universe up with things again and try to find out where you are, right now. Relative to everything else. Let us start at a familiar place and then get increasingly weird. From your perspective the world is seemingly flat and you can move in 3 dimensions. It's what physicists call a 'frame of reference' - the perspective you have of the universe and how you see things moving around you. Where your up and down is. Your frame of reference is correct. For you – but not for the rest of the universe. Around 5 kilometers from you, where the horizon begins, the ground noticeably curves away from you.

If you could see through the ground, you’d see people from below or sideways. But they don't fall 'down' off the planet because gravity doesn't actually pull them down. 'Down' is an illusion of your reference frame. To earth's reference frame, gravity just pulls in. But for humans there is an up and down because within our frame of reference that just makes sense. Which is also why we think that the planet itself has an up and down, north and south. And we made our maps accordingly. But an observer looking at the solar system might disagree. Our maps make sense to us because we are used to them, not because they are correct. Ok.

So this is step one – your position on what feels like a flat surface but is actually a sphere. But this sphere is always moving, never staying in one position. Earth is orbiting a star, the sun at the center of the solar system. While we usually imagine this as pretty orderly, someone looking at us from the outside would see something pretty messy. To see this more clearly we’ll exaggerate all these movements, this is not to scale.

First of all, our orbit really is

An ellipse, so we spend half the year sinking a little bit closer to the sun speeding up, and half the year rising up a bit and slowing down.

And the ellipsis itself changes its shape every 100,000 years too. And in another cycle of 112,000 years, the ellipse itself is drifting – which at least creates a beautiful shape. In the end, we get an orbital path that looks like a wobbly circle with wavy edges. And it gets worse, as the moon now starts to screw things up too.

As the moon is a pretty massive thing, it pulls on earth. Both objects orbit their common center of gravity, that lies around 4700 km off to the side of Earth’s core. In practice this means that as the moon orbits earth, it is jerking earth around a bit, enough to make it jiggle. Ok so you are standing on the surface of a rotating planet, that is jiggling around the sun in an elliptical orbit that changes a bit every year. But who's to say the earth is right? From the perspective of the sun the plane of the solar system is arbitrary, it's defined as the plane the earth orbits in because that is convenient for us. In reality the other planets are just a little bit inclined with respect to our plane. From their point of view, we're the ones with a slightly bent orbit. But this is not it – far from it! The solar system as a whole is orbiting the center of the milky way galaxy. If we look at the milky way, we can clearly make out a galactic plane in which the solar system orbits the center every 230 million years. But of course it is not that simple.

First of all, the plane of the solar system is not aligned with the plane of the galaxy. Nothing really is – just like the planets in the solar system orbit the sun on their own planes, so do all the stars orbiting the galactic center. The solar system as a whole is tilted about 60 degrees towards the galactic plane, speeding through space at almost a million kilometers per hour. Someone in the center of the galaxy would see the orbits of the planets moving through space in a helix shape, which you can imagine as a corkscrew motion, on the tilted plane of the solar system, relative to the plane of the galaxy. This orientation in space means that sometimes the planets are sort of in front of the sun as it orbits around the galactic core. Let us just look at this for a moment – there is a strange and eerie beauty about how our planets and the sun move through space. Do you feel a bit dizzy? It gets worse. This is still not the whole story because the mass of the galactic disk is constantly pulling on the solar system too. Like a drunk dolphin, we're diving down and shooting up hundreds of lightyears through the galactic plane, ten times every orbit, along arcs thousands of light years long. We haven’t mapped this motion out completely, as it takes the solar system tens of millions of years to go up and back once; and, well, humanity is not that old.

Let’s look at your relative position again. On a planet, tilted towards the sun, jiggled around by the moon. In a solar system tilted towards the galactic plane, moving forward in a helical shape, diving up and down through the plane. Hm. But where is “up” in a galaxy? At this point the frame of reference becomes a bit arbitrary and pointless because of the nature of the universe at this scale. The Milky way is part of a galaxy group that appears to be part of greater structures like the Laniakea Supercluster, which itself is part of the gigantic Pisces–Cetus Supercluster Complex and finally a galactic filament that spans hundreds of millions of lightyears in all directions and orientations. Someone looking right at us from that far away will only see the End of Greatness. All stuff appears homogeneous, the same everywhere. Just like with empty space, when everything looks the same, who's to say anyone's view is better than anyone else's? We have reached the end of our little exercise in cosmic humility. Let us make the journey backwards again.

From the indescribably large, to the really large, to our galactic home, to our galaxy, to the solar system diving up and down through the milky way, to the jiggle of existence. And finally, back to you, right now. If this is all a bit much, don’t feel bad. The scale of the universe is brain crushing and trying to keep track of how everything is oriented or decide where the best up and down is, is hard. But it doesn't really matter. Because it doesn't change where you are. You're already in the best spot you could possibly be – right here, right now. For all you care nothing can stop you from being right at the center of your own little universe. Perspective really is everything. That is true for the universe we all inhabit, but even more so within your personal universe.