ELI5 So, is everything in space moving? Like everything and in different directions? Is anything stationary?

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Movement is relative, you have to define motion relative to something. There is no absolute frame of reference.


The closest thing there is to an absolute frame of reference is the cosmic microwave background. This is weak electromagnetic radiation left over from an early phase of the universe where space was filled with dense plasma. You can tell if you are moving relative to this background by looking at the Doppler shift in the radiation.


Stationary to what? The universe has no centre, location is only able to be determined if you compare it to something else. I am sitting on my chair, I am not moving. But the earth is moving around the sun, so I am moving too. But the sun isn’t moving by the looks of it, except it is orbiting the milkyway like other stars. The milkyway is moving through space as it expands. This is what relativity is conceptually. It is the concept that something only has something like motion when you can compare it to something else, but that thing varies based on what you look at it from. It is kind of like sitting in a pitch black room and asking what colour the walls are. If there was a bright red light, the walls would appear red. If the light was green, the walls would appear green. Does that change the colour of the walls? Does a dark room have black walls because the room is black? Does the question even make sense? You could say “hey but the walls are painted white”, but I could just say “white is just the colour you observe when in white light”, the real colour totally depends on how you observe it.


There is no absolute frame, so there is no “stationary” object. Or. every object that wants to consider itself stationary defines a frame of reference which is equally valid.


Yes and no. Motion is entirely relative. What this means is that “stationary” and “moving” don’t make sense in isolation. Imagine a single asteroid, completely alone is the infinity of otherwise empty space. To ask whether or not it is moving is a meaningless question. There’s no way to tell, and it doesn’t make sense anyway. Now imagine there are two asteroids. We can now answer the question in a different way. Is the distance between them growing or shrinking? If shrinking, we can say they are moving towards each other, or vice versa. In this situation, it is impossible to say whether they are both moving toward each other, or if one is stationary and the other moving towards it. Again, in this isolated context it’s meaningless and impossible to say. Finally, imagine a third object, like a star. We can now frame our answers against this reference point. Maybe asteroid A maintains its distance and position relative to the sun, but the distance between the two asteroids is shrinking. Given that A’s distance to the sun is not changing, we can infer that B is the one that is moving. This is still ultimately all relative though, because the sun my night moving too (relative too…) The best way to think of it is as a frame of reference constituting a self contained system. Given the solar system, we can frame all objects within it against a stationary sun. We know that the sun and the whole system is moving around the Milky Way, but this isn’t relevant to our smaller frame of reference.