# What's the reference frame for the universe as a whole?

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1. Sep 23, 2015

### jhus96

I'm just beginning physics and teaching myself classical physics from an intro physics textbook and in the 2nd chapter it described reference frames. This got me thinking. My reference frame is the earth, as that is what I use to describe position, displacement, ect. But what is the reference frame for the universe? I'm not even sure if this is a valid question because we don't know if our universe is moving or not; however we know it's expanding, so mustn't this motion of expansion be in some sort of reference frame?

2. Sep 23, 2015

### phinds

There is no "reference frame for the universe". The expansion is not happening from a point, it's happening everywhere.

Also, technically, your reference frame is not the Earth, your reference frame is YOU. You can take some set of coordinates based on the Earth as a reference frame and measure your motion in that reference frame, or you can take yourself as a reference frame and measure the Earth's motion in that frame. For example, when you are walking, the Earth is spinning under you in your reference frame.

All of that is a bit loose, however. Really, a reference frame is a set of 3D coordinates, so talking about you or the Earth as a reference frame is a bit sloppy.

3. Sep 23, 2015

### jhus96

So what exactly is outside our universe? If anything at all? Also, even if the universe is expanding at different points, it doesn't matter because there's motion happening, and If there's motion there must be some way of knowing there's a motion, thus there has to be a reference frame to decipher the fact that there is motion. Theoretically there can still be a reference frame right?

4. Sep 23, 2015

### jhus96

So is the universe relative to anything? Or do we simply not know enough yet to come to that conclusion?

5. Sep 23, 2015

### jhus96

What is an "unsloppy" example of a reference frame?

6. Sep 23, 2015

### phinds

That's a question that is phrased in such a way as to imply that it has an answer but it is so vague that it doesn't really.

7. Sep 23, 2015

### phinds

As I said in my original response, a 3D coordinate system. Take the edge of your desk. Excellent frame of reference. YOU are not a good frame of reference because you are a wiggly non-rigid body. The universe is not a frame of reference because it has lots of moving parts (that is, parts moving relative to each other)

8. Sep 23, 2015

### phinds

And by the way, there's an issue in your OP that I didn't address because I didn't want to cloud the issue about FOR's and that is that you asked if you universe is moving. For "moving" to be meaningful, you have to specify what it is that you are talking about something moving relative to, since all motion is relative.

For the universe to be moving relative to something, that something would have to be outside the universe, but there IS no "outside the universe".

9. Sep 23, 2015

### jhus96

10. Sep 24, 2015

Staff Emeritus
The universe is, by definition, all that there is. There is no outside.

A reference frame is a coordinate system. You can have as many as you want. There is no "the" reference frame for anything.

11. Sep 25, 2015

### CWatters

12. Feb 28, 2016