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

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Discussion Overview

The discussion centers around the concept of reference frames in relation to the universe as a whole, particularly in the context of its expansion. Participants explore whether a reference frame can be defined for the universe, the implications of motion and reference frames, and the nature of the universe itself.

Discussion Character

  • Exploratory
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • Some participants assert that there is no "reference frame for the universe" since its expansion occurs uniformly everywhere, not from a specific point.
  • It is proposed that technically, a reference frame is defined by a set of 3D coordinates, and using oneself or the Earth as a reference frame is considered imprecise.
  • Questions arise about the existence of something outside the universe to define motion, with some arguing that for motion to be meaningful, it must be relative to something external, which does not exist.
  • Participants discuss the vagueness of the question regarding the universe's motion and whether it can be meaningfully addressed.
  • One participant suggests that a more precise example of a reference frame could be a fixed coordinate system, such as the edge of a desk, rather than a moving body like a person.
  • There is a reiteration that the universe, by definition, encompasses all that exists, implying there cannot be an "outside" to it.

Areas of Agreement / Disagreement

Participants generally agree that there is no singular reference frame for the universe and that motion is relative. However, there are competing views regarding the implications of this and the nature of reference frames themselves, leaving the discussion unresolved.

Contextual Notes

Participants express uncertainty about the implications of motion and reference frames, particularly regarding the need for an external point of reference for meaningful motion. The discussion highlights the limitations of defining reference frames in the context of the universe's expansion.

jhus96
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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?
 
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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.
 
phinds said:
There is no "reference frame for the universe". The expansion is not happening from a point, it's happening everywhere.
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?
 
phinds said:
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.
So is the universe relative to anything? Or do we simply not know enough yet to come to that conclusion?
 
phinds said:
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.
What is an "unsloppy" example of a reference frame?
 
jhus96 said:
So is the universe relative to anything? Or do we simply not know enough yet to come to that conclusion?
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.
 
jhus96 said:
What is an "unsloppy" example of a reference frame?
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)
 
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".
 
phinds said:
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.

I think I see what you're saying.
 
  • #10
jhus96 said:
So what exactly is outside our universe?

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.
 
  • #12
Thank you CWatters! I came here with the exact question your link is answering to!
 

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