[jamesmustain (OP)]

Hi,
Is there anything in the universe that is not in motion? The galaxies are in motion so I would assume that means everything within them is also. Is there any kind of matter in between the galaxies that is not in motion?
Thanks!

[Kryptid]

Motion is relative, so it depends upon your reference frame.

[Hayseed]

It does not matter what frame you are in.   EVERYTHING is in motion.  Everything ALL the TIME.   It's a pillar of physicality.

[Halc]

It does not matter what frame you are in.   EVERYTHING is in motion

Kryptid is right. It is very much frame dependent. For instance, my house is 25m north of Bob's house, and has been so for the entire existence of said Bob's house. My house has never moved in the frame of Bob's house, and neither has Earth, which has remained steadfastly directly below Bob's house the entire time.

[yor_on]

That's definitely true, I use the same idea as Kryptid. Although everything is in 'motion' relative any other frame of reference. If you take two objects, defining it from your own frame of difference (eh, sorry, 'frame of reference'':) you should be able to get to different relative 'motions' versus your own 'standfast' platform of observation. If you find that to be true then 'motion' exist. If two doesn't help you just need to add some more to see that the 'motion' of them will differ.


syntax

[yor_on]

When it comes to what a relative motion is, then that is physics, and according to physics there is no way and no experiment done locally proving to you that f.ex earth 'moves'. A example of it would be measuring red and blue shifts in a so called 'black box' on earth.So if you like you could use earth and define all other speeds from it. The problem with that is that this will hold for any other object in a relative motion too. None of them will be found to 'move', locally defined (experimentally). So the only way to define a motion is relative what you define as your anchor, 'unmoving' and that will be a arbitrarily choice.

so when you see that everything moves, then that is true, but only as defined relative yourself, the solar system, earth, etc.

[William Hardy]

Yes, each and everything in-universe is in motion. Due to Special Relativity, all things are in motion in relation to one another, and none of them have superior frames of reference.

[Colin2B]

Due to Special Relativity, all things are in motion in relation to one another

Incorrect, that isn’t due to SR. SR does not assign a cause to the motion.
Also, as has been pointed out, not everything is in motion relative to all things.

[geordief]

Incorrect, that isn’t due to SR. SR does not assign a cause to the motion.
Also, as has been pointed out, not everything is in motion relative to all things.

Surely even the example given is subject to relative motion over a long enough period of time?

Panta rhei still applies, don't it?
               

https://en.wikipedia.org/wiki/Panta_Rhei

 

[Eternal Student]

Hi,
Is there anything in the universe that is not in motion?

   You've had some good answers and they've made the general statement that there is no special or "ether" frame from which motion can and should be determined.

   I think it's time to look upon things as a bonus, or a benefit not a worry and a concern.

You're worried that everything is in motion.  Stop this.

Be grateful that most objects don't have to be in motion in some frame of reference.  All objects (with mass) have a rest frame - a frame of reference in which they are at rest.  What's more is that their reference frame is "just as good" as any other reference frame.

So, looking at the glass as being half full, not half empty:    Most things in the universe are at rest.

(It gets a bit trickier if you consider massless particles - but hey, let's be positive about this.  A rest frame for a photon is an extremely degenerate case and isn't really "just as good" as some other reference frames).

[Zer0]

Panta rhei still applies, don't it?
               

https://en.wikipedia.org/wiki/Panta_Rhei

Thanks for sharing that Link.
👍
Wasn't aware of it.

Relating to the OP.
Strictly in terms of 4D Fictional Fantasia...maybe, perhaps EveryThing Really is Moving.
(H.G. Wells)
🕰️

If Bob's home is qunatified to exist in a x, y, z, t. Dimensionality...
Then, even if x,y,z. Remaining constant, t is an absolute variable.
For that matter, even Bob, the planet he lives on, & the UnIverse he exists in...Are all Moving!
🏡
(Yep! I admit it's Silly word play & sorry for it. Just a simple Warning shall be Sufficient to cease all the Sillyness)
😊
🙏




P.S. - The Cat that Jumped off the ground, is Different from the Cat that landed back on the ground...& the Cat that was in air, was Different too!
(Meow!)
🐈

[CliffordK]

What I don't think has been fully defined is if there is a universal reference frame.  Some kind of "Fabric of Space" or Aether.

I.E.  In all reference frames, can an entity accelerate to the speed of light equally in any direction?

One reference frame might be the Cosmic Microwave Background Radiation (CMB, CMBR). 

I think astronomers are able to discern motion with respect to CMBR.

[Halc]

In all reference frames, can an entity accelerate to the speed of light equally in any direction?

Relative to a Minkowskian inertial frame, an entity cannot be accelerated to c in any direction.
In non-Minkowskian spacetime or relative to some non-inertial frame in any spacetime, it is quite possible to move at c, sometimes in any direction, sometimes only in limited directions.

One reference frame might be the Cosmic Microwave Background Radiation (CMB, CMBR).

You are speaking of the cosmological frame, which yes, is most easily measured by observation of the CMBR.

I think astronomers are able to discern motion with respect to CMBR.

The CMBR is not an object and doesn't have a location or speed, but the frame is the coordinate system that locally corresponds to the inertial frame in which the CMB appears isotropic. Relative to such a frame, local motion is defined as peculiar velocity, which cannot exceed c, but recession rates (which do not qualify as velocity since they are not vectors) very much do exceed c, no acceleration required.

[Eternal Student]

Hi everyone.

   I've only been using this forum for a short time but it seems you (moderators and regulars) get a few questions that come up time and time again.
   People give answers that are quite often strictly correct but sometimes not all that useful to the person who asked.  I'm surprised you don't have some short-cuts - like links to previously discussed threads, or bookmarked hyperlinks to good explanations from other sources.

CliffordUK - if you fhave some difficulty understanding all of this, then I wouldn't worry.  That probably shows that you are beginining to grasp the complexity of it all.

Aether.

    "Aether frame" has a historical meaning.  People are going to say "no" there is no Aether frame every time you ask the question and use this term.  It doesn't matter if you were leading onto something else or something similar.

 

One reference frame might be the Cosmic Microwave Background Radiation

    There are some research papers that call this  "the CMB frame".   Halc has suggested "the Cosmological frame" which may also be acceptable terminology.   (I don't know Halc,  I've seen CMB frame more often than  Cosmological frame - but I'm not claiming to have read too far and wide on the topic).

I think astronomers are able to discern motion with respect to CMBR.

     Yes.  You (an astronomer) would know if you were at rest in the CMB frame.  If you were at rest in the CMB frame then the Cosmic background Microwave background radiation (CMBR) would be "isotropic".  This just means it would look much the same in every direction.  If you were not at rest in the CMB frame then the CMBR would look redshifted in some direction and blueshifted in another direction.
     Halc went on to describe some complications that arise because the universe is thought to be expanding.  Well this is where it gets complicated. 
It's important to note that there isn't   JUST ONE   CMB frame.     There is a local CMB frame at every point in the universe, so there are a lot of CMB frames.    Under General Relativity we give up on constructing inertial reference frames that can be applied globally  ("universally") - this happens by chapter 2 of most good books on GR.  Instead we accept that inertial frames can only be constructed locally around each point.  If you were at rest in the CMB frame at your point in space x,   then this is not an inertial frame for some other point in space y, so it may appear to be accelerating away from you.   You can send your friend (another astronomer) to the point y in their own spaceship and they can put themselves into a state of being at rest in their CMB frame.   Both you and your friend are now said to be at rest in the CMB frame but you will both seem to be accelerating away from each other, so you wouldn't think you were at rest with respect to each other.
     That's just how it is,  your CMB frame is only a local frame.   Your friend's CMB frame is only a local frame.  They are both called CMB frames but they are not the same frames.  They are LOCAL frames, so their properties (such as being inertial frames and also making the CMBR appear to be isotropic) only apply locally, they are tied down the point in space around which they are centred.    Does that make sense?

   So I suppose we could answer your question this way:  The CMB frame will not serve as a universal frame of reference from which all motion can be measured:  It isn't one frame, it's a collection of local frames and they cannot be extended to cover all of the universe with it's curved geometry without losing some of their properties.

[yor_on]

So the CBR (CMB) is observer dependent? Makes sense to me, if that is what you're suggesting ES.

[Eternal Student]

Hi Yor_on.   I hope you are well.

  Yes, the CMBR is observer dependant.  There will be a dipole in the frequencies of e-m radiation observed according to the motion of the observer.
    Easy reference:   https://astronomy.swin.edu.au/cosmos/c/Cosmic+Microwave+Background+Dipole.

   The CMB frame is an inertial frame where the dipole disappears.

Late editing   Obviously no one has actually travelled to another group of galaxies or moved a bank of microwave receivers at relativistic speeds to see how that affects the dipole.  The earlier discussion(s) refer to what our models suggest and the main empirical evidence we have for this is that the real observations from our part of the universe are consistent with it.

[Halc]

Yes, the CMBR is observer dependant.

I would say No, the same CMB frame would be identified by any observer regardless of their postion or motion. That's what makes it essentially objective and thus a reasonable candidate for any interpretation that asserts a preferred coordinate system.
Unfortunately, while the coordinate system foliates the vast majority of the universe, it fails to foliate all of spacetime, so an interpretation that posits such a preferred frame is forced to deny the existence of regions not mapped despite the fact that relativity has no trouble with such spaces.

The CMB frame is an inertial frame where the dipole disappears.

It is not an inertial frame.  It corresponds locally to one, but pick a different location and the CMB frame corresponds locally to a different inertial frame. No inertial frame foliates even a tiny fraction of spacetime, else light would be able to eventually get from anywhere to anywhere else, which contradicts the existence of an event horizon beyond which light will never reach us.

[Eternal Student]

Hi all.

@Halc
   No major disagreements with what you've said.   The problem with long threads is that things have already been said but no one can read it all.

(concerning the CMB frame)  It is not an inertial frame.  It corresponds locally to one, but pick a different location and the CMB frame corresponds locally to a different inertial frame.

   It can be called an inertial frame.   The word "local" is optional,  most people take forgranted that it can only be inertial locally.  Just in case there was doubt, it was directly mentioned in this thread:

There is a local CMB frame at every point in the universe, so there are a lot of CMB frames.    Under General Relativity we give up on constructing inertial reference frames that can be applied globally  ("universally") - this happens by chapter 2 of most good books on GR.

    Yes, the CMBR is observer dependant.

I would say No, the same CMB frame would be identified by any observer regardless of their postion or motion.

OK again.
    The  CMB frame  is a frame of reference and such a frame can be identified by any observer wherever they may be, regardless of the motion they have at that place.
     The CMBR (with no word like "frame" follwing it) generally refers to the radiation that can be observed and not a frame of reference.  There are differences in the radiation you would observe according to your motion.  This was discussed previously.

   As stated earlier, there are no major disagreements, just a long thread.  Thanks for your time.

[yor_on]

Hi Yor_on.   I hope you are well.

  Yes, the CMBR is observer dependant.  There will be a dipole in the frequencies of e-m radiation observed according to the motion of the observer.
    Easy reference:   https://astronomy.swin.edu.au/cosmos/c/Cosmic+Microwave+Background+Dipole.

   The CMB frame is an inertial frame where the dipole disappears.

Late editing   Obviously no one has actually travelled to another group of galaxies or moved a bank of microwave receivers at relativistic speeds to see how that affects the dipole.  The earlier discussion(s) refer to what our models suggest and the main empirical evidence we have for this is that the real observations from our part of the universe are consistent with it.

:) Np ES, it's a pleasure reading you. And if we consider 'relative motion' we can test it at different locations in space and time. Just by staying on earth, or in its vicinity.

[Eternal Student]

Unfortunately, while the coordinate system foliates the vast majority of the universe, it fails to foliate all of spacetime, so an interpretation that posits such a preferred frame is forced to deny the existence of regions not mapped despite the fact that relativity has no trouble with such spaces.

     I've just re-read what you've written.  This bit is of some interest and probably wasn't mentioned before.  I'm guessing you (Halc) were going back to address some issues in a post raised by ClifforUK some time ago.
   

What I don't think has been fully defined is if there is a universal reference frame.  Some kind of "Fabric of Space" or Aether........
One reference frame might be the Cosmic Microwave Background Radiation (CMB, CMBR).

    If the real universe in which we find ourselves was an FRW universe,  then the set of CMB frames at all points and times would cover the entirety of spacetime.  However, the real universe is not an exact match for the cosmological model that is an FRW universe (it's lumpy with high density regions like stars and planets, rather than a homogeneous cosmological fluid).
    If you (Halc) have some spare minutes then I'd like to hear what you had in mind as a point or region of spacetime where a CMB frame cannot be identified.  If you suggest points in the vicinity of a Schwarzschild black hole then you know what I'm going to say next -  can you really say that General Relativity has "no trouble with such spaces".  Where a space is geodesically incomplete then I'll agree that there can be some "directions" from which no radiation can be received by an observer (but a geodesically incomplete space is plenty of trouble for GR in my opinion).
   If we consider a point in the vicinity of a less extreme gravitational source, the CMBR may be impossible to observe isotropically in intensity (since there would be some bending of light rays making solid angles of the sky according to the observer correspond to different volumes of space) but I think there should always be some motion the observer can be given to observe the radiation isotropically in terms of temperature equivalence from a black body.
   Anyway, what did you have in mind as a point in spacetime where a CMB frame cannot be identified?