# Do we move in the Universe? Or is everything stationary

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1. Aug 20, 2015

### Jaami M.

This isn't too much of a question, but more of an open proposal.
Now... to begin, I know that both you, reading this, and I can agree that our universe is expanding. The universe is expanding from every point within itself, there is no Defenitive reference point, this is a fact.

So for an analogy
- You are in total empty space, a complete void. You are traveling though the void, but you are ignorant to that (the speed doesn't matter). Now in the distance you see another astronaut traveling towards you. From his view he thinks your moving, but from your stand point you think that he moving towards you. Can any of you prove that who's moving? The answer is No. Because there's No point of reference in our universe

This means that we can only tell that we "so call move" because we can track our distance from a body of mass or multiple masses. In Reality "we don't move in our universe" Is this phrase correct? I mean we should define movement as *"changing distance from one mass to another"*. We don't move in our universe, because we can't track ourselves (previous analogy) in it we need other bodies of mass to track our "so called movement"

Can this conclude the idea that nothing in our universe moves according to the universe.

2. Aug 20, 2015

### Staff: Mentor

Yes, it is correct to say that all inertial motion can only defined relative to something else. The statement "A is moving" without saying what that motion is relative to is as meaningless saying that something is "bigger" without saying what the comparison is with. We can say that A is moving relative to B, and we will get the same results whether we analyze that situation as if A is at rest while B is moving or vice versa.

This idea is one of the essential concepts behind Special Relativity, so if you're comfortable with it you're well-positioned to start learning SR.

3. Aug 20, 2015

### Jaami M.

I would love to learn SR, as well as all other physics.

4. Aug 20, 2015

### Jaami M.

But would you agree that Nothing moves according to our universe with measures pertained with the universe, but only measures relative to other bodies of mass.

5. Aug 20, 2015

### Staff: Mentor

Yes, that sounds like what I'm trying to say.

6. Aug 20, 2015

### A.T.

To claim no movement you need a reference. Otherwise movement is undefined not zero.

7. Aug 20, 2015

### Jaami M.

I'm saying because that there is no definitive point of reference, to track movement, there is no motion according to the universe(look at previous analogy). Only changes in distance from separate bodies of mass. Which is what we define as motion

8. Aug 20, 2015

### Jaami M.

So is what your saying that you agree with me? Are you stating that movement relative to other mass can be tracked as distance between the two (defining movement) but movement in the universe itself is undefined Not Zero or Inexistent, but motion in the universe is Undefined(the nature of complete lack of motion)

9. Aug 20, 2015

### Staff: Mentor

Of course I agree with you - this is beginner-level physics that has been well understood for better than a century. Why do you think it's called "Relativity"?

10. Aug 20, 2015

### phinds

"Lack of motion" is the same as "motion" in that it requires a reference point. "Lack of motion" relative to WHAT? To just say "movement is undefined" is wrong. Movement IS defined, but only relative to something else. If you are not moving relative to something then in the frame of reference in which it is not moving, you are also not moving. But you both ARE moving in an infinite number of other frames of reference.

11. Aug 20, 2015

### Jaami M.

Ok I just want to know this... Does anything move according to the universe

12. Aug 20, 2015

### phinds

That is not a meaningful question. Move according to WHAT? To move "according to the universe" there would have to be an absolute frame of reference called "the universal frame of reference" but there is no such thing. The "universe" is not a thing that has a fixed position of its own. Everything "moves" only relative to other things and the "universe" is not such a thing that you can "move relative to".

13. Aug 20, 2015

### Jaami M.

That is my point... There is no definitive point of reference. We only know motion because of the objects around us. So are we(me, you, the sun, the Milky Way) moving at all? Are we just moving amongst ourselves in an expanding void. You say the universe has No ultimate point of reference(I agree) so does that tell us that we don't know we're moving unless another object is presented? Yes. But how do we know that an object is moving and we're not. Or vice versa(SR) This continues for All bodies of mass in the universe. Concluding that we can't say we have motion in the universe alone, we just can't prove it. So we can't say that we "move" in the universe. "Motion" of one mass relative to others can not be proven. Only tracking of distance is pertinent

14. Aug 20, 2015

### Staff: Mentor

We don't. If A is in motion relative to B, we can think of A at rest and B moving; or B is at rest and A moving; or both of them moving relative to some third body C. Trying to say that any one of them is "really" moving or not moving is like trying to say that one of the statements "Moscow is to the east of Berlin" and "Berlin is to the west of Moscow" is more correct than the other.

15. Aug 20, 2015

### phinds

I don't know what "move in the universe" could possibly MEAN. You keep saying it and we keep telling you that it is not a meaningful statement. To say that something is moving is a perfectly reasonable statement, you just have to say moving relative to WHAT and "the universe" is not a thing that you can move relative to. Motion is defined as a change in distance between two objects and "the universe" is not an object that you can put one end of your yardstick on.

16. Aug 26, 2015

### Vmedvil

I will try to explain, if the universe were a void and even if u were traveling at the speed of light as the only object, how would you measure your movement? you would not have a frame of reference. Distance is just the measurement between two objects like you and the sun, without that frame of reference, even if you are traveling at the speed of light. Who is to say that you are were moving since that is your relative frame of reference with nothing to compare it to.

What is to say without that other frame that anything is happening? It Requires three.

The only way to could measure that in a void is you versus radius of the universe I guess.

but even that is doubtful, p would be way different in that universe. if he was moving toward that radius faster than he would be moving quicker where as if you were moving faster toward it, you would be moving faster.

Last edited: Aug 26, 2015
17. Aug 26, 2015

### vanhees71

Unfortunately this thread is marked as level B, although it's clearly at least level I. So I try to answer the question "as simple as possible but not simpler" (Einstein).

The question is, if in any sense we "move" or are "at rest" in the universe. As has been stressed several times in this thread, the first thing you have to do is to find a reference frame, according to which you measure velocities. You must always say in which reference frame you measure velocity, otherwise it doesn't tell you anything.

Now, and that's why I think the "B label" in unjustified, to answer the question we need the general theory of relativity, and that's very hard to explain without math, but I'll try. According to general relativity the geometry of space and time, which together are described as a four-dimensional spacetime geometry, is determined by the energy-momentum content, and the geometry of spacetime is not Euclidean, i.e., it has a curvature, and this curvature describes gravitation. The Einstein field equations determine the geometry of spacetime for a given energy-momentum distribution.

It's also a question concerning cosmology, and in cosmology we have (by assumption!) a preferred frame of reference, because we assume that there's no preferred location or time nor a preferred location in space (cosmological principle). Now math tells us that there is a specific class of spacetimes that fulfill this cosmological principle, the socalled Friedmann-Lemaitre-Robertson-Walker spacetimes (FLRM spacetimes). These are exactly those spacetimes with a maximal symmetry, and there is a preferred frame of reference, where an observer at rest with respect to this reference frame is comoving with the energy-momentum distribution of the universe, which of course must also be homogeneous and isotropic as seen in this reference frame.

But observation tells us that this cannot be true, because we see the stars at the sky, and they are not just one isotropic huge light in the sky but consist of point-light sources. What's homogeneous and isotropic is the large-scale averaged energy-momentum density, and a closer investigation shows that this assumption of a maximally symmetric FLRW spacetime as a model for the large-scale coarse graint few on the cosmos is well justified: There is the cosmic microwave background radiation, which is just the relic soup of electromagnetic radiation from the big bang (which socalled Hubble expansion is by the way also implied by the FLRW-solution of the Einstein equations of GR). In earlier epochs the universe was very hot and dense, and the matter consisted of charged particles: In the very early stages of the elementary constituents of matter, of which we know only a tiny part in terms of the particles in the standard model, the quarks, leptons, photons, weak gauge bosons, and gluons, but that's another story; than a bit later in form of the stable particles known today like protons, neutrons, electrons, etc. but they all were still charged particles. Now electromagnetic radiation is scattered by charged particles and thus this medium of charged particles (plasma) is opaque to radiation as long as it is dense enough. The matter itself is strongly interacting and thus in thermal equilibrium with a definite temperature, but it's cooling due to the Hubble expansion. Now since the photons are scattering also all the time with this dense plasma, it's also in equilibrium forming a socalled "black-body spectrum".

Now at a certain point the universe got cold enough such that the protons and electrons built stable bound states of hydrogen atoms, which are electrically neutral, and from then on the electromagnetic radiation decoupled from the medium, but it's spectrum still stays a black-body spectrum although with ever cooler temperatures the longer the Hubble expansion goes further on.

Indeed, the cosmic background radiation can nowadays be measured very accurately, showing a nearly perfect black-body spectrum with a very isotropic temperature of around 2.73 K, which shows that indeed our visible universe seems to be very isotropic on the large-scale average. On the other hand, the tiny temperature flucutaions of $\delta T/T \simeq 10^{-5}$ provide very important information on the universe and have thus vigorously studied in recent years with a lot of high-precision measurements, most importantly by satellites like COBE, WMAP, and PLANCK, but that's again another story.

Now the last paragraph was a bit simplified, and now I can finally come to your question, whether we are "moving" or are "at rest" in the universe, and this answer makes sense, because we have this preferred reference frame of the FLRW geometry of spacetime, which is defined as the frame, where a resting observer is comoving with the cosmological substrate, and where the cosmic microwave background has a isotropic temperature. Now we can also answer the question, whether we on Earth are moving with respect to this reference frame. Obviously we are, because the earth is moving around the sun and the sun is moving around the center of the galaxy and whatever other "peculiar" motion all the objects in our direct neighborhood make. The important point, however is, how to measure whether we are moving against the comoving FLRW frame or not, and this is possible again by measuring the temperature of the cosmic microwave background in all directions.

In the comoving frame by definition the temperature is isotropic around each point and the background radiation is described as a black-body spectrum, i.e., it looks precisely like the electromagnetic radiation from a perfectly black body at rest relative to the spectrometer. For an observer/spectrometer moving against the so defined restframe of the black-body radiation, sees this radiation blue or red shifted when he measures its spectrum in a direction moving towards or against the direction of its velocity vector relative to this CMBR restframe. Quantitatively it comes out that in each direction such a moving observer measures again a perfect black-body spectrum in any direction, but he finds a temperature, depending on the direction. The temperature shows a systematic variation with the direction of the spectrometer, which is described by a socalled dipole part of the CMBR temperature variations.

Indeed when the satellite COBE meausured a dipole component in the CMBR temperature variations, which indicated that we move with a speed of around 390 km/s in direction of the Leo constellation. So we can say that we indeed move relative to the comoving reference frame of the large-scale averaged FLRW spacetime.

18. Aug 26, 2015

### Jaami M.

19. Aug 26, 2015

### Jaami M.

vanhees71 your answer was marvelous. It was great, I loved it. thank you for taking in consideration the whole universe, and not just a single body of mass. This makes total sense to me. It's such a relief to have someone to actually answer your question from full understanding in consideration of the Whole query. I'm just still in awe of the answer.
Thanks

20. Aug 26, 2015

### Staff: Mentor

Your answer, which boils down to the fact that we can define "moving" as "doesn't see the CMBR as isotropic", is fine as a level B answer. If the OP wanted to see the math behind it and explore in more detail the properties of the FLRW solutions that make this a reasonable definition of "moving", that would be level I. But you don't need to go into all that in order to state and understand the answer itself.