# Is it possible to determine if space is fixed or moving?

1. Jul 8, 2010

### Emu

This is a question about what we can observe about space. This is not a relativistic question, or about the expansion of space or about “aether”.

Some of the aspects of space as I understand it are: Space has volume. Space has the non-zero properties of permeability and permittivity. Space has a single occupancy rule i.e. two objects with the same spin cannot occupy the same space at the same time. As far as we know, the properties of Space are constant and uniform throughout the universe. It also seems that objects and Space are not fixed to each other in that objects (as we say in the vernacular) travel though Space such as galaxies, stars, planets and light. To keep the idea that there is not a special aspect to an observer we could also say that “space moves though us”, just as well as “we move though space”.

So the question is; can we determine if space moves? It seems to me that with the properties of Space being constant and uniform that there should be no change in an object by Space as the object moves though Space. The converse is that if Space were moving there would not be any change in an object. It therefore seems to me that we cannot determine if Space is moving or if Space is fixed.

2. Jul 8, 2010

### mathman

Question of space moving has to be asked relative to what?

3. Jul 8, 2010

### Staff: Mentor

Ie, there is no property of space of motion (or lack of motion) that can be measured. Special Relativity did away with it.

4. Jul 8, 2010

### Emu

We observe that we are moving relative to the sun, and the solar system is moving in the milky way and the milky way is moving with Andromeda and it all is moving relative to the background radiation. Is Space moving in addition to the objects within it moving? is Space spinning (rotary motion)? Or is movement of uniform constants indeterminable?

5. Jul 9, 2010

### Chronos

Space is moving in the sense that it is expanding. There is no evidence the universe is 'spinning'. A superficial question here is relative to what? This rotation idea has actually been explored, but, ruled out. Rotation would induce changes in the CMB that are not observed. Furthermore, rotation would imply locations along the axis of the rotation are somehow 'special', which violates the cosmological principle.

6. Jul 9, 2010

### yogi

The recessional velocity of the nebula suggests that space is either growing or stretching. If one does not have to answer what space is, then a growing volume containing energy could be considered a form of spatial motion. On the other hand, if space is defined as nothing it becomes difficult to envision what is meant by motion. Einstein concluded that both inertial and accelerating frames were relative - this had a significance with respect to the universe as a whole - specifically, an observer would be unable to distinguish whether the inertial reaction of an object was due to its acceleration relative to the universe or vice versa. This would seem to confer a
substantive attribute upon space. If space contains dark energy in some form - then cosmological expansion might be thought of as isotropic momentum flow - perhaps resulting in a detectable a local affect that could be related to motion.

7. Jul 9, 2010

### Ich

From Einsteins Leyden address, where he chose to spacetime "ether" as it has certain local properties:
(my emphasis)
This still holds: the idea of motion may mot be applied to space. [OT]This is one of the reasons why I oppose the notion of "expanding space". It leads people to believe that there is such a property like the velocity of space, when in fact we're talking about a coordinate choice.[/OT]

8. Jul 9, 2010

### Emu

"Rotation would induce changes in the CMB that are not observed." What aspect or property of Space would induce changes?

"if space is defined as nothing" I have a hard time with the word "nothing" since Space can be occupied by mass or energy it seems that a volume would have a density of energy(mass) from very low such as in a vacuum or extremely high such as in a black hole.

The focus in this thread is about Space itself and if the properties of Space can have an effect on objects, (mass and energy) if the properties are constant and uniform, even if Space is moving relative to the object at a velocity many multiples of the speed of light.

9. Jul 12, 2010

### yogi

Hi ICH,

But was Einstein referring to uniform motion or accelerating motion. The entire Layden address was slanted toward the idea of space as a player in the gravitational game - only two sentences qualifying all that preceded the penultimate and final statement. The relativity of non-inertial frames would seem to cloth space with inertial properties; Einstein seemed to retain this notion in many of his later writings

Last edited: Jul 12, 2010
10. Jul 12, 2010

### Ich

That doesn't matter. Einstein points out that space (or spacetime) has some properties, that's why he called it provocatively "ether".
But the relevant point for this thread is that "motion" does not belong to those properties.

11. Jul 12, 2010

### Chronos

12. Jul 12, 2010

### Emu

Hi Chronos, It is hard to determine from the abstract if the article sited is about the objects in the universe or Space. From first glance it seems it is most likely about the objects in the universe.

13. Jul 15, 2010

### yogi

Perhaps, but in 1920 the concept of cosmological expansion had not been developed - so the idea of motion on the global scale would not necessarily be included within the concept of local motion - if that is what Einstein was qualifying. Had Einstein knew about expansion, and particularly accelerated expansion, he might have used more guarded words when referring to the ether as something incompatible with motion.

But we are all free to speculate what Einstein might have done had he had all the facts - what is amazaing is what he did with a few facts and some wrong assumptions e.g., a static, positively curved closed universe

14. Jul 15, 2010

### Ich

I trust that Einstein would not confuse coordinates with physics. It is unfortunate side effect that the expanding space idea led the public (and some physicists) to believe that phrases like "motion through space" bear any meaning outside a particular coordinate system.
The idea of motion may not be applied to space. Whenever people assign a state of motion to "space", they are in reality talking about the motion of a certain class of test partcles.

15. Jul 16, 2010

### yogi

ICH, I think your statement should be qualified as to what people are "doing the assigning." In these matters I see a close correlation to the study of law. There is the majority view, the minority view, and the California rule. Maybe in physics is Majority, Newtonian, and Crack-Pot, whatever.

But I have a difficult time with the notion of accelerating space as insignificant. If most of the matter in the universe does not exist in particulate form, then it must somehow be amalgamated with space - accelerated spatial flow vis a vis cosmological expansion is a problem of physics that can't be transformed away as confused coordinates.

16. Jul 16, 2010

### Ich

Sorry, neither my command of the English language nor my knowlede of US law are suficcient to extract the meaning from this paragraph. Could you rephrase?
I never said anything about acceleration. It's exactly the fact that acceleration can be defined "relative to spacetime" whick made Einstein state that it does have some properties.
We're talking about motion. Motion is not one of the properties of space.
The acceleration can't be transformed away, but expasion can.
Take http://en.wikipedia.org/wiki/De_Sitter_space" [Broken] for example. This is a static spacetime. De Sitter introduced it accordingly in static coordinates. Space is not expanding then.
If you track the motion of test particles in de Sitter space, you'll find that they undergo acceleration. A cloud of such test particles is indeed expanding.
If you then choose coordinates where such (properly prepared) test particles have constant coordinate values, you're in FRW coordinates. Exactly the same static spacetime is then called "exponentially expanding", which sounds like the complete opposite of "static".

Last edited by a moderator: May 4, 2017
17. Jul 18, 2010

### yogi

Thanks Ich. I follow your tutorial. But it seems to me that the superposition of the expanding coordinate system obscures the physics of acceleration - any local effect due to cosmological expansion is lost in the sense that the essence of spatial motion is merged in the stretching coordinates. Since the coordinate distances of the particles do not change, then your interpretation of Einstein's statement would be correct for global space as well as local motion.

Last edited by a moderator: May 4, 2017
18. Jul 21, 2010

### Emu

Per Yogi: "In these matters I see a close correlation to the study of law."

I see a correlation to the laws of physics. The laws of a nation do not change due to a location within the nation, or the speed at which you travel in the nation. You still get \$200 when you pass GO even if you are flying high above the Plains. In the same way the laws of Space remain the same no matter where you are or how fast you are going i.e. the permittivity and permeability of space determine the speed of light regardless of where the light is and how fast the source of the light. The uniformity of Space can then be tested by the uniformity of the speed of light.

19. Jul 21, 2010

### yogi

Quite right - the problem is in the human interpretation. Twenty years ago it was difficult to find an ear for a theory based upon accelerated cosmological expansion - most of the authorities were convinced the universe was decelerating and the big issue was whether it would collapse or continue forever - with many bets on the 1932 critical density Einstein de sitter model. Scientists are no different than any other profession when it comes to opinions - and they can be quite certain of something that is dead wrong. So while the laws of physics don't vary - human interpretation does, and with great authority - those who express the most confidence are frequently wrong. Near the end of his life Einstein wrote: "The present position of science can have no lasting significance"

20. Jul 22, 2010

### Helios

I've always wondered about something. Suppose you're in a fast moving ( but non-accelerating ) spaceship. Wouldn't the cosmic background radiation look bluer to the bow of the ship and redder off the stern? Couldn't an "absolute velocity" be defined wrt. the cosmic background radiation?

21. Jul 22, 2010

### Emu

The background radiation (photons) is a cloud of "test" particles, not the Space that the particles travel through.

22. Jul 22, 2010

### Helios

I am confused why Emu says this. Does it subtly answer my question and I just don't get it? On the surface, it seems to mean that I don't know the difference between space and a photon and if I did know the difference then I would not have asked the questions I did, because if I knew the difference then I would have known the answer to my question, because the answer to my question lies somewhere in the knowing of the difference between space and a photon, because err, wha?

My question was--Suppose you're in a fast moving ( but non-accelerating ) spaceship. Wouldn't the cosmic background radiation look bluer to the bow of the ship and redder off the stern? Couldn't an "absolute velocity" be defined wrt. the cosmic background radiation?

23. Jul 22, 2010

### Ich

So why put much effort in such a theory, if the technology doesn't allow to prove it anyway?
BTW, I happen to have been studying physics in these times: you're dead wrong. While I, as a student, couldn't understand how all these old farts keep annoying us with these dumb possibilities - like neutrinos having a mass or a cosmological constant being included in the calculations, when common senses tells us clearly that both is ******** - they kept the possibilities to the last moment. When evidence came in to show that they were right, not I.
Well, as you see, there's quite a difference in perception here. With the difference possibly being me having had contact to those you're talking about, and you not?

Edit: Great, I didn't know they had an algorithm to turn excrements into stars.
And to Helios: Yes. But this spees doesn't change the laws of physics.

24. Jul 22, 2010

### Emu

"CMBR dipole anisotropy

From the CMB data it is seen that our local group of galaxies (the galactic cluster that includes the Solar System's Milky Way Galaxy) appears to be moving at 627±22 km/s relative to the reference frame of the CMB (also called the CMB rest frame) in the direction of galactic longitude l = 276±3°, b = 30±3°.[64] This motion results in an anisotropy of the data (CMB appearing slightly warmer in the direction of movement than in the opposite direction)[65]. The standard interpretation of this temperature variation is a simple velocity redshift and blueshift due to motion relative to the CMB, but alternative cosmological models can explain some fraction of the observed dipole temperature distribution in the CMB.[66]"

Hope this helps. But these are still the cloud of test particles in space, not Space itself.

25. Jul 23, 2010

### Chronos

Yes, that is what WMAP measured - our velocity relative to the CMB. It is not, however, an absolute velocity, merely a convenient reference frame. Our motion wrt the cmb is a pathetic ~600km per second.