# Universe is at rest?

Discussion in 'Physics & Math' started by hansda, May 6, 2017.

1. ### hansdaValued Senior Member

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We know that a particle is always in motion and never at rest. Rest is a relative concept.

When two particles are moving at same velocity, the relative velocity between them is zero and it can be said that the particles are at rest relative to each other. If the motion of a particle is also compared with itself, the particle can be consider at rest with relative to itself.

We know that our universe is expanding. But if we compare the expansion of our universe, with relative to our universe itself; can we say that, our universe is at rest?

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3. ### karenmanskerHSIRIRegistered Senior Member

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Hansda: IMO, one could apply your suggestion (universe at rest) on a local scale, but since (we observe that) more distant parts of the universe 'appear' to be moving more rapidly, some sort of 'sliding' comparative measuremnt (observational) scale might be applicable to resolve more-distant apparent relative motions with one's own (at rest) FOR (Frame of Reference). Perhaps such a 'sliding scale' derives from GR? . . . . and apparent FOR differences actually are - at rest relative to each other. . . . . thus the apparent observational illusion . . . . . (HAHA!)

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7. ### danshawenValued Senior Member

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Good question.

Photons are created and move at a relative velocity of c RELATIVE TO A REST FRAME that is the same rest frame as the paired electrical charges which provided the acceleration necessary to produce the photons.

You could also go the "matching velocities" route, but this is the only example or definition you will ever need about a rest frame. Rest frames are not unique or preferred for matter or antimatter. Any inertial reference frame is a suitable rest frame, except for one whose relative velocity with respect to any of the other inertial frames in which matter or antimatter exists, =c.

Absolutely, and also with respect to the Big Bang, or whatever it was, assuming it was the center of the expansion.

This would also be true if it were rotating, which we can't really determine without reference to something outside of it that does not share the same angular momentum. As the universe as a whole expands, it's angular velocity nearest the outside decreases. Even more difficult to determine out there, because at cosmological distance, even the speed of light requires an eternity to rotate even by an arc second, from out point of view.

8. ### karenmanskerHSIRIRegistered Senior Member

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" . . . . As the universe as a whole expands, it's angular velocity nearest the outside decreases. . . . . ."

danshawen: Can you provide a factual evidence resource for this statement? Has this actually been "observed", or is it merely spectulation?

9. ### danshawenValued Senior Member

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http://hyperphysics.phy-astr.gsu.edu/hbase/rotq.html

As the radius increases (constant linear velocity, assume = c), omega (angular velocity) decreases

10. ### karenmanskerHSIRIRegistered Senior Member

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Thanks for the link on rotation . . . . . . thought above posts designated the universe as expanding, but not rotating.

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11. ### danshawenValued Senior Member

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Yes, if omega = zero to begin with (no rotational velocity at all from the Big Bang forward), then all expansion would be radial and not rotating.

And where else in nature do we find no rotation at all, even on large scales? About as often as meteors or asteroids fall directly into something else, which is not very often. Just because we haven't observed rotation on the largest scales doesn't necessarily mean it isn't there. I have suggested why this might be the case.

12. ### hansdaValued Senior Member

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Our Universe is expanding. That is true. But this does not imply that our universe is in motion. To decide about the motion of our universe, ideally we should have a reference point(FoR) outside the universe. But this is not possible. Any reference point(FoR) has to be considered, within our universe. This is the only possible case. Our universe has to be considered at rest, with relative to this reference point.

For example consider a tree being planted at some location on the earth. With the gradual passage of time, the tree will grow and expand. But the tree will still remain at the same location on the earth. With relative to the earth, the tree is at rest.

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13. ### karenmanskerHSIRIRegistered Senior Member

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BTW: "Just because we haven't observed it, doesn't mean it isn't there" (paraphrased from Danshawen's recent post regarding Universe rotation on the Sciforums thread: Universe at Rest).

"Science historically has employed the Scientific Method to examine and observe otherwise unexplainable phenomena, and via the Method to determine and explain the underlying causes of such phenomena. But phenomena are seemingly not finite in number. Newly recognized or observed phenomena will require application of the Method to determine new causalities that are perhaps yet unrecognized."

Last edited: May 8, 2017
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15. ### CounterRegistered Senior Member

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The expansion of the universe is simply SPACE. WE exist in space. But in order for space to exist it must be expanding, allowing movement. If we were without space we would not be able to move. The expansion of SPACE is simply it's nature.

16. ### danshawenValued Senior Member

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I keep defining exactly what "at rest" means here and no one seems to get it, or they just ignore it or don't respond that they get it.

"At rest" is both invariant and non-unique. Any frame containing one or more particles may be considered to be "at rest" as long as c measured in any two diametrically opposing directions measures exactly the same. ±c defines an inertial reference frame that is at rest, for any inertial reference frame. No such frame is unique or preferred, nor can any local experiment be performed to determine absolute motion of the frame through inertialess space.

A particle cannot be considered to really be "at rest" with respect to itself unless it is not spinning. A quantum spin of zero is uniquely a Higgs boson, the force carrier of the Higgs mechanism, which provides inertial mass to fermions, W and Z bosons, neutrinos, and their antiparticles.

A fundamental particle such as an electron is a good example of why some particles cannot be at rest. An electron can never really be considered to be "at rest", even and especially if it is bound by atomic structure. For an electron to be "at rest", the uncertainty principle requires it to expand to the size of the known universe, and this is not a joke. What that means is that for an electron to come completely to rest would mean that it would emit all of its EM energy in the form of a single red shifted photon, the wavelength of which would need to be the radius of the known universe. In this way, the uncertainty principle is simply a restatement of the principle of wave-particle duality.

17. ### danshawenValued Senior Member

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In a manner of speaking. The frame from which the "Big Bang" exploded can be considered "at rest" relative to all other motions it started, in the same way that the annhilation of an atom of positronium is at rest relative to the two gamma photons it emits at the end of the process.

In the case of the Big Bang, it doesn't look as though it is likely there will ever be a "Big Crunch", or a series of big crunches at any time in the distant future. I'd rather not speculate about this. It touches on cosmology, for which I currently have low regard.

In the case of annhilation of positronium, it appears equally unlikely that the photons released in opposite directions will ever curve around and meet each other the way a closed universe would. Again, this touches on cosmology, a subject of which I am not overly fond. I prefer to withold judgement about such speculation indefinitely.

I'm sorry if this does not really answer your question about the rest frame. There isn't any way I know of to determine if the Big Bang was a spontaneous release of energy from a single point, or a violent collision between a pair of hypermassive black holes. There is simply no way anyone can nail the origin of a classical greek coordinate system into inertialess space.

18. ### hansdaValued Senior Member

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Say, two photon particles are travelling in the same direction with a time difference t. What will be the relative velocity between these two photon particles?

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19. ### CounterRegistered Senior Member

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That depends. Are they travelling forever?

20. ### hansdaValued Senior Member

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Depends on what?

If they are travelling for ever, will it affect their relative velocity at any instant of time?

Consider two photon particles travelling from the Sun to our Earth in the same trajectory. One photon particle reached the Earth say at 6'O clock in the morning. The other photon particle reached the same location on the Earth one minute later. What will be the relative velocity between these two photon particles, when the are in between the Sun and the Earth?

21. ### The GodValued Senior Member

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The present physics does not envisage rest frame for any photon travelling at c. For that matter we have no frame of reference travelling at c.

22. ### hansdaValued Senior Member

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Seems you are following GR model. May be in GR model, this question can not be answered. Perhaps this can be answered in Newtonian Model.

23. ### The GodValued Senior Member

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SR Vs Galilean...rather Lorentz Vs Galilean.

This is how it is. Say a particle A is moving in left direction at 0.5c and another B is moving in right direction at 0.5c, then the relative speed between the two as seen by any of them is not 0.5c+0.5c = c, but (to create complexity) a stationary observer between the two sees the separation increasing at the rate of c.

Last edited: Jun 23, 2017 at 5:39 PM