A centre of mass of the Universe.

[geordief]

Can this exist?I have been told that it does but I cannot seem to find  any old posts lying around  the internet that  support this idea. 

 

Anyone here want to have a go at the question (it is understood that there is no centre  ,I am asking whether there is a centre of mass ,although it  could not be used as special Frame of Reference)

[StringJunky]

If the distribution is isotropic and homogenous, and the galaxies are moving in random directions as an ensemble, how might there be a centre of mass?

[studiot]

First of all the universe is not a rigid body.

The term centre of mass is only strictly applicable to rigid bodies.

Secondly you have to decide if you are talking in Newtonian or Relativistic terms.

This is because of the time taken for any force applied at one point to act on  another at significant distance.

It is even possible for one part of the universe to have responded to a force before the effect of that force has reached another part.

Here are the newtonian formulae for the coordinates of COM.

Edited June 24, 2018 by studiot

[geordief]

40 minutes ago, studiot said:

First of all the universe is not a rigid body.

The term centre of mass is only strictly applicable to rigid bodies.

 

Yes ,it has the feel of a very half baked question now.

 

If mass is somehow conferred on  matter by this Higgs field  and we have the problem of Dark Matter too  then it seems  a pointless question after all.

 

One definition of a Universal centre of mass might be the direction in which all matter is converging  but  no one has any idea  whether this might ever happen,have they ?(we have to stop expanding first and then we can compare notes  )

 

 

[Airbrush]

Can there be a center of mass to the universe?  That depends on what exactly the universe is.  If the universe is finite in size and expanding, then the center of mass would be roughly the center of the finite universe.  Why would it not?

[studiot]

4 minutes ago, Airbrush said:

Why would it not?

Think about the definition and properties of a centre of mass then come back and ask again.

15 minutes ago, geordief said:

Yes ,it has the feel of a very half baked question now.

No, I think it is a good question as it allows consideration and discussion of the Physics, without the mathematics getting in the way.

 

[Scotty99]

Well this is the beauty of general relativity is it not?

The answer is up to the observer lol.

 

 

[Sensei]

2 hours ago, studiot said:

The term centre of mass is only strictly applicable to rigid bodies.

Not only. Two stars are made of matter in plasma state. And there is center of mass of binary star system. There is center of mass of Solar system etc.

[studiot]

22 minutes ago, Sensei said:

Not only. Two stars are made of matter in plasma state. And there is center of mass of binary star system. There is center of mass of Solar system etc.

Yes only.

 

The concept makes no sense if external forces applied to the system would deform it as well as move it.

The binary stars you mention act as a pseudo non deformable 'body' or system, as does the solar system, as does a galaxy, as does a rocket and its exhaust.

But apply a sideways force to the exhaust once it has left the rocket and tell me what happens to the 'centre of mass' ?

Alternatively, what happens if the rocket collides with an asteroid?

Does it hit with the momentum of the whole system or just the rocket part?

[Butch]

On 6/24/2018 at 11:29 AM, geordief said:

Yes ,it has the feel of a very half baked question now.

 

If mass is somehow conferred on  matter by this Higgs field  and we have the problem of Dark Matter too  then it seems  a pointless question after all.

 

One definition of a Universal centre of mass might be the direction in which all matter is converging  but  no one has any idea  whether this might ever happen,have they ?(we have to stop expanding first and then we can compare notes  )

 

 

Dr. Perlmutter and his associates won a Nobel prize for investigating the expansion of the universe. They wished to determine if the expansion was constant or slowing... Surprise! It seems it is expanding at an accelerating rate!

[beecee]

On 6/25/2018 at 1:56 AM, Scotty99 said:

Well this is the beauty of general relativity is it not?

The answer is up to the observer lol.

And each observer is correct. 

[Neil Obstat]

On 6/24/2018 at 10:45 AM, studiot said:

Yes only.

 

The concept makes no sense if external forces applied to the system would deform it as well as move it.

The binary stars you mention act as a pseudo non deformable 'body' or system, as does the solar system, as does a galaxy, as does a rocket and its exhaust.

But apply a sideways force to the exhaust once it has left the rocket and tell me what happens to the 'centre of mass' ?

Alternatively, what happens if the rocket collides with an asteroid?

Does it hit with the momentum of the whole system or just the rocket part?

But when does the effect of the asteroid begin? When it collides, or before that?

Since the the rigidity or deformation of the system is assessed without collisions taking place, it would seem the asteroid should be part of the system before collision.

Therefore, the asteroid should be considered long before it collides with the rocket, just as the rocket exhaust is considered in your COM calculation. Therefore, it ought to be a COM calculation of rocket, rocket exhaust, and approaching asteroid all considered together! What happened -- someone forgot to anticipate the asteroid? 

 

On 6/24/2018 at 7:39 AM, studiot said:

First of all the universe is not a rigid body.

The term centre of mass is only strictly applicable to rigid bodies.

Secondly you have to decide if you are talking in Newtonian or Relativistic terms.

This is because of the time taken for any force applied at one point to act on  another at significant distance.

It is even possible for one part of the universe to have responded to a force before the effect of that force has reached another part.

Here are the newtonian formulae for the coordinates of COM.

It's too bad that right angle / sideways "L" artifact got included in your z-bar equation because it makes the otherwise neat appearance rather messy.     

[J.C.MacSwell]

On 6/24/2018 at 2:45 PM, studiot said:

Yes only.

 

The concept makes no sense if external forces applied to the system would deform it as well as move it.

The binary stars you mention act as a pseudo non deformable 'body' or system, as does the solar system, as does a galaxy, as does a rocket and its exhaust.

But apply a sideways force to the exhaust once it has left the rocket and tell me what happens to the 'centre of mass' ?

Alternatively, what happens if the rocket collides with an asteroid?

Does it hit with the momentum of the whole system or just the rocket part?

None of this changes the concept of the centre of mass of a system. It just complicates it.

For the bold it is very simple classically (Newtonian). F=ma applies to the centre of mass of the rocket/exhaust system. It does not matter how much or how little of the system the force is applied to.

Now, the whole Universe is another matter. I don't believe it has a centre as we understand it.

Edited June 26, 2018 by J.C.MacSwell

[Mordred]

When you have a homogenous and isotropic mass density distribution, for any application of a CoM one can arbitrarily choose any coordinate as an effective centre for applications of shell theorem. Any coordinate for those purposes under math will be valid. So no there is no centre.

[Neil Obstat]

2 hours ago, J.C.MacSwell said:

Now, the whole Universe is another matter. I don't believe it has a centre as we understand it.

Is this the way scientists think? How does what you believe have any bearing on what is real? 

[studiot]

4 hours ago, J.C.MacSwell said:

None of this changes the concept of the centre of mass of a system. It just complicates it.

For the bold it is very simple classically (Newtonian). F=ma applies to the centre of mass of the rocket/exhaust system. It does not matter how much or how little of the system the force is applied to.

Now, the whole Universe is another matter. I don't believe it has a centre as we understand it.

Please define centre of mass and show how  this applies to the rocket/exhaust system under the conditions where I specified.

[beecee]

4 hours ago, Neil Obstat said:

Is this the way scientists think? How does what you believe have any bearing on what is real? 

Yes, and he is correct  according to the BB model. The BB tells us that it [the BB] was the evolution of spacetime from a hot dense state at least as far back as t+10-43 seconds. In other words all of spacetime, or all of what we can know that existed, existed in a volume smaller then an atomic nucleus.....The BB obviously then happened in all of space at the same time...no center, no edge, no outside, except obviously the center of one's own observable universe, which applies to whoever is doing the measurement and where ever it is being done. It is wrong to view the BB as an explosion emanating from any one point, rather as is highlighted, an evolution of spacetime happening everywhere.

 

 

[swansont]

4 hours ago, Neil Obstat said:

Is this the way scientists think? How does what you believe have any bearing on what is real? 

!

Moderator Note

You are using the wrong definition of belief, and besides, this is off-topic.

 

[Neil Obstat]

1 hour ago, beecee said:

Yes, and he is correct according to the BB model. The BB tells us that it [the BB] was the evolution of spacetime from a hot dense state at least as far back as t+10-43 seconds. In other words all of spacetime, or all of what we can know that existed, existed in a volume smaller then [sic] an atomic nucleus.....The BB obviously then happened in all of space at the same time...no center, no edge, no outside, except obviously the center of one's own observable universe, which applies to whoever is doing the measurement and where ever it is being done. It is wrong to view the BB as an explosion emanating from any one point, rather as is highlighted, an evolution of spacetime happening everywhere.

 

 

Thank you for your reply, beecee. I'm having a hard time understanding this material. I hope you can help me! 

Please forgive me for changing the emphasis of your post I'm quoting here, because I'd like to focus on words other than the ones you had in bold and it got too messy that way.

You said, "...all of what we can know that existed, existed in a volume smaller [than] an atomic nucleus." However, you also said, "It is wrong to view the BB as an explosion emanating from any one point...," therefore, interpreted, you are distinguishing with the greatest possible emphasis between a volume smaller than an atomic nucleus and any one point, correct?

I'm sorry, but that seems to be a very clear contradiction. If you don't think it is a contradiction, can you explain why you think it's not a contradiction? 

I'm asking this question, because I hear you saying that since all of reality was hypothetically contained in this tiny volume smaller than an atomic nucleus, from that axiom we can conclude that therefore there was no "space or location" as we know it outside of those confines "at least as far back as t+10^-43 seconds," consequently, we are (under whatever set of someone's rules, you didn't say) forbidden from equating the "BB" with an explosion, because (am I right?) any explosion, as we know it, is necessarily surrounded by existing "space and location" as we know it, into which the explosion expands (Yes? No?), which was hypothetically not the case with the "BB," since EVERYTHING was within the confines of that very small space which must by all means be distinguished from such a thing as "any one point." Is that correct, or am I somehow off track? 

It would seem, if I'm not off track, that you (and whoever else it is making the rules for this conceptual hypothesis) are not so much interested in the so-called tiny volume (smaller than an atomic nucleus), but everything outside of that -- which, by the way, according to this conceptual hypothesis, was NOTHING at all (since "everything" was INSIDE of that so-called tiny volume. Yes, or no? 

In other words, it is literally nothing, with which you are most concerned. Correct? Yes or no?

It seems to me that the subject of a center of mass of the universe (this thread's title therefore the TOPIC, no?) is not missing here, because, this "tiny volume smaller than an atomic nucleus" which hypothetically was the universe as we know it, certainly would have had a center of mass, would it not?

On 6/24/2018 at 7:39 AM, studiot said:

First of all the universe is not a rigid body.

The term centre of mass is only strictly applicable to rigid bodies.

I have a problem with this proposition, "The term centre of mass is only strictly applicable to rigid bodies." 

Mechanical engineers deal with systems in motion every day of the week, in which they assess the changing location of center of mass. 

In fact, typical examples include but are not limited to aircraft (fuel consumption is regulated so as not to leave one side or the other heavier), ships (whose changing centroid of buoyancy and hypercenter determine whether the ship would be in danger of capsizing), and submarines (whose complex systems of weight distribution must constantly be under intense supervision by engineers on board lest the submarine becomes unstable or rolls to the side, disrupting fuel storage tanks which are open to the salt water environment).

Edited June 26, 2018 by Neil Obstat

[Mordred]

1 hour ago, Neil Obstat said:

 

You said, "...all of what we can know that existed, existed in a volume smaller [than] an atomic nucleus." However, you also said, "It is wrong to view the BB as an explosion emanating from any one point...," therefore, interpreted, you are distinguishing with the greatest possible emphasis between a volume smaller than an atomic nucleus and any one point, correct?

I'm sorry, but that seems to be a very clear contradiction. If you don't think it is a contradiction, can you explain why you think it's not a contradiction? 

 

Think of it this way, Our Observable universe. Key note our observable portion of the universe originated at a point smaller than an atom. However this is just the portion that makes up our Observable portion of the universe today. Now here is the trick that smaller than an atom portion now makes up the entirety of our observable universe today. We reside within that portion and everywhere we look we see within that portion. So that original smaller than an atom is our entire observable universe today. The only centre is the center of our observable portion but how much bigger the rest of the universe is we do not know. We can only measure our observable universe and never beyond it. However we are certain there is a much larger universe we can never see or measure. We can also tell it must have similar conditions in mass density to our own observable portion due to the way a homogeneous and isotropic expansion occurs.

homogeneous no preferred location ie a centre.

isotropic no preferred direction.

An explosion has a specific dynamic in that it radiates from a centre outward. this is inhomogeneous and anistropic. However measurements on how galaxies expand from one another shows no preferred direction to expansion rather it is simply a density decrease in all directions equally.

Now  center of mass. In a homogeneous and isotropic volume where the mass density is uniform, one can choose any arbitrary location. Then measure around that point and measure the same amount of mass surrounding every location. No location will have a higher gravitational potential than any other location, so one can arbitrarily choose any location as a center of mass as there is no difference between any other location in potential strength. Ie there is no discernable center of mass which would be a location of higher potential.

Edited June 26, 2018 by Mordred

[J.C.MacSwell]

4 hours ago, studiot said:

Please define centre of mass and show how  this applies to the rocket/exhaust system under the conditions where I specified.

First you define your system, rocket plus exhaust,..or you and your dog...or the solar system...or all of the solar system except you and your dog etc etc.

It matters not if the systems parts are already in motion with respect to one another, or parts are displaced from one another. 

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

Then choose a coordinate system where the centre of mass is at rest,

Now define your force.

It applies to any external forces. F=ma, Any external force on any part of the system accelerates the centre of mass of the system.

It can obviously become impossible to keep track of the whole thing but conservation of momentum still applies.

 

[studiot]

1 hour ago, Neil Obstat said:

Mechanical engineers deal with systems in motion every day of the week, in which they assess the changing location of center of mass. 

Yes indeed they do, but the issue is change over time which you mention so clearly here.

If you study the mechancis of machines you will study several different instantaneous centres including an instantaneous COM.

So there is no such thing as the COM of the universe.

Consider the following apparatus.

A heavy dumbell is eccentrically attached to a non symmetrical lighter lamina so that it can rotate, independently of the lamina.
The lamina itself can also rotate.
Both are within a fixed frame.

The apparatus is set so the movable parts are spinning.

It can easily be seen that, as you rightly say, it moves about and you can calculate the locus of its movement.

 

Now expand the diagram to  the size of the universe and replot it observing due regard to relativity.

What does an instantaneous centre mean in this case?

 

[J.C.MacSwell]

2 hours ago, Neil Obstat said:

 

It seems to me that the subject of a center of mass of the universe (this thread's title therefore the TOPIC, no?) is not missing here, because, this "tiny volume smaller than an atomic nucleus" which hypothetically was the universe as we know it, certainly would have had a center of mass, would it not?

 

The observable Universe would in fact have a centre of mass, somewhere in the vicinity of where we are.

But the observable universe is not the whole thing and the Universe, even conceptually, is not understood to have one.

If you take the balloon analogy of the expanding universe, there is no centre of mass on the 2D surface (a balloon would have a centre of mass at it's centre inside the balloon, but there is no inside or outside to the Universe that is analogous to that of the model)

[studiot]

9 minutes ago, J.C.MacSwell said:

The observable Universe would in fact have a centre of mass, somewhere in the vicinity of where we are.

 

How can mass that may have ceased to exist millenia ago, be counted in the centre of mass of the present day universe?

The observable universe is not an instantaneous universe.

1 hour ago, studiot said:

What does an instantaneous centre mean

 

[J.C.MacSwell]

6 minutes ago, studiot said:

How can mass that may have ceased to exist millenia ago, be counted in the centre of mass of the present day universe?

The observable universe is not an instantaneous universe.

 

The present day observable universe, however you choose define it, would have a COM. Your (admittedly our) difficulty defining it, or keeping track of it, does not change that. Unlike the Universe, it conceptually would have a COM.

Why do you think it has ceased to exist? Conservation of mass and energy...the equations are different but the same principles apply in relativity as they do in Newtonian physics.