Can an Accelerating Universe Rotate According to GR Principles?

In summary, the conversation discusses the possibility of experiencing centrifugal forces on only one of two rotating planets in a universe with only two stars or planets. It is stated that in this scenario, the other planet would not experience any centrifugal forces. The concept of Mach's principle and its relationship to the theory of general relativity is also brought up, with the conclusion that there is no prohibition in GR against a universe with nonzero angular momentum. The conversation ends with a clarification that acceleration and rotation can be detected and are not relative, but absolute.
  • #1
blue_sky
53
0
I have a doubt.
Let consider an universe of only 2 planets with the form of 2 dishes on the same z axis.
The 2 dishes are rotating 1 respect to the other.
Can we experience centrifugal forces only on 1 of them?
 
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  • #2
Im not sure i understand what you are asking, but i'll answer anyways. If you had 2 planets only, (or 2 stars) you can have a case where one is revolving around the other, or you can have a case where both are revolving around each other, which a lot of binary star systems do. In the first case only one would feel centrifugal forces, while in case 2, both would.
 
  • #3
If in the universe there are just 2 stars, how you can saw:
ArmoSkater87 said:
you can have a case where one is revolving around the other
?
If 1 is revolving around the other, is also true the opposite and who or what can distinguish the 1 star revolving and the 1 still?
 
  • #4
What do u mean? Dont you consider the sun to be stationary in respect to the solar system?? So why can't you have a stationary star, with another revolving around it?
 
  • #5
The Sun and the planets in the Solar System all revolve around their common center of gravity; all theories of gravitation, in particular Newton's and Einsteins's are symmetrical between the gravitating masses. Look up "back reaction".

The Sun is only stationary in relation to the planets to a first approximation.
 
  • #6
I think it's a fair question, as one reading of Mach's principle says that inertial forces are caused by movement relative to the matter in the universe, but this doesn't really feature in any of the mechanical models that we have.
 
  • #7
ArmoSkater87 said:
What do u mean? Dont you consider the sun to be stationary in respect to the solar system?? So why can't you have a stationary star, with another revolving around it?
My question is based on the assumption that the universe is made just by the 2 planet (or stars).
In your answer the solar system is embedded in a larger universe and this make a lot of difference.
 
  • #8
ArmoSkater87 said:
Im not sure i understand what you are asking, but i'll answer anyways. If you had 2 planets only, (or 2 stars) you can have a case where one is revolving around the other, or you can have a case where both are revolving around each other, which a lot of binary star systems do. In the first case only one would feel centrifugal forces, while in case 2, both would.

your answer is right here. :biggrin:
 
  • #9
jcsd said:
I think it's a fair question, as one reading of Mach's principle says that inertial forces are caused by movement relative to the matter in the universe, but this doesn't really feature in any of the mechanical models that we have.

Thanks.
Is the Mach's principle not embedded in the GR?
 
  • #10
blue_sky said:
Thanks.
Is the Mach's principle not embedded in the GR?

Not as I stated it. Imagine an object accelarting against a Minkowslkian background (i.e. a univesre that contains no mass!), you'll still have inertial forces. I think it appears in a more general sense: i.e. the relativity of accelartion.
 
  • #11
blue_sky said:
Thanks.
Is the Mach's principle not embedded in the GR?

Mach's principle influenced Einstein, but you won't find it embedded in GR. One key issue with Mach's principle in general is defining what it means, precisely.

You appear to be thinking about the question as to whether it's possible in principle for a universe to have a total nonzero angular momentum. In GR the answer is "yes".
 
  • #12
pervect said:
You appear to be thinking about the question as to whether it's possible in principle for a universe to have a total nonzero angular momentum. In GR the answer is "yes".

this means that the universe is rotating? if yes, relative to what?
 
  • #13
blue_sky said:
this means that the universe is rotating? if yes, relative to what?

Hmmm. I may well be mistaken, but was not Godels solution a rotating universe ?
I seem to remember that this annoyed his close friend Einstein because it allowed closed world lines, i.e. time travel.
 
  • #14
blue_sky said:
this means that the universe is rotating? if yes, relative to what?

A uniform velocity has no local effects on physics, but acceleration and rotation certaiinly do. If one is in a closed elevator, one can tell that the elevator is accelerating by the apparent inertial forces which cause objects to fall. If one is in a rotating room one can detect the centrifugal and coriolis forces. This is commonly phrased by saying that "acceleration is not relative, it's absolute".

It's certainly possible to postulate a universe with non-zero angular momentum according to the principles of GR. If one has a verion of Mach's principle which attempts to prohibit this, this version of Mach's principle is NOT part of GR. Less formally, a universe in GR _can_ rotate, and it doesn't need anything else to rotate relative to.
 
  • #15
pervect said:
A uniform velocity has no local effects on physics, but acceleration and rotation certaiinly do. If one is in a closed elevator, one can tell that the elevator is accelerating by the apparent inertial forces which cause objects to fall. If one is in a rotating room one can detect the centrifugal and coriolis forces. This is commonly phrased by saying that "acceleration is not relative, it's absolute".

It's certainly possible to postulate a universe with non-zero angular momentum according to the principles of GR. If one has a verion of Mach's principle which attempts to prohibit this, this version of Mach's principle is NOT part of GR. Less formally, a universe in GR _can_ rotate, and it doesn't need anything else to rotate relative to.

That unclear to me (but can be just my fault)
 

What is a centrifugal force?

A centrifugal force is an apparent force that appears to act outward on an object moving in a circular path. It is caused by the inertia of the object, which tends to make it continue moving in a straight line, even as it is following a circular path.

Is a centrifugal force a real force?

No, a centrifugal force is not a real force. It is an apparent force that is caused by the inertia of the object and the centripetal force acting on it. It is only observed in a non-inertial reference frame and does not actually exist.

What is the difference between a centrifugal force and a centripetal force?

A centrifugal force acts outward and away from the center of rotation, while a centripetal force acts inward and towards the center of rotation. Centrifugal force is an apparent force, while centripetal force is a real force that keeps the object in circular motion.

Do all circular motions experience a centrifugal force?

No, only circular motions in a non-inertial reference frame experience a centrifugal force. In an inertial reference frame, the object will continue moving in a straight line and no centrifugal force will be observed.

How does the magnitude of a centrifugal force change with speed?

The magnitude of a centrifugal force is directly proportional to the speed of the object. As the speed of the object increases, the centrifugal force also increases. This is because the inertia of the object increases with speed, making it more resistant to changes in direction and resulting in a larger centrifugal force.

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