# A doubt

1. Aug 15, 2004

### blue_sky

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?

2. Aug 15, 2004

### ArmoSkater87

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. Aug 15, 2004

### blue_sky

If in the universe there are just 2 stars, how you can saw:
?
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. Aug 15, 2004

### ArmoSkater87

What do u mean? Dont you consider the sun to be stationary in respect to the solar system?? So why cant you have a stationary star, with another revolving around it?

5. Aug 15, 2004

### selfAdjoint

Staff Emeritus
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. Aug 15, 2004

### jcsd

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. Aug 15, 2004

### blue_sky

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. Aug 15, 2004

### Nenad

your answer is right here.

9. Aug 15, 2004

### blue_sky

Thanks.
Is the Mach's principle not embedded in the GR?

10. Aug 15, 2004

### jcsd

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. Aug 15, 2004

### pervect

Staff Emeritus
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. Aug 17, 2004

### blue_sky

this means that the universe is rotating? if yes, relative to what?

13. Aug 18, 2004

### mijoon

Hmmm. I may well be mistaken, but was not Godels solution a rotating universe ???
I seem to remember that this annoyed his close freind Einstein because it allowed closed world lines, i.e. time travel.

14. Aug 18, 2004

### pervect

Staff Emeritus
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. Aug 19, 2004

### blue_sky

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

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