# With respect to what does a spinning object maintain its orientation?

#### tmst08

A spinning object such as a gyroscope is known to maintain its orientation. I am wondering *what* it maintains its orientation with respect *to*.

One answer is itself. Something else I have read is "with respect to the distant stars".

If it is not with itself but with "distant stars", one can ask "which distant stars?" since the stars are all in motion with respect to one another.

Of course, there is another answer. And that is that the spinning object, unless acted upon by another force, maintains its orientation with respect to the *universe*. In my very limited knowledge of physics and astronomy, this appears to imply that there actually *is* a universal frame of reference in respect to which all matter and energy is in motion but which itself is motionless.

I'll let the reader ponder the implications of this with respect to the existence of a prime mover unmoved...

-tmst08

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#### D H

Staff Emeritus
Newton's first law of motion says that a moving, constant mass object that is not subject to any external forces maintains a constant velocity (constant linear momentum). The same thing happens with rotation. A rotating object that is not subject to any external torques maintains a constant angular momentum.

Note well: This is only true from the perspective of an inertial frame of reference. An observer who is accelerating or rotating with respect to an inertial frame will see the objects that are not subject to external forces or torques undergo acceleration. So, why are some reference frames inertial frames of reference? How to construct one? The first question is deep. The second question, not so deep.

We use the remote stars, extremely remote stars, to define an inertial (non-rotating) frame of reference. Even if a remote star is moving normal to the line connecting our sun and the remote star, the long distance makes that motion unobservable. Suppose a quasar 6 billion light years away is moving at the speed of light normal to the line connecting our sun and the quasar. That huge velocity represents a proper motion of 34 micro arc seconds per year. It would take decades of observation to see that motion. A less extreme normal velocity is unobservable. Whatever tiny amount of proper motion does exist will average out if we look at hundreds of quasars. The result is our current best guess regarding a non-rotating frame of reference, the International Celestial Reference Frame. Work is underway to make an even better guess, ICRF-2.

Back to the first question: What makes non-rotating frames of reference so special? Your answer was
And that is that the spinning object, unless acted upon by another force, maintains its orientation with respect to the *universe*.
That is essentially Mach's Principle.

#### tmst08

Thanks. I now know the idea has been considered and apparently has been a subject of some contention in various circles, at least if the Wikipedia article I scanned is somewhat correct. That's interesting about Mach's Principle and it being somehow related to Einstein's relativity theories, perhaps even assisting such.

#### tmst08

I neglected to mention my appreciation for D H tying together the concepts of rotational and linear inertia, here. So my question is really subsumed by the idea of an absolute inertial reference frame? It has occurred to me that there's something odd about inertia. Is there something called an initial constant, which is like some arbitrary number used to describe an objects resistance to changing velocity?

-tmst08

#### tmst08

Another post in my quest for truth: I recently ran into some information about something called a sub-quantum or "Zero-point" field. Maybe a gyroscope maintains its orientation with respect to this. Sounds like an "ether" that has been variously postulated.

#### tmst08

...
The result is our current best guess regarding a non-rotating frame of reference, the International Celestial Reference Frame. Work is underway to make an even better guess, ICRF-2.
...
Methinks it might be more accurate to say "the" non-rotating frame of reference, rather than "a", since it *does* exist and the problem is only in detecting it.

#### Cleonis

Gold Member
We use the remote stars, extremely remote stars, to define an inertial (non-rotating) frame of reference.
Are you sure you want to phrase it that way?

Let me make a comparison: we use the triple point of water to define the units of the Kelvin scale of temperature. The choice of water is arbitrary.

But as we know, there is nothing arbitrary in using distant stars; the frame of the distant stars coincides with the local non-rotating frame. (Or equivalently, the local non-rotating frame coincides with the frame of the distant stars.)

(For completeness: I am aware of the Gravity Probe B experiment that was aimed at confirming the GR prediction of a 42 milli-arcseconds per year frame dragging effect. Given how small the frame dragging effect is the general assertion that the local non-rotating frame coincides with the frame of the distant stars is justified.)

#### Cleonis

Gold Member
A spinning object such as a gyroscope is known to maintain its orientation. I am wondering *what* it maintains its orientation with respect *to*.
It seems to me that another forum, the 'General Physics' forum, is the right place for asking above question.
You can re-post the question there (or maybe you can ask the moderators to move the thread.)
I had begun to write an answer, but such general discussion is not what this 'Homework & Coursework questions' section is intended for.

#### D H

Staff Emeritus
Are you sure you want to phrase it that way?
Technically you are correct. The ICRF (or the relatively new second realization of the ICRF) is a quasi-inertial frame. For one thing, it is accelerating (the origin is the solar system barycenter), and for another, it is based on measurements. The ICRF-2 is the best available guess (a very, very good one) as to what constitutes a non-rotating frame.

#### tmst08

Thanks, Cleonis, for the heads up about the ICRF. I wasn't aware that such existed, nor the Gravity B Probe. Reference frame dragging is a curious concept. Can you say anything about how this concept may affect any notion of a postulated universal reference frame? After all, if local (dragging) effects are possible, then isn't it no longer universal? Doesn't it seem like a redundant construct will then have been added to the fabric of space-time?

I'm currently interested in how such might be related to what I've heard of as a sub-quantum, or "zero-point" field that sounds much like the ether.

Yes, it appears I started this thread in the incorrect forum. A moderator is invited to move it to General Physics or Cosmology, as appropriate.

#### tmst08

ICRF-2 is based on the the solar system barycenter rather than fixed stars? Is this only for expediency?

#### Relay

I always thought that a spinning gyroscope maintains its orientation with respect to nothing. The gyroscope just points in it's start direction. Every spinning gyroscope is a direction vector.

#### tmst08

Perhaps that is a satisfactory answer. But then why does it appear to maintain its orientation nearly exactly to the cosmos? Coincidence?

#### Relay

I don't underestand what responce #14 from tmst08 means. How does something appear to maintain its orientation nearly exactly to the cosmos? To even observe this you must be outside the cosmos observing both the cosmos and the gyroscope. Mabe the universe is all dynamic with no single obsolute reference point. Mabe when you consider light year distances the gyroscope is no longer a straight direction vector but an arc. I can understand how a gyroscope is a direction vector but I cannot see the cosmos as a direction vector. In my mind I can't compare the two. How do you get a direction vector from the cosmos?

#### tmst08

So, I am referring to what I now know of as the ICRF. I believe a gyroscope maintains its orientation with respect to this - in fact ICRF may possibly be *defined* by gyroscope orientation. The fixed stars, or cosmos, are, at least to me, conceptually identical with ICRF.

Then there is the matter of centrifugal force in a rotating reference frame, and positings by Einstein regarding coinciding rotating reference frames both possessing it. I mean, isn't it interesting that the one that is relatively motionless with respect to the fixed stars has less of it?

That's a cool concept about the straight direction vector actually being an arc at light-year distances. I'm wondering whether it's actually the case that the shortest distance between two points is, or can be, otherwise than the path light takes. Hmm...

#### Relay

Now I know what the ICRF (International Celestial Reference Frame) is. It can be interpreted as a vector and I can see gyroscopes staying in the same orientation with it for billions of years.

As for the "cool concept" don't read too much into it. I said "Mabe..." because I can't argue for or againt the idea. I simply don't know.

"With respect to what does a spinning object maintain its orientation?"

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