Gravity Probe-B: Results & General Relativity

[magnetar]

Gravity probe-B!

The gravity probe-b mission had finished collection date for a long time .Why not announce any result ??

Do you think that the results will consistent with "general relativity"?

 

[neutrino]

We intend to announce the final experimental results of GP-B through a NASA press/media event towards the end of 2007. At that time it is also our intention to have submitted a number of papers on the GP-B results for publication in peer-reviewed scientific and technical journals.

http://einstein.stanford.edu/

 

[Garth]

But the first results are promised in April, on Saturday the 14th at the APS conference in Jacksonville Florida.

Not long to wait.

Garth

 

[rbj]

But the first results are promised in April, on Saturday the 14th at the APS conference in Jacksonville Florida.

Not long to wait.

do you think we might have heard a hint or a leak if the results were similar to those of the Michaelson-Morley experiment? if the results are not within noise of what GR says they should be, that would be hard to contain, no?

 

[Garth]

They have kept a 'blind' element in the results by not including the proper motion of the guide star against a distant quasar until the last minute.

They have kept their cards very close to their chest!

However we already know the proper motion of the guide star to 0.001"/yr from The stars of Pegasus from the Bright Star Catalogue, 5th Revised Ed. (Preliminary Version) (Hoffleit+, 1991, Yale University Observatory) as distributed by the Astronomical Data Center at NASA Goddard Space Flight Center.

IM PegasiRA J2000 : 22h 53m 2.3s
DEC J2000 : +16° 50' 28"
Proper motion in RA : -0.018 arcsec/y
Proper motion in DEC : -0.024 arcsec/y
mag : 5.64
MK spectral class : K1-2II-III

The Proper motion in RA will affect the E-W precession and the
Proper motion in DEC will affect the N-S precession.

The experiment measures the precession of the gyros to four decimal places of arcsec/yr, so the final convolution of the tracking a precession data will only increase the accuracy of the result in that fourth decimal place.

On the one hand, as you say, it would be hard to contain a non-GR result for so long, but on the other hand the fact that they are taking so long over processing the data might be an indication that they have found an anomaly and are checking and double checking their results before publication.

We wait and see.

Garth

 

[magnetar]

It seems that in 1997 scientists already have evidence of "frame-dragging"effect!
http://web.mit.edu/newsoffice/1997/blackholes.html

 

[Garth]

It's the geodetic effect I am interested in!

Garth

 

[magnetar]

This phenomenon, known as frame-dragging, was first predicted in 1918 as a natural consequence of Einstein's general theory of relativity, which describes the effects of gravity on space and time. But it had been unproved by experiments or observation until recently, when Italian researchers suggested the effect might be present near spinning neutron stars. The MIT team then applied a similar idea to several black holes in our galaxy-------

excerpt from http://web.mit.edu/newsoffice/1997/blackholes.html

 

[pervect]

From http://einstein.stanford.edu/content/story_of_gpb/gpbsty3.html

But what does "the same direction in space" mean? For Newton the answer was easy. Space and time were absolutes. A perfect gyroscope set spinning and pointed at a star would stay aligned forever. Not so for Einstein. Space-time is warped -- and may even be set in motion by moving matter. A gyroscope orbiting the Earth finds two distinct space-time processes -- frame-dragging and the geodetic effect -- gradually changing its direction of spin.

According to Einstein the Earth warps space-time. A second, much larger change in spin direction, the geodetic effect, follows from the gyroscope's motion through this space-time curvature. The phenomenon was foreshadowed in 1916 by W. de Sitter who predicted a minute relativistic correction to the complicated motions of the Earth-Moon system around the Sun -- an effect finally detected in 1988 through an elaborate combination of lunar ranging and radio interferometry data. For a gyroscope the predicted effect is a rotation in the orbit-plane of 6,600 milliarc-seconds per year -- quite a large angle by relativistic standards.

Gravity Probe B will measure the change to 1 part in 10,000 or better, the most precise qualitative check yet of any effect predicted by general relativity.

The geodetic precession is closely related to Thomas precession (if that helps any).

It's of interest to Garth because his dark-horse (though peer-reviewed) theory of gravity, SCC http://arxiv.org/abs/gr-qc/0212111 makes an experimental prediction different from GR that will be tested by gravity probe B.

You can probably find a lot of threads on SCC here at PF if you look - being published in a reputable journal, it's "fair game" for discussion on PF.

 

[MeJennifer]

The geodetic precession is closely related to Thomas precession (if that helps any).

This statement intrigues me.

While Thomas precession is an effect due to the hyperbolic nature of space and is a compensation for the non commutative Einstein addition formula (see for instance: "Ungar A A - Beyond the Einstein addition law and its gyroscopic Thomas precession (Kluwer,2001)(ISBN 0792369092)"), I thought the geodetic precession had something to do with curvature?

No?

 

[Garth]

Yes MeJennifer - though there has been some confusion.

In four-space, SR's Minkowski space-time, an object moving relative to an observer has a four-vector inclined to the four-vector of that observer. Therefore, if a force accelerates a gyroscope, its four-vector gradually leans over relative to its original direction and its spin axis will consequentially precess. This is Thomas precession. It is a SR effect.

Geodetic Precession on the other hand is a GR effect.

Consider the 'curved funnel' model of the space component of curvature around a gravitating mass. At anyone particular distance around the central mass the space curvature can be modeled by a 'slice' of a cone. That cone can be constructed by cutting a thin cake slice out of a circle and gluing the edges together.

The space curvature component of geodetic precession, described by the Robertson parameter \gamma, is the angle of the slice cut out of the full circle.

The other component of the geodetic precession is a time dilation effect.

Another way of describing the GP-B geodetic effect is to say 'it is the amount the gyros 'lean over' into the slope of the curvature of space-time' as they orbit the Earth. If that helps.

In the GP-B experiment SCC predicts the same frame-dragging E-W precession but only 2/3 the N-S precession as GR. We shall know soon!

Garth

 

[pervect]

Yes MeJennifer - though there has been some confusion.

Some of the confusion here appears to be mine, which I apologize for, but I don't want to pass up the opportunity to learn something and hopefully become less confused.

In four-space, SR's Minkowski space-time, an object moving relative to an observer has a four-vector inclined to the four-vector of that observer. Therefore, if a force accelerates a gyroscope, its four-vector gradually leans over relative to its original direction and its spin axis will consequentially precess. This is Thomas precession. It is a SR effect.
ere appears to be some coordina
Geodetic Precession on the other hand is a GR effect.

Suppose we have observer A, in flat space-time, in a powered circular orbit. The centripetal acceleration is supplied by his rocket. Because the direction of his acceleration is constantly changing, he will experience Thomas precession.

Suppose we have observer B, in curved space-time, in an unpowered circular orbit. From a Newtonian viewpoint, the centripetal acceleration is supplied by gravity. From a GR viewpoint, he is simply following a geodesic.

I was under the (possibly erroneous) impression that observers A and B experienced the same amount of precession in the same direction. I really ought to do a calculation to check this, but it will have to wait.

Consider the 'curved funnel' model of the space component of curvature around a gravitating mass. At anyone particular distance around the central mass the space curvature can be modeled by a 'slice' of a cone. That cone can be constructed by cutting a thin cake slice out of a circle and gluing the edges together.

The space curvature component of geodetic precession, described by the Robertson parameter \gamma, is the angle of the slice cut out of the full circle.

The other component of the geodetic precession is a time dilation effect.

Another way of describing the GP-B geodetic effect is to say 'it is the amount the gyros 'lean over' into the slope of the curvature of space-time' as they orbit the Earth. If that helps.

In the GP-B experiment SCC predicts the same frame-dragging E-W precession but only 2/3 the N-S precession as GR. We shall know soon!

Garth

This appears to be the standard explanation of geodetic precession (and standard is good).

I ran across http://prola.aps.org/abstract/PRD/v42/i4/p1118_1 though, which seems to blur the distinction being made here, and to suggest that some of the distinction comes about because of the particular coordinate system being assumed.

Experimentally the distinction is important because the two effects occur at right angles for GP-B. They occur at right angles only for a satellite in polar orbit. In fact the polar orbit was chosen to make this happen.

 

[Garth]

Suppose we have observer A, in flat space-time, in a powered circular orbit. The centripetal acceleration is supplied by his rocket. Because the direction of his acceleration is constantly changing, he will experience Thomas precession.

Note: observer A would experience Thomas precession even if she were accelerating in a straight line - because her 4-vector would be rotating in space-time.

Suppose we have observer B, in curved space-time, in an unpowered circular orbit. From a Newtonian viewpoint, the centripetal acceleration is supplied by gravity. From a GR viewpoint, he is simply following a geodesic.

I was under the (possibly erroneous) impression that observers A and B experienced the same amount of precession in the same direction. I really ought to do a calculation to check this, but it will have to wait.

In that situation the Thomas precession of observer A is equal and opposite in direction to the 'time dilation' component of the Geodetic precession of observer B.

On top of that GR would predict a 'space curvature ("missing slice") component' precession for observer B that was twice his 'time dilation component' precession. (See MTW page 1119 equation 40.34)

So observer B suffers 3X observer A's total precession.

This appears to be the standard explanation of geodetic precession (and standard is good).

I ran across http://prola.aps.org/abstract/PRD/v42/i4/p1118_1 though, which seems to blur the distinction being made here, and to suggest that some of the distinction comes about because of the particular coordinate system being assumed.

Although I do not have access to the paper itself I can understand what they are doing. From an orbiting coordinate system the Earth is spinning even if it is non-rotating wrt the fixed stars. It could be confusing because the two precessions in their case are equal yet have fundamentally different origins. This becomes obvious if the satellite's orbit is not in the plane of the Earth's rotation, as indeed it is not with GP-B.

Geodetic precession will be experienced by a gyroscope orbiting the equator of a non-rotating Earth, but IMHO it is confusing to call this 'frame dragging' .

Experimentally the distinction is important because the two effects occur at right angles for GP-B. They occur at right angles only for a satellite in polar orbit. In fact the polar orbit was chosen to make this happen.

Indeed, they had a launch window of 1 second each day to put it into that orbit!

Garth

 

[pervect]

Note: observer A would experience Thomas precession even if she were accelerating in a straight line - because her 4-vector would be rotating in space-time.

(See MTW page 1119 equation 40.34)

Surely v x a would be zero if v were in the same direction as a? Implying no precession for an observer accelerating in a straight line (where a is always in the same direction as v) as MTW's 40.33b reduces to -(1/2) v x a for this case.

So observer B suffers 3X observer A's total precession.

OK, I can see that now. Thanks for the correction.

 

[Garth]

Surely v x a would be zero if v were in the same direction as a? Implying no precession for an observer accelerating in a straight line (where a is always in the same direction as v) as MTW's 40.33b reduces to -(1/2) v x a for this case.

a is actually the 4-acceleration, in equation 40.34 the components are evaluated in the PPN approximation where the 3-acceleration suffices. See MTW exercise 6.9 page 175.

Garth

 

[pervect]

40.33 (pg 1118) has already been re-written in a 3d vector form - it's not a 4-vector equation (and the cross product only makes sense in 3 dimensions).

One could regard the Lorentz boost as a kind of rotation, but there isn't any spatial rotation in the normal 3-d sense in the case of constant acceleration in the same direction that I can see. And I've looked at exercise 6.9 as you suggest.

I.e. if we accelerate in the x direction with a constant acceleration, a gyroscope with its spin axis in any of the x, y, or z directions will not precess in any spatial direction (assuming the force accelerating the gyroscope is applied at the center of mass).

The most that will happen is that time dilation might affect the apparent angular momentum carried by the gyroscopes, but this is not a 3-d rotational effect.

 

[Garth]

As I said that 3-acceleration expression suffices in the PPN approximation.
From MTW page 118

Equations 40.33 describe in complete generality at the post-Newtonian level of approximation the precession of the gyroscope spin S relative to the comoving orthonormal frame that is rotationally tied to the distant stars.

Where we are confusing ourselves is over two sorts of acceleration.

In the case of a gyroscope fixed to the Earth's surface the significant Thomas precession is due to the gyro's acceleration when carried around by the Earth's rotation.

However, my point in #13 was referring to a linear acceleration that was the subject of MTW Exercise 6.9.

Here a constant force acting through the centre of mass produces a acceleration constant in direction however, the 4-velocity of an linearly accelerating body actually rotates in an inertial space-time frame.

Now, the four-angular momentum S of a rotating body has the property of always being orthogonal to its 4-velocity (eq. 6.21), therefore S has to precess under acceleration \mathbf{a} to keep orthogonal to that 4-velocity \mathbf{u}:

\frac{d\mathbf{S}}{d\tau} = (\mathbf{u} \wedge \mathbf{a})\cdot d\mathbf{S}

Garth

 

[pervect]

I would agree that there will be a "pseudorotation" in the timelike plane formed by a and u, as described in MTW,

The very concept of acceleration, therfore, implies "rotation" of veloctiy 4-vector. How then is the requirement of "no rotation" to be interpreted? Demand that the tetrad change from instant to instant by preciesly that amount implied by the rat of change of u, and by no additional arbitrary rotation. In orther words accept the inevitable pseudorotation in the timelike plane defined by the velocity 4-vector and the acceleration byht (2) rule out any ordinary rotation of the three space vectors.

However, I don't believe that it is been demonstrated (and I don't believe that it is true) that an observer in hyperbolic motion, Fermi-Walker transporting his basis vectors (or equivalently, using a set of gyroscopes he carries with him to orient his basis tetrads) will observe any apparent rotation of the distant "fixed stars".

 

[Garth]

I disagree.

Looking more closely at the case of the Thomas precession of a gyroscope attached to the Earth's surface, in MTW eq. 40.34.

The acceleration concerned is the upwards acceleration \bigtriangledown U of a freely falling frame so that it becomes at rest with the Earth's surface. The velocity is that of the gyroscope being carried around by the Earth's rotation.

Again, has this Thomas precession applied to a spinning electron moving in a circular orbit around an atomic nucleus not been observed? Does it not account for a factor of two in the effective energy of coupling of spin and orbital angular momentum of a atomic bound electron?

MTW page 1145 $41.4:

A spinning object, free of all torque, but undergoing acceleration, changes its direction as this direction is recorded in an inertial frame of reference.

///////////////////////////

How is it possible for "no rotation" to appear as "rotation"? The answer is this: one pure boost, followed by another pure boost in another direction, does not have as net result a third pure boost; instead, the net result is a boost plus a rotation.

Garth

 

[pervect]

Well, let's get specific then.

Suppose one accelerates at a constant rate towards a "guide star", say Sirius, in a flat Minkowski space-time, while using an array of gyroscopes to transport one's basis vectors (the mathematical description of this operation would be Fermi-Walker transporting these basis vectors).

I say that there should be no rotation of the "fixed stars" (assumed, for the sake of this example, to all be stationary relative to each other in some inertial frame).

You appear to be claiming that there will be. What is the axis of this rotation, and given an acceleration 'a', how many radians/second will it be?

For starters, can we agree that the guide star will always be "ahead" of the accelerating spaceship, i.e. that the Fermi-Walker transported basis vector intitally pointing towards the guide star will always point towards the guide star? (I suppose I should add for completeness that I'm assuming the guide star is far enough away that one doesn't reach it).

 

[Garth]

Well, let's get specific then.

Certainly! An illustrative example to think about.

Suppose one accelerates at a constant rate towards a "guide star", say Sirius, in a flat Minkowski space-time, while using an array of gyroscopes to transport one's basis vectors (the mathematical description of this operation would be Fermi-Walker transporting these basis vectors).

I say that there should be no rotation of the "fixed stars" (assumed, for the sake of this example, to all be stationary relative to each other in some inertial frame).

You appear to be claiming that there will be.

I claim that under that acceleration an on-board torque free gyroscope will precess relative to the fixed stars.

What is the axis of this rotation, and given an acceleration 'a', how many radians/second will it be?

My understanding of this case is, if we let the spaceship have a relative 3-velocity initially also in the direction of Sirius parallel to the 3-acceleration vector:

\frac{d\mathbf{S}}{d\tau} = (\mathbf{u} \wedge \mathbf{a})\cdot d\mathbf{S}

The gyroscope's intrinsic angular momentum 4-vector is always orthogonal to the 4-velocity.

It will precess in 3-space orthogonally to the plane defined by the gyro's axis and the ship-Sirius direction 3-vector by an amount (while speeds are non-relativistic):

\frac{a \sin(\theta)}{c} radians/sec

where \theta is the angle between the direction of the gyro's spin axis and the ship-Sirius direction 3-vector and a is the magnitude of the acceleration.

Garth

 

[pervect]

Certainly! An illustrative example to think about. I claim that under that acceleration an on-board torque free gyroscope will precess relative to the fixed stars.
My understanding of this case is, if we let the spaceship have a relative 3-velocity initially also in the direction of Sirius parallel to the 3-acceleration vector:

\frac{d\mathbf{S}}{d\tau} = (\mathbf{u} \wedge \mathbf{a})\cdot d\mathbf{S}

The gyroscope's intrinsic angular momentum 4-vector is always orthogonal to the 4-velocity.

It will precess in 3-space orthogonally to the plane defined by the gyro's axis and the ship-Sirius direction 3-vector by an amount (while speeds are non-relativistic):

\frac{a \sin(\theta)}{c} radians/sec

where \theta is the angle between the direction of the gyro's spin axis and the ship-Sirius direction 3-vector and a is the magnitude of the acceleration.

Garth

Let's introduce a coordinate system, with Sirius lying in the 'x' direction.

y
^
^
^
-------> x (Sirius)

and define vectors using their components (t,x,y,z), in that order

Your claim, as I understand it, is that a gyroscope with a spin axis in the 'y' direction precesses.

Now for a spin axis in the 'y' direction, in a comoving frame we should haveS = \left( 0,0, L, 0 \right)

Let's use your formula (a^u) S to find the initial rate of change of S in a comoving frame.

Then
u = \left( 1,0,0,0 \right)
a = \left( 0, a, 0, 0 \right)

see MTW 6.4 pg 166 for the components of a, which we can see is orthogonal to u as it should be. Then

a \wedge u = \left( \begin{array} {cccc} <br /> 0&amp;-a&amp;0&amp;0\\<br /> a&amp;0&amp;0&amp;0\\<br /> 0&amp;0&amp;0&amp;0\\<br /> 0&amp;0&amp;0&amp;0 \end{array} \right)<br />

We can see that \left( a \wedge u \right) \cdot S = 0

[add]
Why can we see this? Because (a ^ u) only has components (tx and xt). This is because a and u only have t and x components, hence a^u can only have t and x components. Because the wedge product is anti-symmetric, the tt and xx components are 0, only the tx and xt components exist, and they must be negatives of each other.

But the vector that we multiply by has only a y component. The only way to get a nonzero result is from y components in the wedge product. But all such components are zero.

Note that we also expect (0,0,L,0) to boost in the x direction to (0,0,L,0), i.e. we don't expect a boost in the x direction to change the y component of angular momentum given that it transforms as a 4-vector. Thus we don't really expect acceleration in the x direction (which is a series of small boosts) to change the y component of momentum. Which is what we just calculated in more detail.

[end add]

i.e. there is no change in S, a gyroscope with its spin axis initially in the 'y' direction does not precess.

 

[Garth]

Yes, I agree.

This is an interesting problem that I do not have time to do justice to.

My original statement was referring to a precession relative to the distant stars. (As we had been talking about GP-B.)

The tetrad of basis vectors itself undergoes a rotation in space-time relative to an inertial frame of reference.

The effect I am thinking of may, after a substantial velocity change has built up, be related to the relativistic aberration and the relativistic searchlight effect. The gyro would appear to precess relative to the distant stars, which themselves would precess in the frame of reference of the spaceship .

Or am I mistaken?

Garth

 

[magnetar]

I am waiting for the results! (14th April)

 

[Garth]

I am waiting for the results! (14th April)

Yes indeed! See the Alternative theories being tested by Gravity Probe B thread.

Perfect Returning to my previous musings about a constantly accelerating gyroscope observed from an inertial frame of reference.

Does not the its orthogonal tetrad of basis vectors not rotate but look warped from the inertial frame?

The spacecraft would experience this as the relativistic aberration of light from fixed stars by which the gyroscope's precession is being tracked.

In GP-B if the experiment was not in orbit but fell straight towards a non-rotaitng Earth would not the gyroscope still precess relative to the distant quasar?

If so, my question is, "Would it still precess if the experiment had only just begun to fall so it as under 'Newtonian' gravitational acceleration yet momentarily at rest w.r.t. the Earth?"

Garth

 

[pervect]

While I'll certainly agree that the light appears aberrated, I don't see any errors in my argument / calculations that torque-free gyroscopes (modelled as fermi-walker transported tetrads) see no apparent rotation of the fixed stars for the constant acceleration case when the gyroscopes are orthogonal to the direction of motion (ie for the y and z spin axes in the coordinate system I introduced).

The case where the gyroscope points towards the guide star is more difficult mathematically, but it's pretty clear from the above argument that if this gyroscope had no y or z components in its angular momentum initially, it will not have them if it is acclerated. Thus I don't believe that the change in the 4-momentum of a gyroscope with the spin axis in the 'x' direction can't be interepreted as a spatial precession.

So I would say yes to aberration and no to precession for these cases.

I haven't analyzed the case where a gyroscope is at "in-between" angles, I suppose that if the x component of the angular momentum were initially equal to the y component, this would not remain true as the system accelerated - the x component would vary because of the boost, but the y component should remain constant. I suppose this would be regarded as precession.

 

[Garth]

Yes, I agree with you that the torque-free Fermi-Walker transported gyroscope does not rotate relative to the Fermi-Walker transported basis vectors of its constantly accelerating frame of reference. What I should have said was that relative to an inertial frame of reference that accelerating frame of basis vectors looks warped, i.e. not orthogonal to each other.

The traveling frame experiences a relativistic aberration of light from the 'fixed stars', which may be a corollary of the relativity of simultaneity.

That aberration would increase as its velocity built up, thus, relative to a fixed star, the gyroscope will appear to suffer a precession.

Let's do some maths:

Take the case of an observer accelerating uniformly at a > 0 but momentarily at zero velocity (to keep the maths simple on these Forums!).

A star at angle \theta_0 to the direction of travel is observed at angle \theta due to the relativistic aberration of light where:

\cos{\theta} = \frac{\cos{\theta_0} + v/c}{1 + \cos{\theta_0}v/c}

After a short time \delta t, \theta increases to \theta + \delta \theta, and differentiating, i.e. dividing by \delta t and letting \delta t and \delta \theta \rightarrow 0, then setting v = 0 and hence \theta_0 = \theta we obtain

\frac{d\theta}{dt} = - \frac{a}{c}\sin{\theta}

The gyroscope measured relative to this distant star would appear to have a precession with the opposite sense, thus obtaining my formula in post #21.

At Earth g acceleration ~ 10³ cm.sec^-2 this introduces an precession of a star (where the direction of the star is orthogonal to the acceleration) of 13 milliarcsec/sec, quite significant to GP-B!. Note the acceleration is measured relative to the fixed stars and not GP-B's freely falling frame, but it would be cyclic and therefore easy to detect and eliminate.

Garth

 

[pervect]

Yes, I agree with you that the torque-free Fermi-Walker transported gyroscope does not rotate relative to the Fermi-Walker transported basis vectors of its constantly accelerating frame of reference. What I should have said was that relative to an inertial frame of reference that accelerating frame looks warped.

What I was trying to say was that we adopt a particular orthonormal reference frame for our Minkowski space-time (the t,x,y,z coordinate system I set up) which we regard as the "fixed stars". We can furthermore actually imagine that we have fixed, unmoving stars scattered around at constant unchanging coordinates in this particular coordinate system.

We would measure precession as a change in the spin axis of the gyroscope relative to this particular coordinate system.

We have to go further, actually - we have to separate out the space parts of the precession. I have simply chopped off the time component of the 4-vector to do this, I *think* this is a reasonable approach. (In some of the cases we considered, though, the time component turns out not to vary anyway, making the job easier).

If we imagine a relativistic uniformly accelerating traveller, he would "see" the stars start to cluster ahead of him as he accelerates.

http://www.exo.net/~pauld/stars/PD_images_relativ.html

I would regard this as a visual effect though, a change in the paths light took rather than an actual precession. YMMV.

 

[Garth]

I agree, but we were talking about GP-B.

Here the precession of the gyroscope is measured relative to the observed line of direction to IM Pegasi and then IM Pegasi's proper motion relative to a distant quasar. In fact it is the only way of measuring the precession accurately.

There are several sources of observed precession 'noise' for the GP-B team to eliminate, which is why they are taking so long to publish.

But not long now for the first results!

Garth

 

[Garth]

Continuing this line of thought...

Consider no gravitational field and a laboratory A carried by a rocket with three stars x, y, z conveniently placed so they are each in the direction of A's orthogonal basis vectors x, y, z. The rocket motor now ignites accelerating the lab in the z direction.

The x, and y stars now precess towards z at the rate

\frac{d\theta}{dt} = - \frac{a}{c}

and relative to those stars x and y a torque-free gyroscope carried in the lab appears to precess.

If its rotational axis is inclined at an angle \theta to z, then it appears to precess at the rate:

\frac{d\theta}{dt} = + \frac{a}{c}\sin{\theta}

relative to a guide star in the direction of its axis.Whereas to the accelerating observer in the lab the three basis vectors remain orthogonal and the stars x and y precess, to an inertial observer it is the three stars that remain fixed and it is the lab's orthonormal basis tetrad that appears to warp.

The lab's x-y plane appears in the inertial frame to be warped into a cone of which the sides are inclined to the inertial x-y plane by an angle that increases by the rate a/c.

In this inertial frame it is the gyroscope that appears to precess away from the direction of its acceleration.

This precession is due to the rate of change of the lab's velocity and applies as soon as the acceleration begins even while the velocity is momentarily zero.

IMHO It would not apply to a supported, stationary, lab in a gravitational field of equal acceleration.

It is an interesting reflection on the equivalence principle that by simply observing the fixed stars you can distinguish between 'gravitational' and 'velocity' accelerations, even while the velocity of the latter w.r.t. the fixed stars builds up from zero.

Of course the same could be said of an accelerating observer measuring the rate of change of the frequency of an absorption line (increase of blue shift) emitted by a distant star ahead, in line with her acceleration.

Garth

 

[pervect]

I'm also quite curious as to the GP-B results, though I don't have anything riding on it.

I guess april 15 is the magic date, I imagine some people "in the know" already have an idea what will be presented in the way of the preliminary data analysis at the APS conference but of course they aren't talking publically.

As far as your points on precession goes, I do agree with your recent points about the apparent precession of distant stars from an accelerating observer, and I think any remaining issues are semantic.

 

[magnetar]

http://einstein.stanford.edu/

 

[Chris Hillman]

The GP-B press release in plain English?

The gravity probe-b mission had finished collection date for a long time .Why not announce any result ??

Do you think that the results will consistent with "general relativity"?

I'm also quite curious as to the GP-B results, though I don't have anything riding on it.

After more than forty years (!) of development, GP-B was finally launched in April 2004. It recorded data for more than a year, and the Stanford team has been analyzing this data ever since. They will hold a press conference on the 18th but a press release has been made available today. The total cost of the experiment is said to be some 700 million dollars. For comparison, this is less than the cost of a one mile stretch of a proposed six lane traffic tunnel in an American city.

Recall that geodetic precession or de Sitter precession of the spin axis of a gyroscope in a quasi-Keplerian orbit around a nonrotating massive object (think of a Schwarzschild object, treated in weak-field gtr) has already been confirmed to good accuracy by previous work. See http://science.nasa.gov/headlines/y2000/geodetic.htm . The goal of GP-B is to measure a much smaller effect, the frame-dragging precession or gravitomagnetic precession, or Lense-Thirring precession, which takes account of rotation effects when the massive object is spinning about its own axis (think of a Kerr object, treated in weak-field gtr). These effects should not be confused with the precession of the periastria (locus of closest approach) of a small object in quasi-Keplerian orbit around a massive object.

Recall also that there is a large class of metric gravitation theories (certain relativistic classical field theories) which behave much like gtr but are more complicated in various senses. Most of these also predict de Sitter and Lense-Thirring type effects, but with magnitudes differing from the gtr prediction, in any given situation--- but possibly not by very much. So the goal of GP-B and other tests can be understood as asking the question: "Do we need to consider more complicated theories than gtr, or does gtr suffice to account for all the gravitational effects we can directly test?" See http://math.ucr.edu/home/baez/RelWWW/HTML/grad.html#tests for background on the subject of testing theories of gravitation.

To forestall possible misunderstanding:

In the (embargoed) press release at http://einstein.stanford.edu/, the GP-B team says:

1. The preliminary data analysis did confirm the de Sitter precession formula to higher accuracy than ever before.

2. It uncovered two unforeseen sources of error involving classical physics of the experimental equipment--- basically, a telescope in an Earth-orbiting satellite. As I understand it, both new error sources arise from a kind of electrostatic friction in the telescope mounting. Figuring this out took a lot of work. The effect is well known in physics, but when the experiment was originally designed, it was thought that the resulting errors would be self-canceling, but this turns out not to be true.

3. These errors need to be carefully modeled before they can be removed in order to test the Lense-Thirring precession formula.

4. The Stanford team hopes to complete this analysis by Dec 2007.

For a more detailed discussion, try http://twistedphysics.typepad.com/cocktail_party_physics/2007/04/the_not_so_frie.html

 

[marcus]

Thanks for the plain English summary! It saves an interested audience member like myself much possible confusion and bother.

 

[rbj]

Recall that geodetic precession or de Sitter precession of the spin axis of a gyroscope in a quasi-Keplerian orbit around a nonrotating massive object (think of a Schwarzschild object, treated in weak-field gtr) has already been confirmed to good accuracy by previous work. ... The goal of GP-B is to measure a much smaller effect, the frame-dragging precession or gravitomagnetic precession, or Lense-Thirring precession, which takes account of rotation effects when the massive object is spinning about its own axis (think of a Kerr object, treated in weak-field gtr). ...
Recall also that there is a large class of metric gravitation theories (certain relativistic classical field theories) which behave much like gtr but are more complicated in various senses. Most of these also predict de Sitter and Lense-Thirring type effects, but with magnitudes differing from the gtr prediction, in any given situation--- but possibly not by very much. So the goal of GP-B and other tests can be understood as asking the question: "Do we need to consider more complicated theories than gtr, or does gtr suffice to account for all the gravitational effects we can directly test?"
...
1. The preliminary data analysis did confirm the de Sitter precession formula to higher accuracy than ever before.

so, just to be clear, Chris: does the GP-B results, at least so far, confirm the GTR quantitative prediction of the amount of de Sitter precession over the quantitation preditions of the other theories in this "large class of metric gravitation theories"? did they say as much or are they still hedging this?

 

[Chris Hillman]

The "Acceptable Sliver" in PPN Space

Hi, rbj,

does the GP-B results, at least so far, confirm the GTR quantitative prediction of the amount of de Sitter precession over the quantitation preditions of the other theories in this "large class of metric gravitation theories"? did they say as much or are they still hedging this?

Regarding "confirm the GTR quantitative prediction of the amount of de Sitter precession over the quantitative preditions of the other theories", this phraseology seems to miss one of the points I was trying to emphasize. See the review paper by Will on the webpage I cited and look for the diagrams showing regions of "allowed theories" parameterized by the PPN formalism. Observe that in the PPN picture, gtr sits smack dab in the center of a very thin "acceptable sliver" sitting inside a large (but finite dimensional) space of alternative gravitation theories. This sliver represents the theories which are in agreement with current observation/experiment. (Modulo inevitable quibbles over what observations to accept at face value.) However, there are still points in this sliver which are arbitrarily close to the point representing gtr.

The "acceptable sliver" has just been reduced in one dimension, if you like, because GP-B has measured the de Sitter precession to greater accuracy than ever before. Now, gtr still sits in the center of the newly reduced "sliver", but given the fact that limits of accuracy will always be with us, you can never hope to reduce this sliver to a single point! By Occam's razor, gtr is preferred theoretically because it happens to be the simplest of all these theories, which is one reason for paying so much more attention to it than to the other theories in the "acceptable sliver".

As for "hedging", the whole point of my post was to explain why there is no "hedging" from the Stanford team! Rather, they said that they are not yet ready to say anything about the more interesting Lense-Thirring precession. While this is certainly disappointing, they went on to carefully explain why that is the case. They carefully explained why they think they know what they have to do in order to "safely remove" the unanticipated errors in order to finally obtain the desired clean test, and they said they hope to complete this work by December. Science is hard, and sometimes you just have to be patient while people work out some wrinkles.

 

[magnetar]

Gravity probe -B backs general relativity!?
http://physicsweb.org/articles/news/11/4/11/1