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Wally
2004-Apr-28, 01:33 PM
Just curious. Einstein predicted rotating masses would "drag" spacetime around in the direction of the spin, right? And this newly launched satellite will either prove or disprove this dragging. My question is, why would spacetime be affected by the spinning of a mass? To me, it seems the spin would be irrelevent, unless maybe the spin approaches relativistic speeds or something (which is most likely impossible as that would tend to cause the mass to fly apart).

skrap1r0n
2004-Apr-28, 01:42 PM
think of a spinning bowling ball in molasass. what happens to the molasass?

milli360
2004-Apr-28, 02:16 PM
My question is, why would spacetime be affected by the spinning of a mass? To me, it seems the spin would be irrelevent, unless maybe the spin approaches relativistic speeds or something (which is most likely impossible as that would tend to cause the mass to fly apart).
It's true that the relativistic correction is small at low speed, but it's there nonetheless. If you had very sensitive instruments, you could measure it.

The same is true of the Gravity Probe B experiment--the effect is very small, but we've spent forty years developing the technology, just so we could measure that extremely small effect.

Wally
2004-Apr-28, 02:56 PM
Yeah, but. . . (and here he hesitates out of fear. Couple deep breathes. . . ok, here goes!) doesn't that almost imply an ether type thing? (SAM5, stay outa this! I'm just looking for clarification! I do NOT believe in any kind of ether theory!!). I'd think space/time just wouldn't be affected by the spin of a massive body, only by its presence.

skrap1r0n
2004-Apr-28, 03:22 PM
Yeah, but. . . (and here he hesitates out of fear. Couple deep breathes. . . ok, here goes!) doesn't that almost imply an ether type thing? (SAM5, stay outa this! I'm just looking for clarification! I do NOT believe in any kind of ether theory!!). I'd think space/time just wouldn't be affected by the spin of a massive body, only by its presence.

Why do you think this?

I am completely ignorant about any of this, however I find it interesting that there is the possibility that there may be some type of "Friction" between large chunks of mass and the space/time fabric. According to this article (http://www.phy.duke.edu/~kolena/framedrag.html) the Effect has moved LAGEOS I and II sattelites about 6 feet per year ahead of where they were supposed to be. Now 6 feet over 365 days is not a huge variance, but think about it for a second...

We can calculate orbits down to the Nth decimal point. The Earth's Mass SHIFTED space and everything in it ahead 6 feet. I mean if you really think about it, it is mind boggling. The fabric of space was moved ahead of where it was supposed to be. That is a colossal find.

Padawan
2004-Apr-28, 03:38 PM
i ight be wrong, but i think i read an article a long time ago about fram dragging around a massive black whole somewhere in space, if scientist actually did detect it, then why would you send up this Gravity probe around earth?

eburacum45
2004-Apr-28, 03:50 PM
Frame dragging would be very noticeable around a spinning black hole;it would allow angular momentum to be extracted from beyond the event horizon, for a start.

But the phenomenon exists around any spinning mass, although it is quite small. A missing inch in an orbit around the Earth is small but detectable.

Kaptain K
2004-Apr-28, 03:59 PM
i ight be wrong, but i think i read an article a long time ago about fram dragging around a massive black whole somewhere in space, if scientist actually did detect it, then why would you send up this Gravity probe around earth?
Because the mass of the Earth is known to a high degree of precision and the separation of the satelite and the Earth can be measured to a high degree of precision. The parameters of the black hole and the space around it can not be known to a similar degree. The question is not so much whether frame dragging occurs, but to what degree it matches the predictions made by relativity.

Padawan
2004-Apr-28, 04:37 PM
oh, that settles it then ;)

skrap1r0n
2004-Apr-28, 05:18 PM
oh, that settles it then ;)

thats why he is a Bad PhD, and we are only newbies and interns


edit/ :P How Ironic, this post turned me from an intern into a fellow :lol:

JohnOwens
2004-Apr-28, 09:59 PM
Because the mass of the Earth is known to a high degree of precision and the separation of the satel[l]ite and the Earth can be measured to a high degree of precision....
Minor trivial nitpick: The mass of the Earth is not known with a high degree of precision. However, the product of Earth's mass and the gravitational constant is known to a very high degree of precision, and that's close enough for these purposes. (It's actually better than if we knew the mass very precisely but the gravitational constant only as well as we do now.)

Ricimer
2004-Apr-28, 10:57 PM
Just curious. Einstein predicted rotating masses would "drag" spacetime around in the direction of the spin, right? And this newly launched satellite will either prove or disprove this dragging. My question is, why would spacetime be affected by the spinning of a mass? To me, it seems the spin would be irrelevent, unless maybe the spin approaches relativistic speeds or something (which is most likely impossible as that would tend to cause the mass to fly apart).


Basically think of it this way: Space-time is bent by mass, but not instantaneously, it has some "inertia" to it. So if the earth disappeared, it would take some time for the spacetime in the region to snap flat.

Now, take some mass, and set it moving. The spacetime shifts to follow it, but not perfectly.

A rotating planet is nothing but moving mass. As the earth rotates it moves the various chunks of matter. The spacetime shifts, trying to follow that chunk, and so also gets set in rotational motion.

Thats how I understand it anyway.

Kaptain K
2004-Apr-29, 10:30 AM
Because the mass of the Earth is known to a high degree of precision and the separation of the satel[l]ite and the Earth can be measured to a high degree of precision....
Minor trivial nitpick: The mass of the Earth is not known with a high degree of precision. However, the product of Earth's mass and the gravitational constant is known to a very high degree of precision, and that's close enough for these purposes. (It's actually better than if we knew the mass very precisely but the gravitational constant only as well as we do now.)
True. :oops: Good catch. =D> But you've gotta admit, we do know the mass of the Earth to a higher precision than we do that of a black hole several thousand light years away!

Lorentz
2004-Aug-28, 06:58 PM
Just curious. Einstein predicted rotating masses would "drag" spacetime around in the direction of the spin, right? And this newly launched satellite will either prove or disprove this dragging. My question is, why would spacetime be affected by the spinning of a mass?

Actually, one can use special relativity (SR) to show this, without explicitly putting in general relativity (GR).

Accept SR is correct, but not necessarily the law of equivalence between inertial and gravitational mass. A gravitational field then obeys the exact same equations as the electric field, but the gravitational mass replaces the electric charge. Inertial mass can be different from gravitational mass in this SR model. If gravitational mass is different from inertial mass, there does not have to be any no gravitational time dilation (i.e., GR is wrong).

Apply SR the gravitational field and the gravitational force on a test particle. You have to define a Lorentzian four-vector of gravitational force, exactly analogous to the Lorentzian four-vector of the electric field.
Another component of the gravitational field appears, exactly analogous to the magnetic field, with gravitational mass replacing electric charge. The new component of force, analogous to the magnetic field, is called a Thirring force. It pushes any mass moving perpendicular to it in a directiona orthogonal to both the motion and the Thirring field.

Moving gravitational masses generate a Thirring field, exactly analogous to a magnetic field. The tensor of the combined gravitational field and Thirring field is called a gravitoThirretric field (like electromagnetic field, get it?).

Describe the earth as a loop gravitational mass current, exactly analogous to a loop electric charge current. The Thirring field acts on artificial satellites orbitting the planet, causing them to precess. The Thirrring field around the planet is called frame dragging in GR. Frame dragging and the Thirring field are almost the same, except for gravitational time dilation or gravitational length contraction in GR. But the analysis of forces is ALMOST the same.

Gravitational waves become exactly analogous to electromagnetic waves. Call them gravitoThirring waves.

The precession of the satellite won't be proof positive of GR, because it could be explained by the Thirring force using 100% SR. However, if the precession is shown not to exist, one has disproved GR. However, the existence of the Thirring force would have an impact on physics independent of whether GR is exactly correct.

The Thirring field if it exists is very weak under earth type conditions. On earth, it will be MUCH weaker than the magnetic field. You can calculate the Thirring force using the analogy of mass with electric charge. You can pretty much ignore it except in some difficult experiments, or observations of spinning black holes.

George
2004-Aug-30, 03:10 PM
...Another component of the gravitational field appears, exactly analogous to the magnetic field, with gravitational mass replacing electric charge. The new component of force, analogous to the magnetic field, is called a Thirring force. It pushes any mass moving perpendicular to it in a directiona orthogonal to both the motion and the Thirring field.
This sounds analogous to the "right hand rule" where the thumb and next two fingers are all perpendicular. Is this applicable?
Would the Thirring force be in a direction perpendicular to Earth's axis? #-o

Is it incorrect to treat the "draging" as movement of the fabric of space around Earth as in the molasis analogy?

Lorentz
2004-Sep-01, 09:30 PM
[quote=Lorentz]
This sounds analogous to the "right hand rule" where the thumb and next two fingers are all perpendicular. Is this applicable?
Yes. There would even be an anolog to Lenzes law: that the induced Thirring field of a moving mass will resist the motion of the moving mass. Deciding Thirring pole what exactly corresponds to the north magnetic pole may be ambiguous, however.
Would the Thirring force be in a direction perpendicular to Earth's axis? #-o
[quote="George"]
If the satellite circles the equator of the earth, which is the most likely orbit for an artificial satellite, then yes. The satellite in its motion would cut across the Thirring lines, producing a force perpendicular to the earths rotational axis. However, the satellite will spin on an axis independent of the earths axis. If it is spinning fast, at an angle to the force induced by the Thirring field, it will precess.
Like a top. A toy top precesses because the force exerted by the ground is at an angle to the gravitational force caused by the weight of the top.
[Quote=Lorentz]

Is it incorrect to treat the "draging" as movement of the fabric of space around Earth as in the molasis analogy?[Quote=Geeorge]

I think so. Fluid viscosity changes energy from kinetic energy to heat energy. (The difference between kinetic energy and heat energy is another can of worms, but I don't want to discuss that now). If the ether were strictly analogous to molasses, the satellites would slow down, heat up, and eventually lose the orbit. So if there is an aether, it has to have properties very different from molasses.

Both the magnetic field and the Thirring force preserve mechanical energy because the force they eert is perpendicular to the motion. Work is not done against the motion of the satellite. The idea of an ether that exerts a frictional forces that preserves mechanical energy is a bit counter-intuitive to some of us. Lorentz and later Einstein suggested that a modified ether theory may be useful in some context. But it would have to have some unusual nonfluidic properties to actually explain the magnetic force.

An earlier Lorenz pointed out that the ether idea is also inconsistent with the fact that light has two transverse polarization states, not longitudinal polarization states. The ether thus suppresses longitudinal waves, not transverse waves. Known fluids (water, air) do the opposite, they suppress transverse states and propagate longitudinal polarization states.

Again, if you want to assign really strange properties to the ether, you can make up an ether consistent with Maxwell's equations. But even if you do that, the answer would be superfluous. As Einstein showed, if you find one fluid with those properties, you change around the values of those properties and still come up with the same rules for electromagnetic radiation. Thus, you have more than one ether sufficient by itself to explain Maxwell's equations. One ether was enough, but these extra ether's are superfluous. The ether's are each sufficient to model the behaviour of light, but not all of them are necessary to model the behavior of light.

I think that pretty much explains the genius of Einstein's approach over Lorentz's approach. I got this from H. A. Lorentz himself, he explained both his an Einstein's theory really clearly. Lorentz made up a frictional force of the ether that also keeps the electron from falling apart. It was brilliant. Just expressing it took a chapter, and analyzing it took an entire book. But it wasn't the only possible force law for an ether that could explain the theoretical data because the answer is ambiguous. Einstein said, "Forget the detailed force law for an electron, let us start by assumming that the ambiguity in force is unavoidable."
[Lorentz]

George
2004-Sep-01, 11:27 PM
Is it incorrect to treat the "draging" as movement of the fabric of space around Earth as in the molasis analogy?
I think so. Fluid viscosity changes energy from kinetic energy to heat energy. (The difference between kinetic energy and heat energy is another can of worms, but I don't want to discuss that now). If the ether were strictly analogous to molasses, the satellites would slow down, heat up, and eventually lose the orbit. So if there is an aether, it has to have properties very different from molasses.
I suppose I am reaching more for the elastic properties of ether, as well as, some sort of mental picture of this invisible field. Also, an elastic property of an ether might hint as to the value of c.


I got this from H. A. Lorentz himself, he explained both his an Einstein's theory really clearly. Wow. He was known, apparently, for being quite a gentleman. I had to memorize his transformation equations in deriving e=mc^2. [I'll eschew the obvious implication of your statement. :) ]

btw-thanks for the detail, however, in my case, you aren't "preaching to the choir" but to the "janitor". This board has renewed an old flame. 8)

CERDIP
2004-Sep-02, 04:32 AM
So this all implies that gravity has a speed? If there is frame drag, it means gravity isn't instantaneous, right?

Is the speed of gravity constant? Can it accelerate? (no, I don't mean g, either, I mean a change in the speed of gravity over time....).

Lorentz
2004-Sep-03, 02:25 AM
So this all implies that gravity has a speed? If there is frame drag, it means gravity isn't instantaneous, right?
Yes. Supposedly gravity waves in a vaccuum move at the same speed as light in a vacuum.

Is the speed of gravity constant? Can it accelerate? (no, I don't mean g, either, I mean a change in the speed of gravity over time....).

If the speed of light changes, supposedly the speed of gravity waves change with it.