# On the frame dragging effect

1. Apr 20, 2010

### Simonomis

Hi just a quick question, i've been searching online to find out if there is any measurable frame dragging effect on a high rpm massive rotating disc, but havnt been able to find anything. Ive read a bit about gravity probe B but in my limited understanding of the effect and the physics around it, i thought it might be possible to see this effect in the lab provided the effect at this scale is not so negligible as to see no result.

In the experiment would you see any frame dragging if you were to rotate a number of massive discs very close to non rotating discs in a perfect vacuum so as to cancel any drag effects due to air - in this setup the arrangement of discs would look like the multiple discs of a hard drive platter (but much larger and much more massive) where every other disc is made to rotate in very close proximity to a set of non rotating discs (but with high quality bearings so that they can rotate if made to do so)

I imagine that if there was a frame dragging effect the rotating discs would eventually cause the non rotating discs to start to rotate? I'm not sure if there would be other effects that may be stronger than the frame dragging effect but therein lie another of my questions?

any comments would be very much appreciated

Cheers!

2. Apr 20, 2010

### cesiumfrog

Indeed. Read more closely about the lengths they needed to go to, just to detect the frame-dragging of an entire planet.

3. Apr 20, 2010

### Jonathan Scott

The frame dragging field caused by each piece of a moving or spinning object is roughly equal in magnitude to its gravitational field times v/c, where v is its speed. It is not easy to detect the gravitational force between objects in a laboratory situation. To measure a frame dragging effect, you would need apparatus v/c times more sensitive.

So for example if you could make a disk move around at about 300 ms-1, nearly the speed of sound in air and about a millionth of the speed of light, then your apparatus to detect frame-dragging would have to be a million times more sensitive than the apparatus to detect the gravitational field of a laboratory object.

4. Apr 20, 2010

### Simonomis

Thanks very much for your responses it very satisfying when somebody can apply values to the miniscule effect, helps alot with my understanding. One thing that keeps popping into my head is - would the effective mass of the discs be increased as all parts of the discs move through a particular point many times per second - and would energising the discs make any difference (Although I'm sure all sorts of other effects would make themselves known if you energised it)

5. Apr 21, 2010

### Matterwave

You are "energizing" the disc since you are making it spin...you give it rotational kinetic energy.

The mass dilation (even tho I dislike this term) is quite negligible for speeds v<<c.