View Full Version : Discussion: Gravity Probe B Arrives at Vandenberg

2003-Jul-14, 09:45 AM
SUMMARY: NASA's Gravity Probe B arrived at Vandenberg Air Force Base on Friday, July 11 to begin launch preparations. Once launched, the spacecraft will use four ultra-precise gyroscopes to test two predictions of Einstein's General Theory of Relativity: how space and time are warped by the Earth, and how the Earth's rotation drags space-time around with it. If all goes well, the spacecraft will launch on board a Boeing Delta II rocket in late 2003.

Comments or questions about this story? Feel free to share your thoughts.

Al in Virginia
2003-Jul-14, 06:35 PM
"and how the Earth's rotation drags space-time around with it"

That is as interesting a statement as one from an article a few months back about how gravity propagates forward at the same speed as light.

I struggle to understand these things. :blink: and would welcome any comments that try to explain them to me, a very non-scientist...

Webmaster @ Gravity Probe B
2003-Jul-15, 12:55 AM
Hi - You said:

"That is as interesting a statement as one from an article a few months back about how gravity propagates forward at the same speed as light. I struggle to understand these things, and would welcome any comments that try to explain them to me, a very non-scientist... "

I will try to help, though I am not a PhD physicist (I am a mathematician who's been doing database administration for several years, but also a science tutor). Before I begin, please allow me to point you to our Teacher's Guide to GP-B at
http://einstein.stanford.edu/gen_int/educa...uide/Frame.html (http://einstein.stanford.edu/gen_int/education/EducatorsGuide/Frame.html)
It's a non-scientist's guide to the ideas behind relativity.

Einstein's theory of General Relativity (GR) says that spacetime has to have a geometry. Basically, that says that there are rules in space about how things, such as light or force or mass, travel through it. Einstein says that space, as we know it, is not just a shapeless void with no instructions or road rules. This was a new idea at the time it was put forth, 1916.

The old school of Isaac Newton's equations on gravity didn't ever take into account how long gravitational effects might take to reach a given mass. Think about that for a moment. Newton's theory says that F = GmM/r^2. That is, the gravitational force F exerted between two masses (m and M) is equal to G (a small constant number) times the two masses divided by the square of the distance ® between them. So you could figure out how much the gravity there is between the planet Mars and the Sun by plugging in the respective masses and the distance between those two bodies, and putting in the known constant value for G. If the mass of one body changes, the force between them changes. That makes sense for the Sun and Mars, but what if we are talking about two masses in two different galaxies thousands of light years apart? If one of those masses changed, how soon would the force change be felt by the other body? Well, according to Newton, instantly - there's nothing about time in his equation. However, "instantly" presents a problem because nothing travels faster than the speed of light (well proven by Einstein's special relativity theory and supporting experimental evidence). So how could gravitational force travel so fast - instantly across several thousand light years? Einstein believed that it cannot - nothing can travel faster than the speed of light (~186,000 miles per second). He believed that to accomodate that "speed limit" there must be some sort of "road rules", or geometry, to govern the travel of force, light and matter in the universe. That's why the speed of gravity is of interest to today's physicists, for instance.

In 1916, Einstein presented the world with a new understanding of the world- his theory of general relativity. In this theory, space is not an empty void, but an invisible structure called spacetime. Nor is space simply a three-dimensional grid through which matter and energy moves. It is a four-dimensional structure whose shape is determined by the presence of matter and energy. Around any mass (or energy), spacetime is curved. The presence of planets, stars and galaxies deform the fabric of spacetime like a large ball deforms a bedsheet. (This spacetime deformation occurs in four dimensions, so the two-dimensional bedsheet is a limited model. Try visualizing these depressions on all sides of a planet to build a more accurate image of this concept.)

When a smaller mass passes near a larger mass, it curves toward the larger mass because spacetime itself is curved toward the larger mass. The smaller mass is not "attracted" to the larger mass by any force. The smaller mass simply follows the structure of curved spacetime near the larger mass. For example, the massive Sun curves spacetime around it, a curvature that reaches out to the edges of the solar system and beyond. The planets orbiting the Sun are not being pulled by the Sun; they are following the curved spacetime deformed by the Sun.

A few years after Einstein submitted his theory of curved spacetime, Austrian physicists Joseph Lense and Hans Thirring predicted that a mass could deform spacetime in a second way - through frame-dragging (1919). They proposed that the rotation of planets and stars (i.e., any rotating mass) twists the structure of spacetime near that mass. Think about this as if the rotating mass is a basketball being rotated on top of a fishing net. Imagine what would happen at the point of contact between the two - a measurable twist. So, not only is local spacetime curved near the Sun, it is twisted by the the Sun's rotation. Lense and Thirring predicted that this effect would be extremely small, and become smaller farther from the rotating mass, but it would occur around every rotating planet, star, galaxy, or person. Gravity Probe B will try to measure this miniscule effect near the planet earth. We are the first experiment designed to measure such an effect.

I hope this explanation is of some help, and that you will consider visiting our web site, http://einstein.stanford.edu/. I invite you to particularly examine our "Educational Outreach" and "Frequently Asked Questions" sections under "General Interest". The "Press Clips" section has some good articles for the lay-person about science and relativity in particular.

Best regards,
Jennifer Spencer
Web Site Curator
Gravity Probe B
Stanford University

2003-Jul-15, 04:11 AM
There's a really good book called 'Hyperspace' by Michio Kaku. It describes all this and more in a very easy and readable way. I highly recommend it.

2003-Jul-15, 04:25 AM
You know what? I'd like to upgrade that rating from "really good" to "excellent"!! Explains time warps, parallel universes, other dimensions, wormholes, the plausibility and possibility of time travel ... etc etc etc.

2003-Jul-15, 05:26 AM
A big thanks to Jennifer at Stanford for providing an official answer to the question about the Gravity Probe B.

John Broomfield
2003-Jul-15, 06:26 PM
I'd like to "second" that Thank YOU to Jennifer for giving an answer that I actually understand! You must have some lucky students! :)


2003-Jul-16, 02:54 AM
Very interesting... this "twist" that you mentioned. If I understand correctly thats how a black hole comes about - by tearing a hole in the "net" with a severe "twist"? Very StarTrek like... ^_^