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Gravity Probe B

Testing Einstein's Universe

What is the Purpose of Gravity Probe B?

What I am trying to accomplish here is properly frame the response so as to answer the question of GP-B's existence from the standpoints of the average taxpayer, the scientist, the politician, the media writer, and the student. I think the questions of why we are doing what we are doing and why it's worth spending money on is one that can be answered from many viewpoints. It would be best if we could capture them all. Ideally, I will eventually write an entire section on the spinoffs from GP-B, but in the mean time, we need at least a basic answer to the question.

The scientific purpose of this project is to test Einstein's theory of General Relativity.   This theory has only been partially verified and is one of the least tested of all physical theories (for more information on the testing already done and alternate theories of gravity, reference The Story of Gravity Probe B).   Until a theory is thoroughly tested, we cannot accept it as fact, and we cannot reliably base further theory or engineering on its postulates.

Many people ask us why we are doing what we are doing.   Usually, these inquiries are meant to probe beyond the scientific reasons to get at why the government is spending tax money to fund it, and ultimately, why the GP-B project is important to the world at large.   Underlying this question is the deeper issue of whether we as a society should fund basic scientific research.   A peripheral question is, what, specifically, could humanity accomplish if we knew that the theory of general relativity correctly describes the universe?

Should we do basic scientific research such as Gravity Probe B?

This is a challenging question to answer.   One way to put the question into perspective is to think about discoveries in primary areas of science that came earlier and what ultimately came out of them.

Four fundamental forces govern our universe: electromagnetism, the strong and the weak nuclear forces, and gravity.   Einstein himself (unsuccessfully) spent a great portion of his life trying to relate these forces and unify their laws into one set of governing equations that would apply to all. To many people, it seems unlikely that understanding the physics behind these four forces could significantly improve our daily lives. The core-level research seems esoteric and too "out there" to yield something we could all use to our benefit. They ask the fair question "Why should we bother with that when we could be feeding hungry people right now instead?"    A period in society's history when it didn't invest anything in research and only concentrated on today's needs can be referenced for comparison: that time is commonly referred to as the "Dark Ages."    The cost of all U.S. science, space & technology and is less than 1% of the annual federal budget. As a pay-back, the results of basic scientific research over the years have helped humanity in thousands of different situations.   Think of radar during WWII, generators during blizzards, radios used to signal for help on ships in distress and electrocardiograms.   Here are some historical examples of basic scientific research that seemed wasteful to most of society at the time, and some of the results.


Arguably the most practical area of scientific discovery in the last 200 years has been electromagnetism. For a long time, no one knew for certain how magnetism and electricity were related. No one understood the electromagnetic nature of light or the propagation of electromagnetic waves in general (such as radio waves, microwaves or X-rays).

As you may recall, Benjamin Franklin investigated electromagnetic energy in the form of lightning. No one knew what lightening really was (except perhaps that it was something to be avoided whenever possible). Many people thought Ben Franklin was crazy, or that he would probably kill himself. However, he discovered that lighting was actually related to other electrical phenomena (something long suspected by the scientific community, but never proven until Franklin's experiment), and in 1752 Franklin invented the lightning rod. The most evident lesson of Franklin's work is that understanding more about lightning enabled us to redirect lightning strikes and ultimately save lives. For more information, see "The Key & the Kite Experiment" web page.

Far reaching discoveries were made by James Clerk Maxwell in the late 1800s. His theories and discoveries definitively and conclusively explain the relationship between electricity and magnetism, and also tell us how light is propagated. Because these relationships were verified and the phenomena of light explained, we can wirelessly transmit electromagnetic signals. Thanks to his discoveries and the inventors who furthered them, we can watch TV, listen to the radio, use a microwave oven, transmit signals through space from satellites and generate electricity. However, when Maxwell was first studying electromagnetism, people weren't visualizing ahead to the day when we could each have our own light bulbs and computers - and they certainly weren't thinking of satellite TV or rental-car GPS [link here to GPS site] units. In 1888, Professor Heinrich Hertz demonstrated Maxwell's theories by successfully sending a spark from a transmitter in one corner of the room to a receiver in the opposite corner without the use of wire or any physical connection. He was asked by a student if these electromagnetic waves could be of any practical use. Hertz answered "None whatsoever. It's simply an interesting laboratory experiment which proves that Maxwell was right. I don't see any useful purpose for this mysterious, invisible electromagnetic energy."   Clearly, the Dr. Hertz was not a prophet.

Strong & Weak Nuclear Forces:

Fusion & Fission: Based on research in Special Relativity, mankind learned how to harness energy from the elements. Remember Einstein's E=mc2? It was used to learn how to release the energy in the heavy elements (fission) and how to create more energy by fusing light elements together. This technology was first used to create atomic bombs (a controversial choice and one that Einstein was remorseful about), and later nuclear energy plants (also controversial). If research efforts continue, hopefully, one day huge amounts of energy may be provided with harmless helium as a by-product. For better or for worse, nuclear weaponry has completely changed the global political dynamic and the way war is waged.

Quantum Mechanics: Due to basic research at linear accelerators and subsequent advances in quantum mechanics, the MRI and the tunneling electron microscope were invented. Magnetic Resonance Imaging is used in medical scanning equipment and has saved many people from undergoing exploratory brain surgery. The scanning tunneling electron microscope allows us to understand materials at new levels, thus enabling the recent leaps-and-bounds advances in computer chip technology, among other things.

The indirect result of basic research work is that engineers and inventors, like Thomas Edison, were later able to build on what was already known; allowing them to exploit electricity (for example) and channel it into useful things like light bulbs.

Sometimes it is hard to envision where a better understanding of scientific laws and the physical universe will take us. But if we do not gain the understanding, the light bulbs of the future will never be invented. Think of all the great inventors who had the initiative to build on scraps of scientific evidence which to others seemed useless and unconnected to anything practical. If those inventors had not had research evidence available, they would not have had a place to begin their great work.


Gravity Probe B is a basic research project. It is researching one of the most fundamental theorems of science about the nature of mass and space. Mass and space are the building blocks and fabric of our universe from the farthest galaxies to the kitchen stove. If we better understand the nature of mass and space, we may be able to do things previously undreamed of. We cannot possibly foresee all the wonderful things that may come from a better understanding of space-time and mass-energy, but a theorem about these fundamental subjects must be thoroughly examined if we are to use it to our advantage. Perhaps one day many of the things science-fiction authors write about will be possible, but only if we lay the groundwork now.

This brings us to the next question: what, specifically, could humanity accomplish if we knew that the theory of general relativity correctly describes the universe?
We are limited only by our own imaginations when it comes to applications of science. Who knows? If we can understand how space-time and mass-energy work and interrelate, maybe we can travel through space differently than we do today. Maybe we can learn to manipulate gravity as thoroughly as we now manipulate electricity. Think about the possibilities. Theory has it that the extreme energy coming from quasars is caused by gravity. If we could produce that kind of energy just using gravity, think of what we could do with it - produce clean power or perhaps even change the course of a meteoroid heading towards earth. Or maybe we could learn to "defy" gravity and come up with a new type of flying vehicle. Who can say? The purpose of Gravity Probe B is to test whether the physical theory of General Relativity is really true before we embark on any engineering applications.

The work done on Gravity Probe B thus far has already helped society in many ways. By manufacturing and machining the world's roundest objects (our gyroscopes), we have achieved new levels of accuracy of measurement, machining and material purity in engineering. We have educated hundreds of undergraduate students in the fields of aerospace, computer programming and physics through their work with us. Many of these students go on to work in industry, taking their ideas and experience with them. Stanford has awarded degrees to over fifty graduate students, each one writing a ground-breaking thesis researching Gravity Probe B for his or her Ph.D.  l;  Some of our students have invented methods for automated farm-field plowing (check out the article from the Stanford Report) and automated landing of commercial aircraft in their research.

Unlocking the Inner-Workings of Time and Gravity

GP-B is focusing on the very part of Einstein's General Theory of Relativity dealing with time and spin.

A paraphrase of a recent article from the New York Times:
Time is probably the deepest of all enigmas in physics. At the everyday level, it is believed that the "arrow" of time points always in the direction of increasing disorder (or "entropy"). Natural processes run down, order yields to disorder. Dr. John A. Wheeler of Princeton University, the cosmologist and astrophysicist who coined the term "black hole" to describe ultradense objects from which light cannot escape, believes that despite the puzzles and paradoxes posed by time, a fundamental simplicity underlies it. "It's not so much that there's something strange about time," Dr. Wheeler said in an interview a NY Times article by Malcolm Browne. "The thing that's strange is what's going on inside time."

To understand the inner-workings of time could unlock a door to new discovery and reconcile the science of large and small. Cosmic relationships are governed by the laws of general relativity, while particle (subatomic) physics is governed by ordinary quantum mechanics. These have yet to be brought into consonance. To come to terms with such things, physicists need to deal with quanta - the discrete packets of energy that define the microscopic world: electrons, photons, quarks, and so forth. Even empty space is believed to be quantized - subdivided into infinitesimal cells. But so far, despite the best efforts of Albert Einstein and many other theorists, no one has been able to dissect gravity and time into their component quantum packets, if such exist. "We're still children as far as quantum gravity is concerned," said Dr. Daniel E. Holz, a relativity theorist at the Max Planck Institute, Potsdam, Germany. "We don't know how to quantize time," Dr. Holz said. "You can't make heads or tails of it. When you try to quantize gravity, time is what sinks you. When we understand what to do with time in quantum gravity we'll have it done. Or turn it around: When we get quantum gravity, the big revelation will be, aha! So that's the way time works!"