What causes gravity?

Chris OConnor · Posted: Mon Oct 18, 2004 3:10 pm Post subject: Re: What causes gravity?

Today there was an article on Space.com about gravity that might be of interest to participants in this thread discussion.

The Problem with Gravity: New Mission Would Probe Strange Puzzle
By Robert Roy Britt
Senior Science Writer
posted: 18 October 2004
06:33 am ET

Imagine the weight of a nagging suspicion that what held your world together, a constant and consistent presence you had come to understand and rely on, wasn't what it seemed. That's how scientists feel when they ponder gravity these days.

For more than three centuries, the basics of gravity were pretty well understood.

Newton described the force as depending on an object's mass. Though it extends infinitely, gravity weakens with distance (specifically, by the inverse square of the distance). Einstein built on these givens in developing his theory of relativity.

Then more than a decade ago a researcher noticed something funny about two Pioneer spacecraft that were streaming toward the edge of the solar system. They weren't where they should have been.

Something was holding the probes back, according to calculations of their paths, speed and how the gravity of all the objects in the solar system -- and even a tiny push provided by sunlight -- ought to act on them.

Now scientists have proposed a new mission to figure out what's up with gravity.

Staggering possibilities

Pioneer 10 and 11 launched in 1972 and 1973. Today each is several billion miles away, heading in opposite directions out of the solar system.

The discrepancy caused by the anomaly amounts to about 248,500 miles (400,000 kilometers), or roughly the distance between Earth and the Moon. That's how much farther the probes should have traveled in their 34 years, if our understanding of gravity is correct. (The distance figure is an oversimplification of the actual measurements, but more on that in a moment.)

Scientists are quick to suggest the Pioneer anomaly, as they call it, is probably caused by the space probes themselves, perhaps emitting heat or gas. But the possibilities have been tested and modeled and penciled out, and so far they don't add up.

Which leaves open staggering possibilities that would force wholesale reprinting of all physics books:

Invisible dark matter is tugging at the probes
Other dimensions create small forces we don't understand
Gravity works differently than we think
Devoted to the problem

Slava Turyshev at NASA's Jet Propulsion Laboratory is one of a handful of scientists who wrestle mentally with the Pioneer anomaly every day. He is not paid to work specifically on the problem, so he has to juggle the disturbing thought with his regular research, which involves other aspects of gravity and, significantly, whether theories that explain the glue of the whole universe might one day match neatly with those describing the invisible, subatomic world.

"I have been working on [the Pioneer anomaly] for more than 11 years now, and was never funded to do this job," Turyshev tells SPACE.com. "I guess this says a lot about my devotion to solve this mystery."

Data from the Galileo and Ulysses spacecraft suggest the anomaly may have affected them, too. But neither has been far enough from the Sun -- the dominant source of gravity in the solar system -- to firmly distinguish any possible discrepancy from noise in the data, Turyshev says. Galileo was crashed into Jupiter last year, and Ulysses will never go farther than it has.

That leaves two data points -- one from each Pioneer craft. Turyshev pointedly considers the pair as one data point, so as not to inflate the case for strange new physics. He looked at the two Voyager spacecraft, also exiting the solar system, but says their design involved "numerous attitude-control maneuvers" that "can overwhelm the signal of a small external acceleration."

NASA engineers have made their last communications with the Pioneer probes, so the two table-sized robots are carrying the unsolved mystery silently to the stars.

New mission proposed

The Pioneer anomaly was discovered by John Anderson, also of JPL, in the 1980s. For years he didn't publish what he'd noticed. Then he discussed it with physicist Michael Martin Nieto at the Los Alamos National Laboratory. Nieto says he "almost fell off my chair."

Nieto jumped into the investigation, and the two were later joined by Turyshev. They dug deeper into the data, even tracking down retired NASA scientists for some of it.

Unraveling the enigma will require a new mission, the researchers say. NASA, however, doesn't have such a project on its agenda and has not expressed much interest in one. Europeans, for reasons both historic and having to do with a current strong desire to better grasp gravity, seem more interested in investigating the problem.

So Anderson's team recently proposed to the European Space Agency a "mission to explore the Pioneer anomaly" using the latest accelerometers and advanced navigation methods. All possible sources of onboard radiation would be eliminated in "the most precisely tracked spacecraft ever to go into deep space," the group writes in the September issue of Physics World magazine.

The idea has "very high chances" of being chosen for future study, Turyshev thinks. If funded, it could launch as early as 2015.

If the mission were to find a natural, cosmic cause to the Pioneer anomaly, the revelation would rank right up there with other apple-on-the-head moments in the history of physics.

"If the anomaly is due to some new physical mechanism, this discovery would have a truly fundamental impact," Turyshev said.

Exotic candidates

One candidate is dark matter. This unknown stuff seems to infuse the universe and, though invisible, has a collective gravitational impact greater than all known matter, including stars and planets. Dark matter is inferred to exist because, without it, galaxies would fly apart. Every galaxy must be loaded with the stuff, astronomers conclude, based on how stars are bound to orbit the centers of the galaxies.

But dark matter's effects have been presumed to operate across large expanses, both within and between galaxies. There is no evidence of it controlling anything on a scale so small as our solar system.

Another idea is that gravity tugs slightly harder at things farther away. That radical suggestion, if proved true, would force a modification of Einstein's general theory of relativity and might eliminate dark matter as a player.

Yet one more exotic possibility: Dimensions exist beyond the four we know (three directions and time). Models of string theory propose that higher dimensions could provide weak forces that act in ways we don't yet comprehend.

No fancy theory in existence, however, properly explains the Pioneer data.

Drifting journeys

The Pioneer anomaly is not actually a measure of how far the Pioneer probes did or didn't travel.

Instead, scientists bounced microwave signals off each probe and noticed an unexpected drift in the Doppler frequency as the probes got farther away. The technique is akin to noting the sound change in a siren as an ambulance races first toward you, and then away from you. The Doppler effect is a shortening or lengthening of sound waves (or microwaves, or any waves) forced by an object's movement.

The drift showed that the Pioneers were being accelerated toward the Sun (or, rather, decelerated in their movement away from the Sun) by a tiny but inexplicable amount. The level of drift is equal to a gravitational effect 10 billion times weaker than the pull of Earth.

Though tiny, the signal is clear, other scientists agree.

Despite 11 years of devotion to the mystery, Turyshev is the first to admit that the "most obvious explanation" would be an unknown onboard effect. Perhaps excessive internal heat or leaks of propulsion gas are providing a wee bit of thrust that adds up over the years.

Yet despite a lot of testing, "no unambiguous, onboard systematic problem has been discovered," he said. "This inability to explain the anomalous acceleration of the Pioneer spacecraft with conventional physics has contributed to the growing discussion about its origin."

Even if the anomaly is caused by the Pioneer probes themselves, figuring it out will be useful says Turyshev, who is the proposal leader for the U.S. group.

"Finding it would help us to build a better spacecraft for the needs of fundamental physics," he said. "These craft would much more stable, quieter and would allow us to go even deeper in our quests of studying the fabric of fundamental and gravitational physics."

This article is part of SPACE.com's weekly Mystery Monday series.

"Education is the most powerful weapon which you can use to change the world." - Nelson Mandella

ZachSylvanus · Posted: Thu Dec 30, 2004 12:51 pm Post subject: Re: What causes gravity?

Electronegativity on the scale of larger objects such as planets etc. does not function as an attractive force, because the amount of contact able to be made electrically is too small to explain the attractive force observed. Plus, most bodies tend to be electrically neutral, which negates any sort of electromagnetic attraction between two objects.

Van der Waals forces (electromagnetic attraction between two surfaces in close proximity) are a weak force overall, and do not generally aid larger bodies.

ttalkman · Newbie Joined: 03 Feb 2005 Posts: 2 Gender:

Since I was 4 years old, I have grappled with this very question. Ever since an early neighborhood friend pointed to his driveway and said it was "gravel". I thought my god, that must be what gravity is, rocks.

To this day, I have yet to figured out what gravity "is". I have always had the feeling though that it relates to negative charges found in atoms or other charges founded in sub atomic particles. They just act and look differently depending on what level they are acting upon.

It is troubling that it's root cause has not been found. It's overt behavior is so familar and so quantitized mathmatically yet so mysterious.

I have asked others what gravity is and their standard answer always is an "attractive force between two masses". They go on to describe the behavior. When I ask what causes mass to act that way, they end up just starring at me blankly or in bewilderment. They think it's a trick question because everyone surely knows what gravity is.

Some people are perplexed when they realize we really don't know what gravity is after all, only it's behavior. They somehow feel scientists already know.

Is it just me or has this question bugged other people all their lives too?

Interbane · Senior Joined: 09 Oct 2004 Posts: 369 Gender:

I thought electronegativity only worked on small scales?

Also, how does that differ from the Strong or Weak Nuclear forces?

Doc: "In some way, it is a bit ironic that we must try to be more stupid to obtain salvation."

MA: "I can't think of a better way to convince a group of critical thinkers of the worthlessness of faith."

rellimlk · Newbie Joined: 18 Sep 2004 Posts: 4 Gender:

I agree with Interbane that non-fiction books for the lumpen are no substitute for a PhD in physics. While I'm not the sort of person who has 8 years and $100,000 for the degree, I am the sort of person that has a couple weeks and the $28.95 for "The Fabric of the Cosmos." What follows is (my understanding of) the description of the cause of gravity Greene describes based on String Theory.

The short answer is that gravity is caused by 'gravitons' which are the messenger particles of the force of gravity.

The quantum mechanical equations of string theory predict the existence of a messenger particle that is massless with a spin 2 (twice the speed of a photon--the messenger particle of electromagnetic force). With a string of approximately 1 Plank length (10 to the -33 power cm), the force transmitted by the particle would be equal to the observed force of gravity.

I haven't read much in this discussion about String Theory, so I don't know how aware people are of it. I will summarize in a way that I'm sure would infuriate a PhD:

Basically, String Theory is an attempt at a unified theory that explains the qualities of all matter and all forces--it doesn't just describe the qualities of each force and particle the way quantum particle physics does. It also provides a mathematical structure that explains WHY each particle and force is the way it is.

It supposes that both matter and force are composed of vibrating filaments of energy. The length of the string and its vibrational pattern cause each filament to display the properties of the particles of matter and force that we know. A filament of a certain length and vibration will show the properties (mass, charge & spin) of an electron, another an up-quark, down-quark, strange-quark etc. The four forces (Strong, Electromagnetic, Weak and Gravity) are also the result of strings vibrating in a particular pattern.

To use Greene's metaphor: think of a cello string. It is capable of vibrating in many ways--each pattern is heard as a different musical note. Similarly, the filaments in string theory vibrate in different patterns and cause us to observe a particular mass, charge and spin which we have identified in particle physics as electrons, quarks, photons etc.

There are mathematics which are beyond me which would logically predict various kinds of particles of matter and force. Gravitons are predicted, but would be so small that current technology could never detect them directly.

I don't know if this answers "what causes gravity" on a metaphysical level. But according to string theory gravity is "caused" by the movement and emission of gravitons in the same way electromagnetic force is caused by the movement/emission of photons.

...Or so I understand

Ken Hemingway · Intern Joined: 22 Dec 2004 Posts: 188 Gender:

Rellimlk

Thanks for the summary. I think for this to begin to jell I'd need answers to two further questions.

1. How does exchange of particles cause force in the first place? I think it has something to do with the momentum of the exchanged particle (somehow negative for attractive forces), but this is puzzling when the exchanged particle has no mass (as with the photon).

2. How doe the particle exchange theory fit in with the warped space-time theory fo General Relativity? Einstein spent half his life looking for a unification of electromagnetism and gravity with no success. Have the recent advances cast any light on this question?

rellimlk · Newbie Joined: 18 Sep 2004 Posts: 4 Gender:

Quote:
Einstein spent half his life looking for a unification of electromagnetism and gravity with no success. Have the recent advances cast any light on this question?

String theory and M-Theory (a meta-theory that takes different versions of string theory that seem to conflict and reveals a common substructure) is currently the the widest held (perhaps only) theory that unifies gravity with the other 3 forces. The messenger particles for the other 3 forces, by the way, have been experimentally verified (if I remember correctly) in atom smashing.

Quote:
How doe the particle exchange theory fit in with the warped space-time theory fo General Relativity?

I don't have a good answer to that...except that the gravitational field works the same way as any other force field (electromagnetic, for example). Gravitons are simply the smallest, fundamental component. It is almost a misnomer to call them particles--it is more like a packet of force. In some ways, matter PARTICLES with a mass should also be understood to be fields just like forces--since they are really just energy vibrations. But we are very accustomed to thinking in terms of tiny bits of solid stuff that things don't get phrased that way. It is tied up with the E=mc2 which describes the interchangeability of energy and mass.

Quote:
1. How does exchange of particles cause force in the first place? I think it has something to do with the momentum of the exchanged particle (somehow negative for attractive forces), but this is puzzling when the exchanged particle has no mass (as with the photon).

You mention mass as though for something to exist physically (that is have physical effects on things)it has to have mass. This is a very fundamental assumption that can be difficult to get beyond...it is so integral to our experience of the world. When we experience the mass of an object we experience its resistance to being moved (accelerated). But accelerated in relation to what? And what is it that makes a thing need to stay in the same state? What gives a thing MASS?

One answer provided is what they call the Higgs Ocean...a universal force field that is left over from the big bang. All the different vibrating strings behave differently when passing through the Higgs Ocean. A string that has mass (like an electron or bottom quark) are strings that can not move unimpeded through the Higgs field. Mass is simply the measure of that strings resistence to moving through the Higgs Ocean. A photon, gluon or graviton can move through the Higgs Ocean unimpeded. Once one looks at mass in this light, if one assumes that particles with mass can act upon other particles when they come in contact, there is no reason why a force particle would not act upon other particles when they come in contact: both are filaments of energy, the difference being that matter particles are impeded by the Higgs field and force particles are not.

I don't think that answers your question...I think I just gave a new backdrop for it. My point is that it is just as valid a question to ask where does matter get its "matterness" as to ask where does force get its "forcefulness." The answers would seem to be equivalent.

I think it may be time for a PhD to step in: I'm not sure how much more I can add to this subject.

Interbane · Senior Joined: 09 Oct 2004 Posts: 369 Gender:

I've read "The Elegant Universe" by Brian Greene years ago and caught the gist that even though extrapolations can be made of the current theory to predict what a Graviton would be and act like, it is just as good as any other explanation until the predictions are observed.

I haven't read the newer book yet, I may have to do that. Hopefully it sheds a bit more light on things for me. I hear they're building a new collider also, one that may be able to observe previously unobserved particles. I trust Scientific American to keep me up to date there.

Ken Hemingway · Intern Joined: 22 Dec 2004 Posts: 188 Gender:

Interbane wrote: You mention mass as though for something to exist physically (that is have physical effects on things)it has to have mass.

No, I don't think that. But to have momentum without mass seems problematic - p = mv?

grmanny · Getting comfortable Joined: 19 Apr 2005 Posts: 9 Gender:

this interesting


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