Scientists know how gravity works at big distances -- the inter-planetary or inter-stellar range -- but does it work the same way at the inter-atomic range?

A variety of tabletop experiments are trying to explore this issue. Already some theorists say that a departure from conventional gravity behavior could hint at the existence of extra dimensions.

Isaac Newton’s theory of gravity is one of the great stories of science. It correctly showed that the moon’s motion around the Earth, Earth’s orbit around the sun, and the fact that we all remain safely fixed to the Earth’s surface were all manifestations of a single force: universal gravity.

One of the critical parts of Newton’s theory is an equation stipulating that the force of gravity between two objects gets rapidly weaker as the distance between them increases. Called the inverse-square law, it has been tested over the years both by observing the actual movements of the planets and stars and by experiments conducted in labs that examined gravity at the level of a few feet.

A new detection apparatus proposed by the National Institute of Standards and Technology in Boulder, Colo., hopes to explore gravity at the 100-1,000 nanometer level -- larger than atoms but many thousands of times smaller than any previous experiment has achieved. They hope to measure gravity over the shortest range yet.

Their apparatus consists of a tiny glass bead suspended in vacuum by laser beams. A gold rod is swept past the bead, disturbing its place in the laser beams. The bead then oscillates back and forth, and this pendulum-like motion provides a way to precisely measure the force of gravity between the rod and the bead.

One of the difficulties of measuring gravity is that it is so weak.

Of course gravity is strong enough to keep the Earth going around the sun, but that’s only because the sun and the Earth are so big. Compared to the other physical forces, such as the electromagnetic force holding atoms together or the strong nuclear force holding nuclei together, gravity is incredibly weak. Lab tests of gravity have to shield against interfering effects from the kind of stray electric fields that can fill an ordinary lab. Electric fields, in the form of radio waves, bloom from just about any powered device in our vicinity, as well as surrounding microwave towers and radio antennas.

One of the NIST researchers, Andrew Geraci, said that the micro-spheres he plans to use will be carefully prepared to have no net electrical charge, which helps to guard against electric interference. Using light to suspend the spheres eliminates friction, further insulating them from their environment and making gravity measurements more precise.

"The apparatus is an excellent sensor of small forces," said Geraci. "It could be significantly -- 100,000 times -- more sensitive than previous short-range gravity experiments at the micron scale, possibly yielding exciting new discoveries."

One of the discoveries he has in mind is determining whether gravity falls off with distance even faster than Newton specified. Some theorists believe that one reason gravity is so weak is that it bleeds off into extra spatial dimensions that are difficult to measure with scientific detectors, much less with our own human senses.

Thus a study of gravity with a tabletop experiment could help to find such hypothetical extra dimensions. It might also help scientists discover forces never thought of before by theorists. In this way, tabletop experiments can be complementary to the kinds of gigantic experiments performed at particle accelerators, where evidence for extra dimensions and new forces is teased out of the debris left over from the fiery collisions of particles traveling near the speed of light.

Previous tabletop experiments have explored close-in gravity by watching how a carefully suspended weight undergoes a torsion-like swivel in the presence of other nearby weights.

Eric Adelberger, a physicist at the University of Washington in Seattle, has worked on that kind of gravity measurement. He suggests that the NIST researchers have an interesting and novel approach for studying short distance forces, one worth pursuing.

"However, being able to measure small forces does not, in itself, allow one to study gravity at short distances," Adelberger said.

The real problem, Adelberger believes, is getting rid of the systematic influences -- the things, like those stray electric or magnetic fields in the lab, that might confuse a measurement -- which grow rapidly more important as the sizes of the interacting bodies get smaller. This might not be easy to do. For this reason the sensitivity needed for studying gravity over such short distances, even with the NIST design, might be hard to achieve, Adelberger argued.

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## mattytheory

Why use gold instead of some other electromagnetically neutral material? Aren't there oscillating EM fields associated with laser beams that could interact with the gold rod to produce another EM field in the gold rod that might not influence the neutral glass bead directly but could influence the beam itself - by bending it ever so slightly or through some other effect - and therefore influence the motion of the glass bead within the beam? Or am I way off base?

## axemaster

Furthermore, it seems like the trend is towards hashing out a dozen theories, then waiting for evidence to invalidate the wrong ones. But that is a wrongheaded approach. They aren't asking the most important question, which is, WHY does nature choose one consistent mathematical framework over another?

It's as if they think that simply by approximating more and more closely to physical reality, they will find the truth. But no, they will simply approximate too closely and then it will be impossible to refute them. It's really a dire situation.

## holoman

Why is there gravity ?

What energy/force is Gravity made of ?

If these 2 answers could be determined I

think it would be the greatest solution for

mankind.

I am tired of the same old mathematical modeling, hyperbola, and speculation giving us test data defining what ?

That gravity doesn't work past a certain distance or size ? Big Wup.

## daywalk3r

Stating that something is undefinable/unmeasurable - or simply that there is effect without cause - sure looks like an excellent and easy sollution - at least to some, but for others it is equal to just putting your head into the sand while you keep praying your mantras in blind faith..

And as you're having a go at geometry, do you think that "things" like Pi or Phi are nonsense then? And if so, do you refuse to use them in your calculations? ;-D

I'm confused.. please elaborate :-)

Quite ironicaly.. geometry :)

## TDK

## stanfrax

## frajo

Unmeasurable as well is the realm of the large distances where discrepancies between observations (of galactic rotation velocities) and theory (law of universal gravitation) are explained away by assuming the existence of some "dark matter" consisting of some unknown ghostly particles.

## Donutz

## workforit

This by itself should define the Multi-dimesional nature of the Propogation of Gravity. If this were not so, we should be able to simply shield the Acceleration imputed on the objects we are trying to work with.

## axemaster

Geometry is a great tool with lots of descriptive power. My problem is with the fact that ultimately, the geometry used in physics is essentially arbitrary. There's nothing fundamental about a sphere - it's just an imaginary construct with a radius.

For example, many physicists used to think that electrons were spheres. But when they tried to calculate the rotation of the surface, they found it must be greater than the speed of light. Now electrons are considered zero dimensional.

## PinkElephant

A zero-dimensional point is still a geometrical construct. Indeed, anything that ever mentions "dimensions", or indeed any coordinate system at all (as in gauge equations...), is fundamentally geometrical in structure. If you ever talk of distances, vectors/momenta/velocities, polarization, and so on: you're inevitably talking geometry.

@mattytheory,

AFAIK light cannot be bent with EM fields. But there may be a possibility of the beam (especially if it's pulsed) inducing electron oscillations in the bead and gold bar, which may in turn result in exchange of EM radiation and transient forces between those two, with further complications due to possible resonant modes. The smaller the distances involved, the greater the danger of such "contamination". That's probably part of the difficulty mentioned toward the end of the article...

## axemaster

I have to disagree with that... since you cannot have vectors in 0D. You can only have scalars. A dimension is an axes so without any axes you have no dimensions. A point is definitely not a geometric object, since it has no descriptive properties.

## axemaster

To continue from 2 posts ago...

This illustrates my point - if you pretend that the geometry used in your calculations is real, then you are stepping into a dangerous territory. This is very explicitly indicated by quantum mechanics and virtual particles replacing the force fields of classical mechanics. Sadly, many physicists seem to have learned the material but forgotten the lesson.

For example, I am currently working on outlining a theory that has no dimensions at all - a zero dimensional universe. Nevertheless, I have already figured out a simple, emergent way that three dimensions emerge from the construct, as well as all the other relevant geometries. In fact, the most interesting thing is that ONLY three spatial dimensions are possible - it cannot produce more (although it can produce less in certain scenarios).

## PinkElephant

Perhaps I was trying to be too terse (given the character limit.) I did not mean to involve vectors into discussion of points. Rather, with my talk of vectors, coordinate axes, etc. I was trying to point out that geometric notions are pervasive and absolutely unavoidable in physics. Even the wave equations of quantum mechanics are formulated in the context of coordinate systems.

But as far as geometry, a point is indeed a fundamental geometric axiomatic construct. This goes all the way back to Euclid. Indeed, to talk about defining a point in the first place, you end up using concepts like dimensions or volumes. The entity in question can't be defined without using geometrical terminology in the first place.

Sure, you can talk abstract numbers (scalars), but those aren't the same thing as points. And even the concept of numbers (and indeed, all of arithmetic) is essentially geometrical at heart: e.g. being closely related to things like the "number line"...

## AdamCC

## TDK

Albert Einstein: "It is open to every man to choose the direction of his striving: and also every man may draw from Lessing's fine saying, that the search for truth is more precious than its possession."

If it so, it's not so surprising, the physicists aren't looking for truth, but for the opportunity of further research. The contemporary theories are valued not by their predicative or even explanatory value - but by number of physicists, which these theories are able to employ.

## KBK

Ie, live and constantly in motion or a interacting and very high frequencies, ie, quanta. Thus the geometry still plays in here.

## TheQuietMan

## BrianFraser

http://fqxi.org/d...Time.pdf (paper)

http://fqxi.org/c...opic/294 (discussion)

Note how naturally the inverse square force distribution arises. And no "gravitons" are needed.

There are actually TWO forces that have to be considered when studying gravity. See my comments at:

http://cosmiclog....17598040

(The link should take you directly to my comment, but it may take several seconds to load.)

The comment is about gravity on an astronomical scale. Almost the same applies at the quantum mechanical level, but with a weird and surprising twist: gravity becomes a force of repulsion instead of attraction!

The compound spin systems in the atom also have specific gravity-like effects, but are beyond the scope of this comment.

## hypersabah

## Graeme

## CTD1

Actually the mechanism of both these forces appears equivalent, so we can still subtract the Casimir force from the result - but such approach would correspond the proverb "The darkest place is under the candlestick".

## Neodim