(PhysOrg.com) -- One of the givens in physics is the weak equivalence principle. This principle has been considered solid since Einstein proposed that it is not possible to detect the difference between uniform acceleration and a uniform static gravitational field. The uniqueness of freefall allows uniform acceleration, even between masses that are different, according to Einstein's postulate in the theory of General Relativity. The weak equivalence principle is well established amongst the science community, but it has yet to be demonstrated completely. This is where Phillippe Bouyer at Laboratoire Charles Fabry de l’Institut d’Optique, Campus Polytechnique in Palaiseau, France, and his colleagues are attempting to go.

“We are looking to see to which extent this principle holds,” Bouyer tells *PhysOrg.com*. “Does the principle break down at a certain level? And if it does, what is that level? If it breaks down, that opens up a whole range of fundamental questions, such as the existence of new interactions as predicted by many current quantum theories of gravity. We have created a test that we hope will open the field to see whether the weak equivalence principle holds at the quantum level.”

In order to test the weak equivalence principle, Bouyer and his peers Varoquaux, Nyman, Geiger and Cheinet at the Laboratoire Charles Fabry, and Landragin at SYRTE, suggest a technique using a parabolic flight plane outfitted with an atom interferometer. The device would be used to determine whether the acceleration is the same for two different atoms in free fall. Their proposed method is described in *New Journal of Physics*: “How to estimate the differential acceleration in a two-species atom interferometer to test the equivalence principle.”

“If you want to test the acceleration of atoms on the ground, you have very little time before the atoms hit the floor” Bouyer explains. “In parabolic flights, though, you have up to 20 seconds of free fall to see whether there is a difference in acceleration. When dealing with atoms, this is a rather long time indeed.” The atoms would be captured, laser cooled using well known techniques and then dropped in the “free falling” plane. It would then be possible to precisely measure the acceleration of the two different atoms using atom interferometry. “We plan to use rubidium and potassium,” he says, “since they are very different atoms with a difference in mass that is significant. This way, if they have no difference in acceleration, the equivalence principle is demonstrated. If there is a difference in acceleration, we know that it breaks down at the precision level of our measurement.”

One of the problems, though, is that the parabolic flight plane is very noisy. Bouyer and a team from various scientific institutes in France addressed this problem last year. They tested the operation of a specially designed atom interferometer in a free fall plane (results can be found in *The European Physical Journal D*), finding that the sensitivity of measurement was enhanced, allowing acceleration in rubidium to be measured. To cancel out the noise, Bouyer and his colleagues worked out a way to extract the acceleration using Bayesian statistical estimation. “Our dedicated statistical analogy allows us to extract the EP signal out of the noise of the environment,” he explains.

For now, the proposed experiment remains just that: proposed. But Bouyer is hopeful. “We are very close to getting two atoms at the same place, and using our interferometer in a state of free fall should enable us to take measurements that may help us demonstrate the viability of the equivalence principle at the quantum level.”

__More information:__

• Varoquaux, et al, “How to estimate the differential acceleration in a two-species atom interferometer to test the equivalence principle,” *New Journal of Physics* (2009). Available online: http://www.iop.org/EJ/abstract/1367-2630/11/11/113010/.

• Stern, et al, “Light-pulse atom interferometry in microgravity,” *European Physical Journal D* (2009). Available online.

*Copyright 2009 PhysOrg.com.
All rights reserved. This material may not be published, broadcast, rewritten or redistributed in whole or part without the express written permission of PhysOrg.com.*

**Explore further:**
Testing the Equivalence Principle

## Alexa

Basically it means, quantum scale begins at Casimir force scale, which roughly corresponds the wavelength of cosmic microwave background, which roughly corresponds the size of human brain waves.

## Alexa

http://en.wikiped...astrophe

## KBK

Look it up ---if you don't believe me.

## OregonWind

I don't believe you and where then should I look that up?

## barakn

## barakn

KBK's issue is that (s)he doesn't understand that Maxwell didn't use vector notation, instead presenting the equations in Cartesian coordinates. In many cases this forced the use of separate equations for each of the three axes. Modern vector notation reduces the number of equations but is more or less fully mathematically equivalent to Maxwell's original formulations. Try telling that to KBK though.

## sender

## Donutz

## sender

http://projectrho...able.jpg

## Donutz

Website declines to show the webpage without a login. that's just great. Pique my curiosity then yank the rug out.

## nkalanaga

## DuncanI

## rmuldavin

{{Comparing the acceleration of these two different atomic species constitutes a meaningful test of the UFF, as they combine a large mass ratio (almost a factor of two), very different nuclear compositions (37 protons and 50 neutrons for 87 Rb and 20/19 for 39 K) and almost equal laser wavelength and thus interferometer scale factors.}}

[comments-rm: the "20/19" appears reversed. Trying to clear my confusion:

Z/A(K): 19/39

Z/A(Rb): 37/85; 37/87

I'm easily confused, perhaps so was the editor of the article, or the type setter, or a noisy electron circuit? To assure myself I am not doing my counting correctly:

Taking the most relevant abundance Rb(85) at 72.2% or even the half-life given at 4.9x10^10 yrs, Rb(87), then doing prime number separations, and remembering R at Z=86 is not listed on my notebook Atomic Masses

## rmuldavin

A(K@39)=>(19x2)+(1);

A(RB@85)=>(17x5)

Where am I going here? Trying to get some polyhedron structures so the authors can draw their K and Rb free falling differences.

Back when I get more done.

Best, rm

## vacuum-mechanics

May be it is easier way to visualize this matter than to do with a neutron star! Does anyone (on our earth) found stationary electron(s) radiate?

Instead, for one whom familiar with the process of radio communication system, such as one which was used in mobile telephone system, would know that radio radiation signal was created and then radiate from the acceleration (oscillation) process of stream of electrons in the equipment!

## Alexa

Sorry, I didn't get it. Why it should violate equivalence principle, if it does nothing?

## lomed

In fact, an electron dropped from rest in a uniform (or even radial) gravitational field (with that being the only force on the particle) will not radiate either since its velocity and acceleration are parallel.

## flaredone

## lomed