Proposed test of weak equivalence principle could be most accurate yet

May 03, 2010 By Lisa Zyga feature
This illustration shows the two test mass assemblies, which consists of two pairs of aluminum cubes. Four tracking frequency laser gauges measure each cube’s acceleration as it falls to Earth from altitudes of up to 1200 km. Image credit: Reasenberg and Phillips.

(PhysOrg.com) -- The weak equivalence principle (WEP) - which states that all bodies fall at the same rate in a gravitational field, regardless of structure or composition - is one of the key postulates of general relativity. Tests have shown that the WEP is accurate to within one part in 10 trillion, or an uncertainty of 10-13 of the acceleration of gravity. However, a violation of the WEP is suggested by most theories that attempt to unify gravity with the other forces, which is one of the biggest challenges in physics today. Looking for new ways to test the WEP to even greater accuracy and perhaps detect a violation, astrophysicists at the Harvard-Smithsonian Center for Astrophysics have designed a new WEP test to be conducted during free fall in a rocket flight.

As astrophysicists Robert Reasenberg and James Phillips explain in a series of papers, their proposed test is aimed at a very small measurement uncertainty of 10-16 after averaging the results of eight separate free fall drops. If the experiment is successful, it would be the most accurate WEP test to date, although it is not the most accurate WEP test that has been proposed: the satellite test of the equivalence principle (STEP) is aimed at a 100-fold smaller measurement uncertainty (10-18). STEP is a proposed cryogenic experiment in an Earth-orbiting spacecraft.

One way that scientists can test the WEP is by measuring the accelerations of two bodies made of different materials falling in the same . As far back as the 6th century AD, people have tested the WEP simply by dropping two objects of different masses, and observing no detectable difference. Astronauts even performed the test to low accuracy on the Moon with a feather and hammer, which reached the surface at the same time. The most precise tests to date use a rotating torsion pendulum, which can measure the acceleration of different samples toward the Earth, Sun, or center of the Milky Way. However, this technique is approaching technical limitations.

In their proposed experiment, Dr. Reasenberg and Dr. Phillips compare the accelerations of two test mass assemblies dropped inside an experiment chamber that is carried by a NASA Black Brant XII rocket. The experiment is designed to be conducted at altitudes that range from about 800 km to 1200 km, and includes eight drops of 40 seconds each.

Both test mass assemblies consist of a pair of aluminum cubes connected by a short rod or pair of rods. In one test mass assembly, holes are drilled into the aluminum, and lead tubes are inserted into the holes. Although lead is denser than aluminum, the objects are designed so that they both have the same mass, and only the material is different. The two objects are laid in a crisscrossed configuration in the same plane, so that the four aluminum cubes form a square. Above this configuration, four tracking frequency laser gauges are aimed down at the four cubes toward Earth’s center of mass, measuring each cube’s acceleration as it falls to Earth.

“The extreme speed and sensitivity of the tracking frequency laser gauge makes it possible to do this experiment quickly,” Phillips told PhysOrg.com. “A slower or less sensitive measuring device would not allow the experiment to be carried out in the brief time available during a sounding rocket flight.”

Reasenberg and Phillips have received support from the Astrophysics Division of NASA to develop both the sounding rocket test and the laser gauge. The sounding will be launched from the NASA Flight Facility at Wallops Island, VA.

“A central theme of this experiment’s design is to mitigate systemic errors,” Reasenberg explained.

One way this is done is by reversing the orientation of the free-falling experiment between successive pairs of the eight drops. Other design tactics that reduce errors include precisely aligning the lasers to the test mass assemblies using a hexapod motion system (also known as a Stewart platform) and minimizing thermal perturbations to insignificant levels in many ways, such as flying at night to avoid directional solar heating.

Although taking all these pains to achieve extreme precision may seem tedious, the scientists do so enthusiastically because the discovery of a WEP violation would have profound implications for physics, astrophysics, and cosmology. For instance, knowledge of a WEP violation would guide the formulation of a theory of gravity to supersede , which might be a quantum theory of gravity. The quantum theories of being developed now generally predict a violation of the WEP, but most are not yet able to predict the magnitude of the violation. The magnitude of a WEP violation could be within the range of the sounding-rocket experiment, or well below it.

Explore further: How the physics of champagne bubbles may help address the world's future energy needs

More information: Robert D. Reasenberg and James D. Phillips. “A weak equivalence principle test on a suborbital rocket.” Class. Quantum Grav. 27 (2010) 095005 (14pp). Doi:10.1088/0264-9381/27/9/095005

Harvard-Smithsonian Center for Astrophysics: Test of the Weak Equivalence Principle www.cfa.harvard.edu/PAG/index_files/Page1098.htm

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User comments : 12

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JIMBO
3 / 5 (3) May 03, 2010
I seem to recall that the jury is still out on Gravity Probe-B's frame dragging check of gen.rel. However, over the past CENTURY, there have been NO, repeat NO expt. contradictions to gen.rel.
People are searching for a gnats ass & they can't find it. Lorentz invariance tests good to the Planck Scale, & GR tests good to the scale of Galaxy clusters. The WEP is a fact of nature & any variance detected in this expt. will be systemic and statistical, therefore irrelevant.
Seems to me, these scientists are investing a good chunk of their time & career into looking for angels dancing on the head of a pin.
baudrunner
1.2 / 5 (6) May 03, 2010
The weak equivalence principle (WEP) - which states that all bodies fall at the same rate in a gravitational field, regardless of structure or composition - is one of the key redundancies of general relativity.

Why do these eggheads consistently fail to account for inertia, which doesn't appear to be factored into gravitational theories? There is a direct relationship between the mass of an object and its reluctance to allow forces like gravity to effect a change in its velocity, direction, or momentum. The heavier the object, the greater its inertia, so the effective rate at which any body falls to the surface of a larger body is only, and ONLY a function of the larger object's mass, and this is constant for any falling object.
SongDog
2.5 / 5 (2) May 03, 2010
Who'd have believed in the early 60's that the Black Brant (see http://en.wikiped...(rocket) ) would be doing cutting edge physics half a century later?
Doug_Huffman
1 / 5 (1) May 03, 2010
Alizee
May 03, 2010
This comment has been removed by a moderator.
Question
2 / 5 (2) May 03, 2010
Why not conduct the test at the coldest temperatures we are able to achieve? For example, test the falling rate of two objects with the same mass with one at room temperature the other near absolute zero. Hasn't there been some test conducted near absolute zero by scientist trying to achieve anti-gravity?
Husky
3 / 5 (3) May 03, 2010
well, angels dancing on the head of a pin: good analogy, this is exactly what the quantum scale looks like
Alizee
May 03, 2010
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Parsec
5 / 5 (3) May 04, 2010
Why do these eggheads consistently fail to account for inertia, which doesn't appear to be factored into gravitational theories? There is a direct relationship between the mass of an object and its reluctance to allow forces like gravity to effect a change in its velocity, direction, or momentum. The heavier the object, the greater its inertia, so the effective rate at which any body falls to the surface of a larger body is only, and ONLY a function of the larger object's mass, and this is constant for any falling object.

This is easily invalidated. The orbital mechanics of the solar system would be drastically altered for example.
ZeroX
2 / 5 (4) May 04, 2010
..the orbital mechanics of the solar system would be drastically altered for example.
Well, such finding invalidates the WEP as well without any additional measurements...

The people cannot understand, relativity is just an abstract theory extrapolated to behavior of remoted pin-point objects in empty vacuum. Everything outside it is affected by extradimensions and quantum mechanics, which violates the ISL for gravity heavily.

Of course, if we neglect all these effects, we can continue in search for violations of WEP ad infinitum - but this is not a path for general understanding of reality, because we can't see the wood for the trees. We cannot say, only subtle fifth force could violate the WEP - while ignoring all the other forces, which are violating it too from traditional reasons, just for keeping relativity more valid, then it really is.

This is not, what the fair & consistent thinking means.
baudrunner
1 / 5 (1) May 04, 2010
The idea that any "force" violates the WEP is akin to denying the GUT. Everything works together in the GUT, nothing violates anything.
Alizee
May 04, 2010
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Alizee
May 04, 2010
This comment has been removed by a moderator.
Alizee
May 04, 2010
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MrGrynch
1.3 / 5 (3) May 04, 2010
MY gravity theory, published last year explains why the weak equivalence principle is absolute. However, it also proposes a test which will invalidate the equivalency principal. There is no need to disprove weak equivalence to unify forces. Search for "self-movement" and "gravitation" and you will find the article. If I am right, and experimentation is underway, gravitational propulsion is not far behind
tkjtkj
not rated yet May 06, 2010
"Above this configuration, four tracking frequency laser gauges are aimed down at the four cubes toward Earth’s center of mass, measuring each cube’s acceleration as it falls to Earth"

Now, just *which* 'center of mass' of the earth are they intending to use? And how do they find it? Does not the earth's CoM move, eg, by tidal forces, tectonic plate movement, volcanic activity? Although these effects might be miniscule, at the scale of measurements to be use here, this dynamic nature of earth's CoM could matter, indeed.

tkjtkj@gmail.com
tkjtkj
not rated yet May 06, 2010
"Above this configuration, four tracking frequency laser gauges are aimed down at the four cubes toward Earth’s center of mass, measuring each cube’s acceleration as it falls to Earth"


Now, just *which* 'center of mass' of the earth are they intending to use? And how do they find it? Does not the earth's CoM move, e.g., by tidal forces, tectonic plate movement, volcanic activity? Although these effects might be miniscule, at the scale of measurements to be use here, this dynamic nature of earth's CoM could matter, indeed.

tkjtkj@gmail.com
Alizee
May 07, 2010
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