Test of equivalence principle searches for effects of spin-gravity coupling

Jul 16, 2014 by Lisa Zyga feature
Experimental configuration to test the equivalence principle, in which two strontium isotopes are laser-cooled and trapped in a vertical optical lattice. Credit: Tarallo, et al. ©2014 American Physical Society

(Phys.org) —Einstein's equivalence principle states that an object in gravitational free fall is physically equivalent to an object that is accelerating with the same amount of force in the absence of gravity. This principle lies at the heart of general relativity and has been experimentally tested many times. Now in a new paper, scientists have experimentally demonstrated a conceptually new way to test the equivalence principle that could detect the effects of a relatively new concept called spin-gravity coupling.

The study, by M. G. Tarallo, et al., is published in a recent issue of Physical Review Letters.

"Testing the , or the equivalence of inertial mass and gravitational mass, means testing the validity of one of the fundamental principles of general relativity," coauthor Guglielmo Tino, Professor at the University of Florence, INFN, told Phys.org. "In our experiment, we use a quantum sensor to investigate gravitational interaction; this allowed us to search for new effects."

As the researchers explain, there are a variety of ways to test the equivalence principle. These methods include studying the motion of moons and planets, the use of torsion balances, and—more recently—atom interferometry.

In the new study, the researchers have for the first time tested the equivalence principle by comparing the gravitational interaction for a bosonic particle to that of a fermionic particle. For the purpose of the experiment, the important difference between the two particles is that the bosonic particle (a strontium-88 isotope) has no , while the fermionic particle (a strontium-87 isotope) has a half-integer spin.

In order to determine how the differences in spin might affect a particle's gravitational interaction, the researchers performed tests to measure each isotope's acceleration due to gravity. These tests consist of confining atomic wave packets in a vertical laser standing wave, and then using a quantum effect involving delocalization to measure the effects of gravity. The new method improves the measurement precision by more than an order of magnitude over previous methods.

The results of the experiments enabled the researchers to set an upper limit of 10-7 on the boson-to-fermion ratio. The researchers also searched for a dependence of gravity acceleration of strontium-87 isotope on the spin direction, but found no evidence for it.

"There are theoretical models predicting that spin and gravity should couple; that is, depending on its spin a particle should behave in different ways in a gravitational field," Tino said. "We found no evidence for that. Since we compared an atom with spin with one without spin, this is a rather stringent test. Also, in our experiment one atom is a boson and the other is a fermion and, again, we found no difference in their behavior in a ."

The results could have future applications in connection with optical clocks made of strontium, which have already demonstrated impressive stability and accuracy. In the future, it may also be possible to perform an experiment in space using a strontium optical clock and a strontium interferometer to perform stringent tests of and gravity.

"Our result reported in this paper, as well as the one we recently published on the measurement of the gravitational constant with atoms (G. Rosi, et al.), shows the great potential of quantum sensors based on ultracold atoms and atom interferometry to investigate gravity," Tino said. "We want to try new schemes to increase the sensitivity of the atom interferometer; this would allow us to perform still more stringent tests and search for new effects."

Explore further: Scientists find a practical test for string theory

More information: — M. G. Tarallo, et al. "Test of Einstein Equivalence Principle for 0-Spin and Half-Integer-Spin Atoms: Search for Spin-Gravity Coupling Effects." Physical Review Letters. DOI: 10.1103/PhysRevLett.113.023005
Also at arXiv:1403.1161 [physics.atom-ph]

— G. Rosi, et al. "Precision measurement of the Newtonian gravitational constant using cold atoms." Nature. DOI: 10.1038/nature13433

add to favorites email to friend print save as pdf

Related Stories

Scientists find a practical test for string theory

Jan 06, 2014

(Phys.org) —Scientists at Towson University in Towson, Maryland, have identified a practical, yet overlooked, test of string theory based on the motions of planets, moons and asteroids, reminiscent of Galileo's ...

Does weak equivalence break down at the quantum level?

Dec 08, 2009

(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 ...

Physicists predict new state of matter

Jun 17, 2014

A researcher with the Department of Electrodynamics of Complex Systems and Nanophotonics, Alexander Rozhkov, has presented theoretical calculations which indicate the possible existence of fermionic matter ...

Recommended for you

Uncovering the forbidden side of molecules

19 hours ago

Researchers at the University of Basel in Switzerland have succeeded in observing the "forbidden" infrared spectrum of a charged molecule for the first time. These extremely weak spectra offer perspectives ...

How Paramecium protozoa claw their way to the top

Sep 19, 2014

The ability to swim upwards – towards the sun and food supplies – is vital for many aquatic microorganisms. Exactly how they are able to differentiate between above and below in often murky waters is ...

User comments : 8

Adjust slider to filter visible comments by rank

Display comments: newest first

Deuterium2H
5 / 5 (3) Jul 17, 2014
Geez. Couldn't even get past the first paragraph without finding a fundamental error.

"Einstein's equivalence principle states that an object in gravitational free fall is physically equivalent to an object that is accelerating with the same amount of force in the absence of gravity."

WTF?? No. NO. NO. First of all, the Equivalence Principle relates the force on a mass due to the presence of a gravitational field, with that of an identical mass in a non-inertial frame (accelerated). In an idealized, localized laboratory, the effects are completely equivalent. The statement "an object in gravitational free fall is physically equivalent to an object that is accelerating" doesn't make ANY sense. An object in FREE-FALL in a UNIFORM gravitational field DOESN'T DETECT ANY FORCE--it is an inertial frame. What the author must have meant is that the force on an object at rest (not free-falling) in a 1g gravitational field is equivalent to the same object under 1g acceleration.
Reg Mundy
1 / 5 (5) Jul 17, 2014
"There are theoretical models predicting that spin and gravity should couple; that is, depending on its spin a particle should behave in different ways in a gravitational field," Tino said. "We found no evidence for that. Since we compared an atom with spin with one without spin, this is a rather stringent test. Also, in our experiment one atom is a boson and the other is a fermion and, again, we found no difference in their behavior in a gravitational field."

Yet more evidence that there is no such thing as a gravitational field, i.e. gravity does not exist as a force.
Captain Stumpy
5 / 5 (3) Jul 17, 2014
Yet more evidence that there is no such thing as a gravitational field, i.e. gravity does not exist as a force
@reg
You keep saying this but you STILL cannot provide any empirical evidence supporting your conclusions...

Worse yet, the conclusions that you HAVE given, regarding your expansion theory, have all been debunked with common sense, logic and basic physics.

Feel free to start the explanations all over again here, but know that unless you can describe: Orbits, Tides, the mass dependent proportional anomaly which we've addressed in other threads, you are not likely to get anywhere.

As I pointed out in THIS thread:
http://phys.org/n...firstCmt

per your own words, given mass dependent expansion, there would be a disparity in proportion that would be measurable that is NOT observed to day, to which you STILL cannot reconcile any of your explanations.

And furlong has you on the run completely...

feel like expounding?
George_Rajna
Jul 17, 2014
This comment has been removed by a moderator.
swordsman
1 / 5 (1) Jul 17, 2014
Gravitational force is not a force? Ridiculous statements.

The gravitational force is a Coulomb force, and it behaves accordingly.
Pexeso
1 / 5 (4) Jul 17, 2014
Test of equivalence principle searches for effects of spin-gravity coupling
These effects are already known, for example the Dean/Woodward and electromagnetic drives (which indeed all do violate the equivalence principle as well). The contemporary physicists are looking just for effects, which they're ignoring most obstinately many years. Such an approach may appear nonsensical if not ridiculous, but it actually maximizes the occupation and it minimizes the risk of premature lost of jobs for theorists involved as a whole.
Reg Mundy
1 / 5 (2) Jul 18, 2014
@swordsman
Gravitational force is not a force? Ridiculous statements.

The gravitational force is a Coulomb force, and it behaves accordingly.

Nonsense! Coulombs Law deals with Vector forces, and "gravity" is not a vector force (it cannot be negative, for example).
Dr_toad
Jul 18, 2014
This comment has been removed by a moderator.
Uncle Al
3 / 5 (2) Jul 18, 2014
No measurable observable violates the EP. Geometric chirality is easily observed (no S_n symmetries) but it cannot be measured. Optical rotation is not coupled to mass distribution.

Opposite shoes vacuum free falling along non-identical trajectories violate the EP but not any prior observation. Crystallography's opposite shoes are visually and chemically identical, single crystal test masses in enantiomorphic space groups, e.g., P3(1)21 and P3(2)21 alpha-quartz. Twice {20 grams as 4 single crystal test masses} compare 6.68×10^22 pairs of opposite shoes (pairs of 9-atom enantiomorphic unit cells, the cubic array's opposite vertical sides) as a geometric Eötvös experiment. Test spacetime geometry with geometry. Look.

http://www.mazepa...tor1.jpg
swordsman
1 / 5 (1) Jul 21, 2014
Entanglement has been disentangled.

Gravity is indeed a vector force. That should be obvious. You don't need negative gravity to prove that. Gravity is a second order Coulomb force, as I have proved.
George_Rajna
Jul 24, 2014
This comment has been removed by a moderator.