Posted on 11/16/2004 12:53:57 PM PST by PatrickHenry
Physicists in Germany have used an atomic interferometer to perform the most accurate ever test of the equivalence principle at the level of atoms. Sebastian Fray and colleagues at the Max Planck Institute for Quantum Optics in Garching and the universities of Munich and Tuebingen compared the acceleration of two isotopes of rubidium in the Earth's gravitational field (arXiv.org/abs/physics/0411052). As expected the atoms accelerated at the same rate.
The weak equivalence principle is a cornerstone of general relativity and states that, in the absence of other forces, all objects fall with the same acceleration under the influence of gravity. Experiments that involve bouncing laser beams off mirrors on the Moon have confirmed that the Earth and the Moon accelerate toward the Sun at the same rate to better than one part in 1013. However, some theorists have predicted that new physics might emerge when gravitational experiments are performed with quantum objects such as atoms.
A conventional interferometer works by splitting a coherent beam of light and then recombining the different components at a detector. If the two components are in phase when they recombine, the interference is constructive and the two components reinforce each other. However, if the two components are out of phase, they cancel the each other out. This leads to a characteristic pattern of bright and dark fringes, and this pattern can be used as a "ruler" to measure small distances.
Atomic interferometry is similar but relies on beams of atoms rather than beams of light. Moreover, standing optical waves are used to split and recombine the beams. Atomic interferometers have already been used to measure the Earth's gravitation to an accuracy of 10-9.
Fray and co-workers began by capturing about two billion rubidium-85 or rubidium-87 atoms in a magneto-optical trap. Then they used laser beams to accelerate the atoms upwards. When the laser beams were switched off, the atoms fell back down under the influence of gravity. The interferometer allowed the team to measure the accelerations, g85 and g87, of both types of atoms.
The team found that (g85-g87)/g85 = 1.2 x 10-7, with error bars of 1.7 x 10-7, which is consistent with the two accelerations being the same, in accordance with the equivalence principle. Fray and co-workers also found that the relative acceleration of rubidium-85 atoms in two different internal states were the same within error bars. The new experiments are a factor of three more accurate than previous tests of general relativity with atoms.
Also, I had thought the weak equivalence principle was different than what is presented in the 2nd paragraph. Live and learn.
The error bar is larger than the measured value.
An old saying from one of my science professors:
Industry research is like throwing darts at a bull's eye.
Academic research is like throwing darts and painting bull's eyes around the darts.
Good catch. But this can't be as goofy as it sounds.
That's the result! They are trying to measure a value which is supposed to be zero, according to the theory. If the measured value were significantly larger than the error bars, it would be a non-zero result. That's what the researchers were hoping to find, as it would earn them an easy Nobel Prize in physics. But, well, the universe is the way that it is, and not how they wish it would be.
That won't stop the ignorant and the insane from denying his theory.
Thanks. It makes sense now.
At least I didn't have to try to pronounce the Eötvös Experiment.
And yet he too is still dead.
True. Nevertheless, I'm willing to bet that, in due course (long after we're gone), Big Al won't be the ultimate word in such matters. But don't ask me where the corrections will be made. I see no likely lines of attack (unless it's related to the accelerating expansion of the universe), and certainly the special theory seems bullet-proof. Based on what we know at this point (or at least what little I know), only a total goofball would dispute his work. Testing is one thing; declaring him a bozo (as some websites do) is the sure indicator of mental disorder.
Hardly.
http://map.gsfc.nasa.gov/m_uni/uni_101accel.html
"Einstein first proposed the cosmological constant (not to be confused with the Hubble Constant) usually symbolized by the greek letter "lambda" (L), as a mathematical fix to the theory of general relativity. In its simplest form, general relativity predicted that the universe must either expand or contract. Einstein thought the universe was static, so he added this new term to stop the expansion. Friedmann, a Russian mathematician, realized that this was an unstable fix, like balancing a pencil on its point, and proposed an expanding universe model, now called the Big Bang theory. When Hubble's study of nearby galaxies showed that the universe was in fact expanding, Einstein regretted modifying his elegant theory and viewed the cosmological constant term as his "greatest mistake".
Dan Rather can probably help with the subscripts...
As predicted....
(g85 - g87) / g85 = 1.2 x 10-7
Or:
(g85 - g87) / g85 = 1.2 x 10-7
Or (getting fancier):
(g85 - g87) ÷ g85 = 1.2 x 10-7
LOL, well I look at it like this: Big Al did all his homework on a chalkboard. The new school has the benifit of equipment the Einstein could only dream about in his day, with computer models, observation and such. Even Hawking has the benefit of being able to revise his theories. One wonders if Al wouldn't have done the same had he had modern resources.
"The quantitative description of the Universe by General Relativity requires obtaining solutions to what are termed the Einstein field equations. These are 10 equations that must be solved simultaneously; they are notoriously difficult, and only a few solutions are known. (Technically, the equations to be solved are known to mathematicians as coupled, non-linear, partial differential equations; we may take that as precise shorthand for "very difficult to solve"!)
Solutions of the Equations
Among these solutions, there are two general classes:
Static Universes
Dynamic Universes
where static universes have distance scales that are constant in time and dynamic universes are either expanding or contracting over time. It may be shown that the cosmological principle (the assumption that there are no preferred positions or directions in the Universe) is inconsistent with a static universe. That is, the cosmological principle requires that the Universe be either expanding or contracting.
The Cosmological Constant
When Einstein first realized that the solution of his equations subject to the constraints of the cosmological principle led to universes that were not static, he was dismayed..."
Glad to help. Nice post.
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