New Way to Measure Curvature of Space Could Unite Gravity Theory

Article written: 1 Sep , 2009
Updated: 24 Dec , 2015

Einstein’s general theory of relativity describes gravity in terms of the geometry of both space and time. Far from a source of gravity, such as a star like our sun, space is “flat” and clocks tick at their normal rate. Closer to a source of gravity, however, clocks slow down and space is curved. But measuring this curvature of space is difficult. However, scientists have now used a continent-wide array of radio telescopes to make an extremely precise measurement of the curvature of space caused by the Sun’s gravity. This new technique promises to contribute greatly in studying quantum physics.

“Measuring the curvature of space caused by gravity is one of the most sensitive ways to learn how Einstein’s theory of General Relativity relates to quantum physics. Uniting gravity theory with quantum theory is a major goal of 21st-Century physics, and these astronomical measurements are a key to understanding the relationship between the two,” said Sergei Kopeikin of the University of Missouri.

Kopeikin and his colleagues used the National Science Foundation’s Very Long Baseline Array (VLBA) radio-telescope system to measure the bending of light caused by the Sun’s gravity to within one part in 30,000 3,333 (corrected by NRAO and updated here on 9/03/09 — see this link provided by Ned Wright of UCLA for more information on deflection and delay of light). With further observations, the scientists say their precision technique can make the most accurate measure ever of this phenomenon.

Bending of starlight by gravity was predicted by Albert Einstein when he published his theory of General Relativity in 1916. According to relativity theory, the strong gravity of a massive object such as the Sun produces curvature in the nearby space, which alters the path of light or radio waves passing near the object. The phenomenon was first observed during a solar eclipse in 1919.

Though numerous measurements of the effect have been made over the intervening 90 years, the problem of merging General Relativity and quantum theory has required ever more accurate observations. Physicists describe the space curvature and gravitational light-bending as a parameter called “gamma.” Einstein’s theory holds that gamma should equal exactly 1.0.

“Even a value that differs by one part in a million from 1.0 would have major ramifications for the goal of uniting gravity theory and quantum theory, and thus in predicting the phenomena in high-gravity regions near black holes,” Kopeikin said.

To make extremely precise measurements, the scientists turned to the VLBA, a continent-wide system of radio telescopes ranging from Hawaii to the Virgin Islands. The VLBA offers the power to make the most accurate position measurements in the sky and the most detailed images of any astronomical instrument available.

Sun's Path in Sky in Front of Quasars, 2005. Credit: NRAO

Sun's Path in Sky in Front of Quasars, 2005. Credit: NRAO

The researchers made their observations as the Sun passed nearly in front of four distant quasars — faraway galaxies with supermassive black holes at their cores — in October of 2005. The Sun’s gravity caused slight changes in the apparent positions of the quasars because it deflected the radio waves coming from the more-distant objects.

The result was a measured value of gamma of 0.9998 +/- 0.0003, in excellent agreement with Einstein’s prediction of 1.0.

“With more observations like ours, in addition to complementary measurements such as those made with NASA’s Cassini spacecraft, we can improve the accuracy of this measurement by at least a factor of four, to provide the best measurement ever of gamma,” said Edward Fomalont of the National Radio Astronomy Observatory (NRAO). “Since gamma is a fundamental parameter of gravitational theories, its measurement using different observational methods is crucial to obtain a value that is supported by the physics community,” Fomalont added.

Kopeikin and Fomalont worked with John Benson of the NRAO and Gabor Lanyi of NASA’s Jet Propulsion Laboratory. They reported their findings in the July 10 issue of the Astrophysical Journal.

Source: NRAO

12 Responses

  1. William928 says

    Oh Boy, Nancy, you’ve opened up a can of worms with this article. Is that a snake I hear hissing?

  2. IVAN3MAN says

    Excellent article, Nancy!


    Is that a snake I hear hissing?

    We are going to need this.

  3. Lawrence B. Crowell says

    This is an interesting test of parameterized post Newtonian theory. The results are encouraging.


  4. Yoo says

    This is just a more precise verification of general relativity (which is interesting in its own right, but you know what I mean): how is this directly related to unifying general relativity and quantum theory? Albeit it would place restrictions on any unified theory, but when a headline mentions unification, I would have expected something else as well.

  5. Nephish777 says

    I understand there is doubt that the position shift in stars near the sun may no have been seen in 1919, but was for sure in a later eclipse.

  6. ND says

    To get the 1-in-1million precision, do we need a baseline that’s larger than Earth?

  7. ND says

    I just reread this and they mention using Cassini to get a factor of 4. Is this to use Cassini as another receiver as part of the VLBA?

  8. Astrofiend says

    Is that the sound I hear of Einstein’s monumentally beautiful theory being borne out yet again? I’ve been keeping an extremely accurate running tally of the triumphs of the general theory of relativity, vs. the likes of hokey EU-based pseudoscience. The results of this admittedly awesome study are as follows:

    Einstein: 12409783517123

    Crayon Physics: 0

    Science saves the day again. Hurrah!

  9. Lawrence B. Crowell says

    This is a test of general relativity and on the outset it appears to have nothing to do with quantum mechanics. Directly of course it has nothing to do with quantum mechanics. However, indirectly it does. General relativity can be expressed as a series, where the lowest term is Newtoniania, the first post-Newtonian term is a static correction, the next is similar to Maxwell’s equations and so forth. Now for quantum field theory there is also a set of corrections. Two gravitons interacting have an O(1), which is classical, then an O(hbar) term and other higher order up to O(hbar^n) for n large. This perturbation sequence only makes sense in the long run if it converges to a nonperturbative theory, which is string theory or M-theory.

    The gamma here is predicted from the static gravity field of a star. If gamma = 1 exactly then we know that the above stuff converges properly to something which makes sense. This test with observing radio data from dstant quasars lensed by the solar gravity field puts gamma as unity to four orders of magnitude.


  10. Torbjorn Larsson OM says

    Sorry Nancy and Lawrence, I’m with Yoo and Astrofiend in this. Wouldn’t a different value in gamma come from another action, which in turn would putatively yield another nonperturbative theory and other particles?

    Or is there any significance in using gravitons (for free from string theory)? I don’t get it.

    Meanwhile, another test GR passed with relative ease.

    “Bending of starlight by gravity was predicted by Albert Einstein when he published his theory of General Relativity in 1916.”

    Bending of starlight by gravity is (and probably was) also predicted from special relativity, but only half as much as seen, as soon as you have the SR energy-mass equivalence to use on photons.

  11. Lawrence B. Crowell says

    Bending of light is by plain Newtonian gravity using the E = mc^2 from special and E = hbar*omega from de Broglie’s relation. It would bend by half the amount.

    If gamma departs from unity the consequences might be very deep. It frankly could mean that there is no rational schema for gravitation. On the other hand it could signify departures of some other subtle nature, such as the brans-Dicke scalar modification of the tensors in general relativity.


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