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Einstein’s gravitational redshift measured with unprecedented precision

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Atom interferometryAlbert Einstein’s theory of general relativity makes a number of counterintuitive predictions about the workings of gravity, and experimentalists nearly 100 years after the theory was developed continue to confirm those predictions with increasing accuracy. A new paper co-authored by U.S. Energy Secretary Steven Chu measures the gravitational redshift, illustrated by the gravity-induced slowing of a clock and sometimes referred to as gravitational time dilation (though users of that term often conflate two separate phenomena), a measurement that jibes with Einstein and that is 10,000 times more precise than its predecessor.

To confirm the nature of the gravitational redshift, researchers in 1971 flew atomic clocks in commercial airliners; in 1980 another group launched a device to an altitude of 10,000 kilometers on a rocket. The new research, a repurposing of a decade-old experiment, takes a more down-to-Earth approach, tracking cesium atoms, whose oscillations act as a sort of atomic clock, launched on vertical trajectories differing by only 0.12 millimeter.

As reported in the February 18 issue of Nature by Holger Müller of the University of California, Berkeley, and Lawrence Berkeley National Laboratory, Achim Peters of Humboldt University of Berlin and Chu, cesium atoms, which oscillate with a characteristic frequency, are shot through a gauntlet of laser beams. (Scientific American is part of Nature Publishing Group.) The first beam diverts the atom onto one of two paths, one being slightly higher than the other, with equal probability. A second and third laser act to reunite the paths [see diagram above], producing what is known as an atom interferometer.

In quantum-mechanical terms, the cesium atoms can be thought of as a matter wave. The first laser splits the wave into two waves propagating simultaneously along both paths that the atom could take, while the third and final laser splices them back together into a single wave. If the wave components remain unchanged along their respective paths, the wave will emerge whole once it is recombined. But if one of the waves is waylaid on its path, it will be out of phase with its counterpart, yielding destructive interference when the two are recombined. By monitoring the interference after recombination, the researchers can trace the phase difference induced by the waves traveling along different paths.

In the case of the cesium atoms, the researchers found that the waves traveling along the two trajectories oscillated a different number of times. That is just what is predicted by general relativity, which holds that clocks close to Earth tick more slowly than those at higher elevations—even a fraction of a millimeter higher in elevation.

Image credit: Nature

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  1. 1. joeldooris 3:54 pm 02/17/2010

    So does gravity offer resistance to time in the same way as the density of the earths atmosphere offers resistance to, say an airplane?
    Does the density of gravity offer resistance to the atoms and it just manifests it’s self as a time shift?

    Or something entirely different?

    Link to this
  2. 2. hnkelley 4:31 pm 02/17/2010

    I’m sorry… Am I missing something here? The two possible paths for the cesium were, by their very nature, different. The odds of getting the lengths to match in a manner that allows for the non-destructive recombining are slim unless precisely calculated and performed. Was that done? It is not reported here, so it’s an honest question. More details are needed to understand that the experiment was designed to take into account the different times required for the different paths. Otherwise, this proves nothing.

    [I must admit that this piece of theory, gravitational time dilation, etc., leaves me a bit cold anyway. I need much more proof before I fall into line on this one. ;) ]

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  3. 3. rhodinsthinker 7:27 pm 02/17/2010

    hnkelley: For the details, see the February 18 issue of Nature. The reference is given. It may be necessary to buy the article or the magazine.

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  4. 4. hnkelley 8:15 pm 02/17/2010

    rhodinsthinker: Thank you. I’ll look that up.

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  5. 5. Viadd 12:55 am 02/18/2010

    joeldooris: Gravity (in the sense of things accelerating) can be considered as the result of time moving more slowly when nearer to masses such as Earth. Just as the path of light is bent towards the region where it moves more slowly (e.g. when going from air into gas or water) so are waves of matter (or light for that matter) bent towards the regions of ‘slower time’ that are deeper into the gravity well.

    This is not just an analogy, it is a mathematical description that predicts the observed behavior. (The description is called ‘the Theory of General Relativity’)

    The mystery thus moves from ‘why does an apple fall from a tree?’ to ‘why does time slow down in the proximity of mass?’

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  6. 6. debu 8:11 am 02/18/2010

    Rate of change of entropy flows varying with place due to field of varying ether and the energy level marker causing a dilation of time explained in my ether gravity theory of gravitoethertons published in ASTRONOMY.NET . sO BE CAREFUL –TIME is not dimension. TIME flow varies as per gravity or ether concentration of the zone and this can be explained by ether theory taking into account entropy of the operation and ether field exists in that zone. Read my balloon inside balloon theory and accept Einstein is wrong in basic approach as Dr.Roger Penrose explained physics is wrong to attend the PHYSICS OF THE UNIVERSE meeting OF APS IN MARCH THIS YEAR T O CORRECT EINSTEIN.

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  7. 7. theTribster 4:35 pm 02/18/2010

    So time dilation=redshift? I didn’t know these terms were synonymous. Redshift is somewhat anomalous with respect to scale and even some objects (Quasers). How does this measurement correlate to redshift? Here is an article that describes redshift problem I think:

    Link to this
  8. 8. Dynamike83 1:45 am 02/19/2010

    I would like to help to make this pboenomena more easily understandable. As we all know there is a consistant "max speed" dictating the fourth dimension which is the speed of light. Of course that is relative to where in space you are but that’s a personal theory. Anywho, In essence I can only throw a rock from a to b at one speed before it hits this wall of light speed. As we move through space on earth we are going at a single speed. Imagine if you will my rock traveling one direction traveling at let’s say 40 mph. Gravity is borrowing enery from this rock which means really the rock wants to go in two directions. This is the foreward inertia from the throw and downward which is gravity. This rock cannot have more energy than what is given which means the more gravity is forced on the rock, the slower it will become. The same is true with time. Keep in mind time is merely a measurement, and not a thing to be measured.

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  9. 9. mahdi_83 11:14 am 02/20/2010

    I agree that time is a measurement and not a thing to be measured. That is a compromise between us, the human kind, to be aware that the whole universe is in motion, so we are.

    And the tick of time, as I believe, depends on the speed at which we are moving. Normally, we are so fast that the shortest period of time during which we can complete one task is what we have named a second. That’s how fast we are in motion. Imagine we were as fast as electrons spinning around nuclei. Then, we would be able to do more things in our current one second and hence the shortest period of time measurable to us would be far less than the second.

    On other hand, gravity reduces speed and decelerates moving objects. Therefore, the closer you are to masses, the longer it will take for your watch to tick a second and vice versa.

    As a result, if we were living in a universe with absolutely no motion at all, there would never be such a measurement to name "time", because speed would not find a meaning either.

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  10. 10. jwexler 12:07 pm 02/20/2010

    What if you leave out time altogether and just suppose we go for "duration between events" then time does not exist as a "thing" to measure. Then it can be elastic or stiff in any direction or attitude (which it is).
    The span of occurrence between two or more "happenings" can be perceived as a "time" when objectively it is a "non happening" between events and looking for a measurement between these events has caused us to invent time as a quantifiable "thing" so as to help with our ability to comprehend things out of our normal ability.

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  11. 11. seymour_infenergy 9:48 am 02/22/2010

    the math works but reality is the Planet is absorbing friction stating at the center of the mass were the amount weakins / radius cubed. that means there is less friction closer to the mass than further so really you are taking the path of least resistance and are falling to the ground. this is how it is, pass it on. we need to be in space by 2100. do the math terminal velocity will show the exact rate of absorbsion of resistance I am here and I am American & plan to fix your mistake

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  12. 12. jwexler 3:40 pm 02/22/2010

    Yes, I understand your explanation.
    Suppose at one time we reached the speed of light when in the beginning we accelerated to light speed and crossed into that "whatever" dimenson. Everything still "works the same way here as it did in the other dimension. Light still travels at 186,000 miles/sec/sec. What’s the difference between this dimention and the other? Maybe it’s just because we are "here".
    Time came with us and will follow us where ever we go until we decide it really does not exist.

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  13. 13. psrandhawa 12:44 am 02/24/2010

    I too have been stumped on this one.
    The phase shift can be atributed alone to the variation in the paths travelled. Please share if you have already cleared this doubt. But I agree with you, with this information, experiments means nothing.


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  14. 14. WRQ9 2:36 pm 06/18/2010

    If time is faster in space, is it relative to the strongest gravitational force? is that a direct relationship? Is there a possibility of a harmonically resonant region between two gravitational sources? Can time based on a foreign gravitational source be reconciled to our time?

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