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Why is Theory of Quantum Gravity needed

  1. Aug 5, 2013 #1
    Is it possible to explain simply what a Theory of Quantum Gravity is supposed to achieve/explain and why it is needed.
  2. jcsd
  3. Aug 5, 2013 #2


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  4. Aug 5, 2013 #3
    The main appeal to a theory of quantum gravity is that it would encompass all four of the known forces. One of the most spiffy quantum field theories, called the Standard Model, is a very successful quantum field theory which describes the electromagnetic force and the strong and weak nuclear forces. It does not describe gravity. Gravity, on the other hand, is best described by General Relativity, and there have been theoretical difficulties in combining the concepts of quantum field theory with general relativity. Quantum mechanics and quantum field theory have been wildly successful describing our small-scale experiments, such as those performed on a lab bench or in the Large Hadron Collider, whereas general relativity has success with astronomical observations and cosmological models. A quantum theory of gravity would have all those advantages, describing the fundamental behavior of microscopic entities while also giving insight into the history of the universe as a whole. Sometimes people give a quantum theory of gravity very creative names like a "theory of everything" since it would have such vast applicability.

    Really, though, there are no extant experiments/observations that "need" quantum gravity. We still have very few "experiments" that test general relativity (and these experiments are often just astronomical phenomena we have no control over), so any sort of experiment that tests the quantum gravity regime is even further out of reach, and it will stay that way for quite a while (unless you get very imaginative with ideas like the LHC producing microscopic black holes--which never turned out to actually happen.) Right now, general relativity tends to describe the things that are too big and distant for quantum to matter, and the quantum field theories describe small-scale phenomena where gravity is irrelevant, so the two theories are pretty happy sticking to their own separate domains--unless you are a theorist and enjoy speculating beyond what is observable currently.
    Last edited: Aug 5, 2013
  5. Aug 6, 2013 #4


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    Perihelion precession of mercury, light deflection by the sun, gravitational redshift on earth (at lab scale and with satellites) and for stars, gravitational time dilation (again at lab scale and with satellites), gravitational lensing from individual stars and galaxies, Shapiro delay, Frame-dragging of earth, spectroscopy of black hole accretion disks, energy loss in binary pulsars, cosmology. It is not so bad I think.

  6. Aug 6, 2013 #5
    [My apologies for contributing to a thread highjacking.]

    That's a nice list, mfb, but unfortunately the list wouldn't continue very much further beyond what you have. First of all, I wouldn't necessarily say that cosmology has been a success of GR since most of the "successful" results (concentrations of elements generated at big bang, CMB) aren't specific to any one model (For example, some people say the CMB is good evidence for inflationary cosmological models. But Alan Guth developed inflation theory using de Sitter spacetime--which is utterly unphysical since there's a zero energy momentum tensor!)

    GR also has significant trouble with the rotation curve of galaxies. I am not quite happy with the fact that so many relativists immediately accept "dark matter/dark energy" when they should all remember that Einstein's initial reason for developing relativity was that he thought "the ether" couldn't survive Occam's Razor. (At least the ether didn't make up the overwhelming majority of all the mass/energy in the universe.)

    Robert Laughlin even doubts the existence of black holes!

    I am not saying that GR is a bad theory. In fact its predictions have been very successful (even my problem with galaxy rotation can be accounted for, so really there is no conclusive evidence against GR). But when you compare GR to a science like particle physics, it seems like whoever invented GR didn't mind that there was close to zero observational/experimental evidence, and instead he was very fond of "Philosophical Principles" (e.g. the principle of locality, the principle of causality, the principle of general covariance) and mathematical beauty as his basis for the theory. (And those philosophical principles are nonscientific--in fact experiments have been done that seem to disobey the principle of locality). But a theory like the Standard Model grew up, every step of the way, with experiments, and it has made hundreds if not thousands of accurate predictions. Listing them, just for the Standard Model, would put GR to shame.

    My point is that, despite the beauty of GR, I would view it with more caution than a real experimental science. The same is true of other "observational" sciences like geology, meteorology, or macroeconomics: if you can't go to your bench and test your theory, you need to make a much bigger leap of faith that often lands you in trouble.
    Last edited: Aug 6, 2013
  7. Aug 6, 2013 #6


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    Well, particle physics is easier to test in the lab. Sure, compared to particle physics the list of tests of GR is short, and the tests have larger error bars.

    That's how most particles were predicted, too. Symmetries in the theory and Lorentz covariance.
  8. Aug 6, 2013 #7

    Super Physics Smackdown: Relativity v Quantum Mechanics...In Space

    A Massive Pulsar in a Compact Relativistic Binary
    Science 26 April 2013: Vol. 340 no. 6131.

    Confirmation of General Relativity on Large Scales from Weak Lensing and Galaxy Velocities,
    Nature 464 (March 11, 2010): 256–58.

    Detection of B-mode Polarization in the Cosmic Microwave Background
    with Data from the South Pole Telescope


    Evidence for the Accelerated Expansion of the Universe from Weak Lensing Tomography with COSMOS
    Astronomy and Astrophysics, Volume 516, id.A63, 26 pp.

    Cluster Constraints on f(R) Gravity
    Physical Review D 80 (October 15, 2009): 083505.

    Characterizing the cool kois. v. koi-256: a mutually eclipsing post-common envelope
    The Astrophysical Journal Volume 767 Number 2. 2013 ApJ 767 111

    Constraints on Modified Gravity from the Observed X-ray Luminosity Function of Galaxy Clusters.
    Monthly Notices of the Royal Astronomical Society, (December 2009): 699–704.

    Testing General Relativity and gravitational physics using the LARES satellite
    The European Physical Journal Plus, 127:133, pp. 1-7 (2012)

    General Relativity Accuracy Test (GReAT): New configuration for the differential accelerometer
    Advances in Space Research, Volume 47, Issue 7, 1 April 2011, Pages 1225–1231

    Last edited: Aug 6, 2013
  9. Aug 6, 2013 #8


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    Suppose you have a spherically symmetric quantum state and a spherical detector array. The gravitational field of this system is spherically symmetric. Suppose the quantum state decays into several particles which are registered on the detector array. Via this measurement the symmetry of the original system is broken, therefore after the measurement the gravitational field is no longer spherically symmetric.

    In order to explain how the quantum decay induces a change in the gravitational field a theory of quantum gravity is required.
  10. Aug 6, 2013 #9


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    @audioloop: There are hundreds, probably thousands of particle physics measurements, including measurements (electron spin g-factor) with a precision of 12 significant digits. It is nice to see an extension of the GR list, but that does not change my statement.
  11. Aug 6, 2013 #10
    who wish to change your statement.......

    :confused: :eek:
  12. Aug 6, 2013 #11
    to explain the origin of the universe.
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