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How/Why does GR break down in Planck Epoch?

  1. Sep 20, 2015 #1
    Wikipedia says:
    Traditional big bang cosmology predicts a gravitational singularity before this time, but this theory relies on general relativity and is expected to break down due to quantum effects.​

    Although I found other pages, I have been unable to find anything on the internet that gives a more detailed explanation.

    How/why do the quantum effects during the Planck epoch cause GR to "break down"?

    Please see my Message #3 for a rephrasing of my question.
    Last edited: Sep 20, 2015
  2. jcsd
  3. Sep 20, 2015 #2


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    If anyone knew the answer, they would surely win a Nobel prize. We have no theory that successfully unites General Relativity with Quantum Mechanics. This is one of the most important unsolved problems in physics.
  4. Sep 20, 2015 #3
    Hi @phyzguy:

    Thank you for your post. I phrased my question poorly. I am not looking for an explanation worthy of a Nobel prize. I am just looking for a clear statement about why physisists believe that GR breaks down during the Planck epoch due to quantum effects. What are the characteristics of quantum effects that could cause this break down?

    This is probably quite wrong since I have a very limited understanding of QM, but it gives the flavor of the kind of answer I am looking for.
    Space is assumed to be quantized with a quantum about the size of a single Planck unit of distance. During the Planck epoch the numerical density of particles would be so great that there would be multiple particles crowded into a single quantum of space. GR assumes that space is continuous, and the gravitation effects of such particles would not be able to bend space as GR predicts.​

    Last edited: Sep 20, 2015
  5. Sep 20, 2015 #4
    I think the problem is that GR by itself provides nice deterministic descriptions of how things behave at larger scales and is experimentally proven to be very accurate.
    At the quantum scale though things become probabalistic instead.
    Given an initial condition of X we can no longer confidently predict an outcome Y, only that the outcome Y has some statistical liklyhood, as do some other possible outcomes.
  6. Sep 20, 2015 #5


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    Quantum general relativity does work as an "effective field theory" similarly to quantum electrodynamics. In both of these cases, the theory breaks down at high energies or short distances or high curvatures, but is ok for low energy low resolution measurements.

    Here we start with a theory that has a cutoff whose value is unknown. We find that although the cutoff is unknown, we can make some predictions at low energy. But we also find that we cannot (or do not know how to) remove the cutoff (unless, perhaps, we introduce new degrees of freedom), which is why we say that quantum general relativity or quantum electrodynamics fails at high energies or high curvatures, and we cannot extend the big bang back to the "start".

    A reference is:

    The effective field theory treatment of quantum gravity
    John F. Donoghue
  7. Sep 20, 2015 #6


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    There are two main points that are relevant:
    1. Gravity and quantum mechanics disagree with one another.
    2. At the Planck scale, this disagreement cannot be ignored.

    For the first, a simple way to look at it is to look at energy. The thing that separates quantum mechanics from classical mechanics is that objects can exist in a superposition of states, so that it can be partially in energy state 1, and partially in energy state 2. But energy produces a gravitational field, and a gravitational field doesn't exist in a superposition, so what would the gravitational field of a particle in a superposition be? There just isn't any good way to answer it, and the math that we use to quantize the other forces doesn't work properly with gravity.

    The reason the Planck scale becomes important is that if you had particles with this amount of energy, they'd create blackholes. Thus it's no longer possible to describe quantum behavior at these energies without invoking gravity, but since we don't know the proper quantum behavior of gravity, we really can't know what happens there.

    Bear in mind that quantum gravity may become important at energies much lower than the Planck scale, it's just that our current math stops to make sense close to that scale.
  8. Sep 20, 2015 #7
    Hi rootone, atyy, and Chalnoth:

    Thank you all very much for your posts. They have exactly the kind of answers I wanted.
  9. Sep 21, 2015 #8


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    Tofurther compound matters, we are uncertain about the energy level at which GR and QM part ways. Experiments have been attempted to clarify the cutoff, but, none have proven definitive.
  10. Sep 23, 2015 #9
    Is it also relevant that at subatomic scales, the 'fabric of spacetime' becomes chaotic with the creation/annihilation of virtual particles and with energy fluctuations which would suggest a dynamic gravitational field which can never be calculated accurately? Overall, on larger scales, the quantum foam is averaged out to be almost flat.
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