New and updated, because people were misunderstanding what I meant! General relativity describes gravity as the result of....(very roughly) spacetime curvature Newtonian gravity describes gravity as the result of a force between 2 masses What is the cause of gravity in M-theory? In order for me to understand this (I'm a picky 15 year old), please don't label it as too broad, I would really like to understand it, please try and understand it.)

I do not care about the EFEs, or how M-theory reduces to general relativity, I'm wanting to know how M-theory is different from GR in its description of how gravity works.

For example, a mass, B, curves spacetime, so a negligible mass, A, not technically being acted upon by a force, follows the same path it was already following, incidentally curved in the direction of B... This is what we see as the gravitational "force". Likewise, I would like an answer like this (and this is just made up) a field X couples to field Y, causing... So and so to occur. This, in turn, creates gravity. In other words, I want a fresh description of how gravity works and generated in M-Theory.

And please don't just say it predicts a graviton, give me I for on how exactly gravity gets generated according to the theory. Please don't count this as a duplicate, any similar questions I can't seem to find the answer I'm looking for in. I've seen very many posts that do so.

  • $\begingroup$ Related: physics.stackexchange.com/q/1073/2451 , physics.stackexchange.com/q/44782/2451 and links therein. $\endgroup$
    – Qmechanic
    Aug 18, 2015 at 14:38
  • $\begingroup$ How would you like your answer? Because I'm not writing a book sized answer. $\endgroup$
    – Alec Teal
    Aug 18, 2015 at 14:43
  • $\begingroup$ Basically shortish like, "mass A curves spacetime, B follows its ordinary path (because gravity is not a force) which happens to be curved towards the primary mass.... Thus, B appears to "fall in" to body A- producing what we see as gravity".... That kind of stuff $\endgroup$ Aug 18, 2015 at 14:54
  • $\begingroup$ How does gravity take its form in mtheory AND how it limits to general relativity. In what ways is it a generalization of GR? $\endgroup$ Aug 18, 2015 at 14:55

1 Answer 1


Lets start with the definition of M theory:

M-theory brought all of the string theories together. It did this by asserting that strings are really one-dimensional slices of a two-dimensional membrane vibrating in 11-dimensional space.

So the universe which is at the microscopic level quantum mechanical, is composed of these strings/two-dimensional-membranes whose vibrations give all the particles we see in the microcosm and from which the classical nature we observe macroscopically emerges, Newtonian physics, Maxwell's equations etc.

The classical fields emerge from the underlying quantum mechanical level similar to the way thermodynamics emerges from classical statistical mechanics. Classical mathematical models are more efficient for describing the electromagnetic field macroscopically than QED which applies to elementary particles: an enormous number of photon creation and annihilation operators create the classical fields.

Analogously, the graviton, a vibration level in the M-membrane, will build up the classical gravitational field of Newtonian physics.

Here is a good answer by Eric Zaslow on how from the gravitons eventually Einstein equations emerge.

certainly there is no great organizing principle of string theory (yet). One practical principle is that the 2-dimensional (quantum) field theory which describes the fluctuations of the string worldsheet should be conformal, i.e. independent of local scale invariance of the metric. This allows us to integrate over all metrics on Riemann surfaces only up to diffeomorphisms and scalings, which is to say only up to a finite number of degrees of freedom. That's an integral we can do. (Were we able to integrate over all metrics in a way that is sensible within quantum field theory, we would already have been able to quantize gravity.) Now, scale invariance imposes constraints on the background spacetime fields used to construct the 2d action (such as the metric, which determines the energy of the map from the worldsheet of the string). These constraints reduce to Einstein's equations.

  • $\begingroup$ see also Lubos' answer here physics.stackexchange.com/q/44732 $\endgroup$
    – anna v
    Aug 20, 2015 at 10:31
  • $\begingroup$ This did give me a great insight to the problem, but how exactly is this description of gravity diffent from General relativity's? (I should have specified that in the question, may $\endgroup$ Aug 20, 2015 at 11:22
  • $\begingroup$ My bad.. (I'm sorry if I seem like a jerk, I don't intend it btw! 😄) $\endgroup$ Aug 20, 2015 at 11:23
  • $\begingroup$ in the way that thermodynamics describes temperatures and pressures with different formulae than statistical mechanics, but both describe the same thing. To use statistical mechanics mathematics where the simple thermodynamics equations work is not very smart. To use general relativity where Newton's equations work again it is not very smart. To use for general relativity the string/m theory formalism ditto. $\endgroup$
    – anna v
    Aug 20, 2015 at 12:55

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