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http://www.askamathematician.com/2009/12/q-howwhy-are-quantum-mechanics-and-relativity-incompatible/

So I was reading this article because I'm new to this stuff and don't quite understand the ways in which quantum mechanics and GR contradict each other. Why does general relativity require space to be continuous?

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marked as duplicate by Javier, user36790, Kyle Kanos, Daniel Griscom, ACuriousMind Jan 18 '16 at 14:43

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  • $\begingroup$ they don't contradict. QFT physicists had not yet found the way to produce a consistant 2nd quantization of GR. Some try a refundation of GR to get it. What remains is the buzz, just a way to compete for grants. $\endgroup$ – user46925 Jan 18 '16 at 10:29
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From your link:

General relativity needs space to be “smooth”, or at the very least continuous. So if you have two points side by side, then no matter how close you bring them together you can still tell which one is on the right or left.

True: Continuous space time variables are necessary for Lorenz transformations to work, and the validity of Lorenz transformations has been demonstrated innumerable times. They are also a part of General Relativity for flat spaces.

Quantum mechanically you have to deal with position uncertainty. At very small scales you can’t tell which is right or left.

Not true :You can give a probability for all variables under measurement, including left and right. In an accumulation of measurements you can tell right from left.

In addition (as the name implies) QM requires everything to be “quantized”, or show up in discrete pieces.

Not true: Quantization appears under specific solutions of boundary condition problems. Space and time are continuous for quantum mechanics which is compatible with the Lorenz transformations.

In fact already there exist theories which quantize General Relativity and are proposed for particle physics experiments, string theories.

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  • $\begingroup$ If you look at the surface of a spherical planet, there are non - smooth changes in lots of GR fields at the surface. Like at the surface of a neutron star. $\endgroup$ – Tom Andersen Jan 19 '16 at 15:54
  • $\begingroup$ @TomAndersen It is space time that is continuous. The changes in the planets may look non-smooth but in atomic dimensions continuity is also there, thus the space time generated will also be continuous for small dimensions. $\endgroup$ – anna v Jan 19 '16 at 18:53
  • $\begingroup$ Sorry I was thinking of the metric - space time is continuous, its just the metric which is not. - Concerning the Schwarzschild solution: "Clearly then, if our body has radius less than C then the exterior geometry will involve an r = constant hypersurface across which the metric is discontinuous and non-finite." math.unb.ca/~seahra/resources/notes/black_holes.pdf $\endgroup$ – Tom Andersen Jan 21 '16 at 2:41

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