Editing Quantum mechanics (section)

=== Relation to general relativity ===
Even though the predictions of both quantum theory and general relativity have been supported by rigorous and repeated [[empirical evidence]], their abstract formalisms contradict each other and they have proven extremely difficult to incorporate into one consistent, cohesive model. Gravity is negligible in many areas of particle physics, so that unification between general relativity and quantum mechanics is not an urgent issue in those particular applications. However, the lack of a correct theory of [[quantum gravity]] is an important issue in [[physical cosmology]] and the search by physicists for an elegant "[[Theory of Everything]]" (TOE). Consequently, resolving the inconsistencies between both theories has been a major goal of 20th- and 21st-century physics. This TOE would combine not only the models of subatomic physics but also derive the four fundamental forces of nature from a single force or phenomenon.<ref name="NYT-20221010">{{cite news |last=Overbye |first=Dennis |author-link=Dennis Overbye |title=Black Holes May Hide a Mind-Bending Secret About Our Universe – Take gravity, add quantum mechanics, stir. What do you get? Just maybe, a holographic cosmos |url=https://www.nytimes.com/2022/10/10/science/black-holes-cosmology-hologram.html |date=10 October 2022 |work=[[The New York Times]] |access-date=10 October 2022}}</ref>
[[File:String Vibrations.gif|thumb|upright=0.8|In [[string theory]], particles are re-conceived of as strings, with properties such as mass and charge determined by the string's vibrational state.]]
One proposal for doing so is [[string theory]], which posits that the [[Point particle|point-like particles]] of [[particle physics]] are replaced by [[Dimension (mathematics and physics)|one-dimensional]] objects called [[String (physics)|strings]]. String theory describes how these strings propagate through space and interact with each other. On distance scales larger than the string scale, a string looks just like an ordinary particle, with its [[mass]], [[charge (physics)|charge]], and other properties determined by the [[vibration]]al state of the string. In string theory, one of the many vibrational states of the string corresponds to the [[graviton]], a quantum mechanical particle that carries gravitational force.<ref>{{cite book |last1=Becker |first1=Katrin |last2=Becker |first2=Melanie |author-link2=Melanie Becker |last3=Schwarz |first3=John |title=String theory and M-theory: A modern introduction |date=2007 |publisher=Cambridge University Press |isbn=978-0-521-86069-7}}</ref><ref>{{cite book |last1=Zwiebach |first1=Barton |title=A First Course in String Theory |date=2009 |publisher=Cambridge University Press |isbn=978-0-521-88032-9 |author-link=Barton Zwiebach}}</ref>

Another popular theory is [[loop quantum gravity]] (LQG), which describes quantum properties of gravity and is thus a theory of [[quantum spacetime]]. LQG is an attempt to merge and adapt standard quantum mechanics and standard general relativity. This theory describes space as an extremely fine fabric "woven" of finite loops called [[spin network]]s. The evolution of a spin network over time is called a [[spin foam]]. The characteristic length scale of a spin foam is the [[Planck length]], approximately 1.616×10<sup>−35</sup> m, and so lengths shorter than the Planck length are not physically meaningful in LQG.<ref>{{Cite book |last1=Rovelli |first1=Carlo |url=https://books.google.com/books?id=w6z0BQAAQBAJ |title=Covariant Loop Quantum Gravity: An Elementary Introduction to Quantum Gravity and Spinfoam Theory |last2=Vidotto |first2=Francesca |year=2014 |publisher=Cambridge University Press |isbn=978-1-316-14811-2}}</ref>