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Unraveling the Mystery: Relativity, Quantum, and Gravity Interplay

Explore the compatibility of Relativity, Quantum Physics, and Gravity, delving into the challenges and solutions offered by String Theory in bridging these fundamental concepts. Discover the enigmatic world of vibrating strings and extra dimensions through the lens of Physics.

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Unraveling the Mystery: Relativity, Quantum, and Gravity Interplay

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  1. Relativity + Quantum + Gravity • These three concepts are the basis of physics. • They contain the three fundamental constants c,ħ,G,which form a complete systemofunits. • Aretheseconceptscompatiblewitheachother? - Relativity and gravity (=general relativity)are designed to be compatible. - Relativity and quantum physics are made com- patible by quantum field theory. - Quantum physics and gravity are incompatible when trying it with quantum field theory.

  2. The weakness of gravity

  3. String theory A quantum theory of gravity: Various attempts to combine gravity and quantum physics failed. Gravity is very different from the other three forces. It is much weaker,and its boson has spin2. Infinities pose a mathematical problem: Point-likeparticles create mathematical problems when two particles meet. The forces become infinite. This problem is avoided in string theory by stretching a point out into a tiny loop, a string. Ed Witten, string guru

  4. Feynman diagrams in string theory • World lines become tubes in string theory. • They connect smoothly. • That avoids dealing with infinitely small points. point Time (ct) string Space (x)

  5. What happens at the smallest conceivable length ? Quantum physics meets gravity at thePlanck length. The uncertainty relation makes space itself uncertain. String theorists view space as a collection of vibrating strings. Brian Greene: “The elegant Universe”

  6. Strings on Nova

  7. Different vibration frequencies f Different energy quanta (Planck’s E=hf) Different particle masses (Einstein’s E=mc2) Strings can vibrate in many ways A single string can explain a whole zoo of particles:

  8. Results of string theory • A string can produce a boson with spin 2. That’s whatone needs for the graviton, the boson that mediates gravity(analogoustothephoton in electromagnetism). • So string theory produces a quantum theory of gravity. • This is our best attempt to make gravity compatible withquantumphysics withoutgettingintotroublewith infinities.

  9. Extra dimensions • String theory requires 10-dimensional space-time for mathematical reasons. • How do we get from 10 dimensions downto our four- dimensional space-time? • Two proposals: A) Make the extra dimensions tiny by folding them tightly down to the Planck length. B) Make the extra dimensions large and restrict our space-timetoa 4-dimensonalsurface(membrane) of a higher-dimensional bulk region.

  10. Small extra dimensions A huge number of different folding options. Which one ?

  11. Large extra dimensions Our 4-dimensional space-time Gravity is so weak,becauseit livesonadifferentmembrane (= brane) and gets diluted by connecting with us across the 5-dimensional bulk. (Physics Today, July 2007, p. 80) Gravity

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