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Relativity + Quantum + Gravity

Relativity + Quantum + Gravity. These three concepts are the basis of physics. They contain the three fundamental constants c , ħ , G , which form a complete system of units. Are these concepts compatible with each other ? - Relativity and gravity (= general relativity) are

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Relativity + Quantum + Gravity

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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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