# Strings and Extra Dimensions

Chapter #21 of the course “Dark Matter, Dark Energy: The Dark Side of the Universe” by Prof. Sean Carroll. An interesting chapter to talk about these topics:

• String theory: according to Wikipedia’s entry, “String theory is an active research framework in particle physics that attempts to reconcile quantum mechanics and general relativity. It is a contender for a theory of everything (TOE), a self-contained mathematical model that describes all fundamental forces and forms of matter”. I am pretty sure that if you are reading this blog from time to time, you must have heard of String theory (this could be another “coincidence scandal“, be reading this blog, I mean… or this expression is reserved for other stuff? 🙂
• Quantized gravity: “Quantized Gravity or Quantum Gravity is the field of theoretical physics which attempts to develop scientific models that unify quantum mechanics with general relativity”. This model should be able to produce the same results that are currently known in the following two limits: a) when there is weak gravity (when we use QM and not GR), and b) when we use only GR at much larger distances than h bar (Planck’s reduced constant, i.e., when we do not take into consideration quantum phenomena). In addition to this, it should be able to explain physical situations in which both QM and GR are involved. The most famous quantum gravity theories are String Theory and Loop Quantum Gravity. I am pretty sure that I will be posting about them in future entries of this blog.
• QED: relativistic QFT (quantum field theory) of electrodynamics. It describes how matter and light interact and involves both quantum mechanics and the special theory of relativity. The founder (one of the founders) of this theory was Richard FeynmanSurely You’re Joking, Mr. Feynman!
• Planck length: length scale at which the structure of spacetime becomes dominated by quantum effects. In some theories, it is proposed that the structure of spacetime is discrete and its smallest distance is the Planck length.

• M-theory: this refers to the theory proposed by Edward Witten who in 1995 said that the 5 viable string theories were nothing but different aspects of the same theory. Please, don’t ask me what the M stands for. We Physicists don’t know! (mother? magic? mistery? matrix? master?… the W in Witten’s surname but inverted? Choose the one that you like best).
• QCD: quantum chromodynamics is the theory that explains the strong interaction that binds together quarks and gluons to form hadrons (i.e., protons and neutrons). One of the fathers of this theory was Murray Gell-Mann. If you want to see Gell-Mann in action, I strongly recommend you this TED Talk about “Beauty and Truth in Physics“. Gell-Mann, you are an incredible man, you know!
• Branes: mathematical concept that appears in string and related theories such as the M-theory. Enough for the moment…

Although I recommend purchasing the original videos from The Teaching Company, this chapter can be seen on YouTube here (part 1) and here (part 2) and here (part 3).