String Theory's Competition: What is Looping?
String Theory: Phantom Energy and Rogue Universes
Why Is String Theory So Important?

Ten Questions String Theory Should Answer

Any “theory of everything” — whether it be string theory or something else — would need to answer some of the most difficult questions that physics has ever asked. These questions are so difficult that the combined efforts of the entire physics community have so far been unable to answer them.

  • The Big Bang: What Banged (and Inflated)? Currently, physics and cosmology tell us that the universe as we know it started about 14 billion or so years ago, in a singularity at which the laws of physics break down.

    This breaks the question of the universe’s origin into two parts:

    • What were the initial conditions that triggered the big bang?

    • What caused the repulsive gravity of the inflation era to end?

    Even if string theory fails, any theory that attempts to expand beyond the Standard Model of particle physics will need to tackle these questions regarding the early moments of the universe.

  • Baryon Asymmetry: Why Does Matter Exist? After the big bang, there was an early difference between matter and antimatter is called baryon asymmetry (because regular matter, made up of baryons, is called baryonic matter). Presumably a theory of everything would explain why the dense energy of the early universe tended to favor — even if only by a little bit — matter over antimatter.

  • Hierarchy Issues: Why Are There Gaps in Forces, Particles, and Energy Levels? Particles themselves come in multiple varieties — far more varieties than we seem to need — and each variety jumps by large multiples in size. Instead of a smooth continuum of forces, particles, and energy, there are huge gaps. A theory of everything should explain why these gaps exist and why they exist where they do.

  • Fine-Tuning: Why Do Fundamental Constants Have the Values They Do? Physicists hope that a theory of everything would explain the precision of these values — in essence, explain the reason why life itself is allowed to exist in our universe — from fundamental principles of physics.

  • Black Hole Information Paradox: What Happens to Missing Black Hole Matter? The current thinking on the black hole information paradox is that there is a quantum system underlying the black hole, and that this quantum system never loses information, though the system can mix up the finer points in a complicated way.

    Whatever the solution, a theory of everything would have to present a definitive set of rules that could be applied to figure out what’s happening to matter (and information) that falls into a black hole.

  • Quantum Interpretation: What Does Quantum Mechanics Mean? There’s still not a single clear description of the physical principle that causes quantum mechanics to work the way it does. The majority of theoretical physicists don’t seem to believe that it’s possible to determine one interpretation as correct, and don’t even consider it as a question that needs to be answered, even by a theory of everything.

  • Dark Mystery No. 1: What Is Dark Matter (and Why Is There So Much)? Scientists know dark matter exists because they can detect its gravitational effects, but they can’t currently observe dark matter directly. Still, no one knows what the dark matter is made of. String theory contains other ideas that could explain the nature of dark matter.

  • Dark Mystery No. 2: What Is Dark Energy (and Why Is It So Weak)? There’s a lot of dark energy in the universe — about three times as much as visible matter and dark matter put together! A theory of everything would hopefully explain why the vacuum energy contains the value it does.

  • Time Symmetry: Why Does Time Seem to Move Forward? The space dimensions are interchangeable, but time is distinct because it seems to move in only one direction. Time doesn’t run backwards, and a theory of everything would need to explain this discrepancy between the mathematical symmetry of time and the physical asymmetry of time.

  • The End of the Universe: What Comes Next? And, of course, the eternal question of the fate of the universe is another question that a theory of everything would need to answer. Will our universe (and all the others) end in ice, expanding until heat dissipates out across the vastness of space? Will galaxies huddle together in dense clusters, like winter campers around a campfire? Will the universe contract together and perhaps eventually start the cycle of universal creation all over?

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