• Question: what is string theory?

    Asked by charliebam to Chris, Josh, Rebecca, Rob, Susan on 22 Jun 2015.
    • Photo: Chris Armstrong

      Chris Armstrong answered on 22 Jun 2015:

      It’s a model that treats particles as one dimensional “strings” instead of point-like objects as they are treated in most fields.

      The theory in detail is way beyond me, but I believe it explains how these new string objects interact, the vibration of the string, and how they travel, as a way to explain other effects.

      Its still heavily researched and has developed into a large field of its own.

    • Photo: Susan Cartwright

      Susan Cartwright answered on 22 Jun 2015:

      String theory posits that particles are actually tiny loops of vibrating string, with the details of the vibration resulting in differences in the particle properties.

      The advantage of string theory is that it naturally deals with one of the persistent problems of modern physics: the two great advances of early 20th-century physics, quantum mechanics and general relativity, don’t play nicely together. The problem is that when you get below a certain size scale, called the Planck length, quantum mechanical fluctuations totally screw up general relativity’s spacetime geometry, producing a horrible mess called “quantum foam”. In string theory, the particles are not mathematical points, but have a finite size (which, not coincidentally, is indeed about the Planck length), so you never have to consider size scales small enough to cause said mess.

      The disadvantages of string theory are:
      – it needs 10 or 11 space dimensions, or the maths doesn’t work;
      – it is a theory valid at very high energies, and nobody has so far been able to work out how to break it down to get the low energy theory that we see (it will break down OK, but there seem to be almost uncountable ways in which it can do so, and no reason to pick one over any other);
      – consequently, it is not at all good at making predictions that can be tested by experiment.

      The extra dimensions can be dealt with – curl them up really small, so that they aren’t noticeable, just as one doesn’t really notice the thickness of a fishing line or a human hair – but the fact that there are zillions of ways to do this leads to the problem of not being able to work out what the low-energy behaviour of the theory is like.

      Also, there’s no experimental evidence for it whatsoever.

      I am not a big fan of string theory: it’s very pretty, but it doesn’t seem to do much. I tend to regard it as a branch of pure mathematics that thinks it’s physics, rather than as actual physics.

Comments