# Mind-boggled by the string theory

1. Jul 31, 2012

### apedrape

Hi. I am fairly new to physics. I have recently read the first half of a book by Michio Kaku called “Beyond Einstein”, and im really struggling to get my head around the superstring theory.

The superstring theory seems illogical for the following reasons...

1. Nature demands symmetry. So wouldn't it make sense for the unified field to have an infinite number of planes of symmetry - that of a sphere?
2. The superstring theory suggests that we live in a 10 dimensional universe. Is it possible to have symmetry in 10 dimensions?
3. The unified field is everything and everywhere. It is both the smallest and the biggest. It fills up the universe yet it is still one. So shouldn't it be expanding outwards, just as the universe is?
4. If the unified field is expanding then its mass would never be constant. If the unified field had a mass of 0, but still possess relativistic mass, then the centre point of the unified field would have a mass of 0, since it is not moving, and the outer-most part of the field would have a mass of...erm...whatever the relativistic mass is of a mass-less-particle traveling at the speed of light (not sure about that one). Wouldn't it be able to resonate at different frequencies (like the superstring), if the mass of the sphere had a dynamic range that could unify quantum physics and relativity?

Anyway, I can't even find anything on google that clearly shows how the equation works (not that I would understand it) but my question is...

Have physicists actually taken into account that the unified field might have a dynamic mass, which represents the expanding universe?

Please forgive my ignorance, I just find it hard to learn about something that doesn't appear to conform to any kind of logic. Thanks.

Last edited: Jul 31, 2012
2. Jul 31, 2012

### tom.stoer

You shouldn't have done that!

3. Jul 31, 2012

### apedrape

ok. can you shed any light on the subject?
Why a string as opposed to a sphere? Sorry for the newbie questions but im keen to know.

Thanks.

4. Jul 31, 2012

### marcus

As Tom's comment suggests, it is a good idea to stay away from high-profile popularizers. They give a misleading impression of the field.

A college freshman physics textbook is probably a better introduction than Hawking, Kaku, Greene, if you're really keen to know about the physical universe.

I think the String program is both esoteric and in decline (in numbers of active researchers actually doing stringy research, in producing highly cited papers, in new faculty job hires...). It may not prove relevant to nature, and as yet it has no definite unique theory---no set of underlying principles or main equation. People are still asking "What is string theory?" Some other lines of research seem to be making more real progress at the moment, towards tangible/measurable goals.

But it's still an interesting thing to find out about! So if you want an overview of the String program (by a leading elder String proponent: David Gross) watch this video. Gross is enthusiastic and inspirational and optimistic about current String advances and about the program, and its future.
He is customarily chosen to give the final talk at each year's Strings conference. Or sometimes he gives the opening talk. He is the one chosen to give the main overview of the field and the vision of the future.
If you want to know about String, in a positive light, from someone talking to professionals (not to the general public) watch his talk. Here's the link:
http://www.theorie.physik.uni-muenchen.de/videos/strings2012/gross/index.html
He just gave it last Friday (27 July) so it's up to date.
It was the final talk at the Strings 2012 conference.

Last edited: Jul 31, 2012
5. Aug 1, 2012

### tom.stoer

There is a well-known expression called 'Lagrangian' from which usually a theory with all it's equations and solutions (e.g. trajectories through space) can be derived. For point particles in Newtonian mechanics this expression looks like

$$L = \frac{m}{2}\dot{\vec{r}}^2 - V(\vec{r})$$

where the dot indicates the time derivative, i.e. the first term is something like the kinetic energy, whereas the second term in *minus* the potential energy. This expression is in some sense invariant w.r.t. certain symmetries, depending on the potential V; w/o this term the free Lagrangian

$$L_0 = \frac{m}{2}\dot{\vec{r}}^2$$

is fully invariant w.r.t. rotations and translations in space. That does not mean that a specific solution is invariant, but that the whole class of solutions is invariant, i.e. that a symmetry transformation maps a solution into a (new) solution. Rotating the solution "motion of the earth around the sun" does not leave the solar system invariant; it looks different when the orbit of the earth lies in a different plane, but this 'new solar system' would still be a solution of the equations.

In that sense it's not the individual particle or pointlike object which carries the symmetry, but the whole system; in our case the symmetry is encoded in the Lagrangian.

Outlook: all what one does in string theory is to replace the motion of a relativistic pointlike particle by the motion of a string. I can explain this idea once we finished the discussion of this post ;-)

Last edited: Aug 1, 2012
6. Aug 2, 2012

### apedrape

ok thanks for your time :)