# String theory pros and cons

Today I want to talk about string theory.
Not because I think that’s so interesting but because I think it’s not interesting,
and I wish we could stop talking about it.
String theory is an idea that dates back to the 1970s and it was originally proposed as
a way to describe the force that holds atomic nuclei together.
This is known as the strong nuclear force and it did indeed describe some of the observed
behavior correctly.
It turned out pretty quickly though, that a different theory, which is now called quantum
chromodynamics, was better at describing the observations.
String theory, however, turned out to have different benefits.
The idea of string theory is that all the particles are really made of small strings and all the interactions
between them are also exchanged by strings.
This has the consequence that there is a universal force that is acting on all particles.
Now, the only such universal force that we know of is gravity.
And at the time physicists were trying to combine gravity with the other forces in the
standard model and string theory seemed a very very promising way to do this.
This was in the 1980s and it's the background on which for example Steven Weinberg wrote
his book “Dreams of A Final Theory”.
They were thinking that they were close to combining all of the fundamental interactions into
one Theory of Everything that string theory was supposed to be.
However, it did not work out as planned.
The first problem that appeared is that string theory only works in nine dimensions of space.
We have only seen three.
That brings up the question, well, what do we do with the other six?
The idea that string theorists came up with is that they curled up these additional dimensions
to very small radii.
Now, if you want to probe something that is very very small you need a high energy.
So it might just be that we have not been able to produce energies high enough to probe
these small additional dimensions and that explains why we haven’t seen them.
The next problem that showed up in string theory is that it requires a new kind of symmetry,
which is known as supersymmetry.
Supersymmetry adds a partner particle to all of the known particles of the standard model.
Problem is, we have not seen those particles.
So what theorists suggested at the time is that the partner particles are much heavier
than the already known particles.
This breaks the symmetry, but it’s something that you can do.
And again it explains why we haven’t seen them, because it takes high energies to produce
heavy particles, so we might just not have been able to build large enough colliders
to see supersymmetric particles.
Making the supersymmetic particles heavy, however, did not entirely solve the problem
with supersymmetry.
That’s because not only do you have these additional particles, but you also get additional
kinds of interactions that were not seen.
These are called flavor changing neutral currents, and it was known already in the 1990s that
those were not there.
To fix that problem, theorists assumed that there is a new kind of symmetry on top of supersymmetry.
This new symmetry is known as R-parity and it basically forbids these new kinds of interactions.
So, that made the theory compatible with experiment again.
Next problem that appeared is that sometime in the 1990s, astrophysicists went and measured
the cosmological constant.
String theory works best in a space with a negative cosmological constant.
Unfortunately, the cosmological constant in our universe turns out to be positive.
But, well, it only took a few years and string theorists had come up with a fix for that problem.
Another problem that appeared fairly early in string theory is that in contrast to the
original hope, string theory does not uniquely combine the standard model with gravity.
Instead it has a huge number of different solutions, each of which has a different particle
content and has different constants of nature.
For this reason theorists cannot actually use string theory to predict what particles
we see in our universe and what are the constants of nature that we actually measure.
Now, there are some string theorists who say that we can use string theory to predict what
happens to information that falls into a black hole.
Unfortunately, the calculations that they can actually do are for black holes that live,
once again, in a space with a negative cosmological constant.
And that’s not the kind of universe that we actually live in.
How to do the calculation for a space with a positive cosmological constant, nobody really knows.
Now I told you that string theory contains gravity, and that’s true.
But it’s not gravity the way that we know and like it.
Instead it’s gravity plus a large number of additional fields.
And those additional fields would have played a role in the early universe and they could
also presently play a role for the expansion of the universe.
Problem is, we have not seen any evidence for the existence of these additional fields.
Now again, you can make it work within string theory.
You can come up with reasons why we have not seen these fields so far.
But that’s yet another assumption that has to enter the theory to avoid that it runs
into conflict with experiment.
Now the newest problem that string theorists are discussing is that the fix that was supposed
to solve the problem with the negative cosmological constant may not be working after all.
Or maybe there is a problem with the problem for the fix of the problem.
Well, whatever way it turns out, I am pretty sure that string theorists will find a way
to make observations compatible with their theory, because that’s what they’ve been
doing for the past 40 years.
Now those were a lot of negative points and that’s somewhat unfair because string theory
has not been entirely useless.
String theory has, for example, helped to prove certain theorems that relate abstract
mathematical spaces.
So it has made some contributions to mathematics.
It is also the case that string theory is mathematically very closely related to quantum
field theory and quantum field theory is the kind of theory that we use for the standard
model and also to describe large number of particles.
In these areas string theory may help solve certain problems that we have with doing the calculations
in the quantum field theory.
So far, this approach has not be terribly successful, but maybe one day it will be successful.
Even if that is so, however, let us be clear that this would have nothing to do with string
theory as an approach to a unified theory of the interactions and for quantum gravity.
So, string theory may once have been a promising approach to a theory of everything.
It no longer is.
If you want to know more about string theory, I recommend you read Nadis and Yau’s “Shape
of inner space” or Joseph Conlon’s “Why string theory?”
If you want to know more about what is going wrong with string theory, I recommend you
read Peter Woit’s book “Not Even Wrong” or Lee Smolin’s book “The Trouble with Physics”.
And if you want to know generally what is wrong with the foundations of physics, I recommend
you read my book “Lost in Math”.
Und das gibt’s jetzt auch in der Deutschen Übersetzung.
See you soon.