Ask the expert ... (string theory)

For the last few weeks I've been going in to a local secondary school on a Friday lunchtime and attempting to answer students' questions about science and the environment. When I had to be away one week I asked if they could put some down on paper and I'd return the compliment by writing down my replies. What follows is my top of the head response to the first question.

What is string theory?

Two extremely successful theories dominate modern physics. 

One of them, Quantum Mechanics (QM) covers the world of the very small and explains a huge range of phenomena from the spectrum of light emitted by different atoms, the structure of the periodic table (i.e why we have the elements we have and why they react with each other in the ways they do) all the way through to how the semi-conductor devices in our phones and computers manage to do what they do. Remembering that in science a theory isn't just a piece of idle speculation but the best available explanation of the facts, Quantum Mechanics is the most precise theory that has ever been devised (which doesn't mean, of course, that it couldn't be replaced if a more powerful theory came along)

The other, the General Theory of Relativity (GTR) not only explains the warped relationships between space, time and matter that govern the behaviour of stars and galaxies but also makes well tested predictions about the way, for example, the strength of a gravitational field affects time (to put it, simply clocks run more slowly in a strong gravitational field than a weak one). If the time signals from the clocks on the satellites that our phones use to work out our GPS coordinates weren't corrected for this then the system simply wouldn't work.   (As you move away from the Earth the gravitational field gets weaker and the clocks on a satellite run faster). Atomic clocks can now be made that are so precise that you can tell, just by looking at the time, whether they're on the floor of the lab or have been put up onto a bench.

Unfortunately these two theories, one governing the world of the very small the other that of the very large, are fundamentally incompatible. In practice, this doesn't matter very much because there are few places, apart from the inside and boundary of a black hole, where the rules of one domain need to be applied to the other. But for some physicists, those who aren't just interested in getting the sums right but also want to know that they're getting as close to the truth about the Universe as they can, they'd really like to have a single Unified Theory that covered everything. Everything from the smallest fundamental particle right the way up to the largest cluster of galaxies. 

Physicists have had over 100 years to get used to the idea that the three dimensions of space and the one dimension of time can't be considered separately from one another and instead have to be thought of as a unified 4 dimensional entity space-time. Now whilst it can make your head hurt to try to think of 4 dimensions, all a physicist has to do is describe an event using 4 numbers ( x, y, z and t) and then, for example, use the appropriate equations see what would happen to these 4 numbers if the event happened to be observed by someone travelling past Earth in a high speed rocket. 

The most promising attempts to unify these two theories have been built around the concept of a field. Just as the strength and direction of a gravitational field describes the force a mass would experience if it happened to be there, so the strength of an electric field describes the force that a charged particle would experience if it happened to be there (it turns out that magnetic fields arise when charges are in motion and the two fields, electric and magnetic, are actually the same thing but seen by someone who is either stationary with respect to the charge (electric) or moving relative to the charge (magnetic)). 

It turns out that gravity and electromagnetism are not the only forces that apply in nature. There's also the strong nuclear force, that holds an atomic nucleus together and the weak nuclear force that prevents some nuclear decays (e.g stops the neutrons in a nucleus from spontaneously decaying into a proton and an electron (which is what they do if they're left on their own)) Each of these forces is associated with it's own field and the particles, for want of a better word, that transmit the forces can all be regarded as vibrations in their respective field. Unfortunately, whilst this can be done for Electromagnetism, the Strong Force and the Weak force, it can't be done for Gravity where the big problem is explaining why gravity is so weak.*

String theory uses this idea of fields and vibrations, along with all the complicated mathematics, to describe all of the fundamental particles, and not just the force carriers, as vibrations in their own fields. By doing this they have a theory that unites QM and GTR. So, rather than just using 4 numbers to describe an event you need even more to describe the state of all these other fields and hence specify which particles are there and how they're interacting. If you think of each of these fields as being another dimension then the various fundamental particles are simply one dimensional vibrations in these different dimensions. It is proposed that gravity leaks into all these additional dimensions, which explains why it is so weak in the 3 that we're familiar with. And what one dimensional objects are we familiar with, pieces of string.

* I used to do a calculation with A level physics students about what would happen if the charge on an electron was a tiny bit different in size to that of a proton. This would mean that instead of being electrically neutral we'd each carry a net negative or positive charge (depending which was bigger). Since like charges repel I asked them to estimate the force between themselves and their nearest neighbour. For want of doing the sums again I seem top remember that if they differed by one part in a million million million (i.e not a lot) the repulsive force between two typical students 1m apart was of the order of 90 tonnes.

Assume a mass of each person of about 70kg, assume they're made entirely of water, use atomic masses and Avogadro's constant to estimate the number of water molecules and hence the total number of electrons or protons (18 of each per water molecule). look up the charge on a single electron to work out the total negative charge on the person, assume that this differs slightly from the total positive charge and then use Coulombs law calculate the force between two adjacent students.

Three dimensions into two doesn't really go