1. When you jump into the air, you fall back to the ground. Because gravity.
Isaac Newton first described the force of gravity. But he couldn't really explain how it worked.
2. Matter bends space, and that's how gravity is transmitted.
It was Albert Einstein who worked this out, several centuries after Newton's discovery.
3. A mathematician called Theodore Kaluza tried to explain another fundamental force – the interaction of electricity with magnetism – in the same way.
4. He imagined that the universe had four dimensions instead of three.
5. Then he worked out what equations would describe this universe.
6. Three of them were the same as Newton's gravity equations that describe our own universe.
7. And there was one more. The final equation appeared to describe electromagnetism perfectly.
8. Of course, the extra dimension these equations needed to work was nowhere to be seen.
9. Physicists Oskar Klein suggested that the extra dimension was so tiny and curled up that we couldn't see it.
10. Kaluza and Klein showed that the effect of gravity in the tiny, curled-up dimension would appear to us as electromagnetism.
11. Sadly, there was a hitch. The details of the theory didn't work out.
12. So it went away for a few years.
13. And scientists got on with working out what the universe is made of.
14. Right now we think particles like quarks (they’re the ones inside protons and neutrons) and electrons are the smallest things that make up the universe.
15. But string theory says there could be something smaller: vibrating strings of energy.
16. These strings vibrate in different patterns.
17. Each pattern creates a different particle, such as an electron or a photon.
18. String theory unites the tiniest scales of the universe with the biggest.
19. But maths says string theory doesn’t work in a three-dimensional universe.
20. It only works in a universe with 10 dimensions of space and time.
21. Which takes us back to the idea that there are small, curled-up dimensions we can’t see.
22. These dimensions fold in on themselves and intertwine.
23. The shape of the extra dimensions affects how the strings can vibrate.
24. Which affects what particles they are, and what the universe looks like.
25. If these strings exist they're a millionth of a billionth of a billionth of a billionth of a centimetre long.
26. Which means we can't see them with our current particle accelerators.
27. There are several different versions of string theory.
28. In some theories the strings exist in closed loops. In others, they are open.
29. Some string theories require the universe to obey something called supersymmetry.
30. Supersymmetry says each particle we've found so far has a partner particle.
31. Evidence for supersymmetry would be good evidence that some version of string theory might be right.
32. As would finding hidden, tiny, curled-up dimensions.
33. The imprint of extra dimensions might even be visible in radiation left over from the Big Bang.
34. But right now, physicists are divided about whether any this evidence will ever appear.
Anyone for a nap?
With thanks to this talk by Brian Greene and whystringtheory.com.
More on Kaluza and Klein's extra dimension here, and how that applies to string theory's ten dimensions.