## 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.

chronologyproject.com

Five dimensions in total, if you include time.

## 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.

rebloggy.com

Electricity + magnetism = electromagnetism.

## 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.

Via buzzfeed.com

## 13. And scientists got on with working out what the universe is made of.

Via buzzfeed.com

## 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.

reddit.com

It doesn't get any bigger than gravitational interactions between huge astronomical objects.

## 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.

buzzfeed.com

The partners might be heavier, which could explain why we haven't seen any yet.

## 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.