### 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.

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Five dimensions in total, if you include time.

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### 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.

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Electricity + magnetism = electromagnetism.

### 8. Of course, the extra dimension these equations needed to work was nowhere to be seen.

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### 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

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### 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.

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### 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.

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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.

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### 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.

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### 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.

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### 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.

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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.

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### 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.