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Most of what I know about Kaluza-Klein Theory comes from a book I read a while back by Brian Greene called "The Elegant Universe". What he said was that Kaluza had figured out a way to incorporate a fifth dimension of space-time to the equations of relativity and out popped electromagnetism. Although he wasn't really specific, I was enthralled by this idea and figured that it must be the key to the Unified Field Theory (until I realized it was discredited).

Can someone explain this theory in greater detail and why it failed?

(I am not an expert in math, so preferably words over numbers, unless absolutely necessary)

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It is not really discredited,

It is considered to be an important precursor to string theory.

It was attractive in uniting electromagnetism with gravity, but could not accommodate the weak and strong interactions. The standard model of physics developed and the two could not be reconciled (that is the discredited part) until string theories with many more extra dimensions came along.

In the 1970s, as physicists began to realize that string theory contained extra dimensions, the original Kaluza-Klein theory served as an example from the past. Physicists once again curled up the extra dimensions, as Klein had done, so they were essentially undetectable (I explain this in more detail in topic 10). Such theories are called Kaluza-Klein theories.

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You need to be careful to define exactly what you mean by Kaluza-Klein theory. Originally it was the theory developed by Theodore Kaluza in 1919, and subsequently further developed by Oscar Klein, however the term has come to mean any theory involving compactified extra dimensions. In this latter sense it is most certainly not discredited because the idea is widely used in modern theoretical physics.

The original KK theory wasn't so much discredited as abandoned because it didn't seem to be leading anywhere. Quantum mechanics had come along to describe electromagnetism and it wasn't clear how the KK theory fitted into this. Furthermore it wasn't obvious what mechanism controlled the compactification of the extra dimension i.e. what caused it to remain tightly rolled up rather than expanding to a size where it could be observed.

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  • $\begingroup$ I don't know string theory much at all, but does ST actually explain the mechanism of compactification? So far it seems to me that's also put in by hand. $\endgroup$ – Bence Racskó Jun 16 '17 at 12:18
  • $\begingroup$ @Uldreth: I believe thus is what the KKLT mechanism addresses, though I'm not sure whether KKLT is the latest word or whether there are more recent theories. $\endgroup$ – John Rennie Jun 16 '17 at 12:40

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