19 October 2014
We normally consider efforts to unify Einstein’s idea of gravity with different forces as a extra fashionable development. Fashions corresponding to string idea and loop quantum gravity are seen as fashionable concepts. However actually, as quickly as Einstein offered his mannequin there have been efforts to unify it with the opposite identified drive on the time, electromagnetism.
In 1919, the identical yr that Eddington confirmed Einstein’s idea. Theodor Kaluza proposed a generalization of Einstein’s mannequin that had 5 dimensions, somewhat than the 4 (3 area + time) of normal relativity. Though the overall five-dimensional mannequin was fairly advanced, Kaluza discovered that if the fifth dimension was thought of to be considerably impartial of the others (often known as the cylinder situation), then one thing fascinating occurred. The overall equations could possibly be damaged into three elements: a four-dimensional mannequin that was Einstein’s equations for gravity, a separate set of equations that matched Maxwell’s equations for electromagnetism, and a scalar subject often known as the radion. In different phrases, the 5-dimensional idea appeared to unify gravity and electromagnetism right into a single mannequin.
In 1926 Oskar Klein utilized quantum idea (because it was understood on the time) to Kaluza’s mannequin. He proposed that the additional spatial dimension was “compact,” which might clarify the cylinder situation. Klein then discovered that the dimensions of this compact dimension was linked to the quantization of cost. Quickly this mannequin got here to be often known as the Kaluza-Klein mannequin.
On the time, the robust and weak nuclear forces had been unknown, so the Kaluza-Klein mannequin held promise as a single unified idea to explain all of identified physics. Nevertheless it didn’t transform the case. There are a number of causes for that. One is that whereas the simplifed “cylinder” type of the mannequin clearly gave rise to normal relativity and electromagnetism, the overall mannequin was deeply advanced and obscure. One other is that as quantum idea turned extra sturdy, the Kaluza-Klein mannequin was discovered troublesome to quantize. It wasn’t till the Forties that the classical model was lastly understood, a lot much less a completely quantum mannequin. Lastly, the invention of the robust and weak nuclear forces meant that the Kaluza-Klein mannequin was a partial idea at finest. As the usual mannequin of particle physics was developed (unifying electromagnetism with the robust and weak forces), the recognition of the Kaluza-Klein mannequin waned.
In some ways, the Kaluza-Klein mannequin was the string idea of its day. It had some early successes, adopted by a deep complexity that made progress troublesome and gradual. In reality elements of the Kaluza-Klein mannequin corresponding to increased dimensions and compact manifolds additionally seem in string idea and different fashionable fashions.
Though not as in style because it as soon as was, there may be nonetheless some curiosity within the Kaluza-Klein mannequin. The radion scalar subject has some similarities to the Higgs subject, and the radion may have an effect on the way in which the Higgs interacts with photons. So some researchers hope that knowledge from the Massive Hadron Collider would possibly present proof of such an interplay.
However even when such proof isn’t discovered, the Kaluza-Klein mannequin is an fascinating little bit of theoretical physics historical past. It confirmed that gravity and electromagnetism do share a deep mathematical connection, and it launched theoretical concepts which might be central to fashionable fashions.