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When the heat of the universe was much higher it seems that the weak force and the EM-force was combined into the electro weak force.

For a layman the EM-force and the weak force look quite different, doing different things. In a simplified example:

EM-force When two electrons approach each other they emit a (virtual) photon, repelling the two electrons. Or an electron and proton approach each other, they are attracted to each other, probably also by emitting a photon (?).

Weak force An down quark emits a W boson and turns into a up quark. The W-boson decays in a electron and neutrino.

In short, in the first force is a repelling or attracting force but in the second is a decay force changing a particle into another one.

I know there are more sorts of reactions, but for simplicity I use these ones.

The question is now how can these to very different looking reactions be one and the same when the heat of the energy was much higher? It would be nice if this could be explained in a bit of layman terms also.

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  • $\begingroup$ It's not that they become similar, it's that the hidden symmetries of the are no longer hidden. The (unbroken) weak isospin and weak hypercharge interactions are attractive/repulsive interactions as well as (for the charged W bosons), flavor changing interactions. $\endgroup$ – Alfred Centauri Nov 6 '17 at 0:56
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The Quantum electro dynamics (QED) is described by, like the weak interaction, a specific quantum field theory. The model assumes different symmetries and comes to different particle species, like 3 gauge bosons in weak interaction (W+,W- and Z) or one in QED (photon). People 'merge' these theories by merging the symmetries while introducing a scalar field, allowing the theory to break down into 2 sectors at lower energy, this is often called 'spontaneous symmetry breaking'.

From a more particle physics point of view: The interactions are not so different as you say: The photon is also able to convert particles: it can annihilate electrons into their antiparticles. The W boson does a similar thing in the kaon oscillations. In many Feynman diagrams the photon can be exchanged by the Z boson -a weak gauge boson.

People believe that all these interaction we observe (including the strong force) was unified once in a single symmetry, and they are looking for a symmetry that breaks down to the 2 sectors we observe today: The electroweak and the strong force.

Tags that could help you research: Spontaneous symmetry breaking, Grand unified theories, Electroweak unification.

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  • $\begingroup$ To be sure, the unbroken gauge bosons are three SU(2) bosons $(W^+, W^-, W^0)$ and one U(1) boson $(B)$. The $Z$ boson is a certain mixture of the $W^0$ and $B$ bosons and the photon is the orthogonal mixture. $\endgroup$ – Alfred Centauri Nov 6 '17 at 0:59

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