What were the clues that the weak and EM force are the same

[amenhotep]

Hello,
I've been reading a book on particle physics for the general audience, and as you might expect, I was left with a slew of nagging unanswered questions.
There was a chapter on gauge symmetry where the author described that the weak and the EM forces are the same force at some energy (>80GeV) and said that it is because of a symmetry breaking that the two forces appear unconnected today as the temperature of the universe has gone down since the big bang. I was expecting to be told the reason why the scientists (Schwinger and Glashow) thought that these forces were once merged, but the author didn't provide the reason. I was left wondering why only the weak and the EM forces were thought to be the same and not the weak and gravitational forces, for instance.
I have no background in particle physics. I'm reading just out of curiosity and would appreciate if someone can tell me the clues the mentioned scientists had to assume that the two forces were once merged.
Thanks

 

[The_Duck]

I think it was something like this: there was a bit of experimental data on the weak force and theorists were trying to come up with a mathematical theory to explain the data. They had a class of elegant theories ("spontaneously broken gauge theories") that could describe a weak force, but these theories tended to unavoidably describe a weak force *plus* another much stronger force. They realized that this was OK if they could make the extra force be electromagnetism. They found the theory that made this happen and it turned out to match experiment.

The mathematics of gravity doesn't really fit this framework unfortunately, so there was no hope of getting the extra force to be gravity.

 

[mfb]

They are not the same in the same way magnets and electrically charged objects are not the same. They can be described as two different but connected concepts in a single theory describing both.

In the same way, it is expected that the electroweak interaction (=electromagnetic and weak interaction) can be unified with the strong interaction at an even larger energy ("grand unified theory", GUT), and the ultimate step would be a unification with gravity, probably at the scale of the Planck energy.

 

[Chronos]

Unifying GUT with gravity would result in the alleged holy grail of particle physics - the theory of everything [TOE]. That, of course, assumes nature would turn her head instead of tossing us an unexpected bone along the way [as she seems to delight in doing].