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human
What does it mean when we say that the four fundamental forces are "unified" at high energies?

I understand the basic premise is that in the chaotic, high-density, high-energy and low-volume state of the early universe, the fundamental forces are "unified" and are really a single force acting on the universe until energy levels fall. 

But how do we know at which energy levels this unification occurs? And if we know this, why do we need different equations to describe the now-disparate forces? Shouldn't they, in principle, be describable by the same theory even at the lower energy levels we find them in today?

I am having a hard time wrapping my head around this. How does this factor in the search for a quantum gravity theory?
gpt
>What does it mean when we say that the four fundamental forces are "unified" at high energies?

It means that they are all described by a single field theory, in which a certain local symmetry (a gauge symmetry) is restored at a high enough interaction energy.  Above that energy, interactions behave a certain way which respects that symmetry.  Below that energy, the symmetry is spontaneously broken and the interactions now behave differently; they do not respect the (newly-broken) symmetry but may still respect other unbroken symmetries, allowing some aspects of the interaction to be well-described as a different interaction with a different symmetry.  This is the case, for example, in electroweak theory, in which the SU(2) x U(1) symmetry describing an electroweak interaction is spontaneously broken, leaving behind a part of the interaction which still respects a U(1) symmetry (electromagnetism), plus a part of the interaction which doesn't (the weak interaction).

(Edits for correctness.)

>But how do we know at which energy levels this unification occurs?

Experiment is the only way to *know* for sure.  But there can be strong theoretical hints.  In quantum field theory, interactions are characterized by a coupling constant related to the strength of the interaction.  Though calling it a constant is something of a misnomer, because the coupling constants "run" with the interaction energy level, becoming stronger (or weaker) as the energy increases.  One of the hints pointing at a "grand unified theory" that unifies the strong and electroweak interactions is the observation that under certain assumptions, the coupling constants for each of these separate interactions approaches the same value at a much higher energy level -- the "GUT scale" which is currently way, way beyond scales that we have the capacity to probe via experiment.

>And if we know this, why do we need different equations to describe the now-disparate forces?

Because the unified symmetry is broken at low energies; it isn't respected anymore so the character of the forces change.

>Shouldn't they, in principle, be describable by the same theory even at the lower energy levels we find them in today?

They are describable by the same theory.  The same theory just makes phenomenologically different predictions at lower energy levels when the symmetry is broken than it does at higher energy levels when the symmetry is not broken.

Hope that helps!