Conventional sequence of theories [In a chart]
A Theory of Everything would unify all the fundamental interactions of nature: gravitation, strong interaction, weak
interaction,
and electromagnetism. Because the weak
interaction can transform elementary particles from one kind into another,
the ToE should also yield a deep understanding of the various different kinds
of possible particles. The usual assumed path of theories is given in the
following graph, where each unification step leads one level up:
Theory of Everything [ToE]
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Gravitation
|
Electronuclear force(GUT)
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Strong interaction
SU(3) |
Electroweak interaction
SU(2) x U(1)Y |
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Weak interaction
|
Electromagnetism
U(1)EM |
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Electricity
|
Magnetism
|
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In this graph, electroweak unification occurs
at around 100 GeV, grand unification is predicted to occur at 1016 GeV, and unification of the
GUT force with gravity is expected at the Planck
energy,
roughly 1019 GeV.
Several Grand Unified Theories (GUTs) have been proposed
to unify electromagnetism and the weak and strong forces. Grand unification
would imply the existence of an electronuclear force; it is expected to set in
at energies of the order of 1016 GeV, far greater than could
be reached by any possible Earth-based particle accelerator. Although the simplest
GUTs have been experimentally ruled out, the general idea, especially when
linked with supersymmetry,
remains a favorite candidate in the theoretical physics community.
Supersymmetric GUTs seem plausible not only for their theoretical
"beauty", but because they naturally produce large quantities of dark
matter, and because the inflationary force may be related to GUT physics
(although it does not seem to form an inevitable part of the theory). Yet GUTs
are clearly not the final answer; both the current standard model and all
proposed GUTs are quantum field theories which require the
problematic technique of renormalization to yield sensible answers.
This is usually regarded as a sign that these are only effective field theories, omitting crucial
phenomena relevant only at very high energies.
The final step in the graph requires
resolving the separation between quantum mechanics and gravitation, often
equated with general relativity. Numerous researchers
concentrate their efforts on this specific step; nevertheless, no accepted
theory of quantum
gravity –
and thus no accepted theory of everything – has emerged yet. It is usually
assumed that the ToE will also solve the remaining problems of GUTs.
In addition to explaining the forces listed
in the graph, a ToE may also explain the status of at least two candidate
forces suggested by modern cosmology: an inflationary force and dark
energy.
Furthermore, cosmological experiments also suggest the existence of dark
matter,
supposedly composed of fundamental particles outside the scheme of the standard
model. However, the existence of these forces and particles has not been proven
yet.
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