Thread: Theories of Everything

Can someone explain to me the difference between:

-Theory of Everything
-Grand Unified Theory
-Unified Field Theory

Are they all different names for the same idea or are they different? And how does the string theory fit in here?

Thanks

Originally Posted by ScienceWizard

Can someone explain to me the difference between:

-Theory of Everything
-Grand Unified Theory
-Unified Field Theory

Are they all different names for the same idea or are they different? And how does the string theory fit in here?

Thanks

A unified field theory is a one that incorporates two or more other theories. A very simple example is that classical Maxwellian electrodynamics includes both the electric and magnetic fields. A better example is the electro-weak quantum field theory that unifies the elecromagnetic and weak forces.

A Grand Unified Theory would be one that unifies the electroweak and strong forces. No such theory currently exists.

A Theory of Everything is a theory that includes the electroweak force, the strong force and gravity. No such theory currently exists. The various string theories and their successor, M theory, are attempts at a theory of everything.

Thanks. Maybe you can shed some more light on the various unified theories.

As I understand it the unification theories try to unify the four fundamental forces (aka interactions) into more fundamental forces.

The four forces are:

-Gravity (G)
-Electromagnetism (E)
-Strong (S)
-Weak (W)

So far the Electroweak Theory has combined E and W forces into the Electroweak force (EW).

The Grand Unified Theory (GUT) would unite only the S and EW forces.

The Theory of Everything (TOE) would unite all of four forces into one force.

A unified field theory is a one that incorporates two or more other theories.

Does that mean that both the TOE and the GUT are themselves Unified Field Theories?

Also, if we now have a unified understanding of both the E and W forces under the Electroweak Theory, does that mean that Quantum Electrodynamics (QED) covers both the E and W forces? Or did the Electroweak Theory simply combine QED and a quantum theory of the weak force?

Finally I now know that there is a search for a Theory of Quantum Gravity which would combine Quantum Mechanics and General Relativity. Would Quantum Gravity have to be developed before Gravity could be unified with the GUT to form the TOE? And would the development of Quantum Gravity shed any light on the other other theories, such as the Electroweak Theory?

If you see any mistakes or misconceptions, feel free to correct me.

Thanks

Originally Posted by ScienceWizard

Thanks. Maybe you can shed some more light on the various unified theories.

As I understand it the unification theories try to unify the four fundamental forces (aka interactions) into more fundamental forces.

The four forces are:

-Gravity (G)
-Electromagnetism (E)
-Strong (S)
-Weak (W)

So far the Electroweak Theory has combined E and W forces into the Electroweak force (EW).

The Grand Unified Theory (GUT) would unite only the S and EW forces.

The Theory of Everything (TOE) would unite all of four forces into one force.

A unified field theory is a one that incorporates two or more other theories.

Does that mean that both the TOE and the GUT are themselves Unified Field Theories?

Yes, if someone can produce one.

Also, if we now have a unified understanding of both the E and W forces under the Electroweak Theory, does that mean that Quantum Electrodynamics (QED) covers both the E and W forces? Or did the Electroweak Theory simply combine QED and a quantum theory of the weak force?

Feynman, Schwinger and Tomanaga developed quantum electrodynamics.

Then Wieinberg, Salam and Glashow developed the electroweak theory that unified the electromangnetic and the weak force. 'tHooft showed that it was renormalizable. QED is not subsumed under the electroweak theory.

Finally I now know that there is a search for a Theory of Quantum Gravity which would combine Quantum Mechanics and General Relativity. Would Quantum Gravity have to be developed before Gravity could be unified with the GUT to form the TOE? And would the development of Quantum Gravity shed any light on the other other theories, such as the Electroweak Theory?

Most, but not all researchers think that a quantum theory of gravity is necessary to develop a theory of everything. Some, like Roger Penrose, think that it is quantum mechanics that will have to change rather than general relativity. But assuming that the majority is correct, a quantum theory of gravity will have to be part of a theory of everything.

Quantum gravity may simply turn out to be a consequance of some sort of unified theory. That is how string theorists look at it, as gravity seems to just be part of string theories. But they have their problems.

Development of quantum gravity would probably shed light on a lot of things. But nobody really knows since the theory does not exist. Attempts to quantize gravity have run into severe problems -- the theories are not renormalizable and hence are plagued with infinities. A theory that predicts many things are infinite is not good.

At this time there is a lot of research going on, but not a lot of conclusions. So almost anything could reasonably happen before a real TOE is produced, if it is ever produced.

You might want to read a couple of Lee Smolin's books -- Three Roads to Quantum Gravity and The Trouble with Physics for some perspective from a researcher on the front lines. You might also try The Road to Reality, A Complete Guide to the Laws of the Universe by Roger Penrose, but be warned that it is rather demanding for a "popularization".