# Tag Info

## Hot answers tagged unified-theories

52

When we say that the forces were unified, we mean that the interaction was described by a single gauge group. For example, in the original grand unified theory, this group was $SU(5)$, which spontaneously broke down to $SU(3) \times SU(2) \times U(1)$ as the universe cooled. These three components yield the strong, weak, and electromagnetic forces ...

26

The classical theory of electrodynamics can indeed be written as a geometrical theory in a similar way to general relativity. As it happens there is a question and answer addressing just this, but it's in the Maths SE: Electrodynamics in general spacetime. Classical electrodynamics is an example of a class of theories called classical Yang-Mills gauge ...

21

Dirac thought large numbers shouldn't exist at all in physics, on purely aesthetic grounds. After all, "where would such a number come from?" Today this criterion is known as "Dirac naturalness". You might have heard about naturalness in the press, but few practitioners actually use Dirac's original form, because we know it's not very reliable. For example,...

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I’d like to answer by expanding the analogy made by @Charlie. A theory of everything would be like knowing the rules of chess. We could understand all the rules, the pieces, and their moves and interactions. But there would remain many deep mathematical problems: e.g., what’s the perfect strategy in chess? It seems unlikely that this will be solved in our ...

15

[edit: More exposition along the following lines is now at PhysicsForums at: Spectral Standard Model and String Compactifications] The algebraic formulation of geometry as it appears in Connes's spectral formulation of geometry is in fact well-known elsewhere in physics, even if for some reason it is rarely highlighted as being the same (but see the ...

15

On the quantum level, force is not acceleration. The concept of "fictitious force" makes no sense on a QFT level, because forces are interactions between quantum states, not the classical forces you might imagine. Quantum forces are not vector fields in space. The notion of "fictitious force" would mean that, e.g., the strong force is something influencing ...

15

Is there a group of (paid) researchers that work on M-Theory 24/7, hoping that someday they'll finally unify physics? Or is it more like a thing that passionate people do in their spare time? Virtually all serious physics research is done by full time professionals on salaries funded by grants or the institutions they are associated with, or both, or by ...

13

This is one of my favorite subjects, so I'll add some clarification about what we really mean when we way that the EM and weak forces are "unified". In the Standard Model of particle physics, which is part of the foundation for our present understanding of nature, there are three distinct force fields (physicists call them gauge fields). One corresponds ...

10

Well, the so called "electroweak unification" is really more of an "electroweak mixing". I want to show you how the mixing is done, so that you yourself can decide whether you prefer to call it unification or mixing. You do not need to completely understand the equations, I will try to highlight the important points. The standard model is written in the ...

10

To explain the problem properly we need to think not in terms of "quantum mechanics" (by which you probably mean a quantization of Newtonian mechanics), but rather quantum field theory (the quantization of a Lorentz-invariant field theory). The key difference is that QM considers the state of a small fixed number of particles, whereas in QFT each particle ...

10

I would say it's because of history, as each of the four forces were discovered as separate forces. I will also say it's due to how we experience them today. They are currently four separate forces. They become unified at very large energies/temperatures, which were present very soon after the big bang. But once the universe cooled the electroweak force ...

9

Mouseover: "Of these four forces there's one of them that we don't really understand." "Is it the weak force or the strong--" "It's gravity." The xkcd comic above is available under a creative commons license. Keep in mind that saying there are four fundamental forces is basically a heuristic tool to help teach a complicated reality. One of many reasonable ...

8

Yes, classically, we can unify gravity with electromagnetism. The theories that do so are the famous Kaluza-Klein theories. They are theories of pure gravity in $4+1$ dimensions rather than our usual $3+1$ dimensions. When such theories are viewed from a $3+1$ dimensional perspective, the effects of gravity in the fourth unseen dimension appear in the ...

8

Can we have unified theory of universe without solving the problem of turbulence? Yes. Because the "unified" in the name refers to the basic interactions, from which in principle everything could be explained. In practice, a hierarchy of models, with effective theories emerging from lower-level ones (think fluids laws from molecules' interactions, ...

6

We do not validate theories. We fail to falsify them. The whole scientific method is about the crucial fact that we will never have all experimental facts, and that tomorrow can always yield an observation that will invalidate what we have held to be true for centuries. And so science becomes an endless process of thinking up hypotheses, statements that ...

6

"Unification" refers to explaining two sets of phenomena (theories) which were previously urelated, and combining them into a single cohesive description. Eg: electricy and magnetism unified into electromagnetism. While those two sets of phenomena could be approximately treated (in one regime) by neglecting the other, it is important that the two ...

5

A more practical answer is that in many cases it is more useful to consider them separately. You could compare with electromagnetism. If I want to design a motor, it is much easier to work with the magnetic field generated by the coils than to invoke the whole glory of Maxwell's equations. Similarly, if I want to explain the propagation of light waves, ...

5

is it possible to consider also the other fundamental forces [...] to be fictitious forces like gravity in the framework of general relativity? No, because the equivalence principle only holds for gravity. If we want a final unification of all fundamental forces, hasn't this feature of gravity to become a feature of the other forces as well? The other ...

5

Let's take your questions in turn Theories that have more dimensions are taken seriousely, because their predictions match the experimental evidence. Of course, the fact that we live in only 4 dimensions constrains such ideas (although the idea of another large dimension is not ruled out by our direct non-observation of it (cf. Flatland, where a 3D Object ...

5

You are not far off from one of the main research topics nowadays. It is known as quantum gravity (QG), i.e., a marriage of quantum theory's (QT) and General Relativity (GR). This question may already have related questions on this site, but yours merits an answer becuase of some of what you have thought about. More of an explanation below, but the end ...

4

It is indeed possible to break $SU(3)$ to $SU(2) \times U(1)$. To see that we need to check that $SU(2)$ and $U(1)$ are subgroups of $SU(3)$. Its easy to see that $SU(2)$ is a subgroup since the first three Gell-mann matrices are given by, \lambda _i = \left( \begin{array}{cc} \sigma _i & 0 \\ 0 & 0 \end{array} \...

4

In simple language we do not have a Theory Of Everything (TOE) therefore any answer about the ultimate existence of specific laws is a tentative one. What we do have is a set of nested mathematical theories that fit observations mainly in the study of particle physics. These theories extrapolated to the extremely high energies at the beginning of the Big ...

4

The electromagnetic and weak forces have been unified into the theory of the electroweak force. The recent discovery of the Higgs boson put the icing on this particular cake. The strong force is described by the same type of quantum Yang-Mills theory as the electroweak force, however it is not unified with it. There have been several attempts at unifying ...

4

We generally regard a force as fundamental if it is mediated by a fundamental particle. For example the EM force is mediated by photons, the strong force by gluons and the weak force by $W$ and $Z$ particles. We don't have a quantum theory of the gravitational force, but we generally include it in the fundamental forces because we have no reason not to (...

4

1) I'm not aware of a precise definition of unification in QFT. In gauge theories, practitioners tend to mean that the gauge bosons transform in the adjoint representation of a single simple Lie group, for example SO($N$) or SU($N$) or $E_8$. Transforming under a single simple group means they must all have the same coupling strength at sufficiently high ...

4

I believe Pati-Salam model is what you're looking for. Quick overview: It is very different from $SU(5)$ unification. The quark-lepton unification is achieved by adding a fourth color labeled "lilac" for leptons, so e.g. electrons are lilac down-quarks. The strong force symmetry group naturally enlarges to $SU(4)$. At the same time, a Pati-Salam partner \$SU(...

4

You said, “The Law of Inertia states that any physical object resists its change in velocity”. That is basically correct. And it is not just a coincidence that charge and mass follow similar laws. There are many analogies between electrical and other (mechanical, thermodynamic, etc.) laws and properties. Example are the analogy between electrical current ...

4

The main objective in modern physics research is to find a way to unify quantum mechanics and general relativity... Not quite. This is certainly an area of modern theoretical physics research, but the majority of theorists work in areas completely unrelated to quantum gravity. ...what if quantum mechanics and general relativity are not connected/related.....

4

"Pianology" is at heart a metaphor for spectroscopy in a generalized sense, such as, e.g., the inverse problem for Lagrangian mechanics, and Can One Hear the Shape of a Drum? The criticism seems to question whether it is possible to achieve an underlying theoretical understanding from high-energy accelerator experiments, i.e. essentially a criticism of the ...

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At present they are called deterministic or hidden variable theories, and they try to show that the standard model elementary particles are composite and there is a deterministic theory from which the standard model and quantum mechanics emerge, similar to the emergence of thermodynamics to statistical mechanics. As far as I know none have been successful ...

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