# Why did Standard Model never sense a requirement to include gravitational quantum? [closed]

Standard Model is advanced (lorentz invariant) version of Quantum physics. It tried to include everything which came in the way while understanding quantum world. It even didn't bother to include even Higgs Boson which was hypothetical at that time. Did they never find gravitation in the way of other quantum interaction.

Note: I know, there were many unsuccessful attempts to add gravitation with SM to make Theory of Everything. My question: Why didn't Standard Model keep gravitation as raw ingredients (with unresolved relationship with others)?

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## closed as primarily opinion-based by Dilaton, Dan, Waffle's Crazy Peanut, user1504, Manishearth♦Jul 11 '13 at 4:20

Many good questions generate some degree of opinion based on expert experience, but answers to this question will tend to be almost entirely based on opinions, rather than facts, references, or specific expertise.If this question can be reworded to fit the rules in the help center, please edit the question.

Firstly, the concept of the Higgs actually came into play when the SM was being integrated--it fixed a problem with the electroweak interaction. Secondly, the SM wasn't "planned"--it sprouted bit by bit from the unification of forces. Thirdly, it's not as simple to "add" gravitation as a raw ingredient--there's a lot of mathematics behind it, and anyway, like I said, the model was built when the theory behind each force was being unified. – Manishearth Jul 6 '12 at 6:17
Also, keeping gravity with an unresolved relationship with others makes no sense--these models attempt to explain the working of the forces mathematically. "Keeping gravity with an unresolved relationship" is exactly the same as "assuming that some day, gravity will be added". I see no difference, can you clarify on that? – Manishearth Jul 6 '12 at 6:19
I disagree with the first sentence of the last paragraph. It is too early to decide if the attempts for a TOE are successful or not; and they are still work in progress ... – Dilaton Jul 6 '12 at 18:27
The standard model is just defined as the low energy theory excluding gravity. It isn't that people willfully excluded gravity, that's just not what the theory is describing. It is therefore incomplete in this way, but it is complete in every other way (so far). The low energy theory is an important thing to understand, so people weren't wasting time making the standard model. Also, the Higgs was a part since the beginning in 1967. What's the question exactly? – Ron Maimon Jul 6 '12 at 18:35
@SachinShekhar: Huh? What does that mean? The gravitational field is not a ghost. It has positive propagators in the physical part. This is also a completely different question. I am voting to close. – Ron Maimon Jul 7 '12 at 5:03

Read this link to get a framework of where the SM stands as far as interactions go. The SM is a mathematical shorthand of our data for the microcosm of quarks and leptons.

Look at table 1 and you will see that at the level of quarks and leptons the gravitational interaction is so weak that it is completely irrelevant and certainly its effect on the values used in the standard model cannot be measured with our present experimental accuracies.

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In Quantum Electrodynamics, things are simple, because the photons are uncharged, so they themselves do not interact through Electromagnetdism, . Of course, there are still divergencies, and you still do need to renormalise, but things are very simple, compared to...

Quantum Chromodynamics,. Things now get much more complicated. Well, the Lagrangian Density takes almost the same form as in Quantum Electrodynamics, but compute anything, is a horror. Why? Gluons themselves have colour charge. SSo the additional gluon potentials, which ignoring constants, is $A^\mu=\nabla^\mu-\partial^\mu$ in the contra - variant form, have colour charges themselves, and interact through the strong force themselves. And thus, there is a lot of problems with the divergencies, but still, it is renormalisable.

Since everything was renormalisable, strong coupling, weak coupling, and they could make a TOEEG (theory of everything except gravity), called the standard model, they thought the same elegance could be extended to gravity, to general relativity. I mean, the standard model does incorporate special relativity, so why not general?

The most obvious way to do so, was to introduce a gravitational quantum, the graviton. But sadly, it was done in a very naive way. The gravitons themselves contributed to the gravitational field, just like in Quantum Chromodynamics, and whatever it is, the end result diverged. But unlike in Quantum Chromodynamics, for Quantum Gravity, it was non-renormalisable,.

So, a less naive theory of gravitons is required, and such a theory is string theory.

So, thus, it was not about the standard model not sensing a requirement to include gravity, it couldn't. One needs string theory for that; to make everything known to man, and everything that is true, to come out naturally.

There are, of course, other TOQGs (theories of quantum gravity), but the problem is that most of them are lorentz asymmetric (cf. Lubos Motl's criticism to Loop Quantum Gravity), they don't allow for other forces (interactions) other than gravity, they are mathematically inconsistent, etc. Or they just lead to string theory (e.g. supergravity, kaluza - klein theory).

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