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Why can't the graviton be discovered at the LHC? Do we know its mass? Or it's massless? Or...or.. How.. So many questions..

The graviton is massless, in contrast to the excitation i was mentioning. Which is one of the reason it will hide himself at the lhc. In contrast, its excitations are expected to be very massive so that peaks are expected in the observed distributions of various observables.

ahh How can a graviton be in an excited state?

Let me try to answer concisely (and we can then move forward with details if needed).

In extra-dimensional models, gravity lives in all the dimensions. The graviton is thus attached to a field (like any particle) but this field is an extradimensional field. In terms of four-dimensional objects, this field can be seen as a series of four-dimensional excitations of various masses. The lightest of these excitations is the massless graviton, and the other are the so-called massive KK-gravitons.

Does it help?

Yes a little bit. All this quantum world makes me question everything and wanting to know everything about it!... Ohh What are the for dimensions?
You should keep posting about this subjects for the non scientific populating to learn.

The four dimensions are time and the 3D spatial world.

PS: I didn't stop posting. I just took one week of vacation. I will come back with posting next week :)

What about the four spatial dimensions of the original 12 equations theory of Maxwell? Time was not a dimension there I believe. Also, since I am asking, do think of the speed of light as a constant in any situation? thanks for your time !

What is 'the original 12 equations theory of Maxwell'? There are actually 4 of them (this is electromagnetism), and they all depend in time and space.

The speed of light in the vacuum is also constant.