What about gravitation?

Particle's standard model does not include gravitation. A quantum theory of gravitation is today's quest of physics, it would shed light on vacuum energy or extra-dimensions. Till his death A.Einstein has worked on theories able to unify gravitation to electromagnetism but none has been backed by experimental data. Space and time seem continuous but the theory of the loop quantum gravitation theory presents them as tiny discrete entities. On the other hand, the string theory in its attempt to unify physics'law, describes a strange world that allows strings to vibrate in a number of dimensions that is far beyond what we see in our daily life. The latest development of the string theory show that the brief period of very fast expansion that the universe underwent just after the big-bang could be the consequence of the collision of our universe with another one in a gigantic and multi-dimensional world. Another theory explains that gravitation is an illusion in our 3-dimensional world and must be seen as a consequence of particle interactions in a 2-dimensional world. (A.C.)