دوستان سلام

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با تشکر - حسین جوادی

Gravitational Interactions

Gravitational interactions occur between any two objects that have energy. Mass is just one possible form of this energy. (Photons are massless, but they experience gravitational forces.)

Gravitational interactions between fundamental particles are extremely weak, at least thirty orders of magnitude (that is 1/1,000,000,000,000,000,000,000,000,000,000) smaller than the weak interaction. Hence, gravitational effects can be ignored in particle physics processes involving small numbers of particles.

Why is Gravity so Obvious to Us?

The only reason we experience gravity as an important force is that there is no such thing as negative energy and, thus, the gravitational effects of all objects add -- there is never any cancellation.

The earth exerts a much stronger gravitational pull on us than its electric pull. The electric charges in the earth are all balanced out (the positive charges of atomic nuclei screened by the negative charges of the electrons), but the masses of all the atoms in the earth add together to give a large gravitational effect on objects at the surface of the earth.

Quantum Gravity

The carrier particle for gravitational interactions has been named the graviton. However, no fully satisfactory quantum theory of gravitational interactions via graviton exchange has been identified. Thus the combination of gravity and particle physics remains a major outstanding problem. Much work in theoretical physics today is focused on this problem.

If we want to understand the big bang -- the very earliest moments in the history of the universe -- we will need to understand quantum gravity. The universe, at that time, was a very dense fluid of very-high energy particles. Gravitational interactions are comparable in strength to other particle interactions in that environment, so we need a consistent theory that can treat both interactions together to really understand that era.

String Theory

At this moment, the most promising approach to developing a quantum theory of gravity begins with the idea that elementary particles are not point like but rather are small lines or loops of energy, `strings'. It is very difficult to formulate a theory in which elementary particles have nonzero size which is consistent with relativity and with quantum mechanics.

String Theory is a mathematical theory of extended particles which has special properties that allow it to pass this consistency test. These properties predict the existence of force-carrying particles, such as photons and gluons--and gravitons--with the correct force laws. They also imply some more bizarre predictions, for example, the existence of seven extra space dimensions. Since we do not see any trace of them, even in the highest energy experiments to date, these extra dimensions of space must be very small closed loops.

Many theoretical physicists now believe that this radical modification of relativistic quantum mechanics could lead to a unified theory of all interactions. We'll see.