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Gravitation

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The fruits of Newtons work:

From the time that a child learns that the Earth is round, he is generaly perturbed by a point which appears illogical: Why do the Australians/Chinese, who are on the other side of the world, not fall "downwards"?
We are so used to identifying down as being "the place that objects fall to".
This force which sticks us to the ground is called the force of gravitation: gravitation because it's the same force which makes the moon gravitate around the Earth and the Earth around the Sun (and the Sun around the centre of our galaxy).
 

Newtons Law
 

It was in 1687 that the english physicist Isaac Newton made the link between stars which attract each other and familiar objects which fall to earth; like the famous apple which, in falling from a branch onto Newtons head, brought him this revelation...This is the way that wonderful legends are born in science...

The astronomical Universe is dominated by gravitation: In space two objects attract each other so much more strongly when they are massive and closer together.

These objects could be two stars or an apple and the earth. Evidently, in this last case, the earth pulls a lot more strongly upon the apple than the Universe, in reality, the apple also pulls equally on our earth! This enormous difference in mass certainly makes the action which the apple has on the earth negligible (but not nothing)!
All of this occurs as if the mass of objects was concentrated at their centre, called the centre of gravity.

The gravitational interaction, always attractive, has an infinite reach but decreases according to the inverse square of the distance.... Such is Newtons Law which is defined by the celebrated formula above.

It applies to all particles, including to the photon, which all the same has no mass. It is for this reason that the light of a star situated exactly behind another star can be .......seen.
This paradoxical phenomena is only possible because light can be deviated by a massive star. Why?
 

Gravitation and general relativity
 

the theory of General Relativity of Albert Einstein (established in 1915) tells us that a very massive object (star, black hole) deforms space around it and can therefore deviate the trajectory of photons.

The analogy the most current to help understand this curious phenomena consists of imagining our Universe passing from 3 dimensions (length + width + height) into 2 (we suppress height): We flatten the Universe to simplify it (our brain is limited). Space will then resemble a sort of elastic membrane (like a trampoline).

If you roll a small massless ball (symbolising a photon) across this membrane, its trajectory would follow a straight line because the membrane is flat. By contrast, if you put in the middle of this space-membrane a lead ball, this would sink into the membrane and deform it, more or less depending on its weight. This ball represents a massive star. If you launch a small photon-ball in the vicinity of the star-ball, you would see its trajectory curves and deviates; the light-ball could then arrive at a point situated exactly behind the star-ball. If this point corresponds with our eyes, we would thus see the source of this photon even though it was behind the star-ball.
 

Where the infinitely large meets the infinitessimally small
 

Of all of the 4 interactions of the Universe, gravitation is the one which has the weakest intensity: At the atomic scale, the influence of gravitation is negligible because the particles have so little mass. In fact, this force is cumulative, that is to say that it is the gigantic sum of the gravitation of all particles which creates a gravitational force which can be sensed at our scale.

This is only possible because gravitation only acts in one direction: it pulls but it doesn't push (no antigravity, sorry!), this force can only accumulate in contrast to electromagnetic interaction.

Dominant at the scale of the infinitely large, gravitation is though negligible at the scale accessible in particle physics. All the same, it again becomes important, even dominant, at ultra-microscopic scales of the order of  10^-35 m. The length is very particular and is called Plancks distance.
The time it takes light to travel this distance is,  in addition, called Plancks time (the name of the german physicist Max Planck) and is 10^-34s.

These two values are particularly inaccessible to our means of observation and seem to represent the limits for our  physics. Beyond these mysterious boundaries, physicists imagine a very disconcerting universe where space-time could be discontinuous or fluctuating. This universe is the same as that in the first instants of the Big Bang.
The infinitessimally small and the infinitely large seems then to meet in the kingdom of gravitation, like a snake biting its tail...

Quantum theory imagines that all interaction must happen by means of the intermediation of a boson or a mediator particle associated with a gravitational wave of very low frequency, so weak that it has never yet been detected. But this hypothetical boson has already been named a graviton.
One supposes its mass to be null and its energy to be minuscule. Physicists pray that it exists: otherwise, this would signify that quantum physics and general relativity are theories which are insufficient!