Totally felt it getting stronger. I must admit, I'm putting on the mandatory 15 lbs of winter weight. Odd, gravity feels about 15 lbs
Main article: Gravitation
Gravitation is by far the weakest of the four interactions. Hence it is always ignored when doing particle physics. The weakness of gravity can easily
be demonstrated by suspending a pin using a simple magnet (such as a refrigerator magnet). The magnet is able to hold the pin against the
gravitational pull of the entire earth.
Yet gravity is is very important for macroscopic objects and over macroscopic distances for the following reasons. Gravitational force:
Is the only interaction that acts on all particles having mass;
Has an infinite range, like the electromagnetic force but unlike the strong and weak forces;
Cannot be absorbed or transformed or shielded against;
Always attracts and never repulses.
Even though electromagnetism is far far stronger than gravitation, electrostatic attraction is not relevant for large celestial bodies, such as
planets, stars, and galaxies, simply because such bodies contain equal numbers of protons and electrons and so have a net electric charge of zero.
Also, nothing "cancels" gravity, (debatable, considering that gravitons aren't as of yet an agreed upon....) since it is only attractive, unlike
electric forces which can be attractive or repulsive. There are elementary particles, such as neutrons and neutrinos, lacking electrostatic charge. On
the other hand, all objects having mass are subject to the gravitational force, which only attracts. The upshot is that only gravitation matters for
the large scale structure of the universe.
The long range of gravity makes it responsible for such large-scale phenomena as the structure of galaxies, black holes, and the expansion of the
universe. Gravity also explains astronomical phenomena on more modest scales, such as planetary orbits, as well as everyday experience: objects fall;
heavy objects act as if they were glued to the ground; animals and humans can jump only so high.
Gravitation was the first interaction to be described mathematically. In ancient times, Aristotle theorized that objects of different masses fall at
different rates. During the Scientific Revolution, Galileo Galilei experimentally determined that this was not the case — neglecting the friction
due to air resistance, all objects accelerate toward the Earth at the same rate. Isaac Newton's law of Universal Gravitation (1687) was a good
approximation to the behaviour of gravity. Our present-day understanding of gravity stems from Albert Einstein's General Theory of Relativity of
1915, a more accurate (especially for cosmological masses and distances) description of gravity in terms of the geometry of space-time.
Merging general relativity and quantum mechanics into a more general theory of quantum gravity is an area of active research. It is hypothesized that
in a theory of quantum gravity, the gravitational force would be mediated by a hypothetical massless spin-2 particle called the graviton. Gravitons
have yet to be observed.
Although general relativity has been experimentally confirmed on all but the smallest scales, there are rival theories of gravity. Those taken
seriously by the physics community all reduce to general relativity in some limit, and the focus of observational work is to establish limitations on
what deviations from general relativity are possible.