All the forces in the universe stem from four basic forces. These forces dictate just about every interaction that is undergone in the universe from the largest to the smallest scale in some way. Once we describe the four forces, we can look at how and why there is a question as to the Theory of Everything. It really boils down to one basic problem that, although exceedingly simple, pretty much avoids any solution we have dreamed up so far, at least experimentally.
Everything in the universe attracts everything else with a force that is a proportional to the products of their masses and inversely proportional to the square of the distance between them. That generally means that the more massive they are, the more they attract each other, at a force that depends on their distance away from each other. More simply, Mass and distance determine the force of gravity. Or even more simply, things attract each other but the further they go away from each other, the less they attract to each other. I can put it more simply, but do I really have to? Here’s something to think about: You attract the earth a little, tiny, bit to yourself. Yep. That’s right. When you jump up in the air you are attracted back to the earth by the force of gravity but a teensy, weensy, bit, you are attracting the earth up toward you. Like I said, everything in the universe attracts everything else with a force… You get the point.
Isaac Newton is the father of the modern theory of gravity. He realized that gravity on earth was the same force that dictated the motion of planets. Every piece of matter attracts every other. In Philosophiae naturalis principia mathematica he explained his theories of celestial and terrestrial mechanics using older geometric methods despite having figured them out using the Calculus. It was like using longhand, I guess. Among his other contributions to science were the Laws of Gravity and Motion, beginning work in the Calculus and amazing enough, he discovered that white light is actually a composite of many colors and that light was made of tiny units of corpuscles. Later his theory of corpuscular light was thrown out in favor of the wave theory of light but then it was recombined when Quantum Theory was put forth. Newton is considered one of the greatest scientists who ever lived. He was also quoted to have said the famous phrase, “If I have been able to see further, it was only because I stood on the shoulders of giants.” Additionally, he held the Lucasian Chair of Mathematics at Cambridge, the same Chair that Stephen Hawking holds today, a great honor.
Back to Gravity. An object responds to a force by accelerating in the direction of the force by an amount that is inversely proportional to the mass of the object. So you are not only attracted to the earth but the earth’s attraction makes you fall to the earth at a speed that increases in proportion to your mass. You can also say that in a miniscule amount the earth accelerates toward you. We see this a little better when we look at the planets.
Johannes Kepler lived from 1571 to 1630. He was a great Mathematician and he was an Astrologer, like a Renaissance Jean Dixon. In Kepler’s time Astrology was as important as weather forecasting today and probably a little more accurate. To supplement their income most Astronomers were also in the business of Astrology, although Kepler pretty much disdained the types of Astrologers who made predictions solely based on fashion to appeal to kings and people in power instead of basing their predictions on "fact." Kepler’s interest in figuring out how heavenly bodies affected earthy concerns was the inspiration for his scientific study. See, although he would never engage in the Psychic Network type of forecasting popular today, he was a firm believer in the Astrology of his time and wanted to improve it.
When Tycho Brahe invited Kepler to his observatory to become his assistant and study from his vast collection of observational data, Kepler used that data to develop his three laws of planetary motion. It should be pointed out that Tycho Brahe, for all his faults, built and maintained the most sensitive observational equipment of his time. It was providence that brought together the foremost observational astronomer and the genius of Kepler’s analytical mind in both location and time; an observation that Einstein himself would call fortunate given the pliability of the space-time continuum, or perhaps it is the anthropic quality of the universe that these serendipitous occasions occur. In any event, after Tycho’s death—resulting from holding his pee too long at a dinner party—Kepler was left to study the data at last in peace. He developed his three laws of planetary motion:
1. Kepler's elliptical orbit law: The planets orbit the sun in elliptical orbits with the sun at one focus. An Ellipse is a foreshortened circle. In other words, take a circle, put on its edge on the floor and sit on it. It becomes slightly flattened on the top and bottom and rounder on the sides.
2. Kepler's equal-area law: The line connecting a planet to the sun sweeps out equal areas in equal amounts of time. Put simply, it goes something like in this diagram. Here is a direct quote I found on Wikipedia regarding this image and Kepler’s Second Planetary Law: “Kepler's equal area law. If the time interval taken by the planet to move from P to Q is equal to the time interval from R to S, then according to Kepler's equal area law, the two shaded areas are equal. The reason it speeds up, as later found by Newton, is that the planet is moving faster during interval RS than it did during PQ, because as it approached the sun along QR, the Sun's gravity accelerated it." I think that pretty much sums it up, don’t you?
3. Kepler's law of periods: The time required for a planet to orbit the sun, called its period, is proportional to the long axis of the ellipse raised to the 3/2 power. The constant of proportionality is the same for all the planets. That means that the further the planet is away from the sun, the slower it rotates.All this from studying the observational data of Tycho Brahe! Tycho kept some tight records, that boy did. I forgot to mention that Tycho Brahe had his nose cut off in a duel and usually wore a copper prosthetic in it’s place. What does this have to do with Gravity? Well the sun keeps the planets in place by its gravitational pull. What does Tycho's nose have to do with gravity? It fell off didn't it? I think we've already established that the earth was forever changed by the gravity of a part of Tycho. Literally.
Galileo Galilei is credited with formulating the concept of inertia and experimenting with it. He said that anything that is in motion should continue with that motion unless another force acts upon it to change that motion. So if you roll a ball it should never stop rolling unless there are other forces acting upon it. It should roll forever, that way! But we know that’s not true. There are many forces including friction, gravity and so forth acting on every object on earth. But what about in space?
Think of centrifugal force. Think about it. As a matter of fact go and get a ball and attach a string to it. Then swing it around your head. While you’re doing that I’ll explain to everyone else what’s happening. See how the ball goes around and around. That’s similar to the force that controls the motion of planets around the sun. Here’s how it works. Inertial forces make the planet (or the ball) want to keep going in a straight line that is tangent to the circumference of the circular orbit. But the force of gravity (here represented by the string) pulls the planet toward the sun. The combination of forces makes the ball (or the planet) go around the sun which is the object exerting the force at the center of the orbit. So the inertial force makes the ball want to escape the orbit in a straight line but the gravitational force holds the ball in place. Combined, the two forces produce an orbit, which is the simplest way to explain why the planet earth doesn’t shoot off into space.
So what is gravity? This other guy, named Albert Einstein once wondered such things but he did it in German while working at a Swiss Patent office. He formulated the Special Theory of Relativity. It was a grand theory that among other things postulated that the Speed of Light was constant no matter what the motion of the observer relative to the source of light.
This is what he meant: If you are traveling on a train and walking toward another traveler holding a flashlight, and there is a person outside the train as it passes by, the speed of the light will be measured as the same by all observers. There is an aspect of light, because it's a wave that does change according to the observer, that's the red-shift. If a light source is moving away from you the wavelength of the light moves toward the longer, redder spectrum. If a light source is moving toward you, then the light wave is compressed toward the shorter, bluer end of the light spectrum. This is called the Doppler Effect and is similar to how a siren whines higher pitched when it's approaching and longer and lower when it's moving away from you. In this way, Hubble, deduced that the light of galaxies, shifted to the red spectrum, is moving away from us. The red shift was evidence that the universe was expanding. This odd quirk of the law of the Speed of Light in the universe also poses other problems.
Einstein also postulated that the physical laws of the universe should be the same to any observer moving at a constant speed. Because of this, and the nature of the speed of light Einstein came up with some of the wildest discoveries in his theory. Time and length relative to an outside observer’s frame of reference changes as something speeds up. For instance, the clock of a subject moving at a high but constant speed ticks slower than the clock of an observer not moving relative to the moving subject's clock. It’s complicated because both clocks are moving relative to an observer standing off the earth accounting for the rotation and orbit of the earth. Also, when measuring an object moving very fast the length of the object will seem to get shorter as it speeds up. These are called length and time dilations. Of course if you are the object going at the high rate of speed, everything around you seems to move at a normal pace. You experience time and length as normally.
Additionally, as an object speeds up its mass increases. This is why it is impossible to go the Speed of Light, because, as anyone knows the more massive an object the more energy it takes to move it and at some point when you reach the Speed of Light, mass becomes infinite so the energy needed becomes infinite. Photons, the particles of light, are massless, their entire mass is kinetic energy and since Energy equals Matter times the square of the Speed of Light, its mass is all energy.
In actuality there is no such thing as a definite stationary observer and that is one of the fantastic things about the Theory. You can only be stationary relative to an enclosed space where everything is moving at constant acceleration. You see this phenomena everyday because everything on the earth is moving relative to each other and the rotation of the earth. Because it is so large and we are so small we do not feel this movement. The few reference points that will tell you that you are moving are the celestial objects and they are large and far away too so they provide a very small point to measure against. Any observer will realize with enough time that the earth is not stagnant and that it moves in relation to the stars, sun and moon. (Except the Catholic Church.)
The Special Theory of Relativity has been proven to a fine degree. A jogger or a race car driver is not going to experience time much different than you are because the difference is miniscule and can be all but ignored on our scale. You can't live longer by running fast all the time although it is an interesting idea. We’re talking very fast. Like almost light speed (which as we know is just about impossible).
What does this have to do with gravity? That’s exactly what Einstein thought. “What does this have to do with gravity?” In German. Instead of sitting there scratching his head like you are, he developed the General Theory of Relativity to include Gravity into the mix. Einstein wondered what a person would feel if he fell off a roof. Fun right? I think he meant before the person landed. He postulated that in mid-air the person would feel weightless. Like a person in a free falling elevator, that person would not feel his own weight, until he crashed. Then, ho-boy look out, I mean, like, GRAVITY!! Comin' at ya'.
This moment, Einstein said later, was the happiest moment of his life because he realized that he could now link Gravity to his Special Theory of Relativity. In the Special Theory of Relativity it was impossible to distinguish an experiment in a uniformly accelerating frame of reference from one done in a non-accelerating reference frame in an equivalent gravitational field. That means that in a small space, accelerating at a constant speed where all the laboratory equipment in the room is accelerating at exactly the same speed and there is no other reference point to measure against, then the experiment should yield the exact same results in any laboratory on earth in a gravity field. This theory is very complicated and has many other factors, but this is the most basic explanation.
There are other variations that need to take into account the fact that a strong gravity field is not near the accelerating laboratory and there are no tidal forces affecting the space. This would distort the pure effects of the reference frame. A tidal force is when a gravity field pulls you in one direction stronger than another at the same time or at different times. The moons of Jupiter feel tidal forces as they revolve around the planet stretching the structure of the moon heating it up. The tidal forces stretching and pulling the moon keep lava flowing and the surface and core hotter than they would be normally so far from the sun’s rays. The energy of the Gravity causes friction and heat. An interesting effect of gravity.
On a more dramatic scale, the immensely strong gravitational pull of a black hole would stretch your feet out first and then your body and head as you fell in past the event horizon. Digressing even further into the zaniness of the force of gravity is a black hole itself. Interestingly, the French originally distained the term "black hole" because it sounded so risqué! Imagine that! This from the people who invented tongue kissing! They caved when it was pointed out that a black hole was just that, an area of blackness that nothing, not even light can escape. Cooler heads prevailed, and then we sent them Jerry Lewis.
What makes a sun burn is fusion, forming complex particles by combining hydrogen atoms. What prevents a sun, a very massive object, from collapsing on itself, is nuclear forces, basically propping up the weight of the sun from collapsing on itself. When a sun’s energy is burned out it does collapse on itself and one of a few things can happen.
1. It becomes a white dwarf and continues to burn slightly and then burns out.
2. A heavier star will collapse and then explode into a supernova.
3. An even heavier star will collapse into a neutron star.
4. Even further, a very heavy star will continue to collapse under its own weight and become a singularity.
A singularity is an odd result of physics where the gravity and size of the object becomes infinite. An area around the black hole is a sort of point of no return called an event horizon where gravity is so strong that even light cannot escape. Escape velocity is the velocity an object needs to reach to escape a gravitational field. Rocket scientists know this quite well because it is the speed a rocket ship must reach to escape the earth’s atmosphere and reach space. The escape velocity of a black hole is that of the speed of light. Nothing goes faster than the speed of light hence, black hole. Nothing escapes.
If you have ever seen the movie, The Black Hole, then you might think that a black hole is a swirling hole like a whirlpool. Not true exactly. A black hole is a point of intense gravity like a sphere because the sun that collapsed to cause a Black Hole was a sphere of sorts. Black holes “catching” matter in its gravity causes an accretion disk to surround the event horizon. Much like a solar system surrounds the sun but even more crowded and full of energy. The energy is produced as the matter speeds up and heats. This is said to reduce the energy of the black hole, slowing its rotation a bit. Quantum effects also are said to produce an effect that causes the black hole to shrink over an extremely long time.
This brings us back to General Relativity. Einstein, by expanding Special Relativity to include gravity, opened up the idea that light, because of E=MC2, has mass and even a photon, whose mass is entirely inertial energy, will be affected by the force of gravity. As well, the force of gravity on light escaping a gravitational field will be stretched or red shifted. Because light is theorized to be affected by gravity it opens up all kinds of strange and wonderful physical effects in the universe, like black holes, as discussed. Originally Black Holes were theoretical but science has now found evidence that a Black Hole may be at the heart of just about every galaxy.
What is the nature of the force of gravity? Einstein said that the universe is made of four dimensions: The three usual directional dimensions and time as a fourth dimension, described as space-time. A massive object, like the sun, actually bends space-time around it in much the same way a bowling ball stretches a smooth mattress toward it when placed on a bed. The surface of the bed is like space and the bowling ball is the sun. Now when any smaller object is placed on the bed in the vicinity of the bowling ball that smaller object is drawn toward the bowling ball because the bed is bent toward the center of the ball. Think of this on three dimensions, and time because of the pliable nature of time in a constantly accelerating reference frame, and a gravitational field, since according to Einstein both are indistinguishable. In a very strong gravitational field, like a black hole, time will slow to a stop, presenting a paradox not able to be resolved by physics as yet in a singularity. Gravity is the bending of space-time! The structure of the universe is actually bending under the mass of a very large object.
Gravitational lensing is another example to the strange nature of gravity in the Einsteinian universe. A star’s light can be redirected and intensified by an intervening massive object. The gravity of the massive object bends the light of the distant star around it and focuses it, if we happen to be looking in a direct line of the refracted light. This effect can cause a light source to be bent around a massive object like a galaxy and be refracted around it on two or more sides. This way we see a duplicate of the star on either side of the massive object. Astronomers have found evidence of gravitational lensing and use it to observe distant objects. The space around a massive object is bend thereby making the light follow a bent path through real space called a geodesic. An idea has been put forth to use the sun as a sort of lens to increase the light of distant stars in far future telescopic observations. This would require instruments to be sent far out into space to produce the effect, effectively.
Quantum Physics. Even though Einstein’s formulas helped create this whole wonderful and mysterious field of science called quantum physics, it became a monkey on his back. Up to the day he died, Einstein tried to merge the ideas of Quantum Physics with Relativity although I think he only concerned himself with combining electromagnetism and gravity. The reason for this problem is that on a Quantum scale, which is of the very minute, smaller than microscopic, atomic scale, gravity has negligible effect. How does one resolve gravity with the three other forces of the universe that have such strong effects? Why is gravity such a non-factor in Quantum Mechanics? Also, how do we combine a theory of three of the four major forces that depend on particle fields, quanta, to describe their effects? Electromagnetism is carried by photons, the W and Z Bosons carry Weak Nuclear Force and the Strong Nuclear Force is carried by Gluons. What particle carries the force of gravity? According to Relativity, nothing! Gravity is the result of space bending to the shape of massive objects. According to Quantum Mechanics, an as yet undiscovered particle called a Graviton carries the force of Gravity.
In some calculations, a graviton is needed because Quantum Mechanics requires a static time background to Quantum interactions and Relativity predicts a pliable space-time background.
You can see how this can’t be resolved easily. This is one of the most pressing problems in physics now. There are competing theories and ideas that will find the answer to Quantum Gravity and produce the holy grail of physics, the Theory Of Everything (T.O.E.).