Space and time are the two basic components of the entire universe. The concept of space and time is important to the theory of relativity, which states that the speed of objects in space and time is the same; that is, manipulating one changes the other. According to Albert Einstein, "space and time are two dimensions of a unit". They are also related because time can be measured in space. Since space is infinite, it seems that space also has infinite volume. But according to Einstein, "space is always defined by a bouncing surface." Therefore space is not really infinite; instead, it is infinite only within certain parameters.
What constitutes the space and time continuum? According to Einstein, "Space is what you don't see around you." Therefore, space contains all the physical things in our world. It also contains all the information that is in our world. Time also plays an important role in our universe. Time is measured in seconds, minutes, hours, days, weeks, and—if we had infinite time—perhaps forever. In addition, Einstein theorized that time does not exist independently of the space in which it exists. Instead, both time and space are dimensions of the universe. This means that both dimensions can be modified or changed without affecting each other.
How does the universe appear to be expanding? Space is infinite, so it seems to stretch on forever. According to Einstein, "the universe is constantly expanding." The further the object is from us, the faster it appears to be moving away from us. This phenomenon is called relativity, and it describes how all objects in our universe seem to move away from each other at the same speed. Because different objects move at different speeds, some objects may appear to move faster than others over the same distance. This phenomenon was discovered by Hubble in 1929 when he saw galaxies receding from each other despite constant research and technological advances. He also noticed that galaxies got smaller as they moved away from us. Since galaxies contain billions of stars, this suggests that our universe is expanding faster than ever before.
Why is it important to be aware of your surroundings? According to Aristotle's theory of the celestial sphere, the stars are the divine sparks of God; these are examples of His inherent goodness and beauty as He continually dances through outer space. Each star is connected with a certain sphere of the sky, inhabited by the angels of God; each angel oversees a specific realm and directs heavenly events as needed. In addition, both Galileo and Copernicus revolutionized scientific thinking by proposing heliocenters, where the stars illuminate the sun instead of the Earth. Both men realized that our world—like all other worlds—is expanding explosively due to natural phenomena such as natural fusion or natural reactions (which include gravity). As our world expands, so does everything in it; it also includes the residents themselves, who must remain connected to their surroundings or risk falling completely. The world consists of both space and time; It connects us to our environment and determines how we interact with the world around us. The existence of both space and time made Einstein theorize that they are closely related because they correspond to his relativity. In addition, maintaining contact with the environment facilitates the movement of people and things in space and time. It would not be possible without the space or time around it.
The question "what is gravity?" examined in non-technical terms from Kepler's days, through Newton and Einstein's efforts, right to modern quantum gravity research.
Even today, gravity is still a mysterious force, if it is a force at all. We all know the effects of gravity and how difficult it is to work against it. Think about climbing a long staircase. What is it that tries to pull you down? Obviously it is the mass of Earth, but how does mass accomplish this pulling down? This article gives an overview of the mainstream development of the theory of gravity over the last few centuries.
Kepler's Gravity (1605)
Johannes Kepler's discovered his three laws of planetary motion in 1605, by studying the precise measurement of the orbits of the planets, done previously by Tycho Brahe. Kepler found that these observations followed three relatively simple mathematical laws, i.e.
· The orbit of every planet is an ellipse with the Sun at one of the two focus points.
· A line joining a planet and the Sun sweeps out equal areas during equal intervals of time.
· The squares of the orbital periods of planets are directly proportional to the cubes of the major axis (the "length" of the ellipse) of the orbits.
However, the physical explanation of this behavior of the planets came almost a century later, when Sir Isaac Newton was able to deduce Kepler's laws from his laws of motion and his law of universal gravity.
Newton's Gravity (1687)
In 1687 Isaac Newton published his 'Principia', including the famous three laws of motion and his law of universal gravitation, which can be briefly stated as:
· An object in motion will remain in motion unless acted upon by a net force.
· Force equals mass multiplied by acceleration.
· To every action there is an equal and opposite reaction.
· The force of gravity is proportional to the product of the two masses and inversely proportional to the square of the distance between the point masses.
Newton was uncomfortable with his own theory of gravity and in his words, never "assigned the cause of this power". He was unable to experimentally identify what produces the force of gravity and he refused to even offer a hypothesis as to the cause of this force on grounds that to do so was not sound science.
It is now known that Newton's universal gravitation does not fully describe the effects of gravity when the gravitational field is very strong, or when objects move at very high speed in the field. This is where Einstein's general theory of relativity rules.
Einstein's Gravity (1916)
In his monumental 1916 work 'The Foundation of the General Theory of Relativity', Albert Einstein unified his own Special relativity, Newton's law of universal gravitation, and the crucial insight that the effects of gravity can be described by the curvature of space and time, usually just called space-time curvature.
It is reasonably easy to accept that space can be curved – after all, we all know that a sphere has a curved surface, but how can time be 'curved'? The secret lurks in the combination of space and time into space-time. Normally, a space-time diagram is drawn with a straight horizontal spatial axis and a straight vertical time axis. Just bend the two straight axes a little and we have curved space-time.
Just like there are space geodesics (great circles) on Earth's curved surface, representing the shortest possible path between two points, there are spacetime geodesics through the curved spacetime of the universe. All material objects are always on the move through spacetime (time never stands still) and those movements are along spacetime geodesics.
As you are sitting in front of your computer, the gravitational fields of the Sun, Earth, Moon and planets, actually the matter of the whole universe, define your natural spacetime geodesic. The chair you are sitting on is pushing you ever so slightly out of your natural spacetime geodesic by applying a force to your body. Gravity is not the 'force' – it is the chair and Earth's surface that are applying forces onto you.
Remove all the forces and you enter into free-fall, which means you are following your natural spacetime geodesic until you hit the floor. This is what is happening in orbits – the International Space Station (ISS) follows its natural spacetime geodesic closely and hence suffers negligible forces.
Einstein's general relativity and its geodesics give us 'handles' on all but the most extreme gravitational situations. When gravity gets extreme, like is theorized for inside black holes and just after the big bang, even Einstein's theory of gravitation is thought to break down. This is where quantum gravity should take over.
Quantum Gravity
The latest developments attempt to unify general relativity, a theory coping well at macroscopic level, with quantum mechanics, a theory of the microscopic level and smaller. The two most promising directions seem to be 'string theory' and 'loop quantum gravity' (LQG for short).
String theory postulates string-like objects that vibrate in different modes and give rise to the elementary particles and the basic forces of nature, including the graviton, which is a virtual particle that can describe the effects of gravity. One of the problems with string theory is that it assumes a fixed background spacetime, which is somewhat in conflict with relativity theory.
Loop quantum gravity is an effort to formulate background-independent quantum gravity. It preserves many of the important features of general relativity, while at the same time employing quantization of both space and time at the Planck scale. Quantization basically means that there is a fundamental 'packet' of something that cannot be broken down further. Planck time and Planck length are the two fundamental packets of time and of space respectively.
There have been difficulties with LQG, amongst others that it has one crucial free parameter that has to be chosen in order to give result compatible with both general relativity and quantum physics. It would be better if the theory predicts the value. LQG does however give rise to gravitons, and allows gravitons to interact as expected, reproducing Newton's law of gravity.