
Newton's view of gravity Sir Isaac Newton described gravity as a force that attracts things that have mass. His famous formula (F=Gm1m2/r^2), along with some help from Johannes Kepler, has been successfully used for hundreds of years to describe the (approximate) orbits of the planets around the Sun, and the (approximate) trajectories of things like spacecraft. This formula is also used to calculate a person's weight on a given planet. Einstein's view of gravity After Albert Einstein released his paper on Special Relativity (in 1905) he was bothered by the fact that he had to use the word "special". The special condition was that the things he was describing were only valid when zero net forces existed on the objects under discussion. In the years between 1905 and 1916, Einstein had a revelation. He came to the conclusion that if a person was being held stationary to a large mass (i.e. weight is felt) then the force that this person feels and the environment he or she experiences under these conditions is the same force and same conditions that this person would experience if he or she were out in deep space AND were being forced along by a lit rocket ship (i.e. being pushed by a rocket's engine). Also, Einstein concluded that the conditions that a person experiences when they're out in deep space and NOT being forced along by a lit rocket (i.e. floating out in space) would be the same conditions that this person would experience when near but NOT being held stationary to a large mass (i.e. free falling in a gravitational field). These two concepts are what Einstein called the "Principle of Equivalence" are really the basis for his revolutionary views of gravity. How Newton's and Einstein's view of gravity differ Newton taught that masses attract each other with a force. Here are two examples: 1  Newton's idea was that planets are held in their orbits around the Sun by a force due to the mass of the Sun and the planets, and that force is always pointing inwards (i.e. planets get pulled in the direction of the Sun, and the Sun gets pulled in the directions of the planets). 2  Newton said that a person standing on the Earth is being held to the Earth by a force due to the mass of the Earth and the mass of the person, and that force is always pointing inwards (i.e. a person gets pulled in the direction toward the center of the Earth, and the Earth gets pulled in the direction of that person). Einstein, on the other hand, taught that all things with mass (including the Sun, Earth, and the rest of the planets) cause spacetime to curve and that this curvature is an alteration the geometry of spacetime. The direction of the curvature tends to point toward the center of the largest nearby mass. The closer to this mass the larger the degree of curvature in that local spacetime. Additionally, Einstein taught that if there are no external forces acting upon an object (even those without any mass such as photons) then that object will simply follow its natural path (geodesic) through the altered geometry of the local spacetime  regardless of the degree of curvature of that local spacetime. This implies that if an object is somehow kept from following its natural path (geodesic) then a net force does exist and is measurable on that object. Here are two examples: 1  Einstein's idea is that planets are merely following the curvature of spacetime caused mostly by the mass of the Sun and to a lesser degree the mass of the planets, and that no forces are involved in keeping the planets in their orbits. 2  Einstein said that a person standing on the Earth experiences an upward force (in a direction pointing away from the center of the Earth) and that force is what keeps that person from traveling in his/her natural path (geodesic) through the curvature of spacetime. This curvature is caused mostly by the mass of the Earth. Which view of the universe is the correct one? Einstein had the luxury of being born about 200 years after Newton was born. Einstein predicted that stuff that consists of photons (which have zero mass) would be affected by the curvature of spacetime. This prediction turned out to be true and is an important tool in the field of astronomy. Newton's formula implies that only things with mass are affected by gravity, which is incorrect. For more information on this subject, search for "gravitational lensing". There are some really cool pictures of this on the web. There was a difference observed between the predicted orbit of the planet Mercury and its true orbit. This problem went away when Einstein's method of calculating curved spacetime was applied to the orbit of Mercury. For more information on this subject, search for "precession of the perihelion of Mercury". There are many other aspects of Einstein's Relativity Theory that have not been discussed here but are further proof of its soundness. For example, Einstein predicted that clocks that had a net external force on them (such as those sitting on the Earth) would run more slowly than those that had less or no force on them (such as clocks at a higher elevation and those out in space). This was experimentally verified and the Global Positioning System (GPS) compensates for this effect in order to maintain its extraordinary accuracy. 
Gravity For The Masses Site map: Introduction Definitions Newton's and Einstein's view of the universe Curvature of spacetime Summary, contacts, website revisions 
From this guy's point of view, standing on a ledge is equivalent to being propelled by a rocket ship that's out in deep space. 
From Astronaut Marsha Ivins' point of view, floating out in deep space is equivalent to free falling towards the Earth 
Newton's view of what keeps the planets in orbit around the Sun 
Einstein's view of what keeps the planets in orbit around the Sun 
Newton's view of the direction of forces when something is resting on a planet 
Einstein's view of the direction of forces when something is resting on a planet 