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The field of philosophy is the study of all that it logically possible.  Philosophers aim to discern the nature of anything and everything that is logically feasible.  Science is a very particular form of philosophy that works to deduce only what is physically possible, that is to say, only things that can possibly happen in the real world.  Keeping this in mind, science differs from philosophy as a whole in that philosophy concerns itself with things that are logically possible but not necessarily physically possible.  Indeed, thought experiments are pertinent to both science and philosophy as a whole because they tend to lead to experiments in the lab or in the field.  For instance, Einstein’s thought experiments into the nature of relativity were purely philosophical until scientists could devise tests that would eventually vindicate his assertions and his philosophical ideas could be confirmed scientifically. Einstein’s theory of general relativity would go on to synthesize Sir Isaac Newton’s law of universal gravity with his own previous theory of Special Relativity.  But before Einstein’s theory could be vindicated, scientists were scrambling to set up experiments to catch up with the predictions made in Einstein’s thought experiments.  These classical tests of general relativity are cited as compelling evidence for Einstein’s theory, but future tests have also pointed out limitations in general relativity.

When Einstein attempted to reconcile Newton’s Law and observed reality with his theory of special relativity, he saw that special relativity only made sense when one excluded gravity he realized the equivalence principle that could help make sense of a general relativity that could account for the effects of gravity.  Einstein came to the realization that someone’s frame of reference would essentially be the same if they were held by the gravity of a massive object or accelerating at a sufficiently high speed.  The thrust of a rocket accelerating at 9.8 m/s/s could mimic the experience of the pull of gravity on Earth.  Einstein also surmised that gravity warps the fabric of space-time.  But if this were true, it would also also mean that light does not necessarily travel in a straight line through space-time.  When light travels through the curvature of space-time caused by the gravity of a massive object, its path will bend around the object along the curvature of space-time.  This is called a gravitation lens and it was first verified during a solar eclipse in 1919 by Sir Arthur Eddington when he observed the light from stars passing close to the Sun was slightly bent, so that stars appeared slightly out of position.  Experiments around the world were also set up to confirm Einstein’s hypothesis.

General Relativity predicted that gravity bends the light of a star, a phenomenon that astronomers have verified observationally.

The Two-Body Problem in General Relativity describes the movement of two objects in a system that pull each other by their own gravity.  Newton was able to show that two point masses attracting each other would each follow perfectly elliptical orbits; Newton’s law of universal gravitation explains that the larger of the two masses would move in a smaller elliptical orbit than the other mass.  Two Body Problem will move about in a smaller ellipse and if the difference between masses of the two bodies are immense enough, scientists can essentially ignore the the larger body’s ellipse.  For instance, when we compare the mass of the Sun to the mass of Mercury, the mass of the Sun is so great that we can ignore the elliptical movement of the Sun that comes from the gravitational pull of Mercury.  But when astronomers viewed the elliptical movement of Mercury around the Sun, they found that something unusual.  Mercury’s orbit around the Sun was off-kilter a bit; its orbital precession was faster than predicted.  Among other possible theories to explain Mercury’s wobble, some scientists thought our solar system was missing a planet that could be influencing Mercury’s gravity.  It turned out general relativity introduces a third force that additionally attracts Mercury to the Sun, successfully explaining Mercury’s odd precession.  Future tests on the orbits of Venus and Earth solidified the theory general relativity by discovering the force of general relativity acting on their orbits as well, albeit to a smaller extent.


General relativity is able to explain the increased rate of precession in the perihelion of Mercury’s orbit.

Einstein’s idea were so ahead of their time that in some cases it took technology a while to catch up.  The final classical test of Einstein’s theory of relativity was set up in 1959.  Robert Pound and his graduate student Glen A. Rebka Jr. sought out to  confirm the gravitational red shift of light Einstein predicted as a consequence of the equivalence principle in 1907. When an atom moves from an excited state to back its base state, it emits a photon with a specific frequency.  And when that same atom in its base state is struck by a photon with the same frequency, it will absorb that photon and jump to the excited state.  If the photon’s frequency and energy is different by even a little, the atom cannot absorb it.  When the photon travels through a gravitational field, its frequency is reduced, or red shifted, so the atom couldn’t absorb that photon.  But if the atom moves with the proper speed relative to the photon travelling through a gravitational field, this will cancel out the gravitational red shift and the atom will be able to absorb the photon.  Pound and Rebka set up gamma ray emitters at the top and the bottom of the Jefferson Tower on Harvard’s campus.  When the emitted photons from the top of the tower were measured at the bottom, their wavelengths were decreased, or blue shifted, by a small amount and when photons emitted from the bottom were measured at the top, their wavelengths were red shifted by the same amount.  This minute change in frequency is due to gravity, as predicted by general relativity.

So after 96 years, general relativity is still standing, but clearly has limitations that need to addressed.  General Relativity appears to be incompatible with quantum mechanics, though, the leading physical theory that explains sub-atomic phenomena.  Physicists are attempting to reconcile the two theories through a proposed particle called the graviton.  This elementary particle is purely hypothetical, though.  String theory predicts that the graviton should be a massless boson that carries the force of gravity.  If a massless particle with the necessary spin of 2 is ever discovered, it would have to project a field indistinguishable from the force of gravity.  Such a particle would be the elusive graviton physicists are searching for and such a discovery could reconcile the two worlds of what Brian Greene calls the very big and the very small.