The Fabric of the Cosmos. Space. Time. and the Texture of Reality Alfred A Knopf 2004
In 1905 Einstein overturned the almost mathematically perfect world of Newton by proposing a mathematical explanation for why the speed of light is always the same, no matter what the perspective is of the observer. A constant speed of light does not fit the Newtonian world. Newton’s theory predicts for example, that if you are standing on the ground and throw a baseball at 50mph at an observer, the baseball approaches the observer at 50mph. If the thrower is riding toward the observer on a bicycle at 10mph and then throws the ball at the same speed as before, baseball travels away from the rider at 50mph, and approaches the observer at 60mph.
What was unraveling the Newtonian world around the turn of the last century was the disturbing fact that experiment after experiment was measuring the speed of light at one constant speed, 670 million mph, no matter what the perspective or speed of the observer. For example, if a comet is traveling toward the earth at 1 million mph, a photon (particle of light) would appear to someone standing on the comet to reflect off the surfce off the comet and head toward earth at 670 million mph. However, to someone on earth that same photon would also appear to approach the earth at the same speed, 670 million mph. The photon would not appear to the observer on earth to travel at 671 million mph toward the earth as Newton would have predicted.
To restate: One hour after that photon reflects off the surface of the comet it would travel 670 million miles and an observer on earth could see that photon, according to time as measured by the earth observer. However, that same photon appears to travel away from the rider on the comet, by the rider’s measurement of time also at 670 million miles per hour, even though the comet rider is traveling 1 million miles per hour in the same direction toward earth. Were it possible (and according to Einstein it is not possible) for the rider on the comet to simultaneously start stop-watches on both the comet and on earth the instant the comet rider sees photon leave the comet and stop both stop-watches one hour later when that photon strikes the surface of the earth, these two stop-watches would have different times. The watch on the earth would appear to run faster. Time for each observer is literally a different value. For both observers the following equation would be true: speed of the photon (670 million mph) = time the photon took to travel from the comet to the earth (1 hour) / distance between the comet and earth (670 million mph). This would be true because that 1 hour is literally different for the comet rider and the earth observer. In a very real sense, When the photon strikes earth the Earth observer would be a little bit older than the Comet rider.
In Newton’s world time is absolute, a constant value, the same thing for all observers. The problem arises when one asks, If I throw a baseball at 50mph, what is the absolute speed of that baseball. 50mph? Well, it may be going 50mph away from you, but since the earth on which you are standing is traveling through space circling the sun at an enormous speed, to an observer on the sun the baseball is traveling far faster than 50mph. So Newton invents an absolute point of reference from which to measure the absolute speed of the baseball, whcih he calls ‘aether’. Aether is Space as in Outer Space. Think of it as absolutely motionless ‘stuff’ that the planets, stars, etc are floating in. With this concept you can now measure the ‘real’ speed of the baseball using a time reference that is the same for everyone. With the calculus Newton developed, you can now add the speed of the earth to the speed of the baseball (allowing for the circular direction of the earth around the sun), and figure in the speed of the sun as it rotates around the galaxy and you can arrive at an absolute rate of travel of the baseball, through and in relation to this unchanging ‘space-aether.’ Therefore that photon should strike earth at 671 mph as measured in relation to the unchanging, motionless ether that exists between the comet rider and the earth observer. But it doesn’t. Neither observer would ever measure the speed of the photon at anything other than 670 million mph.
Albert Einstein’s first paper ”On the Electrodynamics of Moving Bodies’ published in 1905 was Einstein’s solution, his ‘Theory of Special Relativity’. Einstein simply decided that since when you observe the speed of a photon it is always 670 million mph, never a bit slower or never a bit faster, and that no one could ever measure or observe Newton’s space aether, then ‘aether’ did not exist and that light always travels 670 million mph relative to the observer of that photon. For Einstein, for both the Comet rider and the Earth rider, no matter what their speeds are relative to one another the following equation is always true:
670 million mph (speed of light) = 670 million miles/ 1 hour.
Time, Einstein reasons, must be the variable in the equation. 100 years of experiments show that this is in fact the case. Time, as measured in relation to the speed of light, is not the same for everyone. The concept of an unchanging, unmoving universal aether from which to measure time and make time the same for everyone, doesn’t describe or predict reality. The speed of light is the absolute, not Newton’s aether. The speed of light is always 670 million mph relative to you, or to any observer, object of thing, no matter what the direction or speed. To Newton, the absolute speed of something is it’s speed relative to the motionless ‘spacestuff’.
Einstein declares, that the speed of an object’s motion through space combined with it’s motion through time is always equal to the speed of light. (p.49). It’s a sum total game, the faster you go through space, the slower you go through time and vice versa. Therefore nothing can travel faster than light, according to Einstein, because if you are moving through space at 670 million mph, there is nothing left for the ‘hour’ in the equation for motion through time. A watch worn by a particle of light would never ‘tick’.
Gravity and Accelerated Motion
Newton’s law of universal gravitation states that every point mass in the universe attracts every other point mass with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between them. This law contradicts Einstein’s dictum that nothing is faster than light. According to Newton’s law, should the moon suddenly disappear, then the tides on earth would be instantly effected. If there was a delay to allow for the transmission of this effect then planetary and stellar orbits would be effected and fail to fit neatly into Newtons equation. According to Einstein, whatever gravity is, it would travel no faster than the speed of light taking at least a second and a half after the moon vanished to reach earth and effect the tides.
Einstein’s first paper, the Theory of Special Relativity, the ‘special’ is only about what happens to bodies moving in constant velocity. In part to address the above contradiction with Newton’s theory, and account for gravity, Einstein developed his theory of General Relativity and describes accelerated motion. Whatever gravity is, it is indistinguishable from accelerated motion. If two space travelers are moving toward each other at constant velocity in the blackness of space, since they are at constant velocity they feel nothing and would be unable to know if they are standing still and the other traveler is rushing at them or vice versa. If however, one space traveler is accelerating toward the other, that traveler feels it, and that traveler knows he or she is moving. Standing on the earth feels exactly like standing on the floor of an accelerating spaceship in outer space. Thus Einstein reasoned gravity IS accelerated motion. In other words if you feel gravity, then you are accelerating and vice versa. You can’t shield yourself from the ‘force’ of gravity any more than you can shield yourself from feeling acceleration. This is his ‘principle of equivalence’ p69. If you do not feel acceleration, then you are motionless. This neatly provides a real objective reference point for discussing, or referencing, motion that dispenses with the need for Newton’s aether. In Einstein’s theory if you are in free fall off a tall building once you have reached terminal velocity, you feel no motion, and thus it is the earth that is moving upward toward you.
It is at this point (p69) that Greene faces the really difficult challenge that all Science writers face in trying to explain Einstein’s Theorys: It down to this. Space, in the vicinity of a mass space is curved. Thus an object such as a space ship, or a photon, traveling in straight line in empty space, changes direction to follow this curve that occurs in the space near the mass. Changing direction is motion you can feel, accelerated motion, or as Einstein would say, gravity. The problem is describing what exactly ‘curved space’ is. Or rather, what force, if any, is exerted by the mass that warps or curves space. No one has measured a ‘gravity wave’ yet.
Quantum Mechanics
Once you accept that the speed of light is absolute it becomes possible to measure the speed and direction of travel of the stars and galaxies from earth. A photon is unique in that it has no mass