♣♣♣♣♣♣♣♣♣ Theory of Relativity ♣♣♣♣♣♣♣♣♣ In the early decades of the 20th century, a young Swiss patent clerk named Albert Einstein published the theory of relativity and changed the face of physics and astronomy forever. The theory of relativity is perhaps the most successful development in the history of science in terms of its agreement with experimental results and its ability to predict new phenomena - only quantum mechanics can claim to compete with its success. Einsteins theory immediately explained some of the major problems in the physics and astronomy of his day, and it has continued to explain new developments that were not even hinted at 90 years ago, including the existence of black holes and recent observations in cosmology. Yet, accepting the theory of relativity requires us to throw out almost all of our previous notions about the universe, as well as most of what we would call common sense. Space and time, which to humans locked on planet Earth appear to be a fixed, unchanging background upon which the events of the cosmos play out, are in fact anything but. Empty space can contract, expand, or curve depending on how close you are to a massive object, and the rate at which time passes can change as well. Space and time can even change depending on who is measuring them; the hands on a clock will look smaller and tick slower the faster the clock is moving with respect to you. ♣♣♣♣♣♣♣♣♣♣♣ Applications of Relativity ♣♣♣♣♣♣♣♣♣♣♣ The theory of relativity is required whenever we study objects that are either (a) moving in a strong gravitational field, or (b) moving near the speed of light. If (b) is true but not (a), we can get away with using a simpler version of the theory called special relativity; historically, this is what Einstein developed first, while the more encompassing theory of general relativity came later. In everyday life on Earth, neither (a) nor (b) is true, so we usually dont have to worry about relativity at all. Nonetheless, its effects can still be important when extremely high precision is needed; for example, one of the most crucial applications of relativity involves the Global Positioning System (GPS), which wouldnt work at all if we didnt take relativistic effects into account. If youve ever used a GPS receiver, youve benefited directly from Einsteins theory of relativity! ♣♣♣♣♣♣♣♣♣♣♣♣♣♣♣ Moving in a Strong Gravitational Field: ♣♣♣♣♣♣♣♣♣♣♣♣♣♣♣ One of the most amazing aspects of the theory of relativity is that it completely changes the way we understand gravity. Scientists have known for a long time that gravity is unusual. Take a bunch of wooden blocks, some big and some small, and sweep them off a table; they will all fall at the same speed and hit the ground at the same time. Glue a piece of metal to each and attract them with a magnet, though, and they will move at different rates; try to pull them with a rope, and youll have to pull harder to get the bigger objects up to speed. Why is it that gravity, and gravity alone, is able to adjust itself to pull everything towards the Earth at the same rate? Einstein answered this question in a revolutionary way. According to Einstein, gravity is not a force which pulls on things; rather, it is a curvature of space and time caused by the presence of a nearby massive object (like the Earth). (★See the attached image of the blue planet making a depression in the fabric of space-time★) When something comes along and moves past the massive object, it will appear to be pulled towards it, but in reality, it isnt being pulled at all. It is actually moving along the same straight line that it was moving along in empty space, but this straight line will now look like it is curved, due to gravitys warping of the underlying space-time continuum. ♣♣♣♣♣♣♣♣♣♣♣♣♣♣♣ Curved space: a simple analogy ♣♣♣♣♣♣♣♣♣♣♣♣♣♣♣ If the above paragraph doesnt make sense (and it is unlikely to!), consider the following analogy having to do with a curved space you are more used to: the surface of the Earth. Suppose you are in Ithaca, New York (home of Cornell University) and want to travel to Rome, Italy, which is approximately due east of Ithaca and a quarter of the way around the globe. (★See the 1st attached image★) You might think the best way to get there is to start off heading east and keep going straight until you reach Rome, as shown in the red path on this map:In fact, though, if you start off heading east and continue to go straight, carefully putting one foot in front of the other, you will wind up taking the blue path; by the time youre as far east as Rome, youll be somewhere in western Africa, near the equator! (If you dont believe this statement, try it out with a globe and a piece of string. Stretch the string tight so that it is forced to be straight, then place it east-west across New York. The rest of the string will pass through Africa and cross the equator, just like the blue path in the above map.) Whats going on here? Nothing too complicated, really. As we all know, the surface of the Earth is round, but when we try to represent it on a two-dimensional map we have to flatten it out. In the process of this flattening, it turns out, things get screwed up, and some lines which are actually straight (like the blue path) look curved, while some lines which are actually curved (like the red path) look straight. According to Einstein, the same thing happens near a massive object, only the curvature happens to something that has four dimensions (the space we live in plus one dimension of time) rather than two dimensions (the surface of the Earth). Space and time near a massive object are curved, but we are unable to perceive this directly since we are limited to seeing things in three dimensions. Our brains therefore assume that space is flat, and in the process of making this assumption, things get screwed up. Objects which are actually moving along straight lines appear, in the map we construct inside our heads, to be traveling along curves and to be pulled by the massive object nearby. Once you get used to it, this new way of looking at gravity is actually very natural! Have you ever seen astronauts in orbit around the Earth? Do they look like theyre being pulled by anything? No, they dont; they experience weightlessness, and if they didnt look out the window to see the Earth below, they could reasonably conclude that their ship was floating through empty space, far away from the Earths gravity. According to Einstein, this is a perfectly reasonable conclusion because the two situations are equivalent! Whether floating through space or orbiting the Earth, the astronauts are moving along the same, straight line path. In fact, we could experience weightlessness too, if it werent for the surface of the Earth which keeps us from falling on our straight line path to the Earths center. It is not gravity we feel, Einstein says, but simply the ground pushing up on our feet.
Posted on: Sun, 20 Jul 2014 04:55:11 +0000
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