<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>My understanding is that the Cosmological Constant now fits, because it showed that the universe was expanding. Also, if I am not mistaken, NONE of Einstein's equations were based on observation. btw, the source is Michio Kaku on the Discovery Channel program about this topic. <br />Posted by job1207</DIV></p><p>The answer to your question depends on what you mean by "based on observation", and what you mean by Einstein's equations.</p><p>Einstein was an intuitive researcher who strongly believed in an essential order to the laws of physics. His personal inspiration in developing relativity was not strongly rooted in experiment, but more so in the beauty of the laws themselves. He developed his laws based on a few simple principles, which were and are consistent with observation. For instance the observation that a person in a falling elevator senses no gravitational effects (until impact) is the basis for the "equivalence principle" which the heart of general relativity. In that sense general relativity is based on the most simple of observations. Special relativity is somewhat simpler and can be derived from just two basic observations: 1) Light propagates at a fixed velocity in all inertial reference frames and 2) the laws of physics are the same in all inertial reference frames.</p><p>While Einstein was not an experimentalist, his theories have been tested many times by very good experimentalists and have been found to produce astoundingly accurate predictions in all but the most extreme conditions -- conditions in which quantum effects should be very important.</p><p>It is also sometimes not understood that general relativity is a theory of gravity and not a theory of cosmology. It is central to cosmology simply because the large-scale behavior of the universe is dominated by the gravitational force, and because general relativity provides the framework to understand the effects of gravity. It is also important becaue the nature of general relativity provides and explanation in terms of the structure of space-time itself, and the complete space-time manifold IS the universe.</p><p>General relativity only provides a framework for cosmology. It can be formulated with a cosmological constant or without one (i.e. a cosmological constant of 0). It does a good job of predicting the behavior of the universe based on observed conditions or assumed initial conditions. For instance it can be used to show that, based on the observed conditions now, that the universe began some time ago in an extremely compressed state -- the theory actually shows a singularity which may be simply an indication of the breakdown of the theory under circumstances in which quantum mechanics becomes important. It cannot explain the mechanism behaind the initial expansion of the universe -- that requires assumed initial conditions or something like inflation that is oustide of general relativity. But general relativity itself, the theory of gravitation based on the geometric properties of space-time as determined by the Einstein field equations has been extensively tested experimentally and found to be consistent with all observations thus far.</p><p>The cosmological constant was initially used by Einstein not to explain an expanding universe, but rather to provide for a static (non-expanding universe) by delicately balancing the contractive effect of gravity wiith a ounter-effect provided by the cosmological constant. When Hubble's work showed the universe to be expanding Einstein dropped the cosmological constant. More recent observations (roughtly in the last 10 years) indicate that not only is the universe expanding, but that the rate of expansion is increasing. The acceleration of the expansion has resulted in the revival of the notion of the cosmological constant, or equivalently the postulation of "dark energy", to provide a repulsive force in the Einstein field equations that describe gravitation. Whether or not this is an accurate reflaction of nature remains an open question, but many astronomers support the idea.</p><p>Einstein also developed theoretical explanations for the photoelectric effect (which is the work for which he received the Nobel Prize) and for Brownian motion, both experimentally observed effects. That work was directly based on experiment and served to promote the development of quantum mechanics (photoelectric effect) and to provide a link between observations and the atomic theory of matter (Brownian motion). </p><p> </p> <div class="Discussion_UserSignature"> </div>