From: jason 13/06/99 1:26:00 Subject: e=mc^2 post id: 17622 the theory of relativity, what the hell is it?? im doing a business degree and know very little about physics or whatever it has to do with, but im very interested to find out what this is all about, if someone could explain (simply). i would greatly appreciate it From: Brendan 13/06/99 2:30:51 Subject: re: e=mc^2 post id: 17624 E (energy) M (mass) C (speed of light) this is a constant. E=MC^2 (energy equals the mass times the speed of light squared). Is that what you were after or do you want more detail? From: Chris W (Plebeian) 13/06/99 12:07:28 Subject: re: e=mc^2 post id: 17629 Pretty big is one thing the two theories, Special and General Relativity, are. Is there a particular area that you are interested in Jason? Did something in particular prompt your question? From: jason 13/06/99 15:28:29 Subject: re: e=mc^2 post id: 17639 general relativity i spose, nothing really triggered my question i was just interested to know what it was, and how it worked im still a bit confused with that whole equation can u give an example how that can be put into practice or am i just asking stupid questions now?? From: James Richmond (Avatar) 14/06/99 10:18:24 Subject: re: e=mc^2 post id: 17682 The equation E=mc2 quantifies the fundamental equivalence of mass and energy. Under the right conditions, it is possible to convert mass to energy and energy to mass. Since c (the speed of light) is a very large number, 299792458 metres per second, a small amount of matter is equivalent to an enormous amount of energy. The equation is very important when we talk about nuclear reactions. For example, the sun fuses hydrogen nuclei (protons) into helium nuclei (each of which is made of 2 protons). It turns out that if we add up the masses of the protons at the start of a fusion event, the total mass is slightly greater than the mass of the the helium nucleus at the end of the process. Where does the left-over mass go? It is converted to energy in the fusion process. This is known as binding energy, for obvious reasons. The amount of binding energy released when hydrogen fuses to helium is given by E=mc2, where m is the mass difference between the final and initial states. This energy appears mostly as heat, which is why the sun is so hot. Different chemical elements (atoms) have different characteristic binding energies. When elements lighter than iron are created by fusion processes in stars, energy is released. When elements heavier than iron are built from lighter elements, energy is absorbed. Since E=mc2 works both ways, it is possible to get the absorbed energy back by splitting heavier elements into lighter elements. This is what happens in a nuclear fission reaction. For example, in a nuclear reactor a heavy element like Uranium, with 92 protons and 143 neutrons, is split into two lighter elements. The combined mass of the products turns out to be lighter than the original nucleus. The difference in mass is converted to energy (mostly heat), which is used in the reactor to boil water, which turns turbines and generates electricity. Of course, to get a good idea of how much energy is produced in nuclear reactions you can't go past the atomic bomb. For example, the bomb dropped on Hiroshima, powered by the (in this case rather inefficient) conversion of mass to energy, had an explosive force equivalent to 15,000 tons of TNT. The explosion of TNT does not involve nuclear reactions, but simply a chemical rearrangement of atoms. The Hiroshima bomb was a fission bomb, using the same splitting of Uranium as occurs in nuclear reactors. In contrast, many nuclear weapons today are fusion weapons which use a fission bomb merely as a trigger for a much more powerful fusion reaction. Unfortunately, through our understanding of E=mc2, we now have the ability to destroy every major city on the planet several times over. There is much more to relativity than E=mc2. Relativity is the theory which replaced Newton's laws of motion. It accurately predicts the behaviour of massive objects travelling at high speeds - something that Newton's laws do not do. The explanation comes at a price though. We are forced by the theory to re-evaluate our "common sense" ideas of time and space; this is why so many people have conceptual difficulties with relativity. I hope this helps. JR. From: Terry Frankcombe 14/06/99 18:21:12 Subject: re: e=mc^2 post id: 17708 This is known as binding energy, for obvious reasons. JR, a comment. I don't think that this really is that obvious. I certainly didn't find it obvious that creating a chemical bond (a related phenomenon) releases energy. It is obvious when you think about it, but certainly not at first glance. From: Martin B 15/06/99 11:08:13 Subject: re: e=mc^2 post id: 17781 The theory of relativity begins with the concept of inertia. This is the property that all bodies have to resist changes in their motion - once something is moving, it will keep moving in a straight line until some force acts on it to speed it up, slow it down or make it change direction. This was quite a novel idea when it came up with Galileo. Before that it was thought that forces caused motion, not changes in motion. An example of inertia: if you are on a sailing ship and someone drops a ball from the top of the mast, where will it land? At the base of the mast? Or will the ship have moved forward since the ball was dropped? The answer is a), though again, most people thought this was ludicrous when first proposed (and many people today still do!). The ball retains the forward motion that it had when it was dropped and continues to move forward with the ship all the way down to the deck. This is known as Galilean Relativity: A reference frame that is moving at constant velocity with respect to an inertial frame is also an inertial frame, and the laws of motion are of the same form in both frames - there is no mechanical experiment which is able to distinguish between the two ie if two frames are moving with constant velocity with respect to each other there is no way you can say absolutely that one is moving and the other is still. Isaac Newton took this princile of relativity and produced some mathematical laws that expressed it precisely. These laws formed the basis of Newtonian mechanics which underpinned our understanding of the physical world for nearly 250 years. The problem came along when Maxwell produced his theory of electromagnetism. This theory also had a precise mathematical formulation and, amongst other things, predicted that light was a form of electromagnetic radiation (which it is). Maxwells equations were very successful. The problem was that they didn't fit with Newtonian mechanics. Most physicists of the time thought that Newton was right, and that Maxwells equations must be slightly wrong and tried to fix them up. One way of doing this was to propose that Maxwells equations were 'really' with respect to an ether which was the medium of light waves. Once the motion of the ether was taken into account, Newtonian mechanics would be restored. Unfortunately, no-one could find this ether. A lot of time and effort went into ways of maintaining the ether hypothesis against the evidence. Einstein had another way of solving the contradiction between Newton and Maxwell. He assumed, daringly, that Newton was wrong and Maxwell was right. He made two assumptions: (1) the principle of relativity held for electromagnetism as well as for motion (2) Maxwells equations correctly described the propagation of light which was independent of the motion of the source These are the foundations of special relativity. Using only these assumptions, many surprising results can be deduced. These include: all observers detect the same speed of light regardless of there relative motion; E=mc^2; and observers in relative motion to each other will measure time and distance differently. Unfortunately, special relativity did not answer all the problems with Newton's theory. Specifically there was no way of incorporating Newton's theory of gravity into special relativity. There was also no way in special relativity to treat accelerated motion, only constant velocity motion. These two problems were found to be the same. There are two 'kinds' of mass appearing in the laws of physics. One is inertial mass: the amount that bodies resist motion. The second is gravitational mass: how strong a gravitational force a body produces. Newton produced his theory of gravitation by assuming that inertial mass = gravitational mass but there is no reason why this should be the case. It was one of the mysteries of Newtonian physics which was swept under the carpet. The consequence of this is that all bodies have the same acceleration in a gravitational field: (in the absence of air resistance) all bodies fall at the same rate. Einstein used this observation to resolve the problem. He realised that there was no way (locally) to distinguish between uniform acceleration of the reference frame and the presence of a gravitational field. Gravity, he hypothesised was a result of the behaviour of the reference frame of space-time and not the result of an action-at-a-distance force. The result was general relativity, the theory that says that gravity is a result of the curvature of space-time. Everything obeys the law of inertia - things still move in straight lines unless acted upon by a force. But the fact that the space-time in which bodies move is curved gives the appearance of curved motion. The curvature of space is produced by mass-energy. General relativity is often summarised as "space tells matter how to move; matter tells space how to curve." Sorry this is a bit long-winded, This forum is un-moderated. The views and opinions expressed are those of the individual poster and not the ABC. The ABC reserves the right to remove offensive or inappropriate messages.