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Do you think Einstein's concept of general relativity will be found to be an inadequate theory? |
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Certainly, it will have to be superseded by some theory of quantum gravity. |
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Does GR eliminate the need for gravitons? If gravity isn't really a force so much as a landscape, then why are gravitons still being sought? |
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We are not certain of the true nature of gravity because it must be quantizable, but we have no theory of quantum gravity. Why does gravity still appear to be so geometrical? A major question. |
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What current research is being done to extend Einstein's theory of relativity, and what practical applications does it have to alternative sources of power? |
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Major areas of research include deducing what Einstein's theory predicts (although we have the fundamental form of the equations, we still don't know all the solutions) and trying to determine how to combine GR with quantum mechanics to develop a theory of quantum gravity. As to practical applications of these theories, none immediately come to mind. (A civilization could use a spinning black hole to generate scads of power in principle, but it doesn't seem feasible in practice.) On the other hand, Maxwell probably didn't anticipate all the applications of his equations, so perhaps some practical application may be found someday. |
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Is there any way gravity can be used to propel a spaceship? |
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"Gravity thrusters" are a popular way to propel ships in science fiction. It would seem to require some sort of antigravity (which is impossible.) Slightly more sophisticated sci-fi posits ships that can locally affect space-time curvature and use that to propel themselves along. One effect that does work in real life and has been exploited is to dive down into some gravitational field and fire the engines, leaving the fuel mass deep in the gravitational field and thus increasing the boost by the gravitational potential energy of the fuel. If you found a rotating black hole you could also tap into the hole's spin energy. |
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Why is the sky blue? It's not due to gravitational blueshift is it? |
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No, otherwise the Sun would also be blue. The blue color comes from scattering of light off gas molecules in the sky. The amount of scattering (Rayleigh scattering, for those who want to know more) is strongly dependent on the wavelength--specifically, it is proportional to the inverse fourth power of wavelength. Blue light is scattered much more than red light. We see the sky lit up by the blue light scattered toward us from all over. At sunset the sky is red because the blue light is strongly scattered out of our line of sight to the Sun due to the thick layer of atmosphere that the light must traverse near the horizon. |
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The media constantly portray gravity and weightlessness incorrectly. Why don't astronomers correct them? |
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Well, they are tricky concepts to understand, and most people have no personal experience with weightlessness, and know only that things fall down. People making movies often care less about whether something is correct than whether the audience will like it or accept it. Most people know there is no sound in space, for example, but they still expect to hear big booms when spaceships blow up. The factual media has less of an excuse for screwing up. Ultimately the problem is you can only teach people who are willing to learn. (I would be happy to help George Lucas get the science right in his next movie.) |
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Is it possible that gravity is a fictitious force created to deal with and understand the curvature of spacetime? |
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Not quite sure what you mean. But gravity is our name for the force (acceleration) we experience because we are not moving along a freefalling (inertial) trajectory. In other contexts such forces (accelerations) are often called "fictitious" (e.g. centrifugal force). |
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Why are time dilation and length contraction reciprocal in special relativity, but not when due to gravitational fields? |
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They are reciprocal in SR because all inertial frames are equivalent. (By the relativity principle.) All accelerating frames are not equivalent. Accelerations vary and are produced by physical effects (forces) proportional to their magnitude. |
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According to Mach would there be distance in a universe that contained only one object? |
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No. Distance can be defined only in terms of a spatial relationship between one object and another. |
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What is the equivalent of a photon for gravity? |
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The graviton. |
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Copyright © 2005 John F. Hawley |