Physman Posted August 17, 2009 Share Posted August 17, 2009 Assume you have a rocketship in outer space pointing in an irrelevant direction, and will start to accelerate instantaneously with the creation of an enormous mass. It would travel at a tangent with the mass's limit of gravitational attraction or a random far ut point within the mass's gravitational reach. Now, according to general relativity in contrat to Newtons theory of Gravity light travels faster than gravity, which confirms that gravity has a finite velocity. We also assume that the rocketship and mass are perfectly calibrated to have the rocket pass the mass without being affected by the gravity i.e. general relativity. So if the situatioon held the rocketship would be able to pass the mass without ever being effected by the body's gravitational attraction. So in theory, couldnt earth cease to exist for a very small amount of time without lossing the moon from its gravitational field? Link to comment Share on other sites More sharing options...
Klaynos Posted August 18, 2009 Share Posted August 18, 2009 Light travels faster than gravity? That's a new one on me, I thought most if not all current thoughts on the speed of gravity put it at the speed of light. As for your final question, the answer is quite simply, if you ignore the laws of physics then you can draw any conclusion you like. If the earth disappeared for 1second there would be a 1 second gap in the gravitational field spreading outwards at the speed of light when this hit the moon it would stop orbiting. and carry on in a straight line. Link to comment Share on other sites More sharing options...
swansont Posted August 18, 2009 Share Posted August 18, 2009 Assume you have a rocketship in outer space pointing in an irrelevant direction, and will start to accelerate instantaneously with the creation of an enormous mass. Why would you assume this? Now, according to general relativity in contrat to Newtons theory of Gravity light travels faster than gravity No. Link to comment Share on other sites More sharing options...
Physman Posted August 18, 2009 Author Share Posted August 18, 2009 General Relativity explains that space time is somewhat like a fabric within the Universe. Planets Orbits are much like grooves that they follow in space time. I realize that there would be an after effect but what I mean to say is to consider the very immediate effects. You must take in mind that the speed of light is the limit, and gravity would seem to be instantaneous to the naked eye although it does have a finite speed. The concept I am putting forth obeys all laws of relativity. I am just sharing this idea to show that gravity could be 'ran away from'. Link to comment Share on other sites More sharing options...
Klaynos Posted August 18, 2009 Share Posted August 18, 2009 no, gravity travels at the speed of light. Link to comment Share on other sites More sharing options...
Physman Posted August 18, 2009 Author Share Posted August 18, 2009 I'm sorry, your missing my point, i am not saying that gravity travels slower than light (which it doesnot), I am saying that it has a finite velocity. Therefore if the situation was set up correctly then the ship would not ever feel the gravitational attraction of the mass. Link to comment Share on other sites More sharing options...
Sisyphus Posted August 18, 2009 Share Posted August 18, 2009 I don't really understand the scenario you set up. However, I should point out that gravity propagates at the same speed as light, and you can never "outrun light." Link to comment Share on other sites More sharing options...
insane_alien Posted August 18, 2009 Share Posted August 18, 2009 so... in reality and scenario in which the objects can see each other via light then they can interact gravitationally too. nothing new and/or interesting here. lets move on. Link to comment Share on other sites More sharing options...
swansont Posted August 18, 2009 Share Posted August 18, 2009 I'm sorry, your missing my point, i am not saying that gravity travels slower than light (which it doesnot), I am saying that it has a finite velocity. Therefore if the situation was set up correctly then the ship would not ever feel the gravitational attraction of the mass. No, because you would require violating conservation of energy to make a mass just pop into existence. Link to comment Share on other sites More sharing options...
Physman Posted August 19, 2009 Author Share Posted August 19, 2009 Swansont, I am not trying to go against the funamental laws of physics, nor is that relevant to my question, what I am trying to do is to show through example that gravity has a finite velocity Link to comment Share on other sites More sharing options...
insane_alien Posted August 19, 2009 Share Posted August 19, 2009 well you can't do that in a thought experiment. you really need to go out and do an experiment to show that. and we have. however accuracy issues mean we can only put it at between 0.8c and 1.5c theory predicts c Link to comment Share on other sites More sharing options...
Physman Posted August 19, 2009 Author Share Posted August 19, 2009 What I am trying to say is that light has a finite velocity, which is correct. Light is the limit therefore gravity must be as faster or slower thatn light causing it to have a finite velocity. After using this logic we can now apply it to the circumsatnces at hand. Link to comment Share on other sites More sharing options...
Sisyphus Posted August 19, 2009 Share Posted August 19, 2009 Yes, gravity has a finite velocity. I still can't tell what you're talking about, though. You can't outrun light, and you can't outrun gravity. The rocketship will be caught by the gravity of the created mass. If the mass of the Earth disappeared for one second, the Moon would travel in a straight line for one second, then go back into a (very slightly different) ellipse. Link to comment Share on other sites More sharing options...
Physman Posted August 19, 2009 Author Share Posted August 19, 2009 No, your right you can not outrun light there fore you can not out run gravity. although this is the case when it has a certain direction and/or propagation. let me rephrase the example (to a different degree). If you stand next to a wall, with a hose pointing straight at the wall, 10 feet to your left. As soon as you start to run the hose turns on, if you run fast enough and your a far enough away you will pass the point of the hose without getting sprayed. Replace the water with gravity and the the hose with the mass, and yourself with the rocket ship. Link to comment Share on other sites More sharing options...
swansont Posted August 20, 2009 Share Posted August 20, 2009 If you could have mass just pop into existence, then yes. But you can't, so it's moot. This is just the reverse of the "what if the sun disappeared?" problem. Link to comment Share on other sites More sharing options...
Physman Posted August 20, 2009 Author Share Posted August 20, 2009 I am just using that as a factor to make the thought experiment possible, although it is not possible. THere are many other examples that could be used. Link to comment Share on other sites More sharing options...
gre Posted August 20, 2009 Share Posted August 20, 2009 I am just using that as a factor to make the thought experiment possible, although it is not possible. THere are many other examples that could be used. What is the value of a thought experiment that isn't possible? Link to comment Share on other sites More sharing options...
Physman Posted August 20, 2009 Author Share Posted August 20, 2009 To prove a point that is independent of if it is possible or not. Link to comment Share on other sites More sharing options...
insane_alien Posted August 20, 2009 Share Posted August 20, 2009 thought experiments cannot be used to prove anything though. only experimentation in reality can. Link to comment Share on other sites More sharing options...
Physman Posted August 20, 2009 Author Share Posted August 20, 2009 I completely agree, bout tell me the next time you see an experiment on this forum that isnt a thought experiment... Link to comment Share on other sites More sharing options...
SH3RL0CK Posted August 20, 2009 Share Posted August 20, 2009 I completely agree, bout tell me the next time you see an experiment on this forum that isnt a thought experiment... An internet forum is not designed for experimentation, but rather for discussion and debate. Nevertheless, there are many experiments discussed which aren't thought experiments; try using the search function. One which comes to mind is: http://www.scienceforums.net/forum/showthread.php?t=40351&highlight=tire Link to comment Share on other sites More sharing options...
Physman Posted August 22, 2009 Author Share Posted August 22, 2009 Yes, but there are also many experiments that work as an examle but could never happen, although they still provide a good example. Link to comment Share on other sites More sharing options...
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