Daniel Patrick Fisher Posted March 14, 2014 Share Posted March 14, 2014 Imagine an astronaut in a space ship which, once it has left the earth's gravitational field, begins to accelerate continuously at 9.8 m/s2. I am imagining the ship's engine is efficient enough to accomplish this. By my calculation, he would be traveling at the speed of light, relative to his point of origin (earth), after 347.125 days. Assuming our space ship has a sufficiently sustaining life support system, our astronaut should be quiet happy and healthy, having spent all of that time in a simulated gravitational field. Ironically, he would have the sensation of being a body at rest the entire time. What law of physics makes this imaginary scenario impossible? Link to comment Share on other sites More sharing options...
swansont Posted March 14, 2014 Share Posted March 14, 2014 Relativity. The amount you can change the speed is not given by naive application of the classical kinematic equation v = at; this is an approximation which begins to noticeably fail once you get to a reasonable fraction of c. You would not be able to sustain the acceleration you originally had. The kinetic energy of your ship is not given by ½ mv2. This, too is an approximation that is valid at speeds below ~ 0.1c Link to comment Share on other sites More sharing options...
Janus Posted March 14, 2014 Share Posted March 14, 2014 Thus applying Relativity to the problem it turns out that the ship, accelerating at 1g as experienced by the astronaut, will reach 70% of c in 347.125 days as measured by Earth, and 75% of c in 347.125 days as measured by the astronaut. Link to comment Share on other sites More sharing options...
Endercreeper01 Posted March 15, 2014 Share Posted March 15, 2014 Special relativity treats acceleration differently then classical physics does. In special relativity, the velocity of an object after moving for a time t at an acceleration a is [latex]v=\frac{at}{\sqrt {1+\frac{a^2t^2}{c^2}}}[/latex] This is to ensure that the velocity never exceeds the speed of light, even in the presence of acceleration. Link to comment Share on other sites More sharing options...
Delta1212 Posted March 16, 2014 Share Posted March 16, 2014 Also, as he is accelerating, he wouldn't have the sensation of being a body at rest. He'd have the sensation of being in an Earth-equivalent gravitational field, but those aren't exactly the same thing. Link to comment Share on other sites More sharing options...
Daniel Patrick Fisher Posted April 28, 2014 Author Share Posted April 28, 2014 Still, it is a difficult thing for me to visualize how two objects traveling at 99% the speed of light, in opposite directions, still can't reach the speed of light relative to each other. I know that the rule is that the speed of light is constant regardless of the motion of it's source, but the relative speed of the objects themselves is where I get confused. Link to comment Share on other sites More sharing options...
swansont Posted April 28, 2014 Share Posted April 28, 2014 Still, it is a difficult thing for me to visualize how two objects traveling at 99% the speed of light, in opposite directions, still can't reach the speed of light relative to each other. I know that the rule is that the speed of light is constant regardless of the motion of it's source, but the relative speed of the objects themselves is where I get confused. You and a lot of people. It's outside of our normal experience, but the theory says it's so, and it's backed up by tons of experimental evidence. It's how the universe behaves. 1 Link to comment Share on other sites More sharing options...
ACG52 Posted April 28, 2014 Share Posted April 28, 2014 Most of relativity is difficult to visualize. Link to comment Share on other sites More sharing options...
ajb Posted April 28, 2014 Share Posted April 28, 2014 Still, it is a difficult thing for me to visualize how two objects traveling at 99% the speed of light, in opposite directions, still can't reach the speed of light relative to each other. I know that the rule is that the speed of light is constant regardless of the motion of it's source, but the relative speed of the objects themselves is where I get confused. You can say that they have a separation speed greater than the speed of light, that is no problem at all and does not break any rules of relativity. However this is not the relative speed. No observer can interpret this separation speed as a relative speed. In particular it is not the relative speed that one of the objects measures the other object to have, which will always be less than the speed of light. 1 Link to comment Share on other sites More sharing options...
Moontanman Posted April 28, 2014 Share Posted April 28, 2014 You can say that they have a separation speed greater than the speed of light, that is no problem at all and does not break any rules of relativity. However this is not the relative speed. No observer can interpret this separation speed as a relative speed. In particular it is not the relative speed that one of the objects measures the other object to have, which will always be less than the speed of light. Can not a third party observer see them as separating at faster than light? Link to comment Share on other sites More sharing options...
swansont Posted April 28, 2014 Share Posted April 28, 2014 Can not a third party observer see them as separating at faster than light? It's right there in the post. Separation speed is not limited to c. Link to comment Share on other sites More sharing options...
Moontanman Posted April 28, 2014 Share Posted April 28, 2014 It's right there in the post. Separation speed is not limited to c. My mistake, i read it wrong, i see it now... Link to comment Share on other sites More sharing options...
Orodruin Posted April 29, 2014 Share Posted April 29, 2014 Just to be complete: Separation speed *is* limeted to 2c as long as we stay in special relativity so that space itself cannot expand. Link to comment Share on other sites More sharing options...
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