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Friday, August 21, 2015

Speed of Gravity

Aug 20, 2015: Dione with Rings and Shadows - Dione hangs in front of Saturn and its rings, captured during Cassini's close flyby of the moon. Saturn is only about a hundred times bigger than this tiny moon.

Nobody understands how gravity works, or if they do, they haven't convinced anyone else that they do. Some people think there are particles involved, kind of like magnetism and neutrinos, but no one has ever figured out what they were.
    I read a science fiction story once about a guy who had invented a graviton conductor. It conducted gravitons much the way a wire conducts electricity. You could clip it onto something and let it hang down to the floor and all the gravitons would drain out of the object. The more gravitons you drained out, the lighter it became. Eventually it would become so light it would just float away. The guy who invented it didn't have much imagination because he used it to give his trucking company a boost. He installed a paddle wheel on the driveshaft of his truck. Once his truck was loaded, he would connect his graviton cable to the load and dangle it above the paddle wheel. Gravitons draining out of the load would strike the paddlewheel and give his truck a boost. It also made his load lighter.
    He couldn't deliver ten tons of corn if it only weighed ten pounds, so just before he got to his destination, he would stop by the base of a cliff and drain some gravitons from the cliff back into the load. It wasn't long before it got out of hand. You'd go to the store to pick up a ten pound bag of potatoes and it would weigh a hundred pounds. That cliff he was draining gravitons out of to reweight his truck loads? It eventually fell into the sky. It was a great story.
    Back to the real world. There is one aspect of gravity that is a little curious. Does gravity have a velocity? The example that is usually used is how soon would the Earth notice if the Sun suddenly vanished? Would it immediately leave orbit, eight minutes before the sunlight disappeared, or would would it wait until there was no more light? The idea is that nothing can travel faster than light, so gravity, if it has any physical substance at all, couldn't travel faster than light, so as long as there is sunlight, the Earth would stay in orbit.
     I always thought that this was a particularly useless example. Matter doesn't just vanish and even when it does (like in an atomic reaction) so much energy is released that any kind of observation you hoped to make is going to be obliterated.
     But then I got to thinking. We are getting pretty accurate with our observations of objects in orbit in our solar system. Maybe we could measure and calculate our way to a conclusion.
    All the objects in our solar system are all pulling on each other all the time. They are also in motion all the time. So when the moon pulls on the Earth, is the pull coming from where it was a second and a half ago, or is it coming from where it is now? The moon is moving pretty quick, but in a second and half it's only going to a go about a mile. We can we tell the difference in the direction of the moon's pull, but can we determine it that accurately? I don't think so.
    On the other hand we have supercomputers and precision instruments and measuring techniques, so maybe running a simulation of the solar system could tell us whether gravity has a velocity or whether it is instantaneous.
    I tend to think gravity is simply a distortion of space. It's kind of like the landscape. It doesn't move, it's just there. But running the simulation might provide some interesting results.

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