Spacetime question?


My current picture of how gravity works is this (its probably wrong because it doesnt work for light. I’d like a better explanation from you guys):
objects have to move in the speed of light in space time, so when they encounter a position in space in which time is slower (this is the result of gravity. and my point of misconception probably), they have to compensate with movement in space . but this doesn’t make sense for light. it travels in the speed of light in space always, and therefore does not advance in time (from its point of view). so there is no reason for light to change its course in space.

if this is wrong i would very much appreciate a better explanation?

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General Physics RvTDLR 5 years 1 Answer 714 views 0

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Answer ( 1 )

  1. First of all, the speed of light has nothing to do with light itself, we just call it the speed of light because light was the first object we discovered that traveled at this speed, but now we know of other things that travel at or near this speed too. It’s best to simply think of this as the universal speed limit, of which light happens to be one of the objects that follow this rule. Other particles that follow this rule are Gluons from the Strong Nuclear force, and neutrinos which happen to travel at or just slight below this universal speed (so close to this speed, that we can’t tell the difference).

    Second of all, this is a special speed because this happens to be the speed at which the time dimension is flowing past the space dimensions. So objects that travel at this speed are basically being dragged along by time, and they find themselves always in the same location in time, because they are flowing with it. This is the same as a boat flowing unimpeded on a river current, it will always see the same position with respect to the water, even though it is moving with respect to the riverbank.

    Now, how does gravity affect time? Continuing on with the flowing water analogy, gravity is like a depression in the ground, and water and objects in the water are going to naturally want to flow towards it. In the case of a black hole, the depression in the ground is as steep as a cliff, and water flows over it and down completely. When water flows down the cliff, it becomes a waterfall. It has plenty of movement in the vertical direction, but this also means that it is no longer moving in the horizontal direction, while it is flowing over the cliff. We happen to measure our time like the horizontal flow of water, and when the water is flowing vertically, it isn’t a type of flow that we are interested in. All of our important work is done by the horizontal flow of water, not by the vertical flow (we’d rather have boats floating horizontally, rather than vertically). Also without the extreme case of a black hole or a waterfall, even less extreme cases like water flowing down a gentle incline like a hill, shows that there is both a vertical and a horizontal component of flow. The steeper the incline the more of a vertical component there is, and the less of a horizontal component there is. Again the important flow to us is the horizontal component of the flow. So you can think of the Earth and the Moon’s gravity as less inclined gentle foothills than that of a black hole’s cliff.

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