We, the crew of the spaceship shown on the right, are floating freely in space, far away from all major sources of gravity. Michael Foale aboard the ISS, together with two floating grapefruits: As an example, the following picture shows the astronaut-scientist C. Those astronauts haven’t escaped the earth’s gravity – they’re experiencing a very special kind of free fall, a free-falling orbit around the earth. Most readers will have seen footage showing situations like this, involving, for instance, astronauts aboard the international space station ISS. In both situation, she would float, weightlessly, in the elevator, as would all objects around her. Imagine a scientist in a small elevator more precisely, in a small, windowless compartment that looks like an elevator cabin) That scientist has great difficulty to tell whether she is in free space, far from all sources of gravity, or in free fall in a gravitational field. This seemingly harmless property has far-reaching consequences. At least in a vacuum (where there’s no air resistance), objects you place at the same location fall with the same acceleration – the mouse or the elephant, the feather or the cannonball. One central feature of gravity is that it makes no distinctions. Overall, gravity is intimately connected with the geometry of space and time. In part, it is associated with a quantity called “curvature”. In curved spacetime, its worldline is curved into the space direction, which means that the particle is beginning to change its position due to gravity.What is gravity? Einstein’s general theory of relativity has an unusual answer to that question which will be explored in this spotlight text. Like a straight line upwards in a spacetime diagram. For example the world line of a particle at rest goes from past to furture, but not in a spatial direction. The combination of space and time.Įvery point in space, like a planet or so, is represented by a line in spacetime (its "worldline"). There is one aspect missing that these pictures can't convey: It's not space that is bent, it's spacetime. Nothing to do with something falling in the funnel, it'd work exactly the same if the funnel were bent upwards.ĭon't take it too literal as explaining gravity. That's what you can learn, in curved space geodesics ("supposed straight lines") are curved. You end up with a curved line, which is calles a geodesic. The standard procedure is to lay an arrow on the plane and move it always in the direction it is pointing, without deliberately rotating it. Now draw a straight line on it (or at least, try to do so). I'm actually not sure in every case what they are supposed to show but here's what I think they should show. These analogies are misleading sometimes. What is making the particle accelerate on that curvature? Why isn't it happy staying in the spot its placed even though the spot might be curved in towards the larger mass? Place it in the "curved" area of space-time around a large mass. Take a stationary particle, of very low mass. So to sum it up, can someone answer or explain this question: Why would that mass want to move down the curve if its at rest? In the analogies, its actually gravity that is implied to move the object along the curve so they're using gravity to explain gravity which doesn't make sense. My confusion is why do the objects move along the curvature? Say you took a stationary mass and placed it on the curved surface. Then the more exact 3-D and 4-D models are given, but its still the idea that an object will "fall" along the curve towards the mass causing the curve. Its also given in the analogy of a funnel with a coin rolled down the funnel. And so we are told a mass placed in this curvature will "fall" down the curve and that's how objects are moved in gravity. In 2-D it is usually a plane with field lines, and the surface of the plane is curved around an object. I've always been confused by the typical analogies I see when gravity as a space-time curvature is explained.
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