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Dec 4, 2023 · Newton’s theory of gravity, gave way to Einstein’s theory of general relativity (GR), which holds that gravity is not a force in the usual sense. Instead, heavy objects such as the sun, bend the fabric of spacetime in such a way that causes the earth to revolve around it.
- Important Concepts
- Non-Inertial Frames and Fictitious Forces
- The Force of Gravity: Newton vs. Einstein
- The Apple Falling
- Spacetime Geometry
- Curved Spacetime and Tidal Gravity
- The Surface of The Earth
- Summary and Key Points
Spacetime: the combination of 3 dimensions of space and 1 dimension of time into a unified 4D “manifold”.Coordinate systems: mappings between events in spacetime and 4 numbers called coordinates. Usually, the coordinates involve a 1-time coordinate and 3 spatial coordinates.Proper acceleration: the acceleration measured by an ideal accelerometer. This is the acceleration that you physically “feel”.Coordinate acceleration: the 2nd derivative of position in some given coordinate system.Physics is traditionally taught using inertial frames, however, there is no restriction that prevents the use of non-inertial frames (e.g. a rotating reference frame). In a non-inertial frame inertial objects undergo coordinate acceleration, and in order to use Newton’s 2nd law in such frames additional “fictitious forces” (e.g. the centrifugal and...
In Newtonian mechanics, gravity is considered to be a real force, despite the fact that it shares the first two properties of fictitious forces listed. This makes Newtonian gravity a bit of a strange force. You cannot determine if a given reference frame is inertial or not simply by using accelerometers, you have to additionally know the distributi...
As mentioned above, an apple freely falling to the ground is considered to be inertial in GR. If we make a coordinate system where the apple is at rest, it is an inertial coordinate system. In this reference frame, the apple does not accelerate, instead, the ground accelerates upwards at g and slams into the apple. This corresponds to the fact that...
Now consider a “spacetime diagram” of the apple, where time is plotted on one axis and the apple’s vertical position is plotted on the other axis. In such a diagram the apple can be represented by a line (called a “worldline”) which shows its position at each point in time. If we take the frame where the apple is at rest then the apple travels alon...
While this is all well and good for a single apple falling in a uniform gravitational field, what happens when we consider tidal gravity (gravity that varies over space) such as two apples falling on opposite sides of the world? Let’s suppose that there is a hole completely through the Earth and no atmosphere so that we can neglect the ground and t...
In the previous section we neglected the ground, but now let’s consider the ground. If the ground is accelerating upwards and the direction corresponding to “up” changes around the globe, then it seems that the surface of the Earth should be expanding, with the distance from one point to another continually increasing. This reasoning is incorrect i...
In GR the force of gravity is considered to be a fictitious force, and inertial reference frames are free-fall frames where falling apples have straight worldlines and the ground continuously accel...In order to model tidal gravity (where gravity varies from location to location), we must use curved spacetime.This allows for two apples to each have straight (geodesic) worldlines but still, accelerate relative to each other.It also allows for the ground to continuously accelerate upwards without the Earth expanding.Nov 23, 2015 · In a kind of cosmic GPS, GR has had a profound effect on our ability to map the location of matter in the Universe. In particular, gravitational lensing – the bending of light caused by massive objects curving space – has allowed astronomers to “see” the invisible by making maps of dark matter.
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Dec 3, 2015 · The Galileo satellites will be testing general relativity, a theory that was published 100 years ago by rock-star physicist Albert Einstein. What is general relativity? Read this anniversary article from NASA for some help.
General relativity, also known as the general theory of relativity and Einstein's theory of gravity, is the geometric theory of gravitation published by Albert Einstein in 1915 and is the current description of gravitation in modern physics.
Oct 11, 2011 · Explainer. Theory of General Relativity. Albert Einstein. Register now. It’s the year 2100. You wake up alone in a small, windowless room. The only other thing in the room is a small ball. Maybe ...
Sep 9, 1997 · Published in 1915, general relativity proposed that gravity, as well as motion, can affect the intervals of time and of space. The key idea of general relativity, called the...
