Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: melaniejs on 19/02/2020 13:18:23
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Tony asks:
I have yet to understand gravity or hear anyone who can describe this all pervading force. Or is it a force?
Could you possibly try and explain gravity and how it works?
Anyone want to give it a go?
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Newton's version: Every mass attracts every other mass in the universe. The force decreases with distance.
- This force allows planets to take on stable circular or elliptical orbits around their star.
Einstein's version: Every mass deforms spacetime, forming a "gravitational well". Mass tends to travel in the shortest path on this deformed spacetime, which bends the path of a planet into a stable orbit around its star.
- If gravity is strong enough (in a black hole), not even light can escape
- If two black holes orbit each other, some of their energy is radiated away in the form of gravitational waves.
Quantum version (still under development): Gravitational waves consist of very many, very tiny subatomic particles called "gravitons".
- Gravitational attraction between masses occurs because of the exchange of (virtual?) gravitons.
The Common version: Gravity sucks!
All of these produce the familiar result of objects falling to Earth, and the planets orbiting the Sun.
See: https://en.wikipedia.org/wiki/Gravity
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Gravity isn't a force. A gravitational acceleration behaves the same as a uniform motion, locally measured. And a uniform motion is something that you locally are free to define any speed you like to, or none. That doesn't state that a apple falling doesn't develop a kinetic energy versus the ground it hits. But in relativity you are free to use earth as the one moving, or both earth and the apple moving, or just the apple. It depends on what frame of reference you use for it. You can call it a 'system' acting on itself.
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Gravity is strange. It deforms space and time so that adjustments have to be made to pythagoras's theorem. This can be seen when drawing a triangle onto a sphere. The angles do not add up to 180 degrees.
The correction for this is part of the Einstein field equations. The fact that this correction has to be made underlies the curvature of spacetime.
A straight line on a sphere is not exactly what you would call straight. However it describes the path of least action between two points on the sphere.
Is gravity a force? What else would you call it?
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A force is anything that accelerates a mass. You can observe the acceleration as an apple falls from a tree or a planet orbits a star, so gravity is indeed a force, and if you don't like moving objects, you can measure it with a spring balance.
Problem is that it appears to be a unique force in that it is generated by and affects all massive objects equally, in direct proportion to their mass, and unlike say electrostatic force, it always attracts - we haven't found a source of negative gravitation or any means of shielding from it.
So on the one hand it is the most familiar force in the universe, and on the other, it seems to be the least understood.
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I think the clue is in it's relation to time