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Gravity is not a force – free-fall parabolas are straight lines in spacetime

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Re: Gravity is not a force – free-fall parabolas are straight lines in spacetime

#111

Post by amelius »

fouronnes3 wrote: post #66
sohkamyung wrote: post #36

Earlier quoted context omitted.

Off topic, but in reality, the trajectory of a ball thrown on Earth is not a parabola, but an ellipse [1]: > under the laws of gravity, a parabola is an impossible shape for an object that's gravitationally bound to the Earth. The math simply doesn't work out. If we could design a precise enough experiment, we'd measure that projectiles on Earth make tiny deviations from the predicted parabolic path we all derived in…

It's a parabola in a uniform gravity field, an ellipse in a circular gravity field coming from a point mass. So if you want to be really pedantic, it's never an ellipse because the Earth is not a point mass. It would be equivalent to a point mass if the Earth were a perfect sphere of uniform density, but it isn't. In reality it's a potato like mass blob that's approximated by what geodesists call the "geoid". So in o…

> an ellipse in a circular gravity field coming from a point mass.

Even if you consider the point mass to move because of the mutual attraction?

Re: Gravity is not a force – free-fall parabolas are straight lines in spacetime

#112

Post by freedomben »

pmcollins wrote: post #103

Wonderful lecture series on Audible by Benjamin Schumacher: https://www.audible.com/pd/Black-Holes-Tides-and-Curved-Spac...

Can second this. I was worried that course would be over my head since I never took physics in college, but he does a great job prepping you to understand the end of the course.

Re: Gravity is not a force – free-fall parabolas are straight lines in spacetime

#114

Post by xwdv »

GuB-42 wrote: post #88

My enlightening moment about general relativity: apples do not fall on the ground, instead, the earth is inflating, and the inflation of the earth is accelerating at 9.8 m/s^2. Eventually, the ground catches the apple. Of course, you are going to tell me that the earth is not inflating, obviously, because it is still the same size after so many years. But here is the trick: the earth is inflating at the same rate as…

Does this mean some sort of artificial gravity may be possible after all by causing some kind of inflation?

Re: Gravity is not a force – free-fall parabolas are straight lines in spacetime

#115

Post by amelius »

crazygringo wrote: post #90

I'm curious, how does the "gravity is not a force" viewpoint relate to the hypothesized graviton particle [1]? Are they incompatible viewpoints, or just different perspectives on the same thing? (E.g. are gravitons hypothesized to disappear depending on frame of reference?) I'm assuming they're incompatible (that we need the theory of everything [2] to reconcile them) but would love to know if there's something I'm m…

As you may know, one of the great problems in physics is to unite relativity and quantum mechanics. It happens to be that the graviton is a concept from QM and "gravity is not a force" is a concept that lives in the theory of relativity.

Re: Gravity is not a force – free-fall parabolas are straight lines in spacetime

#116

Post by lucb1e »

avmich wrote: post #91
willswire wrote: post #75

Veritasium just put out a great video on this: https://www.youtube.com/watch?v=XRr1kaXKBsU

A great video :) . Makes you wish he'd include more explanations of mathematical details...

I'm happy they didn't, making it a great video for me as well :)

Not that I'm afraid of math, I just don't feel like I need to see equations when it's a concept that is being explained rather than how to numerically calculate something. Math helps me as much as a code implementation does to explain the concept of a variable: I can observe its behavior but it doesn't necessarily teach me the concept.

Re: Gravity is not a force – free-fall parabolas are straight lines in spacetime

#118

Post by herodoturtle »

Smaug123 wrote: post #14

I heard an interesting question at one point: "how come, when you throw a ball up on Earth, the parabola is so strongly curved? Spacetime is nearly flat, so how can a straight line become such a steep parabola?" I'll answer this question as I understand it, but I only took four lectures of General Relativity before I gave it up in favour of computability and logic, so if there is a more intuitive and/or less wrong an…

I never studied this stuff, so the genius of your intuitive explanation is appreciated.

My intuitive response to your intuitive explanation: This ball is moving through spacetime relative to the earth, which is in turn also moving through spacetime relative to the sun - and so light is being deflected off of this ball at each point in its position in spacetime relative to the sun for much longer than light is being deflected off of this ball at each point in its position in spacetime relative to the earth - have I got that right?

Light is a hitman Perpetual driveby shooter Never missing As we dance through the night

Re: Gravity is not a force – free-fall parabolas are straight lines in spacetime

#120

Post by tedunangst »

klodolph wrote: post #110
asdfasgasdgasdg wrote: post #109

Earlier quoted context omitted.

> Spherical mass can be replaced with a point mass I'm not a physicist, but that doesn't sound right. One example is that if you are inside of the sphere, at least some of the mass will be pulling you away from the point at its center. I think what you mean is that a point mass closely approximates a spherical mass, but the degree to which that is true becomes less and less the closer you get to the center. I don't t…

In Newtonian physics, a sphere and point mass are exactly interchangeable as long as you are not inside the sphere. If you are outside the sphere, the equivalence is exact, regardless of distance. Proving this is a classic problem in undergraduate physics.

We are inside the atmosphere at least...
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