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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

#101

Post by Ancapistani »

cylon13 wrote: post #34
vbezhenar wrote: post #24

Earlier quoted context omitted.

Are we moving through time with constant speed? Or we're constantly accelerating through time?

We're moving through spacetime with a constant speed, so the faster you move through space, the slower you move through time.

I just had another thought.

The word “dimension” in this context is overloaded. We think of space being three dimensions but really it’s only one - velocity relative to a specific reference frame. Thinking of it this way, the word “spacetime” makes sense; it’s a two-dimensional system: “spatial velocity” (S) on one axis and “temporal velocity” (T) on another. Both velocities are always measured against a reference frame, and their sum is c (c=S+T).

This would mean that time travel is impossible not because of a “speed limit”, but because c is a dimensionless physical constant.

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

#102

Post by Ancapistani »

Smaug123 wrote: post #82
Ancapistani wrote: post #71

Earlier quoted context omitted.

If I’m understanding, a theoretical ball thrown at 1c would follow the same path as a photon?

Yes, although you can't actually get anything with mass to travel at 1c (subject to our current understanding of physics).

As I understand it, the only thing that can travel at c are:

* photons

* spherical, massless cows in a vacuum

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

#104

Post by Sharlin »

perl4ever wrote: post #89
libraryofbabel wrote: post #83

Earlier quoted context omitted.

> The earth is pretty sphere-ish, locally speaking. Kinda. There are mountains, they have their own gravitational attraction, and it can be measured... even in the 18th century! https://en.m.wikipedia.org/wiki/Schiehallion_experiment It all depends on how pedantic one wants to be :)

The first time I heard of the concept, it wasn't actually that, it was how in the 60s, they had to correct for mascons in the moon while orbiting.

Yeah, the moon's gravitational field is quite lumpy compared to Earth's, due to its smaller size and the way it is believed to have formed. Plus you can orbit much closer to the surface due to the lack of atmosphere, so the lumps are steeper.

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

#105

Post by [deleted] »

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…

[deleted]

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

#106

Post by dreamcompiler »

sixo wrote: post #52
dreamcompiler wrote: post #41

Earlier quoted context omitted.

I think of it as being because the ball has mass and photons don't. So Newton's Gm1m2/r^2 = 0 for photons, and you have to use Einstein to measure the "force" on a photon. But because massive objects also cannot move as fast as photons, we're probably both saying the same thing from two different perspectives.

Curvature due to gravity does not depend on mass, which is kind of hinted at when mass cancels in F=ma for that force.

Curvature due to gravity depends entirely on mass. No mass = no spacetime curvature. In this case, the mass of the Earth is causing the curvature.

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

#107

Post by kwargs »

How come on half of the earth's sphere people don't get ejected out into outer space while the other half of the sphere it pushes onto the people giving them sense of acceleration?

If earth is pushing equally on all of the people around the globe how come it doesn't swell up and burst?

And how do you explain force of attraction weakening the higher in altitude you go away from earth?

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

#108

Post by abetusk »

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…

So if light were traveling through "slow glass" where it's speed through the medium was significantly slowed down, we would see it go in a parabola like the ball?

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

#109

Post by asdfasgasdgasdg »

avmich wrote: post #73
fouronnes3 wrote: post #66

Earlier quoted context omitted.

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…

Spherical mass can be replaced with a point mass. Earth however is not spherical (biggest deviation is polar flattening). And even then, Einstein's model, unlike Newton's, says it's not an ellipse even for a point mass. So, moral of the story - we have to be not too pedantic.

> 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 think you have successfully contradicted what the person you responded to said.

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

#110

Post by klodolph »

asdfasgasdgasdg wrote: post #109
avmich wrote: post #73

Earlier quoted context omitted.

Spherical mass can be replaced with a point mass. Earth however is not spherical (biggest deviation is polar flattening). And even then, Einstein's model, unlike Newton's, says it's not an ellipse even for a point mass. So, moral of the story - we have to be not too pedantic.

> 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.

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