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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

#341

Post by pa7x1 »

mrfusion wrote: post #20
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…

Are we flat in the time dimension? Or what is our time size?

Interesting way to phrase the question, the answer is... depends on relative motion. The notion of simultaneity which is the technical term for what you describe as "being flat" in the time-direction is not an absolute thing in relativity. With respect to your own reference system you are essentially "flat" in the time direction [1], but with respect to someone that moves at a certain speed with respect to you you have certain size in the time direction.

The best way to picture it is via Minkowski diagrams, you can find some neat visualizations in the wikipedia article: https://en.wikipedia.org/wiki/Relativity_of_simultaneity

Simply picture an extended object in the x direction, Lorentz transformations are going to rotate it slightly in the time direction causing it to become extended in the time direction.

[1] If you take it to the extreme you may consider that different parts of your body move with respect to each other and thus they are not in the same reference frame. In which case, not even with respect to yourself are totally "flat" in the time direction.

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

#342

Post by mocmoc »

immmmmm wrote: post #315

Physicist here, gravity is a force, just a different one. Also, like everything else in physics: it depends how you observe it. For instance, electromagnetism comes from the curvature of a U(1) bundle over space time, the (local) U(1) symmetry yields electromagnetic interactions. For gravity the symmetry is the (local) Pointcaré (SO(1,3) + translations) symmetry and curvature of spacetime itself. Also gravity on Eart…

No , is not a force

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

#343

Post by immmmmm »

serial_dev wrote: post #339
immmmmm wrote: post #315

Physicist here, gravity is a force, just a different one. Also, like everything else in physics: it depends how you observe it. For instance, electromagnetism comes from the curvature of a U(1) bundle over space time, the (local) U(1) symmetry yields electromagnetic interactions. For gravity the symmetry is the (local) Pointcaré (SO(1,3) + translations) symmetry and curvature of spacetime itself. Also gravity on Eart…

Physicist (BSc) turned software developer here, I understood around 2% of your comment :) (Thanks for the effort, though, it might just be me, in the end I changed profession for a reason)

Ah sorry, i did 8 years of GR and strings..

I was replying to the headline: 'umm, yes and no...'

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

#344

Post by vl »

immmmmm wrote: post #315

Physicist here, gravity is a force, just a different one. Also, like everything else in physics: it depends how you observe it. For instance, electromagnetism comes from the curvature of a U(1) bundle over space time, the (local) U(1) symmetry yields electromagnetic interactions. For gravity the symmetry is the (local) Pointcaré (SO(1,3) + translations) symmetry and curvature of spacetime itself. Also gravity on Eart…

Original page kinda doesn’t explain the idea in simple terms. This Veritasium video gives detailed “popsci” explanation:

https://youtu.be/XRr1kaXKBsU

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

#345

Post by abdullahkhalids »

jstanley wrote: post #330
immmmmm wrote: post #315

Physicist here, gravity is a force, just a different one. Also, like everything else in physics: it depends how you observe it. For instance, electromagnetism comes from the curvature of a U(1) bundle over space time, the (local) U(1) symmetry yields electromagnetic interactions. For gravity the symmetry is the (local) Pointcaré (SO(1,3) + translations) symmetry and curvature of spacetime itself. Also gravity on Eart…

> Physicist here, gravity is a force, just a different one. Isn't gravity an acceleration, and weight a force? Otherwise gravity would accelerate heavier objects more slowly.

You are making a semantic argument, so let's clear it up.

Gravity is the natural phenomena, or experimental observation, whereby bodies with mass appear to gravitate towards each other. One model and explanation of this phenomena is Newton's Law of Gravity used within the framework of Newton's dynamics (or laws of motion). The Law claims that gravity is a force, with a very specific formula we are all aware of.

If you ignore Newton's third law (about action-reaction pairs) for the gravitational phenomena of small bodies near a planet, and assume that the planet doesn't move at all, then we call the force on the small body its weight.

Now, returning to your question, in daily lingo, physicists use the word gravity to refer to the phenomena, the force law, or the weight. You have ascertain from context. In summary, gravity is a force is a perfectly valid and understandable statement within the slang of physics.

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

#346

Post by andrey_utkin »

jstanley wrote: post #330
immmmmm wrote: post #315

Physicist here, gravity is a force, just a different one. Also, like everything else in physics: it depends how you observe it. For instance, electromagnetism comes from the curvature of a U(1) bundle over space time, the (local) U(1) symmetry yields electromagnetic interactions. For gravity the symmetry is the (local) Pointcaré (SO(1,3) + translations) symmetry and curvature of spacetime itself. Also gravity on Eart…

> Physicist here, gravity is a force, just a different one. Isn't gravity an acceleration, and weight a force? Otherwise gravity would accelerate heavier objects more slowly.

A person who has studied physics in a good school here.

Weight is the force a body exerts on its support.

Force of gravity is a force exerted by each body in a system onto every other body.

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

#347

Post by tsimionescu »

ben_w wrote: post #337
jstanley wrote: post #330

Earlier quoted context omitted.

> Physicist here, gravity is a force, just a different one. Isn't gravity an acceleration, and weight a force? Otherwise gravity would accelerate heavier objects more slowly.

Heavy objects are made of many light objects[0], so no, they would still accelerate at the same rate. [0] with the exception of light, which is very light[1], but made of light particles rather than light particles. [1] Homographs ahoy

There are heavy objects that are not made of lighter objects as far as we can tell, such as the Higgs boson or quarks. As far as we currently know, they should also fall at the same speed near the earth.

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

#348

Post by [deleted] »

srtjstjsj wrote: post #203
willis936 wrote: post #195

Earlier quoted context omitted.

I don’t think this is correct. I think the shell theorem says that the gravitational forces cancel if you are on the inside of a hollow sphere and all mass is on the surface. A perfect sphere of uniform density would not meet the shell theorem assumptions.

Yes, it would. A solid ball (filled sphere) is just a union of many shells (hollow spheres), so the theorem still applies. en.wikipedia.org/wiki/Shell_theorem

[deleted]

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

#349

Post by anotheryou »

The first explanation that actually worked for me: https://youtu.be/jlTVIMOix3I?t=55

(also nice engineering on the physical "space time stretcher" he built)

I understood it rationally before, but only with this video it also feels right.

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