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

timhutton.github.io

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

#361

Post by Smaug123 »

msiyer wrote: post #353
andrey_utkin wrote: post #346

Earlier quoted context omitted.

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.

Physics does not magically become different outside the confines of a good school.

On the other hand, the Internet is black with the ink of people who think they understand physics but do not. Credentials do help you to distinguish someone who has a chance of knowing what they're talking about.

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

#362

Post by meditative »

rsp1984 wrote: post #351

When standing on Earth we experience a frame of reference that is accelerating upwards, causing objects in free-fall to move along parabolas, as seen in the accelerating frame of reference on the left. I'm trying to understand what is meant by this. When I drop an item on the floor it goes there in a straight line, not a parabola. Same if I drop something from a helicopter. Obviously I'm missing something here. Can s…

it will look like a parabola (or ellipsis if you read elsewhere in this thread) if you add another dimension to your graph of the balls position.

That could be a physical dimension, such as throwing a ball to your friend across the room and plotting x vs y. Or even plotting the height of the ball against time.

https://imgur.com/a/ccBDZf6

consider the x-axis of this image the distance between you and your friend, or time.

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

#363

Post by bombadilo »

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)

I'm currently doing a BSc in Physics too.

However, when I've graduated I don't think I would ever call myself a physicist like yourself unless I actually went on to do research in physics as a career, which likely means following the traditional academic path of doing a PhD (since I'm not Freeman Dyson).

Sorry for the nitpick but such titles should be earned don't you think?

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

#364

Post by sizt »

immmmmm wrote: post #343
serial_dev wrote: post #339

Earlier quoted context omitted.

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...'

Physics is the study of making simple things complicated.

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

#365

Post by Communitivity »

Gravity is a force, because if it is not for this reason then probably neither are the other forces. Many physicists believe we will achieve grand unification of the forces, which means that in some way they are all related to the curvature of spacetime and each force can be viewed as applying acceleration in a straight line. It's just that the geometry defining that line, the frame of reference, is different for each force.

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

#367

Post by tzot »

jiggawatts wrote: post #256
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…

Some interesting alternative viewpoints I've seen: Matter continuously destroys spacetime at its location, sucking in the fabric of the universe. The Universe isn't expanding; matter is shrinking. Light isn't redshifted on the way to us, it's just that our sensors are getting smaller relative to the unmodified wavelength of light.

> Light isn't redshifted on the way to us

Indeed; light is blue-shifted as its source is on its way to us (and is red-shifted as its source is on its way away from us).

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

#368

Post by zkmon »

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…

Correct. And just making the trajectory to appear straight by modifying the reference grid doesn't change anything. For example, once can see even exponential curve appear in a logarithmic grid. Force is a relation between the reference frame and the object. It is possible to consider either one as straight or flat.

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

#369

Post by koheripbal »

Communitivity wrote: post #365

Gravity is a force, because if it is not for this reason then probably neither are the other forces. Many physicists believe we will achieve grand unification of the forces, which means that in some way they are all related to the curvature of spacetime and each force can be viewed as applying acceleration in a straight line. It's just that the geometry defining that line, the frame of reference, is different for eac…

Yeah, Tim's argument is fundamentally semantic one.

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

#370

Post by nsonha »

the_arun wrote: post #319
nsonha wrote: post #274

Earlier quoted context omitted.

in Newtonian physics only a force can cause acceleration

Something I learnt in 8th grade - if an object is free falling from sky of mass "m" the force it applies when it hits on the ground is m * g, where g is the gravitation.

I think you are mashing up different lessons:

- when you stand on the ground you are the subject of 2 forces: gravitational force m * g and the opposite reaction force of the ground surface, they cancel each other so you stand still

- when you free fall your acceleration is g and the gravitational force is m * g, this is according to Newton's 2nd law

when you hit the ground depending on your velocity and... deformation, the force of impact will be different because you will de-accelerate to 0 or even bounce back very quickly much faster than g. It's gonna be much stronger than g * m

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