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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

#421

Post by yetanotherjosh »

hinkley wrote: post #403
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 me if I'm wrong please, but the 'curved space' strategy describes the velocity of two objects in a 2 body scenario, but it doesn't describe the behavior when the two objects have zero velocity relative to each other, right? The 'two spheres in a box' experiment for testing the gravitational constant has no relative velocity at all, so how could 'curved space' describe the force between them?

As I understand it, even objects with no velocity in space still have velocity in time, and the curvature of spacetime shifts it from the time component into the spatial components. In other words an object's velocity is a 4 dimensional vector, and the curvature of spacetime causes the angle in 4d spacetime of that vector change, while still preserving total momentum, thereby accelerating the object in space while decelerating it in time. So as an object falls into a gravity well, it actually moves more slowly through time, transferring more and more of its total momentum from temporal motion and into spatial motion.

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

#422

Post by raducu »

lovegoblin wrote: post #395
raducu wrote: post #283

Earlier quoted context omitted.

So, do gravitons exist or not?

Unknown. They are only hypothetical.

My point was -- if gravity is not a force, it does not require a force carrier so there is no point for gravitons to exist.

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

#423

Post by shawnz »

antonvs wrote: post #264
abeppu wrote: post #140

I've seen this explained elsewhere, and it does look very cool when displayed this way. But perhaps someone with more background can explain to a lay person -- what even is a force? Why does the existence of a transformation that makes movement under a supposed force actually follow a straight line mean it's not really a force? For the other forces (e.g. electromagnetism) can we say that there's _no way_ to exhibit a…

An intuitive way to tell if something is a "real" force (as proposed to a pseudo- or fictitious force) - can someone subject to that force feel it, or equivalently, can an accelerometer measure the acceleration it produces? When you accelerate or decelerate in a vehicle, you feel it. But when you jump out of a plane and accelerate towards the ground, you don't feel a force. Noticing this fact was a big part of what l…

If all the particles in your body were made magnetic and you jump out of a plane on a weightless magnetic planet, how would it feel different than jumping out of a plane on Earth?

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

#424

Post by Rapzid »

edna314 wrote: post #379
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…

I was always wondering if there is a way of distinguishing a straight line in a curved space from a curved line in a flat space, given that you cannot 'look' at space time from the outside. Would that be possible?

The usual suspects; lasers and mirrors. That's how gravitational waves were detected.

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

#425

Post by immmmmm »

jfengel wrote: post #390
immmmmm wrote: post #388

Earlier quoted context omitted.

correct! but if you introduce a derivative which is covariant under diffeomorphisms and u(1) transformations D = d + \omega + A you see that gravitation (connection \omega) enters on a somewhat same footing as the electromagnetic potential (A). there are nuances ofc, but the view (force/curvature) depends from where you're looking. also in real life particles don't real exist... they're excitations in a field

"Come now, do you really expect me to do derivatives that are covariant under diffeomorphisms and u(1) transformations in my head while strapped to a centrifuge?" "No, Mr Bond, I expect you to die." Obligatory https://xkcd.com/123/

hahaha thanks!

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

#426

Post by imtringued »

phkahler wrote: post #53
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've always wanted to know why a ball doesn't follow a beam of light if they are both following straight lines in spacetime. But even more important, if light beams are reversible under relativity (reflected off a mirror they will backtrack the same path) then light can not enter a black hole because its reversed path could allow it a way out. But then there's that whole thing of objects falling in appear to slow and…

> I've always wanted to know why a ball doesn't follow a beam of light if they are both following straight lines in spacetime.

Because light is much faster. Throw a ball at the speed of light and you will see exactly the same path.

Lets say you shoot rifles with different calibers. Each time the bullet takes one second to hit the ground but the faster bullets travel longer distances. Light is so fast it escapes from the planet but if the planet were big enough even light would hit the ground like a bullet.

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

#427

Post by Smaug123 »

tsimionescu wrote: post #400
mocmoc wrote: post #381

Earlier quoted context omitted.

Before downvoting , prove Einstein wrong

This is like that silly discussion of the centrifugal force. Forces are mathematical models, not fundamental properties of reality (we don't know what the fundamental properties of reality are).

It's one of a general class of problems: "we are each using a word to mean something different". Taboo the word "force" and see if you still disagree :P

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

#428

Post by immmmmm »

bombadilo wrote: post #363
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)

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?

i did a phd, got highly cited and was promised to a successful career. i did not continue tho because:

1) the high-energy field is in deep crisis, unlike in Dyson era there is almost no new experimental data.

2) seeing people trying to get a temporary position looked pretty much like a pack of dog on a single bone with few left scraps of meat. for getting to the bone marrow (aka permanent pos) one basically had to kill all the other dogs then wait the bone crack open (i can tell you the resulting science is not always of high quality)

3) i wanted to do sth useful for mankind.

so yes i was not masochistic enough to become a career physicist, even if i am a respected expert in my field. also i don't think you can compare present day physics to Dyson era, science careers these days are more about social skills and PR.

so you're correct: i'm an aborted / half-backed physicist, who is highly p* of what the field has become, converted to computer sciences, just to realise my new field is as scientifically crooked as the first one.

i'll try to get it right next time :)

... at least this time i didn't get down voted too much on HN

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

#429

Post by TheRealDunkirk »

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…

> So if you want to be really pedantic

Based on what I've seen over the years I've been around here, I'm quite certain that's a primary motivator for a non-trivial number of commenters here.

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

#430

Post by pdonis »

CamperBob2 wrote: post #414
pdonis wrote: post #409

Earlier quoted context omitted.

> You can go argue with Neil deGrasse Tyson Show me an actual textbook or peer-reviewed paper Tyson has written where he makes this claim. Pop science videos don't count. (Tyson is by no means the only one; Brian Greene is notorious for the same thing.) You won't be able to because there aren't any. No scientist who talks about a photon "experiencing zero time" in informal contexts will try it in a textbook or paper.…

Dude, this is basic theory at the high school level. Here's how it works: spacetime is 4-dimensional, and a vector in spacetime is always c units long. You can restrict your travel solely to X, Y, Z, or time if you like. If you do that, the other three components are going to be zero. Photons put it all into the X, Y, and Z components, leaving nothing for the t component. They experience a change of position in space…

> Dude, this is basic theory at the high school level.

No, Tyson's claim is not "basic theory at the high school level". It is a particular interpretation of a theory (Special Relativity) that does not work, for the reasons I gave.

> Photons put it all into the X, Y, and Z components, leaving nothing for the t component.

Wrong. The spacetime vector that describes a photon's trajectory does have a t component.

> Seems that a fellow named Maxwell got a lot of mileage out of the concept

Dude, if you think the concept Tyson described is the same as the concept that Maxwell got a lot of mileage out of, then you are the one who needs to learn more about "how it works".

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