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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

#251

Post by fantispug »

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…

Kaluza-Klein theory is an attempt to exhibit a transformation that charged particles travel on "straight" lines in a 5-dimensional space. The fifth dimension is supposed to be a very small circle. There are some experimental implications, but the circle is so small that we can't detect them (it's similar with string theories).

String theory tries to do the same thing with other forces; so it's not clear whether there's such a transformation. Forces are technically simpler than embedding extra dimensions that there are no evidence for.

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

#252

Post by Jweb_Guru »

leptons wrote: post #241
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…

I'm not sure how that hypothesis works out with things like black holes and gravitational lenses, friction, and a lot of other established physics. Somehow I figure people that believe in the expansion hypothesis will have some kind of workaround for those.

https://www.mathpages.com/home/kmath077/kmath077.htm is a good overview of why trying to make the expansion explanation even work with Newtonian gravity ultimately leads you to general relativity.

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

#253

Post by SiVal »

kens wrote: post #190
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…

> So if you want to be really pedantic, it's never an ellipse because the Earth is not a point mass. This turns out to be not pedantic but very important if you're guiding an ICBM. And when landing on the Moon, Apollo had to deal with irregularities in the Moon's gravity due to mass concentrations, called mascons. (If you're interested in missile guidance, take a look at the book Inventing Accuracy . Among other thin…

(Ah, yes, pedantry. It has its own gravity. You can't tell me it's not a force. I can't resist its pull.)

It wouldn't be an ellipse even if Earth were a point mass. The gravity of the Moon and Sun, the gravitational lumpiness in the sky, has the same effect as the gravitational lumpiness underground. The combined result may be no closer to an ellipse than it is to a parabola.

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

#254

Post by nimish »

yunruse wrote: post #96
nimish wrote: post #70

Gravity is a force in the same way the centrifugal force is a force An artifact of rotating coordinate frames in one, the curvature of spacetime in the other

Mm, just like we have wave-particle duality, one can also speak of some sort of “force-coordinate duality”, as it were. Just as we let the sine function be linear for small angles, quasiparticles with forces in Cartesian space are wonderful abstractions. Good physics – especially applied physics – is about handling this balance between precision and mathematical/computational simplicity. Mind, research physics can be…

I'm unconvinced that treating spacetime as a quantum field is the right approach -- just a gut feeling -- given gravity "feels" so much different than the quantum theories and general relativity is just so beautifully geometric.

I'd be very sad if we had to lose that and deal with gravitons.

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

#256

Post by jiggawatts »

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.

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

#257

Post by miles »

dan-robertson wrote: post #166
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…

Asking what a force is is a very good question with no good answer. One eighteenth century philosopher (if only I could remember who) said that we do not know what forces are but “time has domesticated them”. At around the time of Newton, what we now call physics changed from being something based in an ontology—a theory of what the world was made of to explain why it worked a certain way—into something mathematised…

> One eighteenth century philosopher (if only I could remember who) said that we do not know what forces are but “time has domesticated them”.

Interestingly, I could only find one other reference to that phrase[1]:

One 18th century natural philosopher wrote that forces were incomprehensible but "time has domesticated them."

[1] Why do we study geometry https://www.dpmms.cam.ac.uk/~piers/F-I-G_opening_ppr.pdf

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

#258

Post by Jweb_Guru »

perryizgr8 wrote: post #205

If I throw a ball straight up, it loses speed. The speed at the top of the parabola is exactly zero. So how is the change in speed explained if there is no force involved? Even in a straight path, a change in speed implies a force acting on the object.

My understanding here is that the ball (like everything else) is moving at a constant "speed" through spacetime. From the Earth's reference frame, when the ball is frozen in space, all of that speed is in the time component (as is yours). The reason its path is parabolic is due to the very slight curvature in spacetime induced by the Earth; if you instead stand in a "flat" reference frame (e.g. freefall) and watch the ball, it will indeed look like it's going in a straight line (but the Earth will follow a curved path in spacetime, allowing it to "catch" the ball). You can verify this using the link to https://timhutton.github.io/GravityIsNotAForce/.

I won't push my luck by pretending to understand how this changes when your spatial velocity is nontrivial (intuitively I feel like it should not even possible to differentiate between spatial and temporal components except relative to your own reference frame, but I could easily be wrong there). But in the case you describe it seems pretty clear.

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

#259

Post by fosspowered »

hhjinks wrote: post #224

If gravity is not a force, how is spaghettification a thing? If you are stationary when you're weightless, how can different stationary parts of your stationary body move at the different speeds required for spaghettification to take place.

The way I try to understand this is that the stationary body is not a single particle but a collection of particles. Since the black hole distorts the space-time gravity in a humongous manner each of the particles in the body at different points in space-time have significant different curvature in space-time and hence would experience the different tidal "forces" which would account for spaghettification.
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