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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

#291

Post by jayd16 »

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…

How do things in orbit work if everything is inflating and space is contracting but nothing falls?

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

#292

Post by p1mrx »

herodoturtle wrote: post #118
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 never studied this stuff, so the genius of your intuitive explanation is appreciated. My intuitive response to your intuitive explanation: This ball is moving through spacetime relative to the earth, which is in turn also moving through spacetime relative to the sun - and so light is being deflected off of this ball at each point in its position in spacetime relative to the sun for much longer than light is being d…

> have I got that right?

Probably not, because "the sun" appears in your response. If you're watching a ball move on Earth, the sun is an irrelevant variable.

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

#293

Post by exDM69 »

SiVal wrote: post #253
kens wrote: post #190

Earlier quoted context omitted.

> 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.

But on low earth orbit, the L2 term of earth's oblateness dominates by an order of magnitude compared to the moon and the sun, and the rest of the planets are negligible.

Source is a table in the first chapters of Fundamentals of Astrodynamics.

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

#294

Post by raverbashing »

General Relativity may be a theory with an excellent prediction power but to me it is the one that's lacking in realism (as in the philosophical definition)

It's not a fundamental problem, nobody has abandoned Maxwell's equations for EM except for the most specific cases, but it's a similar case.

There's probably a better explanation than just "distortion of space time" which is a great way of viewing it, but it's a bit of a stretch (pun intended)

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

#296

Post by lopmotr »

pdonis wrote: post #277
lopmotr wrote: post #268

Earlier quoted context omitted.

But is there any physical way to distinguish these fundamentally different situations? If not, then perhaps the fundamentalness of it is just an artifact of the formulation. I'm thinking of solar neutrinos which, for a while, we weren't sure if they were massless or not. We had to observe them experiencing a duration of time to conclude they were massive. If we didn't find that, maybe it was just an even shorter dura…

> is there any physical way to distinguish these fundamentally different situations? Are you asking if there is a way to distinguish a timelike object from a lightlike object? Of course there is. The fact that, for something that has a very, very small invariant mass, it might be practically difficult does not change the fundamental principle. Also note that the reason it was difficult, for example, to tell whether n…

It still doesn't sound physically distinct any more than distinguishing any continuous quantity as being zero or nonzero. If we measure something that looks like 0, we can't be sure if it's just below the sensitivity of our instruments.

For neutrinos, even if we accelerated an rocket and somehow checked if a neutrino was at rest relative to it, we might find that it's not. That means we won't know if we need more speed or if it's impossible. I suppose it's a bit easier than that because we only have to accelerate the rocket fast enough that the neutrino's speed becomes measurably less than c, rather than 0. But still, what if we can't even get it to go fast enough for that? No way to prove that it's travelling at c, it seems.

I'd like to add that even photons have a nonzero upper bound to their possible rest mass. At least they used to. Is there any way, in principle, to show that it's exactly zero, and thus falls into this distinct category?

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

#297

Post by Ace17 »

hartator wrote: post #284
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…

Except that the Apple is exerting the same force on earth.

From what I understand, once you accept that gravity isn't a force, the usual symmetry between the earth and the apple doesn't hold anymore.

The apple doesn't curve space-time as much as the earth does.

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

#298

Post by rawoke083600 »

lostmsu wrote: post #160
Ancapistani wrote: post #102

Earlier quoted context omitted.

As I understand it, the only thing that can travel at c are: * photons * spherical, massless cows in a vacuum

Not sure about cows. I bet a thing traveling at c might not exactly have 3 spatial dimensions.

Agreed ! The "spherical cow", has to first become a circular cow /s

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

#299

Post by Ace17 »

WClayFerguson wrote: post #208

Here's another 'though experiment' I like which some people disagree with, by not understand reference frames: Light always travels in straight lines. Even when light is experiencing a gravitational lensing and looks to us from earth that it's bending around a star or whatever, from the perspective of the light beam itself, it's moving in a straight line. It's entire reference frame is bent compared to ours (relativi…

> Also if the light wasn't moving straight that would mean it's changing direction, which is the same as an acceleration, and a beam of light traveling thru a gravitational field feels no acceleration, because it's not accelerating.

You could have both a deviation (i.e tangential acceleration) and a constant speed.

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

#300

Post by jhoechtl »

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…

To extend the pendantry: Don't we have to account that the onserved object isn't a point mass either?
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