Gravity is not a force – free-fall parabolas are straight lines in spacetime • Hacker News Board

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

#131

Post by abhishekjha » Sun, Oct 18, 2020, 11:51 PM UTC

Ancapistani wrote: post #101

cylon13 wrote: post #34

Earlier quoted context omitted.

We're moving through spacetime with a constant speed, so the faster you move through space, the slower you move through time.

I just had another thought. The word “dimension” in this context is overloaded. We think of space being three dimensions but really it’s only one - velocity relative to a specific reference frame. Thinking of it this way, the word “spacetime” makes sense; it’s a two-dimensional system: “spatial velocity” (S) on one axis and “temporal velocity” (T) on another. Both velocities are always measured against a reference fr…

By c do you mean the speed of light? How is it dimensionless?

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

#132

Post by umvi » Sun, Oct 18, 2020, 11:51 PM UTC

vecter wrote: post #80

umvi wrote: post #22

Earlier quoted context omitted.

I don't think that is quite right. If you travelled near the speed of light and then slowed down again it would look (from your perspective) like you jumped 10 years into the future. So high gravity environments "moving slower" shouldn't result in a jump to the future upon exit. Instead I think what is happening is that massive objects actually stretch the fabric of spacetime somehow so that the closer you are to the…

> the slower you travel through both space and time No. You are always traveling at a constant velocity through spacetime. [0] Spacetime: https://en.wikipedia.org/wiki/Spacetime

Ok then in that case I think of it as massive objects tend to compress space the closer you get to them. So though your velocity is constant, you are travelling through more space the closer you get to a massive object (since the space is now compressed) which means you are travelling through less time (in order to maintain same velocity).

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

#133

Post by sudosysgen » Sun, Oct 18, 2020, 11:51 PM UTC

asdfasgasdgasdg wrote: post #109

avmich wrote: post #73

Earlier quoted context omitted.

Spherical mass can be replaced with a point mass. Earth however is not spherical (biggest deviation is polar flattening). And even then, Einstein's model, unlike Newton's, says it's not an ellipse even for a point mass. So, moral of the story - we have to be not too pedantic.

> Spherical mass can be replaced with a point mass I'm not a physicist, but that doesn't sound right. One example is that if you are inside of the sphere, at least some of the mass will be pulling you away from the point at its center. I think what you mean is that a point mass closely approximates a spherical mass, but the degree to which that is true becomes less and less the closer you get to the center. I don't t…

The intuition we came up with when we had to solve this issue in undergrad physics was interesting.

For each point of any given distance, you can find a disk of points at the plane of the same distance whose gravitational pull will be equivalent to a single point at their center.

You do this for all distances x, and you will find an equivalent rod going from the attracted objected to the center of the sphere, of a non-uniform but symmetrical density.

We find that the density is linearly correlated to the area of the corresponding section, and we then take the closer half of the rod, multiply the density by 1/r^2, and find that it is constant!

For the far half of the rod, we cannot do this, so instead we divide it into two halves of equal pull, then divide those to halves and so on, and find that it this reduces to the equivalent of a point.

Now that we have two points in-line with the object, we find the point with the same total mass that exerts and equivalent force, and lo and behold it is at the center.

I'm sure there is a much simpler way to intuit it, though :)

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

#134

Post by handol » Sun, Oct 18, 2020, 11:52 PM UTC

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?

If you're walking at 1 meter per second with a 1 meter stride. The space-time interval between your footsteps is: sqrt( (299,792,458m)^2 - 1m^2))

299,792,458m is for the distance traveled by light in one second. The -1m is for the distance between your feet.

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

#135

Post by xt00 » Sun, Oct 18, 2020, 11:53 PM UTC

zestyping wrote: post #119

This video really cleared up a lot of misunderstandings for me: https://www.youtube.com/watch?v=wrwgIjBUYVc I had never visualized it this way in my head before, and it all makes a lot more sense now. Highly recommend!

Thats a great video

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

#136

Post by logicallee » Sun, Oct 18, 2020, 11:56 PM UTC

I was taught that gravity is a force (like electromagnetism, or the strong and weak nuclear forces.)

The title says "gravity is not a force". In the article's perspective, why does a kitchen scale report a higher number when I place an object on it?

(In my perspective there is gravitational attraction between the object on the scale and the rest of the Earth, and the the scale, assuming it is "level" (set perpendicular to the direction of gravitation toward the center of the Earth) reports the magnitude of that attraction.)

If gravity isn't a force, then when the object and Earth are not in freefall or moving, what does the kitchen scale measure?

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

#137

Post by tim_hutton » Sun, Oct 18, 2020, 11:57 PM UTC

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…

In another comment zestyping posted this video which also has spacetime contracting, in a sense: https://www.youtube.com/watch?v=wrwgIjBUYVc

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

#138

Post by nealabq » Sun, Oct 18, 2020, 11:57 PM UTC

amelius wrote: post #111

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…

> an ellipse in a circular gravity field coming from a point mass. Even if you consider the point mass to move because of the mutual attraction?

One foci of the ellipse is the earth+ball center of gravity. The earth is also travelling in a much smaller ellipse around that same foci.

Assuming a uniform+spherical earth. And also a uniform+spherical ball I supposed.

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

#139

Post by thesz » Sun, Oct 18, 2020, 11:59 PM UTC

That brought up the verse from Pushkin [1]:

    One bearded sage concluded: there's no motion.
    Without a word, another walked before him.
    He couldn’t answer better; all adored him
    And all agreed that he disproved that notion.
    But one can see it all in a different light,
    For me, another funny thought comes into play:
    We watch the sun move all throughout the day
    And yet the stubborn Galileo had it right.

[1] https://ruverses.com/alexander-pushkin/motion/

Both are right, in short. We observe parabolas and we are right counting on them - we have great successful experience using these prediction. GR theory predicts lines, they are right too, but in different context.

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

#140

Post by abeppu » Mon, Oct 19, 2020, 12:01 AM UTC

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 transformation that causes charged particles travel on "straight" lines?