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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

#201

Post by Koshkin »

vikramkr wrote: post #51
paulpauper wrote: post #17

Earlier quoted context omitted.

>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?" Air resistance, wind, and horizonal acceleration. Over long vertical distances, these perturbations in the x-axis cause an arc. Nothing to do with general relativity.

When you're tossing a ball into the air by hand, gravity is going to have a far more dominant effect on things than air resistance and friction. Things still fall on the moon...

Depends on the density of the ball, of course.

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

#202

Post by Koshkin »

manigandham wrote: post #164
paulpauper wrote: post #17

Earlier quoted context omitted.

>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?" Air resistance, wind, and horizonal acceleration. Over long vertical distances, these perturbations in the x-axis cause an arc. Nothing to do with general relativity.

There wouldn't be an arc at all without gravity/relativity because the ball wouldn't return.

To be fair, you could make it return by using other forces of nature, e.g. by blowing said air from above.

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

#203

Post by srtjstjsj »

willis936 wrote: post #195
pdonis wrote: post #179

Earlier quoted context omitted.

> if you are inside of the sphere, at least some of the mass will be pulling you away from the point at its center If the mass is spherically symmetric, this will not be the case; all of the Newtonian forces from the masses further away from the center than you are will cancel out. This is called the "shell theorem", and it turns out to hold even in General Relativity.

I don’t think this is correct. I think the shell theorem says that the gravitational forces cancel if you are on the inside of a hollow sphere and all mass is on the surface. A perfect sphere of uniform density would not meet the shell theorem assumptions.

Yes, it would.

A solid ball (filled sphere) is just a union of many shells (hollow spheres), so the theorem still applies.

en.wikipedia.org/wiki/Shell_theorem

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

#204

Post by CamperBob2 »

pdonis wrote: post #189
umvi wrote: post #77

Earlier quoted context omitted.

The difference is that light travels through space, but not time (similar to how a vertical line does not travel the x axis, only the y axis). The ball travels through both space and time. The faster you go, the less you travel through time. Thus, if the ball were travelling at the speed of light, it would not travel through time either and would follow the same path as light.

> The difference is that light travels through space, but not time This is a common pop science statement, but it's not correct. A correct statement is that the concept of "speed through spacetime", which is what has to be split into "speed through space" and "speed through time" in the pop science statement, does not apply to a light ray. In more technical language, the tangent vector to the light ray's worldline is…

I wouldn't say it's incorrect at all. From the point of view of a photon, no time elapses between its the origin and destination endpoints.

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

#205

Post by perryizgr8 »

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.

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

#206

Post by marcosdumay »

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…

> 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?

Gravity is peculiar in that there is only one type of charge and it's exactly equal to the inertia quantity. If you try to do something similar to the other forces, you'll get really complicated models, with hidden dimensions and things that don't interact the same way with them.

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

#207

Post by srtjstjsj »

nealabq wrote: post #138
amelius wrote: post #111

Earlier quoted context omitted.

> 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.

*One focus, of the the two foci.

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

#208

Post by WClayFerguson »

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 (relativity) but nonetheless the correct view is that the light is still moving 'straight' in it's own reference frame.

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. Again from this view you can say light is moving straight and experiencing no acceleration, just like an object in free-fall doesn't 'feel' any acceleration, even though they are accelerating from the perspective of some other reference frame other than it's own.

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

#209

Post by jamemuraca »

Has any papers been written describing/modelling spacetime as a fluid? You also see diagrams of space time as a plane with the gravity coming from the dip in that plane. But that model would hold in a 360 degree view, so I think we should model spacetime as a fluid with the density of that fluid going rise to drag and therefore gravity effects

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

#210

Post by gabia »

shawnz wrote: post #181
gabia wrote: post #159

Earlier quoted context omitted.

The effect of a spacetime transformation isn't just to redefine straight lines along which particles move. It means measurements (e.g. lengths, areas, time intervals) are different depending on where you are in the spacetime. The are forces don't come with these "extra" effects - an EM field doesn't stretch and contract space. However, there are a lot of parallels between electromagnetism and relativity! Quite often…

Also a layperson here. Can you give an example of how we can tell that EM fields don't "stretch" space, but gravity does? Is it just about how light behaves in those fields or is there something more to it?

As mentioned below, one way to think about it is that EM only affects charged particles (and depends on their charge), whereas because gravity is acting on the underlying space-time it has a universal effect on everything (including light).

We can pretty much boil EM down to: like charges repel, unlike attract, strength is charge1*charge2/distance^2. What about magnetic field, photons, QFT etc?? None of this exhibits effects which could be described as stretching space-time either.

But we cannot do the same with gravity. An explanation like the above but for gravity (which is traditional Newtonian) leaves out many, now observed effects such as:

-> time dilation (GPS relies on this calculation) (measures time stretching and contracting)

-> gravitational waves (LIGO) (measures space stretching and contracting)

How do we _know_ any of this? People propose theories, those theories are then tested against experiment. AFAIK to date there is no experimental evidence suggesting EM stretches space, and no theory proposed that includes such an effect and correctly matches experimental data. That's the most holistic answer (but unfortunately one you just have to believe unless you have a lot of spare time!)

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