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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

#151

Post by bananabreakfast »

sfink wrote: post #148

Ok, great opportunity for me to ask a dumb question that's been bothering me for a while, for practical reasons I won't go into. How is gravity like a force at all, even in Newtonian physics? It seems like mismatched units. Gravity is an acceleration, not a force. F=ma, right? So if gravity were a force, it would produce an acceleration that was dependent on the mass, and it doesn't, so it seems to me like the only s…

Gravity is a force that acts due to mass just as the electromagnetic force acts due to charge.

Since gravity acts in a magnitude directly proportional to the mass being acted upon, it just so happens that it manifests as an acceleration since the mass cancels out, unlike EM where acceleration due to charge does not cancel out the mass.

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

#152

Post by lostmsu »

wch wrote: post #141
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…

On human scales, the time dimension is much "bigger" than the space dimensions... This is really interesting, and it made me wonder how to convert between space and time. I mean, one meter up is equivalent in magnitude to one meter forward, is equivalent to one meter to the right. Is _c_ the conversion between space and time? In other words, is 300 million meters equivalent in magnitude to one second of time?

This is right, yes. The distance (aka interval) formula for the flat 4D spacetime is

  dist^2 = (c * seconds)^2 - meters^2
So to compensate increase in 1 second you indeed need _c_ ~= 3e8 meters.

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

#153

Post by aflag »

sfink wrote: post #148

Ok, great opportunity for me to ask a dumb question that's been bothering me for a while, for practical reasons I won't go into. How is gravity like a force at all, even in Newtonian physics? It seems like mismatched units. Gravity is an acceleration, not a force. F=ma, right? So if gravity were a force, it would produce an acceleration that was dependent on the mass, and it doesn't, so it seems to me like the only s…

According to Newton's equation, gravity's force is F=(GmM) / r^2. Those are the gravitational masses. The mass in F = ma is the inertial mass. The fact that they are the same is remarkable. If it weren't for that, different mass objects would feel different accelerations due to gravity.

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

#154

Post by GuB-42 »

tim_hutton wrote: post #137
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

It is actually that video that helped me!

The thing is, I watched the French version and I didn't know an English version existed, that's why I didn't post it here, French speakers are, I believe, a minority.

And BTW, the French channel has an 8 part explanation of the maths behind general relativity that is the best I have ever seen. It is on a level above most pop science video since it actually shows the equations, tensors, etc... but the explanations are actually quite accessible.

https://www.youtube.com/watch?v=sg2BBldgKpo

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

#155

Post by ncallaway »

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…

Newton's first law provides a guide: "Every object persists in its state of rest or uniform motion in a straight line unless it is compelled to change that state by forces impressed on it."

So, the definition of a force as something that causes an object to change its movement from a "straight line" comes to us from Newton's laws.

> 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?

It's not just the fact that the transformation exists that means we don't really consider gravity to be a force. It's that the transformation exists, and provides useful predictions about the universe that turn out to be backed by experiment.

Under the theory of general relativity gravity isn't a force, because the fundamental premises of general relativity assume that gravity is actually a distortion of spacetime caused by mass.

We say that "gravity" isn't a force simply because the predictions made by general relativity have been validated by a number of experimental observations—at least on the macro scale.

> 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?

I'm certainly not an expert, so I can't really comment on this. My best guess is that there doesn't exist any such transformation that causes charged particles to travel on "straight" lines that makes experimental predictions as well as our current scientific theories.

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

#156

Post by manigandham »

vbezhenar wrote: post #4
umvi wrote: post #3

Earlier quoted context omitted.

Great Veritasium video. So some flat earth arguments are actually correct if general relativity is correct, namely that gravity is an illusion and that the real reason we are stuck to the earth is that the earth is accelerating toward us at 9.8 m/s^2

I don't understand it. How can it accelerate towards anyone on its surface? Where does it get energy to accelerate? We generally need to burn fuel to accelerate something in the space.

This is about relativity. We're going straight in space-time, but space-time itself is curved because of the heavy mass nearby (the Earth). This is visualized by the rocket curving towards the planet in the video, and the bent sheet experiment where the balls spiral towards the center.

So we're curving in towards the center of the mass of the Earth, but the reason we don't end up in the core of the Earth is because the surface stops us. The Earth is "pushing" us away from the center, and that's the acceleration. It's accelerating you off your straight line path, and this is the deviation from the geodesic.

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

#157

Post by caf »

sfink wrote: post #148

Ok, great opportunity for me to ask a dumb question that's been bothering me for a while, for practical reasons I won't go into. How is gravity like a force at all, even in Newtonian physics? It seems like mismatched units. Gravity is an acceleration, not a force. F=ma, right? So if gravity were a force, it would produce an acceleration that was dependent on the mass, and it doesn't, so it seems to me like the only s…

In Newtonian physics it's a force with a magnitude that is proportional to the mass of the body.

For two different objects at the same point in the same gravitational field, the difference in gravitational force due to their differing mass exactly cancels out the difference in acceleration due to their differing mass, so they both accelerate at the same rate.

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

#158

Post by avmich »

avmich wrote: post #87

A pretty good model. Thank you. The model shown is for 1D space and 1D time, and this 2D spacetime is curved, remaining 2D. Can you though show us a 3D model of curved 2D spacetime? Maybe as a rotatable, scaleable 3D scene, where more complex phenomena, like planetary motion around a star, would be possible to show?

Read TODO list on Github :) . Looks like it's harder than appears...

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

#159

Post by gabia »

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…

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 relativity effects are introduced with an EM analogy, e.g. gravitational waves (which have polarisation) and electromagnetic waves ie photons (which also have polarisation). Note though it really is an analogy - they are fundamentally different things in both reality and mathematical form.

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

#160

Post by lostmsu »

Ancapistani wrote: post #102
Smaug123 wrote: post #82

Earlier quoted context omitted.

Yes, although you can't actually get anything with mass to travel at 1c (subject to our current understanding of physics).

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.
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