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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

#161

Post by Misdicorl »

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?

Correct, the speed of light is the conversion factor

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

#162

Post by manigandham »

k__ wrote: post #149
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 would spacetime know to contract right to the center of earth?!

The mass of the Earth curves space-time.

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

#163

Post by marta_morena_28 »

ncallaway wrote: post #155
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…

So what you are saying is that if we assumed that all particles, including light are magnetic, and everything that has a mass, emits a corresponding magnetic field with a strength relative to its mass, we could not form a similar theory of "general magnetic relativity" in which the frame of reference under magnetic fields would behave in a similar way it does for gravity?

That seems kinda odd. What exactly would provide this difference? How is magnetism different from gravity? Is it that gravity doesn't have a mass, but magnetic fields do?

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

#164

Post by manigandham »

paulpauper wrote: post #17
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 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.

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

#165

Post by mensetmanusman »

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?

We don’t know, because we don’t know if the past or future are objectively real, or if we ride the wave so to speak.

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

#166

Post by dan-robertson »

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 where the equations accurately predict the behaviour of the world but there is no ontology other than that the universe is a universe where those equations hold. Modern physics still fundamentally works this way (see Maxwell’s equations, quantum physics, etc). Compare to, for example, Descartes with his theory of corpuscles and (totally wrong) billiard ball mechanics, or just about any other ontology from before him.

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

#167

Post by manigandham »

xwdv wrote: post #114
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…

Does this mean some sort of artificial gravity may be possible after all by causing some kind of inflation?

If we can somehow control mass then yes. But it's not artificial, it's just gravity.

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

#168

Post by manigandham »

c06n wrote: post #28
pininja wrote: post #2

The video for this page provides great context https://youtu.be/XRr1kaXKBsU

I do not understand how you can have acceleration without changing position (at 10:06). Acceleration is the derivative of speed, which is the derivative of position change. If the position change is zero, how can the acceleration be non-zero?

Your position in spacetime is changing. You're going straight in spacetime, but spacetime is curved by the mass of the Earth so you're following that curve into the center.

The surface of the Earth keeps you from actually falling in, and is therefore pushing you away or upwards from the center. This is the acceleration acting on your straight line path through curved spacetime. This is the deviation from your geodesic.

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

#169

Post by karlicoss »

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…

Doesn't directly answer what is a force, but perhaps you'll find this perspective interesting: https://en.wikipedia.org/wiki/Static_forces_and_virtual-part...
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