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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

#281

Post by Agentlien »

hunterloftis wrote: post #185
atoav wrote: post #15

Earlier quoted context omitted.

This is in tune with what happens when you change the timescale in a game engine with proper physics.

I'm not aware of any game engines that simulate general relativity / 4D spacetime. I think what you may be noticing is that, as you reduce the tick frequency of a newtonian physics simulation, parabolas become less accurate as integration error accumulates.

Along those lines: during my university years, this is what I saw in my homemade engine before I had a good understanding of numerical stability.

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

#283

Post by raducu »

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…

So, do gravitons exist or not?

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

#284

Post by hartator »

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.

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

#285

Post by nsonha »

caf wrote: post #176
ars wrote: post #76

Would not an electron and a positron trace out an identical line/parabola? Would that mean electromagnetism is not a force either? What is the difference between the two that makes one a force and one not a force?

You can't create a similar transformation for an electric field under which the motion of a proton, electron, muon etc are all straight lines. The indicator that gravity is special is the fact that 'inertial mass' and 'gravitational mass' are the same thing.

> gravity is special 'inertial mass' and 'gravitational mass' are the same thing.

That's undecidable and gravity is not special in that way

https://physics.stackexchange.com/questions/390540/what-is-t...

> the gravitational field and acceleration are inductive pairs (similar to the electromagnetic field and electric current.)

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

#286

Post by pdkl95 »

est31 wrote: post #55
Smaug123 wrote: post #43

Earlier quoted context omitted.

Good point; this becomes more obvious if you imagine throwing the ball up and then immediately collapsing all the mass of the Earth into a single point at the centre. What path does the ball follow now? It's probably following a path we would more usually call an "orbit", and it sure looks a lot like an ellipse. Now just put the mass of the Earth back where it was, and notice that the ball hits the ground before it c…

Despite both being conic sections, cutting up an ellipse won't yield you parabolas. An ellipse has two focal points to which the sum of the distances is constant, while a parabola has a focal point and a directrix line to which the difference of the distances is constantly 0. Two different things.

Parabolas are just a special case of ellipses where one focus is infinitely far away.

Vi Hart has a nice explanation: https://www.youtube.com/watch?v=v-pyuaThp-c

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

#287

Post by js8 »

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…

Hm, I think visualizing this would be a nice topic for a 3Blue1Brown's video or something..

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

#289

Post by bawolff »

aeternum wrote: post #65
didibus wrote: post #40

I'm a bit confused, how does a 2D graph with a curved X axis work? Does this just mean to say that if we saw things distorted with an outward curve, then something that is moving in a curve would look to be moving in a straight line? And what's the point of such an observation?

The polar or cylindrical coordinate systems are examples of graphs that can be though of as having a curved x-axis.

Sure, but that is irrelavent since the topic at hand is neither a polar nor cylindrical graph.

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

#290

Post by Jweb_Guru »

Yuioup wrote: post #278

So ... what is terminal velocity then?

Terminal velocity is entirely a product of air resistance (or in general, resistance from any fluid medium). That's mostly caused by electromagnetic interactions (AFAIK) which despite physicists' best efforts is still best explained as a "real" force. Hence why objects with different shapes or masses can have very different terminal velocities on Earth (and the same is true if you vary the atmospheric composition, or consider objects falling in water). But they will all fall at the same rate in a vacuum, unless the object is massive enough for gravity to warp the trajectory of the object onto which it's falling.
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