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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

#51

Post by vikramkr »

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.

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...

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

#52

Post by sixo »

dreamcompiler wrote: post #41
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 think of it as being because the ball has mass and photons don't. So Newton's Gm1m2/r^2 = 0 for photons, and you have to use Einstein to measure the "force" on a photon. But because massive objects also cannot move as fast as photons, we're probably both saying the same thing from two different perspectives.

Curvature due to gravity does not depend on mass, which is kind of hinted at when mass cancels in F=ma for that force.

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

#53

Post by phkahler »

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've always wanted to know why a ball doesn't follow a beam of light if they are both following straight lines in spacetime.

But even more important, if light beams are reversible under relativity (reflected off a mirror they will backtrack the same path) then light can not enter a black hole because its reversed path could allow it a way out. But then there's that whole thing of objects falling in appear to slow and stop as they approach the event horizon, so maybe light doesnt enter after all.

My conclusion is that you cant really understand it without serious study of under someone who already gets it.

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

#54

Post by j1vms »

Gravity is not a force. The surface of the Earth is moving up to the object in free-fall at an acceleration of 9.8 m/s^2. The force pushing the surface, and the pressurized atmospheric shell, upward is a result of the processes occurring within the Earth (likely, in particular, those within the the core).

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

#55

Post by est31 »

Smaug123 wrote: post #43
sohkamyung wrote: post #36

Earlier quoted context omitted.

Off topic, but in reality, the trajectory of a ball thrown on Earth is not a parabola, but an ellipse [1]: > under the laws of gravity, a parabola is an impossible shape for an object that's gravitationally bound to the Earth. The math simply doesn't work out. If we could design a precise enough experiment, we'd measure that projectiles on Earth make tiny deviations from the predicted parabolic path we all derived in…

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.

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

#56

Post by saagarjha »

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?

If you watch a bit more, there's another term that is added.

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

#57

Post by ithkuil »

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?

"All tragedies are finished by a death, all comedies by a marriage." —Lord Byron

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

#58

Post by nobody9999 »

chrisseaton wrote: post #47
nobody9999 wrote: post #44

Earlier quoted context omitted.

>How do you get away from all gravity? Aren't you subject to gravity from all other matter in the universe at all times? Yes. But the effect of the distortion of space-time that we call "gravity" is subject to the inverse square law[0]. This means that, as Newton described: "The gravitational attraction force between two point masses is directly proportional to the product of their masses and inversely proportional t…

> but not eliminated So... you can't get away from it. That's what I said isn't it?

Yes, you did. However, it depends on how you define "getting away" from it.

At some point, the effect is so small that it either can't be measured or even if it can, the effect is so small that any impact is irrelevant in practical terms.

If that's the case, you're effectively "getting away" from it.

I guess it's a matter of perspective.

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

#60

Post by jeremyjh »

phkahler wrote: post #53
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've always wanted to know why a ball doesn't follow a beam of light if they are both following straight lines in spacetime. But even more important, if light beams are reversible under relativity (reflected off a mirror they will backtrack the same path) then light can not enter a black hole because its reversed path could allow it a way out. But then there's that whole thing of objects falling in appear to slow and…

Light is moving so much faster. In the same two seconds it crosses a much larger distance in space.

The line is only straight in spacetime, not in space.

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