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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

#261

Post by skykooler »

hhjinks wrote: post #224

If gravity is not a force, how is spaghettification a thing? If you are stationary when you're weightless, how can different stationary parts of your stationary body move at the different speeds required for spaghettification to take place.

It's only a thing in an extremely high gravity gradient, such that one end of you experiences far higher gravitational forces than the other. This means that while you are in freefall "on average", overall there is a stretching effect as parts of you closer to the source of gravity try to accelerate faster and parts farther away are lagging behind.

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

#262

Post by skykooler »

kubanczyk wrote: post #21
a3w wrote: post #12

Somewhere I read that a free fall parabola does not even take into account earth's curvature. Although I cannot remember, what kind of function describes the reference system specific ``path, as a function of time''. Could this be named more correctly: Gravity is not a force – free-fall hyperbolas are straight lines in spacetime (timhutton.github.io) ?

The planet's curvature is not very relevant to the article, but, yes, if an object is in a free fall at a slow speed (I mean non-orbital) its trajectory is: - parabola if you assume flat&infinite ground, - ellipsis if you assume a spherical planet (an ellipsis is crossing the planet's surface). This is Newton, not GR.

Of course, a very very eccentric ellipse approximates a parabola quite well. You only start to notice errors when dealing with trajectories dozens of miles long (on Earth).

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

#263

Post by strken »

manigandham wrote: post #156
vbezhenar wrote: post #4

Earlier quoted context omitted.

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 t…

I thought that the Earth curved spacetime, and since an inertial observer approaching the earth would follow a straight line through curved spacetime, they'd appear to be following a curved line through space towards the earth.

What's confusing me here is the notion that when two objects collide, they accelerate into each other. Why and how is force constantly applied after the collision? My intuition is falling down here, and none of the resources I've looked at so far have explained why the acceleration happens.

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

#264

Post by antonvs »

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…

An intuitive way to tell if something is a "real" force (as proposed to a pseudo- or fictitious force) - can someone subject to that force feel it, or equivalently, can an accelerometer measure the acceleration it produces?

When you accelerate or decelerate in a vehicle, you feel it. But when you jump out of a plane and accelerate towards the ground, you don't feel a force.

Noticing this fact was a big part of what led Einstein to his equivalence principle and General Relativity.

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

#266

Post by thaumasiotes »

abetusk wrote: post #108
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 if light were traveling through "slow glass" where it's speed through the medium was significantly slowed down, we would see it go in a parabola like the ball?

> we would see it go in a parabola like the ball?

That is a valid path for the light to follow, hypothetically.

But if that's what the light was doing, you wouldn't see it. You can only see things when light enters your eyes.

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

#267

Post by dllthomas »

Koshkin wrote: post #180
centimeter wrote: post #72

Earlier quoted context omitted.

> it's never an ellipse because the Earth is not a point mass. At least classically, a sphere is indistinguishable (gravitationally) from a point mass while you're outside it. The earth is pretty sphere-ish, locally speaking.

Locally speaking, the earth is flat.

But if you're speaking globally, you've already defined yourself out of calling the earth flat.

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

#268

Post by lopmotr »

pdonis wrote: post #242
umvi wrote: post #232

Earlier quoted context omitted.

It's impossible for an object with mass to go the speed of light. But you can observe what happens as you approach it: Let's say I put you in a spaceship and accelerate you to 50% the speed of light toward the sun. From an inertial viewer's perspective you are travelling toward the sun at half the speed of light and it takes you ~16 minutes to crash into the sun. But from your perspective it only took ~14 minutes to…

> you can observe what happens as you approach it Yes, but you cannot extrapolate from this to say that the time lapse for a photon would be zero. A photon is not the limit of objects with mass going closer and closer to the speed of light, because "closer and closer to the speed of light" is frame-dependent, but a photon's speed being c is not. I can find an inertial frame in which each of your objects is at rest, a…

But is there any physical way to distinguish these fundamentally different situations? If not, then perhaps the fundamentalness of it is just an artifact of the formulation.

I'm thinking of solar neutrinos which, for a while, we weren't sure if they were massless or not. We had to observe them experiencing a duration of time to conclude they were massive. If we didn't find that, maybe it was just an even shorter duration, not the absence of one and we would never be able to tell the difference.

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

#269

Post by basicplus2 »

antonvs wrote: post #264
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…

An intuitive way to tell if something is a "real" force (as proposed to a pseudo- or fictitious force) - can someone subject to that force feel it, or equivalently, can an accelerometer measure the acceleration it produces? When you accelerate or decelerate in a vehicle, you feel it. But when you jump out of a plane and accelerate towards the ground, you don't feel a force. Noticing this fact was a big part of what l…

What about when you splat on the ground? Is that not a force? Where did it come from?

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

#270

Post by colordrops »

drdeca wrote: post #200
colordrops wrote: post #98

Earlier quoted context omitted.

Ok so that makes sense, but if we are accelerating by standing on the ground, isn't that implying that we are continuously increasing in energy? Couldn't that be harvested for a perpetual motion machine of some sort?

First, "it all adds up to normality". Second, hm, I'm not sure. Well, the glass on the table next to me is "accelerating", but in my frame of reference (which is the same as that of the glass) it is also stationary...? If I drop a ball above the glass, and we consider things from the frame of reference where the ball is stationary, which (neglecting air resistance) is an inertial reference frame. In this frame, the g…

The layman descriptions are probably incomplete or rough abstractions missing nuance. Reminds me of the idea of a rubber membrane with balls on it representing the curvature of spacetime explaining how gravity works - I've always disliked that example because you still need plain old gravity to pull the balls into the membrane.
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