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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

#271

Post by antonvs »

basicplus2 wrote: post #269
antonvs wrote: post #264

Earlier quoted context omitted.

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?

That involves a force which comes from encountering an object that's not in free fall with you.

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

#272

Post by manigandham »

strken wrote: post #263
manigandham wrote: post #156

Earlier quoted context omitted.

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…

1st sentence is correct and is the same as what I said, sorry if I wasn't clear.

Remember that spacetime = space + time dimensions. The object is always travelling through time, and the curvature of spacetime is converting some of that speed through time into speed through space. That's what you perceive as motion (caused by gravity).

Time and space are linked together. The faster you go through space, the "slower" you go through time (as in you experience it slower). This is very measurable and even used to alter timings for satellites GPS readings. You can take an atomic clock on a plane and age slower than someone who just stayed on the ground.

So the spacetime curvature is continuously converting some of your temporal motion into spatial motion, until that's stopped by the surface of the Earth which is constantly "accelerating" to stop you from going further.

As to why we always move through time, that's beyond my understanding at this point but it's a fundamental axiom of physics.

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

#273

Post by gilgoomesh »

colordrops wrote: post #98
gilgoomesh wrote: post #78

Earlier quoted context omitted.

The falling object doesn't accelerate. You, standing on the ground are the one that's accelerating. You see the object as accelerating but that's an illusion due to frames of reference. As evidence: which object feels a force on it? You can feel the force the ground continually pushes up at you. The ground is accelerating you up. The falling object is completely idle in its inertial frame and feels nothing.

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?

Energy is Force times Distance. You're not moving (even though you're accelerating) so there's no energy expended.

See Veritasium's video at 10m17s:

https://youtu.be/XRr1kaXKBsU?t=617

He addresses that basic science makes this confusing because there's a curvature term that is usually left out of acceleration equations which balances out when you're accelerating along your curvature (e.g. against gravity), instead of along your spatial coordinates.

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

#274

Post by nsonha »

the_arun wrote: post #193

Don't we always consider gravity as "acceleration"? In f = m * a, for objects falling from sky, it will be f = m * g where g is gravity, m is mass & f is force Sorry, I think I am missing the point of the article.

in Newtonian physics only a force can cause acceleration

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

#275

Post by nsonha »

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?

I believe quantum field theory explores these

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

#276

Post by shahar2k »

umvi wrote: post #77
phkahler wrote: post #53

Earlier quoted context omitted.

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…

The difference is that light travels through space, but not time (similar to how a vertical line does not travel the x axis, only the y axis). The ball travels through both space and time. The faster you go, the less you travel through time. Thus, if the ball were travelling at the speed of light, it would not travel through time either and would follow the same path as light.

does that mean that velocity is constant "speed of light" and only direction in spacetime changes?

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

#277

Post by pdonis »

lopmotr wrote: post #268
pdonis wrote: post #242

Earlier quoted context omitted.

> 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 dura…

> is there any physical way to distinguish these fundamentally different situations?

Are you asking if there is a way to distinguish a timelike object from a lightlike object? Of course there is. The fact that, for something that has a very, very small invariant mass, it might be practically difficult does not change the fundamental principle.

Also note that the reason it was difficult, for example, to tell whether neutrinos have mass or not is that we can't just do the obvious and straightforward thing and find an inertial frame in which they are at rest (by, for example, taking a rocket and accelerating it in the direction of a neutrino to see if we can bring it to rest relative to the rocket). So we have to resort to indirect methods. But, again, that's a practical limitation that doesn't change the fundamental principle.

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

#279

Post by sergeykish »

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…

Flat Earth constantly accelerated by rocket.., that's colonization ship!

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

#280

Post by drivers99 »

basicplus2 wrote: post #269
antonvs wrote: post #264

Earlier quoted context omitted.

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?

Specifically the electrons in the atoms of the ground and the electrons of the atoms of yourself repel each other when they come together.

“So pushing just two atoms close to each other takes energy, as all their electrons need to go into unoccupied high-energy states. Trying to push all the table-atoms and finger-atoms together demands an awful lot of energy – more than your muscles can supply. You feel that, as resistance to your finger, which is why and how the table feels solid to your touch.“ https://theconversation.com/if-atoms-are-mostly-empty-space-...

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