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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

#411

Post by colordrops »

gilgoomesh wrote: post #273
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?

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…

Nice, that's it, thank you

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

#412

Post by politician »

Hasn't it been shown that mass originates from coupling with the Higgs field and it's mass that deforms spacetime? Given the large size of the Higgs boson, I don't understood why the lack of gravity in QM is viewed as a problem. Why does gravity have to be fundamental? What if it just shows up right around the same time as electroweak symmetry breaks down?

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

#413

Post by WClayFerguson »

Ace17 wrote: post #405
WClayFerguson wrote: post #312

Earlier quoted context omitted.

Any change in direction is an acceleration (by definition). Even an object moving in a perfect circle at constant radians per second is nonetheless undergoing a constant non-zero acceleration just due to change in direction. Acceleration is any change in a velocity vector, including simply a change in direction, and requires a force (if the object has mass)

Yes, obviously. But the message seemed to say that the constant speed of light implied that the derivative of the velocity with respect to time had to be zero.

In physics speed means the magnitude of the velocity vector. Velocity is the first derivative of position with respect to time, and it's a vector in 3D space (technically 4D). If you change the orientation of that vector (direction change) that is always an acceleration even if the magnitude of the vector doesn't change.

So when light goes thru a gravitational field it doesn't change direction (EVEN though gravitational lensing is happening, from the perspective of an observer). If it did change direction that would be synonymous with an acceleration, and light never accelerates.

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

#414

Post by CamperBob2 »

pdonis wrote: post #409
CamperBob2 wrote: post #332

Earlier quoted context omitted.

(Shrug) You can go argue with Neil deGrasse Tyson, it's over my pay grade. https://www.youtube.com/watch?v=5ELA3ReWQJY

> You can go argue with Neil deGrasse Tyson Show me an actual textbook or peer-reviewed paper Tyson has written where he makes this claim. Pop science videos don't count. (Tyson is by no means the only one; Brian Greene is notorious for the same thing.) You won't be able to because there aren't any. No scientist who talks about a photon "experiencing zero time" in informal contexts will try it in a textbook or paper.…

Dude, this is basic theory at the high school level. Here's how it works: spacetime is 4-dimensional, and a vector in spacetime is always c units long. You can restrict your travel solely to X, Y, Z, or time if you like. If you do that, the other three components are going to be zero.

Photons put it all into the X, Y, and Z components, leaving nothing for the t component. They experience a change of position in space, but not in time. What's so hard to grasp about this?

Another point is that if this concept were actually scientifically useful, somebody would be using it in a textbook or peer-reviewed paper.

Seems that a fellow named Maxwell got a lot of mileage out of the concept, even if he didn't know what was really going on.

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

#415

Post by caf »

nsonha wrote: post #285
caf wrote: post #176

Earlier quoted context omitted.

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.)

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

How so? You can have different electric charge to inertial mass ratios, but that is not so for gravitational mass to inertial mass.

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

#416

Post by caf »

sfink wrote: post #402
caf wrote: post #157

Earlier quoted context omitted.

In Newtonian physics it's a force with a magnitude that is proportional to the mass of the body. For two different objects at the same point in the same gravitational field, the difference in gravitational force due to their differing mass exactly cancels out the difference in acceleration due to their differing mass, so they both accelerate at the same rate.

Sure, but the phrase "it's a force with a magnitude that is proportional to the mass" is like me asking "how heavy are you?" and you replying "75 liters", expecting me to know that humans are basically the density of water. My point is that it really seems like "gravity" is not a force, it's an acceleration, but there is something you could call "force due to gravity" that you reverse-engineer from the known accelera…

Sure, but the phrase "it's a force with a magnitude that is proportional to the mass" is like me asking "how heavy are you?" and you replying "75 liters", expecting me to know that humans are basically the density of water.

I don't see how. A "force between two bodies with a magnitude proportional to the product of their masses and inversely proportional to the square of the displacement between them" seems no different in principle from a "force between two bodies with a magnitude proportional to the product of their electric charges and the inversely proportional to the square of the displacement between them".

My point is that it really seems like "gravity" is not a force, it's an acceleration, but there is something you could call "force due to gravity" that you reverse-engineer from the known acceleration, and that means you need to multiply by the mass. Clearly different masses will just cancel, so the resulting acceleration is the same.

We're talking Newtonian physics, a model of the world where an acceleration is the result of an unbalanced force applied to a mass. You don't have to "reverse-engineer it from the known acceleration", you can calculate it as Gm₁m₂/r².

I'm fine with saying "force due to gravity" or even "gravitational force". Which is what you're describing in your first sentence, and I have no disagreement with that. "Force of gravity" starts to sound a little off, and I bet if I ask "what is gravity at Earth's surface?" I'll get back "9.8 m/s^2", which is an acceleration not a force.

You can equally describe the magnitude of the gravitational field as 9.8 Nkg⁻¹.

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

#417

Post by ars »

caf wrote: post #415
nsonha wrote: post #285

Earlier quoted context omitted.

> 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.)

That's undecidable and gravity is not special in that way How so? You can have different electric charge to inertial mass ratios, but that is not so for gravitational mass to inertial mass.

Gravitational attraction is a function of both objects, so the force between them is not a constant, same as for the electron.

Also, what if you have a charge so high that the electrical-escape velocity exceeds the speed of light? A strange kind of black hole?

I ask these things because I do not understand why gravity is called "not a force", while electromagnetism is, when I see no real difference between how particles act.

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

#418

Post by nsonha »

caf wrote: post #415
nsonha wrote: post #285

Earlier quoted context omitted.

> 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.)

That's undecidable and gravity is not special in that way How so? You can have different electric charge to inertial mass ratios, but that is not so for gravitational mass to inertial mass.

the link mentions that it's not proven that intertial mass and gravitational mass are the same, they are related. If gravitation mass contributes to the majority of intertial mass it would look like they're the same.

But let's just say they are the same, then gravity and "spacial" inertia are intertwined. Gravity is only special if intertia is also considered special. Inertia seems special because it's explainable in terms of motion in space. There might be other kinds of motion in other dimensions that could explain the randomness of quantum space.

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

#419

Post by j1vms »

j1vms wrote: post #54

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).

Here's [0] a bit more thorough explanation of this concept for those who are interested.

[0] https://www.youtube.com/watch?v=XRr1kaXKBsU&t=504 (Veritasium on Youtube: Why Gravity is NOT a Force @ 8:24)

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

#420

Post by caf »

ars wrote: post #417
caf wrote: post #415

Earlier quoted context omitted.

That's undecidable and gravity is not special in that way How so? You can have different electric charge to inertial mass ratios, but that is not so for gravitational mass to inertial mass.

Gravitational attraction is a function of both objects, so the force between them is not a constant, same as for the electron. Also, what if you have a charge so high that the electrical-escape velocity exceeds the speed of light? A strange kind of black hole? I ask these things because I do not understand why gravity is called "not a force", while electromagnetism is, when I see no real difference between how partic…

Gravitational attraction is a function of both objects, so the force between them is not a constant, same as for the electron.

The way that varies is always in proportion to the object's inertial mass though, which has the result that the path traced by an any object with the same starting position and velocity in a given gravitational field is going to be the same.

Contrast this with charge, which can vary independently of inertial mass. This has the result that the paths traced by objects with the same starting position and velocity in a given electrical field will vary.

This also means there is no fixed "electrical escape velocity". More massive objects with the same charge can escape with a lower velocity.

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