Veritasium just put out a great video on this: https://www.youtube.com/watch?v=XRr1kaXKBsU
A great video :) . Makes you wish he'd include more explanations of mathematical details...
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
#92You might enjoy my special relativity interactive illustration: https://xixixao.github.io/cheetah-paradox/
Play with the accelaration to see some spooky seemingly faster-than-light movement.
Re: Gravity is not a force – free-fall parabolas are straight lines in spacetime
#93Someone watched Veritasium...
but the real question is: can you counter your own arguments? Because that's where absolutely fascinating science happens.
Re: Gravity is not a force – free-fall parabolas are straight lines in spacetime
#94Veritasium just put out a great video on this: https://www.youtube.com/watch?v=XRr1kaXKBsU
It's linked at the bottom of this article.
Re: Gravity is not a force – free-fall parabolas are straight lines in spacetime
#95True gravitational force is something that can’t be transformed away by an arbitrary choice of frame (even an accelerating one). As a brief example, consider two objects in downwards free fall toward the centre of some massive object. Since they head towards the centre, in a free falling frame the two objects actually get closer to each other until they collide as they reach the centre. This is known as the tidal eff…
I'm super layman in all this, but I think the word force here refers to the Newton's force. Which experimentally was shown to not properly predict the effect of gravity, where as seeing gravity as a distortion of spacetime instead of as a acceleration force applied to the objects, did predict accurately the trajectory in experiments. You can call both a force in the generic dictionary definition of force. Because obv…
If you choose to view the universe in one particular coordinate system (e.g. one centred on your eyes), then you'll see some things mysteriously happening. For example, you might find that if you release something from your hand, it will mysteriously move towards your feet. Eventually you'll realise that a bunch of things can cause things to mysteriously move, and you'll start using the word "force" to describe "tendency to mysteriously move".
But some of these forces are not observed by other people who are watching; they arise as by-products of the fact that you are the one doing the measuring. For example, if you're falling in a lift, you won't observe the force that causes a ball to fall towards your feet; but I, standing on the ground outside the lift, will observe that actually you and the ball and the lift are subject to this force.
In fact there's an underlying reality which we can't directly observe but which we can infer from the paths of objects. That reality is a curved spacetime, not a flat one (as it appears to be). This curvature means that "straight line" is actually not quite what you're used to; things follow straight lines, but those straight lines don't look straight to us, because of the underlying space's curvature: we can't see the whole of spacetime, only small segments of it, so we can't see enough to get a proper sense of the curvature. But since our limited frames of reference have their own notion of "straight line" which is incompatible with that of the global spacetime, we observe a mysterious tendency of things to deviate from what we think is the straight line they should be following.
So gravity "is a force": it's a mysterious tendency of things to move, because we are limited in what we can see and our own observation frames are subtly incompatible with the global structure. But it's also "not a force": if we were somehow able to take a fully global view of the universe, there would be no mysterious movement, only a huge number of things moving at exactly the same speed in perfect straight lines through a curved spacetime. (Assuming General Relativity is 100% accurate.)
Re: Gravity is not a force – free-fall parabolas are straight lines in spacetime
#96Gravity is a force in the same way the centrifugal force is a force An artifact of rotating coordinate frames in one, the curvature of spacetime in the other
Mm, just like we have wave-particle duality, one can also speak of some sort of “force-coordinate duality”, as it were. Just as we let the sine function be linear for small angles, quasiparticles with forces in Cartesian space are wonderful abstractions. Good physics – especially applied physics – is about handling this balance between precision and mathematical/computational simplicity. Mind, research physics can be the opposite; a lot of nuclear involves (often special) relativity as combined with QM. Until we have a unifird theory, however, the graviton and its force ain’t dead yet.
Re: Gravity is not a force – free-fall parabolas are straight lines in spacetime
#97Speaking of reducing the dimensions of physical reality, a physics professor shared a very interesting video saying we should be thinking of photons and other massless objects as existing in 2d "areatime" as opposed to 3d "spacetime." I'm wondering if a revolution in simplifying the math behind physics is brewing.
https://www.youtube.com/watch?v=cb_PCuv0vcY&feature=emb_logo
Re: Gravity is not a force – free-fall parabolas are straight lines in spacetime
#98How is it not a force though? Regardless of curvature, a ball starts moving if you let it go without applying any force. Curvature alone can't account for that could it?
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?
Re: Gravity is not a force – free-fall parabolas are straight lines in spacetime
#99Earlier quoted context omitted.
> The earth is pretty sphere-ish, locally speaking. Kinda. There are mountains, they have their own gravitational attraction, and it can be measured... even in the 18th century! https://en.m.wikipedia.org/wiki/Schiehallion_experiment It all depends on how pedantic one wants to be :)
The first time I heard of the concept, it wasn't actually that, it was how in the 60s, they had to correct for mascons in the moon while orbiting.
There are very few stable orbits close to the lunar surface. Basically a couple of polar orbits with very specific parameters, and that's it.
The rest get so perturbed by gravitational anomalies that they fall out of orbit after a few months or years--faster than low Earth orbit where there is atmospheric drag!
https://www.lpi.usra.edu/lunar/documents/NTRS/collection2/NA...
Re: Gravity is not a force – free-fall parabolas are straight lines in spacetime
#100I'm curious, how does the "gravity is not a force" viewpoint relate to the hypothesized graviton particle [1]? Are they incompatible viewpoints, or just different perspectives on the same thing? (E.g. are gravitons hypothesized to disappear depending on frame of reference?) I'm assuming they're incompatible (that we need the theory of everything [2] to reconcile them) but would love to know if there's something I'm m…
It is likely that both the "fabric of spacetime" idea and the Graviton are parts of the whole picture, like with particle-wave duality. In the end, particles, waves, and spacetime are really just abstractions on top of the true, possibly unknowable rules of the universe.