How Gravity Makes Things Fall
Why Physics Stack Exchange recommends it
Physics Stack Exchange users link it when they want a concrete visual of objects following straight paths through spacetime, as an alternative to the rubber-sheet analogy.
Recommended by 2 people across 2 different months, 2014 to 2021.
What it is
A 4-minute demonstration by EdwardCurrent using a homemade "Spacetime Stretcher" to show gravity as the warping of spacetime. It shows how a falling object's path trades travel through time for travel through space, with the speed of light as the conversion factor.
What people said
Recommends Martin · 2015-11 · Physics Stack Exchange · on “Is the popular explanation given for gravity in General Relativity misleading?”
The problem is that no "intuitive" explanation can capture what gravity is actually about, because if it could, then general relativity should itself be intuitive. The rubber-sheet analogy is in my view not a totally misleading analogy for what it wants to show (namely that masses curve spacetime), but it tackles the wrong problem - the main problem being that our intuition about how motion happens in curved space is missing. How can you make it right? Maybe combining a few methods can help? Start with Newton's first axiom. If there is no force, the motion of an object remains in constant velocity. This is something many people can grasp nowadays (it wasn't intuitive before Newton though). A
Recommends trent · 2021-02 · Physics Stack Exchange · on “Would Foucault's pendulum work on the moon?”
@JakobKS Objects follow straight paths through space**time**, not through space. How Gravity Makes Things Fall [this video] has a great visual demonstration of this fact that is better than the typical "heavy ball on a rubber sheet" metaphor.
Cites Derek · 2014-11 · Physics Stack Exchange · on “How to explain centripetal force in terms or relativity”
At the end of a video of dropping a ball and feathers in a vacuum, Brian Cox explains that the Ball and Feathers, as understood in terms of General Relativity, aren't falling. (apologies I can only post one link) There was lots of discussion on a Reddit thread and consensus seemed to be that objects, while travelling in straight lines, warp space time which cause them to move together over time. There was a nice practical demonstration in this video [this video] . My question is how does spinning an object counteract the effect of gravity? If I am on the surface of an object[edit: sorry by object here I meant something like a planet!] and (this is the tough bit) all frames of reference are equall
Cites 317070 · 2015-01 · Physics Stack Exchange · on “Is the surface of a heavy sphere bigger than $4 \pi r^2$ due to general relativity?”
I am unfortunately not familiar with the mathematics behind general relativity. However, on a heavy planet (say a sphere) gravity will bend space-time in a way that an object initially in rest, will experience over time, more and more of its time-dimension as a space-dimension towards the planet, hence causing it to accelerate towards the planet. (Video: [this video] ) If I generalize this line of thought conceptually, this would mean that on the surface of the planet, there is more space (and less time) than an outside observer assuming a flat space-time would expect. Is this correct? Is the surface of a heavy spherical planet bigger than $4 \pi r^2$? This does seem to match with the rubber shee