This is one of many attempted explanations for near Earth gravity using General Relativity. Relativists can't agree on this because none of their explanations make any sense.
"The apple falls because it's future points downwards"
Not even remotely possible, since even in relativity time is not a spacial dimension so it cannot point something "downwards" or at an object in space. That's beyond stupid stacked on top of an already stupid model of reality.
The last 6 minutes talk about a fictional "black hole".
Space and time are supposedly so "curved" within it, that nothing escapes. That's undefined nonsense because these are terms with no physical understandings attached to them.
Someone first needs to define what a "non-curved" space physically is and what a "curved" space is. What are their properties? What are the forces it exerts on an object and why? Why does a non-curved space seem to exert no forced on anything, but a "curved" space does? How does it exert a force on both light and matter? If a weight is put on a spring, how does "curved" space make the apple push down the spring?
And once again, time can't "point" to the center of a blackhole because in GR the "time" axis is orthogonal to space. It doesn't point AT individual objects within space.
I decided to ask physicists on reddit again and here is the best they've come up with so far. It still defines spacetime as a mathematical fantasy land and not a physical model.
[QUOTE]
"What then does that space curvature physically mean for space in that box?"
There are two main interpretations of curvature:
1- Geodesic deviation: usually, we expect that objects with zero acceleration are in uniform, rectilinear motion with respect to one another. This is not the case in curved spacetimes. Curvature tells us precisely how they are accelerating with respect to one another.
(We have to be careful with the meaning of "acceleration", but this is the gist of the idea).
One of the key insights during the development of General Relativity was that free falling objects can accelerate with respect to one another when the gravitational field is non-uniform. We call this effect tidal acceleration. So, if we wish to interpret free fall as inertial motion, we better have a way to make inertial objects "accelerate" with respect to one another.
Anyways, this leads to a beautiful correspondence between curvature and tidal effects.
2- Parallel transport around a closed loop (have a look at the geometric meaning section): suppose we pick an arrow and start moving around with it, doing our best not to deform nor to change its direction. Eventually, we go back to our starting point. By how much would our vector have changed?
We expect that it wouldn't have changed at all. And we would be right if spacetime were flat. In curved spacetimes, though, our arrow might change. Once again, curvature tells us precisely how it changes.
what do we mean by "space" anyway?
In the context of General Relativity, spacetime is a structure with three main aspects:
1- A manifold structure: we are able to map events via coordinate charts, just like a cartographer maps places on the surface of our planet.
2- It has a metric structure: we are able to measure lengths and time intervals.
3- It has an affine structure: we can tell by how much vectors change from one point to another.
If we are able to slice spacetime in a very specific way, we can call these slices "space".
And since this is relativity, I'd also ask the question "what is it straight or curved relative to exactly?"
Not everything in Relativity is frame dependent. An essential part of the theory is to explain which things are not coordinate dependent. Besides, something may be "relative" in a trivial sense: it is defined in a coordinate dependent manner, but it has the same values in arbitrary coordinate systems.
He defines curved space (potentially) by acceleration of objects in that space. But that still brings us back to what is space and what is curving? What is acting on what and how?
His other (potential) definition for curved space is pure mathematics, treating everything as a vector in a coordinate system. Well that is just math and not a physical model. In a proper physical model an object needs to be acted upon by something in some way.
His definition of space are just some general characteristics of a mathematical coordinate system. So just more math and nothing physical.
Just watch the first 6 minutes of the video.
This is one of many attempted explanations for near Earth gravity using General Relativity. Relativists can't agree on this because none of their explanations make any sense.
"The apple falls because it's future points downwards"
Not even remotely possible, since even in relativity time is not a spacial dimension so it cannot point something "downwards" or at an object in space. That's beyond stupid stacked on top of an already stupid model of reality.
The last 6 minutes talk about a fictional "black hole".
Space and time are supposedly so "curved" within it, that nothing escapes. That's undefined nonsense because these are terms with no physical understandings attached to them.
Someone first needs to define what a "non-curved" space physically is and what a "curved" space is. What are their properties? What are the forces it exerts on an object and why? Why does a non-curved space seem to exert no forced on anything, but a "curved" space does? How does it exert a force on both light and matter? If a weight is put on a spring, how does "curved" space make the apple push down the spring?
And once again, time can't "point" to the center of a blackhole because in GR the "time" axis is orthogonal to space. It doesn't point AT individual objects within space.
I simply ask the relativists - what is spacetime made of?
I decided to ask physicists on reddit again and here is the best they've come up with so far. It still defines spacetime as a mathematical fantasy land and not a physical model.
[QUOTE] "What then does that space curvature physically mean for space in that box?"
There are two main interpretations of curvature:
1- Geodesic deviation: usually, we expect that objects with zero acceleration are in uniform, rectilinear motion with respect to one another. This is not the case in curved spacetimes. Curvature tells us precisely how they are accelerating with respect to one another.
(We have to be careful with the meaning of "acceleration", but this is the gist of the idea).
One of the key insights during the development of General Relativity was that free falling objects can accelerate with respect to one another when the gravitational field is non-uniform. We call this effect tidal acceleration. So, if we wish to interpret free fall as inertial motion, we better have a way to make inertial objects "accelerate" with respect to one another.
Anyways, this leads to a beautiful correspondence between curvature and tidal effects.
2- Parallel transport around a closed loop (have a look at the geometric meaning section): suppose we pick an arrow and start moving around with it, doing our best not to deform nor to change its direction. Eventually, we go back to our starting point. By how much would our vector have changed?
We expect that it wouldn't have changed at all. And we would be right if spacetime were flat. In curved spacetimes, though, our arrow might change. Once again, curvature tells us precisely how it changes.
what do we mean by "space" anyway?
In the context of General Relativity, spacetime is a structure with three main aspects:
1- A manifold structure: we are able to map events via coordinate charts, just like a cartographer maps places on the surface of our planet.
2- It has a metric structure: we are able to measure lengths and time intervals.
3- It has an affine structure: we can tell by how much vectors change from one point to another.
If we are able to slice spacetime in a very specific way, we can call these slices "space".
And since this is relativity, I'd also ask the question "what is it straight or curved relative to exactly?"
Not everything in Relativity is frame dependent. An essential part of the theory is to explain which things are not coordinate dependent. Besides, something may be "relative" in a trivial sense: it is defined in a coordinate dependent manner, but it has the same values in arbitrary coordinate systems.
[END QUOTE]
So
He defines curved space (potentially) by acceleration of objects in that space. But that still brings us back to what is space and what is curving? What is acting on what and how?
His other (potential) definition for curved space is pure mathematics, treating everything as a vector in a coordinate system. Well that is just math and not a physical model. In a proper physical model an object needs to be acted upon by something in some way.
His definition of space are just some general characteristics of a mathematical coordinate system. So just more math and nothing physical.