If gravity is not a force, what is holding us down?

[Gaussian97]

I particularly like the following video:

 

[stevendaryl]

Anyway, I have heard the explanations of how mass bends spacetime, creating a geodesic path for objects to move through, but what I don't get is how this actually creates gravity. What is actually pushing the object to move through the path in the first place?

Have you studied Newton’s laws? Objects do not need a push to move. They need a push to change their velocity. No force is required for an object to continue at constant velocity.

The point that you are making is that curvature doesn’t create motion, it only bends it. But every object already has motion through spacetime, so that’s good enough.

 

[Ibix]

I particularly like the following video:

There are some nice diagrams around 7 minutes, but it seems to me like there's a lot of fluff apart from that. I did skip quite ruthlessly, so possible I'm missing something.

 

[paradisePhysicist]

I was actually thinking of the book. It has been so long since I read it that I do not remember whether it was Flatland or Sphereland. But it got me to the point where I could wrap my head around curved space and higher dimensional geometries.

Oh. So far I have watched Flatland, but the movie Sphereland seems elusive, website is down. I see a physical copy of it for $24.95 on Amazon but maybe there is someway to rent it.

If you have access through your college / other, familiarise yourself with the geodesic hypothesis.
https://aip.scitation.org/doi/10.1063/1.522416

Hmm it won't let me access without a login.

It is tautological. This by @A.T. is much better:

Seen that. Seems tautological, it just replaces space movement with time movement, but doesn't explain what is causing the forward momentum in time in the first place.

I particularly like the following video:

Idk. I seen that a while ago, but still was confused. I watched that for 8 minutes before I gave up lol. I'll watch it again later.

Also this gives me an idea, you can represent 4d dimensions by just doing 3d haptics and creating different amounts of sensations per 3d point.

Assuming this video is true, my hypothesis is that consciousness is what causes gravity. The object deciding to accelerate is caused by consciousness going forward.

In any case, that is only my hypothesis. I don't know if that's true, but also some other things about the videos don't make sense to me. If time is 1 dimensional, how can it be curved? Also, in order to turn an object (like a car on a turn) you need fuel. A turn is simply when motion in one dimension is converted to motion in a different dimension. So in order for an object to convert its time momentum to spatial momentum, there seems like fuel is required. But idk and give up for now.

Orbiting means that its location changes as a function of time.

My original statement was implying that time is the result of location.

If spacetime were flat, then yes. It is the curvature of spacetime that allows different parts of the Earth to be continually accelerating away from each other without getting further away from each other. Did you read the link I cited earlier? I really tried to walk through the origin of these concepts. No math is involved, but it is a verbal description of the actual theory and not a handwaving “rubber sheet” thing.

I read some of that but I will try to read it again more in detail later.

 

[ergospherical]

mate my advice would be to go buy/borrow/steal yourself a nice general relativity textbook, or if that's not possible then read through the notes of tong/carroll/blau/etc. (i.e. you've got the enthusiasm, but at some point you've got to back it up with a solid understanding of the basic principles )

 

[paradisePhysicist]

mate my advice would be to go buy/borrow/steal yourself a nice general relativity textbook, or if that's not possible then read through the notes of tong/carroll/blau/etc. (i.e. you've got the enthusiasm, but at some point you've got to back it up with a solid understanding of the basic principles )

I don't have the energy for that right now but maybe later when I have more free time. Was hoping for also an easier explanation that didn't require hundreds of pages...

 

[PeterDonis]

Assuming this video is true, my hypothesis is that consciousness is what causes gravity. The object deciding to accelerate is caused by consciousness going forward.

No. Consciousness has nothing to do with it. (Nor do I understand how you could get the idea that it does from that video.)

First: All objects move through spacetime into the future. They don't have to be conscious. Rocks do it.

Second: Objects that are not being pushed on by something (the Earth's surface, a rocket engine, etc.) move through spacetime on geodesics, i.e., freely falling worldlines. Such objects feel no weight; they are weightless, like astronauts (and all other objects) on the International Space Station. (This, btw, shows that not all geodesics near the Earth are falling towards the Earth; there are others that are orbits around the Earth.) Note that such objects are not "accelerating" in the sense in which that term is used in GR (the more precise term is "proper acceleration").

Third: Objects sitting at rest on the Earth's surface are being pushed on by something: the Earth. That's why such objects feel weight--because they aren't moving through spacetime on geodesics. (Such objects have nonzero proper acceleration, in GR terms: in other words, "nonzero proper acceleration" is the same thing as "feels weight".) That's what prevents them from falling towards the center of the Earth, which would be their "natural" motion (the geodesic motion, the motion they would follow if nothing was pushing on them).

So the proper question to ask is not what is holding us down, but what is holding us up? And the answer is, the Earth.

 

[PeterDonis]

If time is 1 dimensional, how can it be curved?

It's not time that is curved, it's spacetime. Spacetime has 4 dimensions.

"Time curvature" is just a way of saying that the effects of spacetime curvature for an object sitting at rest on the Earth are the result of moving through spacetime in the time direction, not one of the spatial directions.

lso, in order to turn an object (like a car on a turn) you need fuel. A turn is simply when motion in one dimension is converted to motion in a different dimension.

Your basic intuition here is correct, but you need to change your definition of what constitutes a "turn". In GR, a "turn" is non-geodesic motion, i.e., non-freely-falling motion (motion in which the object feels weight), which is the correct generalization to the spacetime case of the intuitive notion of "moving in a straight line" that you are using here. So what requires a "push" on the object is non-geodesic motion through spacetime.

So in order for an object to convert its time momentum to spatial momentum, there seems like fuel is required.

If what is pushing on the object is a rocket engine or something like that, then yes. But in the case of the Earth, what is pushing on the object is a huge static configuration of matter in hydrostatic equilibrium. Such a configuration can push on an object on its surface without having to expend any fuel.

 

[stevendaryl]

Assuming this video is true, my hypothesis is that consciousness is what causes gravity.
...
My original statement was implying that time is the result of location.

Are you trying to understand General Relativity, or had you rather just make up your own theory? Physics Forums is not the place for the latter.

If you are trying to ask questions about how a theory works, then don't interject your own theory. That's just muddying the waters. And a general rule of thumb is that you try to understand existing theories before you venture to make up your own. You don't make up your own theory because you can't be bothered to understand what people have already learned.

 

[stevendaryl]

Seen that. Seems tautological, it just replaces space movement with time movement, but doesn't explain what is causing the forward momentum in time in the first place.

How is that tautological? No, it doesn't explain why we are always moving forward in time. But you're asking too much if you want a theory to explain everything about why the universe is the way that it is. General Relativity is only describing how gravity works.

It starts off with the assumption that objects are moving through 4 dimensional spacetime. It can't prove that assumption. Physical theories are not provable, they are only falsifiable: you use the theory to make predictions and then do experiments to see if those predictions are born out.

 

[Dale]

If time is 1 dimensional, how can it be curved?

It takes at least two dimensions for curvature. But it isn’t time that is curved, it is spacetime that is curved. Spacetime can curve in the time direction, but it is still 4D spacetime that is curving

Was hoping for also an easier explanation that didn't require hundreds of pages.

Try my explanation then. It is only a few pages. In the meantime please don’t speculate. The whole consciousness thing is silly.

 

[mitochan]

but I imagine even it is geodesic, it is line, why does it have to be just one way on the geodesic, even my free falling stopped?

Not one way, reverse process is possible. Say you throw a ball upward, it goes slower, stops at the top, goes down faster and comes back to your hand. All through the trajectory is geodesic and called free fall even when it is going upward, or called, more properly I think, in free motion.

 

[weirdoguy]

Not one way, reverse process is possible.

Still, even in that case - geodesic is a curve in spacetime, not space, and everything is "moving" only one way along it.

 

[paradisePhysicist]

Second: Objects that are not being pushed on by something (the Earth's surface, a rocket engine, etc.) move through spacetime on geodesics, i.e., freely falling worldlines. Such objects feel no weight; they are weightless, like astronauts (and all other objects) on the International Space Station. (This, btw, shows that not all geodesics near the Earth are falling towards the Earth; there are others that are orbits around the Earth.) Note that such objects are not "accelerating" in the sense in which that term is used in GR (the more precise term is "proper acceleration").

Third: Objects sitting at rest on the Earth's surface are being pushed on by something: the Earth. That's why such objects feel weight--because they aren't moving through spacetime on geodesics.

My original understanding (or lack thereof) was that the object was moving on the geodesic, through space time, then when it collides with the Earth most of the object is still moving or at least trying to move, except for the parts of the object touching Earth.

It's not time that is curved, it's spacetime. Spacetime has 4 dimensions.

"Time curvature" is just a way of saying that the effects of spacetime curvature for an object sitting at rest on the Earth are the result of moving through spacetime in the time direction, not one of the spatial directions.

A curve is produced when a function uses x input to change the y output. Are you saying that some dimension of space, let's say z, changes the t output of time? Or maybe that t changes the output of z? Z refers to the vertical dimension of earth, the surface normal pointing up.

Your basic intuition here is correct, but you need to change your definition of what constitutes a "turn". In GR, a "turn" is non-geodesic motion, i.e., non-freely-falling motion (motion in which the object feels weight), which is the correct generalization to the spacetime case of the intuitive notion of "moving in a straight line" that you are using here. So what requires a "push" on the object is non-geodesic motion through spacetime.

A plane follows a geodesic to save on fuel. I guess what's confusing is when a plane chooses not to follow a geodesic, the plane doesn't automatically experience a force compelling it to follow the most geodesic trajectory, which would rubberband it back to the most optimized trajectory. Or at least that's of my current understanding of how planes operate.

Are you trying to understand General Relativity, or had you rather just make up your own theory? Physics Forums is not the place for the latter.

It was merely a hypothesis I said. The hypothesis I said, that consciousness caused time momentum, was meant to support the Einstein gravity theory.

If you are trying to ask questions about how a theory works, then don't interject your own theory. That's just muddying the waters. And a general rule of thumb is that you try to understand existing theories before you venture to make up your own. You don't make up your own theory because you can't be bothered to understand what people have already learned.

Perhaps this did muddy the waters, but I was only trying to offer a suggestion as to why time moves forward. I was not trying to contradict the Einstein theory but offer an explanation of why it works. My first statement, that time was simply the result of location, was meant to merely offer a new perspective of time, not to be presented as a proper theory. But in the future I shall not post any more speculations on here about such matters, if that's what you desire.

It takes at least two dimensions for curvature. But it isn’t time that is curved, it is spacetime that is curved. Spacetime can curve in the time direction, but it is still 4D spacetime that is curving

Try my explanation then. It is only a few pages. In the meantime please don’t speculate. The whole consciousness thing is silly.

I plan on reading this explanation of yours today. I shall also not post any more speculations on here about such matters.



I watched this video again, the animation seems to show gravity causing gravity, like the Earth is using free energy and pulling the fabric of space inwards. This is not meant as a speculation or saying that the free energy is actually happening in the video, but just what the video looks like to me when watching it.

 

[weirdoguy]

I was not trying to contradict the Einstein theory but offer an explanation of why it works.

How can you offer any meaningfull explanation if you don't even know and understand this subject on a technical level? What for? It's a waste of time - primarily yours. You could use this time to try to learn technical details.

 

[paradisePhysicist]

How can you offer any meaningfull explanation if you don't even know and understand this subject on a technical level? What for? It's a waste of time - primarily yours. You could use this time to try to learn technical details.

I am going to look into reading Dave's theory on the matter, then watch all of the Flatland movies, then maybe read one of the chapters of the books mentioned, after that if I still don't get it I plan on maybe giving up on this for a few weeks.

 

[PeterDonis]

when it collides with the Earth most of the object is still moving or at least trying to move, except for the parts of the object touching Earth.

During the collision, yes, the object will in general not be rigid; its shape will deform. If the collision is hard enough, the object's shape when it comes to rest on the Earth will still be deformed, but once it has come to rest its motion in the new shape will again be rigid; it won't deform any further once it has reached its new equilibrium shape resting on the Earth's surface.

Also, "move" as you are using the term here means "move through space", and "space" is not an invariant; what "space" is depends on how you split up spacetime into "space" and "time". You are implicitly doing that in the way that is most natural for an observer at rest relative to the Earth (which is the way we usually do it intuitively in our everyday lives), but that is not the only possible way to do it.

A curve is produced when a function uses x input to change the y output. Are you saying that some dimension of space, let's say z, changes the t output of time?

Yes; the closer you are to the center of a large massive body like the Earth, the slower time flows for you. Or, to be more precise, the slower your proper time (the time according to the clock you carry with you) "ticks" as compared with the proper time of someone very, very far away from Earth. (I am assuming here that both observers are at rest relative to the Earth.)

Don't be confused by the fact that your worldline, sitting on the surface of the Earth, doesn't "look curved". It is not curved in space (in space, as we are defining "space" here--see my comments above on that--your worldline is just a point, not even a curve); it is curved in spacetime. It "looks straight" in a particular kind of coordinates because those coordinates are representing a curved geometry.

when a plane chooses not to follow a geodesic, the plane doesn't automatically experience a force compelling it to follow the most geodesic trajectory

Neither does an object in spacetime. Gravity is not a force in GR.

Try the analogy this way: suppose you have a plane that has no global navigation system; all it knows how to do is pick out the path that is locally straight (say by shining a laser beam pointing straight ahead from the nose--we'll ignore wind and all the other complications of actual flight and assume that the plane always moves in the direction the nose points if its controls are set at neutral). If the plane just flies directly along that locally straight path, leaving the controls in neutral all the time, it will end up following a geodesic (great circle) around the Earth. This is the analogue of an object freely falling in spacetime; in spacetime, falling freely (feeling no weight) is how you pick out the locally straight path; in a plane, flying "straight and level" (controls in neutral, no inputs) is how you do that.

Now suppose the pilot of the plane decides to turn, say by banking to the left (left rudder and left ailerons). How will he know he is turning? Because the plane doesn't move in the direction the laser beam points. It moves to the left of that direction. (The direction of the laser beam itself will change as the plane turns, but it will lag behind the actual direction of flight. Just as, if you are in a rocket whose engines are firing, the direction in spacetime that you would go if the rockets suddenly shut off changes, but it lags behind the actual direction of flight of the rocket.) There is no force compelling the plane to "try" to move in the direction the laser beam points. But if the pilot puts the controls back to neutral--stops the turn--the plane will again start moving in whatever direction the laser beam is pointing. Just as, if you are in a rocket whose engines are firing, and they stop firing, the rocket starts moving in whatever direction in spacetime is the geodesic (free-fall) direction.

So it's not right to think of geodesic motion as something objects have to be "forced" back to. Geodesic motion is just the "natural" motion that all objects undergo if nothing is pushing them (no rocket engine in spacetime; no control inputs in the plane).

 

[PeterDonis]

I was not trying to contradict the Einstein theory but offer an explanation of why it works

You should not try to do this with a theory you don't understand. Before you even try to explain why a theory works, you first need to understand how it works.

 

[stevendaryl]

I watched this video again, the animation seems to show gravity causing gravity, like the Earth is using free energy and pulling the fabric of space inwards.

The idea isn't that gravity causes the "fabric of space" (once again, it's spaceTIME, not space, that is curved) to bend by pulling on it. According to General Relativity, gravity simply IS curved spacetime. It's not that gravity causes spacetime to bend, any more than high temperature is what causes something to be hot.

And there is absolutely nothing anywhere in the video that mentions "free energy". I don't know where that idea came from.

 

[PeterDonis]

this video

Unfortunately I don't think this video's final representation, the one it claims is new and improved, is useful; rather, it is misleading. The best representation the video actually gives is in the middle, the one with the grid all around the Earth and clocks at each point of the grid. It does not, however, show the clocks closer to the Earth ticking slower than the clocks further away, which is the crucial point of looking at things that way. All you would need to add to that is a rule that the worldlines of objects in free fall bend towards the region where the clocks tick slower.

The later animations that seem to show the grid being "sucked" into the Earth are misleading because they wrongly imply that the grid is somehow dynamic; but it isn't. The "grid" around a massive object like the Earth is static; it doesn't change with time. This video misleadingly suggests that it does.

 

[mitochan]

Still, even in that case - geodesic is a curve in spacetime, not space, and everything is "moving" only one way along it.

Say a ball falls and bounce back from the floor to make a reverse motion. Falling ball is along a geodesic and the going up ball is along another geodesic because momentum is given from the floor and free motion is interrupted then. This shows that reverse motion is not going backward along the geodesic, from future to past, but transfer to another geodesic starting with reversed velocity.

 

[A.T.]

I particularly like the following video:

It's good that they point out the issues with the rubber sheet analogy. But I have issues with this video too:

- The animation at 7:00 makes it look like the spatial geometry is changing with time, which is wrong.

- The video concentrates too much on spatial geometry, which is not really that important for gravity (except for fast objects like light)

- The way the spatial geometry is presented, is also misleading, as I explained once when similar pictures were posted:

You cannot correctly show the curved 3D space around a mass with a distorted 3D-grid that is embedded in non-curved 3D space (the illustration). The shown distorted 3D-grid still encompasses the same total volume as would a non-distorted 3D-grid with the same outer boundary. But in actual curved 3D-space around a mass there is more spatial volume enclosed than in flat space of the same outer boundary.

So basically the 2D sheet with a dent or hill is actually a better representation of the spatial geometry than those distorted grids, because it has more surface area than a flat sheet would have. But spatial geometry doesn't explain gravity.

 

[jbriggs444]

Oh. So far I have watched Flatland, but the movie Sphereland seems elusive, website is down. I see a physical copy of it for $24.95 on Amazon but maybe there is someway to rent it.

Movies... *pffft*. Kids these days.

 

[stevendaryl]

It's good that they point out the issues with the rubber sheet analogy. But I have issues with this video too:

- The animation at 7:00 makes it look like the spatial geometry is changing with time, which is wrong.

Yeah, the animation shows a distorted 2D grid, but I don't have any idea what that grid is supposed to mean.

The video seems way overly complicated. To get across the main point, that an apple falls to the Earth because of curved spacetime, it seems to me that you could just suppress one spatial dimension to make the problem 3D, with a time axis and 2D spacelike slices. The surface of Earth would be represented approximately as a cylinder centered on the time axis. The geodesics of a small object (with a nonzero angular momentum) would be represented as a helix around the time axis. The apple crashes into the surface of the Earth when the helix representing the apple intersects the cylinder representing the surface of the Earth.

 

[paradisePhysicist]

Movies... *pffft*. Kids these days.

On a more serious note though, I think the movie will be easier to visualize since its already visual. On the other hand, the book may be so well written that it is easy to visualize also.

 

[jbriggs444]

On a more serious note though, I think the movie will be easier to visualize since its already visual. On the other hand, the book may be so well written that it is easy to visualize also.

Understanding is the goal. A visual understanding is not.

 

[MikeeMiracle]

As someone not familiar with the math's I find Vsauce's youtube video below very informational for those trying to get a grasp on why no forces are involved. I would recommend you watch it all though for a quick snippet check out timeframe 16:15 - 17:25, this demonstrates how straight paths come together on a curved surface. Possibly continue watch through to 20:40.