I have been reading a book on hyperspace and again the author has gone

Main Question or Discussion Point

I have been reading a book on hyperspace and again the author has gone back over
EINSTEIN,s space gravity thing.
And i have this problem that reoccur,s everytime this happens which is many times over many years
and its this.
All the authors seem to be saying that when a planet gets to the dip or slope in the sheet(this is the
cannon ball in the miidle of a sheet thing)and goes over the edge it has done this becuase its following warped space
.Now it is the "it has done it because" that i am unsure. Which one they are suggesting it is.
1. It is really following a track like scalextric track that is in space or a force that again is in or is space.
2. Now this is the one i think they are trying to say is whats happening but i dont buy it.It is that the planet is some how just rolling
over the edge and starts to roll down "because (and this is the bit that gets me)" thats what happens on earth now thats the way it would happen
if you rolled a ball across a sheet with a dip in it.this would be using our pre exsiting ideas of what gravity is
to illustrate Einsteins gravity. this is surely wrong.

Also in the idea "planets are following a track which is in space or is space" ,i still have a
thing i dislike we have gotten rid of gravitons with Einsteins gravity but have replaced it
with something else that has to be found. the tracks or force of what is changing the direction of the planet. And when you
think that the gravitons where controlled by the mass .Now what do we find "space" is CONTROLLED BY MASS but really when we say space we mean the tracks
or whatever it is .If you notice these 2 ideas tracks in space and gravitons both controlled by mass one is seperate from space
the other is space.they become almost the same thing.Its ok, but with these books the authors seem to suggest we have got rid of the need for a force
and the geometery of space deals with it.I dont see it.Can someone set me straight.

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phinds
Gold Member
2019 Award

Perhaps it's just me, but I find your post to be mostly incoherent. Perhaps you could make it shorter and focus on a single point, simply stated.

Without using math, the simplest statement to what I THINK your question is, is this --- mass tells spacetime how to shape itself and spacetime tells mass how to move.

How what is the Mechanism that space use,s to tell mass move.
is it a force or a track or is it just a planet rolling over the edge of the warp but not using any Mechanism or force atall.
If it not using a force ,than we must be using our old ideas (Newtons)of gravity .to make the planet roll down.ie: a force
so the geometry of space is not telling mass how to move unless the word space is redifined to include the fact it has a force in it .

phinds
Gold Member
2019 Award

I still can't quite tell what you are getting at.

In General relativity, mass warps spacetime and mass follows a geodesic when moving in that spacetime. There is no force involved (in GR) in keeping planets in their orbits, they are just following the path of least resistance.

Bill_K

the planet is some how just rolling over the edge and starts to roll down "because (and this is the bit that gets me)" thats what happens on earth now thats the way it would happen if you rolled a ball across a sheet with a dip in it.this would be using our pre exsiting ideas of what gravity is to illustrate Einsteins gravity. this is surely wrong.
I agree completely, and you see this unfortunate explanation given all over the place. the trouble is that people often take this picture too literally and start to ask questions like: What's the vertical direction called? What's the shape of the bedsheet? Can it vibrate? Where's the black hole?

Well, the sheet is curved, and it makes things go around in a circle, and that's about all it has in common with general relativity!

phinds
Gold Member
2019 Award

Well, the sheet is curved, and it makes things go around in a circle, and that's about all it has in common with general relativity!
Yes, that's well put. The rubber sheet analogy is EXTREMELY simplistic and you HAVE to keep in mind that is is just an analogy, not any kind of representation of reality.

A.T.
phinds
Gold Member
2019 Award

if you rolled a ball across a sheet with a dip in it.this would be using our pre exsiting ideas of what gravity is
to illustrate Einsteins gravity. this is surely wrong.
Which part of the word ANALOGY do you not understand ?

A.T.

Which part of the word ANALOGY do you not understand ?
I think he understands the word pretty well. "Rolling into dents of a rubber sheet" is not an analogy of anything in GR. It is circular and highly misleading handwaving.

cepheid
Staff Emeritus
Gold Member

Notice that there is no gravitational force in this model. The motion of an upwards-thrown apple is determined by the shape of the spacetime together with the law of straight lines.
i still feel a force is needed.
or if a planet is just coming to the edge of the warp it should just carry on .surely it needs a force even if this is tracks in space. To alter its direction.
or how does something change direction in space with out a force to me this applies if the space is curved space .what difference does this curved space make you still need a force.To change its direction.

or enougher question is
1.is there a force needed(even if that force is the same as space or is space)
2.or isn,t there any force needed.
surely its one or the other.

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phinds
Gold Member
2019 Award

i still feel a force is needed.
Well, Newton would have agreed with you, but Newton has been found to be wrong.

Nugatory
Mentor

i still feel a force is needed.
Imagine that you and your brother are standing ten meters apart at the equator. Now you both start walking due north. Nothing interesting will happen at first, but as you get closer to the north pole, you will start to feel as if a force is shoving the two of closer together until you collide at the pole itself.

That's curvature at work - you and your brother are moving straight ahead, yet you feel a force that is driving you closer together.

OK, so now you'll say that that's because you're MOVING on a curved surface ... But what about a stationary object, such as a weight that's just sitting on a table yet being pulled towards the ground, so that if the table weren't there it would fall?

The answer is that we're talking about curved space-time, not curved space. Even an object at rest in space is moving forward in time. The natural straight path of the weight through space-time will intersect the natural straight path of the surface of the earth, just as the natural straight northwards paths of you and your brother will intersect at the north pole. The table is shoving the weight off of its natural straight path, stopping it from meeting the surface of the earth. You and your brother could produce a similar effect in your trek towards the north pole by holding on to the ends of a ten-meter rod, so that you never moved closer - the rod would shove one of you off track so that you'll still be ten meters apart when you get to the north pole.

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this depends on what the 2 brothers where doing before they get to the point where you started. If they "had" to change direction at any point due to the north pole .then surely (unless your going to say all things are always on a straight line but are at the same time always on a curved spacetime and to this i would ask if a mass ever influnces enougher mass by warping space .i would say it still would need a force) force is required .if a mass is warping space and a object has to change its direction.

why are the 2 brothers following this straight line because they want to, dont want to or dont know its curved . if the north pole hadnt curved the space then we would of walked straight on and never get closer together.so the north pole has warperd the space now why would we follow this path we have said we could of gone straight on if the north pole hadnt warped space.so why have we got to follow this curve.because we are being Influenced and this Influence is the same as a force

you are imagining a straight line in curved space and yes they can meet at a point .thats not it .im saying that a object has to change from one curve say its in deep space when it gets near a big sun that makes its curve more curved and its at this point that a force is needed from a smooth curve to a tighter curve.

there are tighter curves i take it in curved space.the tighter the cuve the quicker we get to the point right.but surely there are still mass influencing each other .and that influence amounts to force.doesnt it.

Drakkith
Staff Emeritus

If the effects are identical, then does it matter if you call it a force or not? Any force would have to obey the laws of relativity, so I don't see that it matters in the end.

A.T.

Notice that there is no gravitational force in this model. The motion of an upwards-thrown apple is determined by the shape of the spacetime together with the law of straight lines.
i still feel a force is needed.
No force is needed to follow a straight line in space-time. This is true in Newtons model and in General Relativity. But in curved spacetime the line is only locally straight (a geodesic).

how does something change direction in space with out a force to me this applies if the space is curved space .
It's not just curved space. It's curved spacetime. As for the "how?", try it out! Find some trumpet or cone shaped surface, and draw your space-time axes on it, like in the example. Then stick adhesive tape around it, without tearing or folding the edges of the tape: roll it straight out to create a locally straight line. You will get world-lines of free-fallers, which change their direction in curved spacetime because they are straight and force-free.

A.T.
thanks that was a earlier link.but i didnt see its meaning.then and i still dont
i think its when you pull the tape of and flatten it out i will find a straight line on it.but it came of a curved surface .anyway i will try it out .my first thought is "no dont like it" .but i will have to give it a try.

A.T.