Which way is "space time" bent?

[xelasnave]

We have considered nothing and I hope that thread took a few minds away from the weather.

I know so little about the concept of space time I spend a lot of time trying to work out what it says .... it seems to be geometry to me however so many speak about it as if it is a mystery and dont buy into some of the propositions used to expalin it... I think I understand it a little but have a problem with some examples I see in mags and sites that seek to explain it to the masses ..of which I am but a small part.

I have seen an example of how space time bends or warps space by the use of a ball on a rubber sheet or blanket...I am sure that anyone familar with the subject have had the example used when the concept of space time was first put to them..and indeed may have used the example to explain space time to others.
I look at that example and suspect it is misleading... to me general relativity would have the ball under the blanket. If the blanket represents a grid line I feel it is entirely placed in the wrong position...

I have placed this in general chat and hope someone can see what I am driving at and put me straight. And although moderators may feel this post should be placed in the science section I submit that given the fact I am not a scientist any discussion in the science forum could be seen as inappropriate by those frustrated by laymen trying to understand matters that may well be beyond their educational resources.

I notice that so many artist impressions follow this approach when attempting to show gravitational lensing... and to me the representation is entirely incorrect..you may notice in the many artist representations that light bends around an object where as I think general relativity would see the light bending in and then out again... that is space is bent in toward an object and bends out having passed...

Back to the ball and blanket... if we use the blanket and ball example it would mean that in a 3d (leave out the 4th as it is not relevant at this level I feel but if it is the reason the example seems around the wrong way please point it out)..sorry in a 3d representation the ball would be pushing space away and that the space time graph must therefore necessarily end at the surface of the object..planet Sun or ball..which suggests that general relativity must stop at an objects surface which I doubt is the case..

I know if I knew more I would not have to ask however I feel that such a fundamentally incorrect example should be exposed...that is on the assumption that others see the point I make and agree with my humble and simple observation or disagree and can point out where I have missed the point...
alex
alex

[DJDD]

Originally Posted by xelasnave

I notice that so many artist impressions follow this approach when attempting to show gravitational lensing... and to me the representation is entirely incorrect..you may notice in the many artist representations that light bends around an object where as I think general relativity would see the light bending in and then out again... that is space is bent in toward an object and bends out having passed...

light can be bent both ways.
But, with lensing what we see is the light that would otherwise have streamed past be bent towards the earth.

[xelasnave]

I have seen artist impressions showing the beam going around the object seemingly in an effort to fit the blanket example...it just seems they follow the example and not the theory...as I said this is my impression and my interest is to see if anyone sees the point I make...and I accept I probably come over as incoherient but that is simply my inability to express myself in strictly scientific terms... however hopefully it is clearer than mud.

Thank you very much for your input.
alex
alex

[sjastro]

Alex,

Remember these are visual aids and nothing more. Mathematically the shape of a beam of light passing through a gravitational field can be anything provided the light takes the shortest distance. It depends on the geometry of space-time being used. If it's Euclidean the light travels in a straight line, if it's spherical it travels in the shape of an arc.

With the exception of gravity free fields in which case the geometry is flat or Euclidean, the geometry of space-time can be of any non-Euclidean form. For low gravity potentials Euclidean geometry is a good approximation.

The solution for Black Holes, the gravitational bending of light and the perihelion advance of Mercury's orbit all assumed that the geometry of space-time was spherical, not because space-time is actually spherical, but the field equations were easily solved in this system.

Regards

Steven

[xelasnave]

Thank you Steven for such a well considered reply.
I accept the ball and the blanket is an example of a visual aid but I need to know if my view is correct.. simply it means I have some sort of grip on the concept .. i say that the example if to be correct would be more representative of the situation if the ball were under the blanket such that it were stretched toward the ball..

I guess what I suggest is that the grid can only bend toward an object.. such that the squares of the grid become "smaller" and I associate smaller squares with increased gravity...and in an effort to bend we get out arcs.. that is obvious I guess...I dont know what smaller squares suggest other than time must be the thing that does it... I dont know if I am the right track but I imagine that space is compacted the closer it is to matter.

Sorry I am so bewildered by all this I have difficulty in accepting anything I can not explain to myself with absolute confidence...and with all this I am not confident that I understand it...as I said all I see is geometry ..I read someplace that one could think of it as a pythagorean therom with an addition of a negative time line...now just because I read it takes me little further than visualising a grid that has curves which represent variations in gravity.

I simply hope the concepts are not so abstract that I am unable to visualise for the purpose of my limited understanding but maybe if I keep at it I will achieve a better understanding.

But so far I see general relativity as a geometry to describe space time...and I at this point think that space time has been derived from the principle of equivelence..(I know there is more to it and there are many contributions of others before Dr A but it seems to me that acceleration has been related to the effect of gravity... I guess that is where T got involved ...however I feel there is something fundamentally wrong about the approach as we use a human experience to describe something more complex than such approach admits... but not being absolutely at ease with the subject I can only speculate and grasp at thread I see bear...

Now I am not saying Dr A is wrong by any means and please dont think that of me..he is one of my heros, my main one really..he seemed like a very decent man one whos manners one would be happy to emulate...in fact the reason I got into the gravity push thing was my belief that Dr A had something similar in mind when he put forward the cosmological constant... he seemed to give up on that idea when Hubble presented his observations and belief that the Universe was expanding... I think he was on the right track so I have developed an unhealthy pursuit of giving weight in my mind to his concept of an overall pushing force.

Anyways thanks again for helping me..I will think about all you have said.

I do read heaps but I dont feel I have a grip on all of it...it cant be as simple as I believe or as complex as many seem to present it..not you Steven your writtings are enjoyable and free flowing and I do appreciate that.


alex

[xelasnave]

Also Steven....Do you follow the gravity b probe mission and its attempts to show frame dragging may I ask?

alex

[Jen]

my head hurts

[xelasnave]

Now now it is simple when you understand that I / you dont understand and others dont understand but its better than thinking about nothing I think..
alex

[sjastro]

Originally Posted by xelasnave

I guess what I suggest is that the grid can only bend toward an object.. such that the squares of the grid become "smaller" and I associate smaller squares with increased gravity...and in an effort to bend we get out arcs.. that is obvious I guess...I dont know what smaller squares suggest other than time must be the thing that does it... I dont know if I am the right track but I imagine that space is compacted the closer it is to matter.

You're partly right Alex. The more convergent the grid lines the higher the gravitational potential or the strength of the field at that point.

Space time diagrams are much more complicated than that. The angle of the grid lines to the X or spatial axis as they dip into the depression determines if a particle can follow this trajectory. For example if the angle is less than 45 degrees no particle can follow this trajectory as causality is violated and the particle travels faster than light.

This is difficult to explain in a few sentences. You might want to look at these.

http://en.wikipedia.org/wiki/Schwarz...27s_paraboloid
http://en.wikipedia.org/wiki/Minkowski_diagram

Sorry I am so bewildered by all this I have difficulty in accepting anything I can not explain to myself with absolute confidence...and with all this I am not confident that I understand it...as I said all I see is geometry ..I read someplace that one could think of it as a pythagorean therom with an addition of a negative time line...now just because I read it takes me little further than visualising a grid that has curves which represent variations in gravity.

I simply hope the concepts are not so abstract that I am unable to visualise for the purpose of my limited understanding but maybe if I keep at it I will achieve a better understanding.

Unfortunately the physical sciences have gone well past the visualization stage for understanding and a command of mathematics is essential.

But so far I see general relativity as a geometry to describe space time...and I at this point think that space time has been derived from the principle of equivelence..(I know there is more to it and there are many contributions of others before Dr A but it seems to me that acceleration has been related to the effect of gravity... I guess that is where T got involved ...however I feel there is something fundamentally wrong about the approach as we use a human experience to describe something more complex than such approach admits... but not being absolutely at ease with the subject I can only speculate and grasp at thread I see bear...

Quantum mechanics is a much more esoteric subject than GR (and mathematically much more challenging). It is contary to human experience by bringing into question the concept of reality. Yet it has contributed to major technological changes in the 20th century.

Regards

Steven

[xelasnave]

Thanks Steven.. I like all that.. formulas scribe the graphs I see the graphs easy or the scales or grids whatever... that math doesnrt seem difficult I will have a good look...and play with them..thanks very much I really appreciate your help.

alex

[Astro78]

Anyone know of any lensing examples that can be seen through a modest 8"?

[xelasnave]

I dont know I did read about someone getting onto something with an 18 inch..can not recall object or observer was in a US mag.

alex