Bending space and space time - what is the difference?

[Premanand]

Sorry... This question may be very basic. As a self taught, I understand that the mass will bend the space around it such that any other mass entering the bent space will immediately be pushed towards the center because of the bent space. Higher the mass, higher will be the bent (if that is the right word)... and since speed of light is constant, the 'experience' will also vary accordingly as all our nerves and all other biological activity happens by electromagnetic waves... Thus, time and space are interconnected... However, can I just say that space gets bent by presence of mass... Because somewhere I read that it is incorrect to say space gets bent because it is the space time that is bent... What is the difference? Because to me adding mass to bend the space is perfectly OK and time is just a side effect of it.

 

[Orodruin]

What is the difference?

Space time curvature has a very precise meaning and it is perfectly possible to have a curved space time with a flat space. GR is about curved space-time, not about curved space. Space may be curved as well, but you reqlly cannot neglect the time part.

Because to me adding mass to bend the space is perfectly OK and time is just a side effect of it.

Then you would be wrong.

 

[Premanand]

Space time curvature has a very precise meaning and it is perfectly possible to have a curved space time with a flat space. GR is about curved space-time, not about curved space. Space may be curved as well, but you reqlly cannot neglect the time part.Then you would be wrong.

Thank you... Could u answer one more question please? Does adding mass to the object bends the space? Is the bent space a reason for increase in force of attraction when mass is increased?

 

[harrylin]

Sorry... This question may be very basic. As a self taught, I understand that the mass will bend the space around it such that any other mass entering the bent space will immediately be pushed towards the center because of the bent space. Higher the mass, higher will be the bent (if that is the right word)... and since speed of light is constant, the 'experience' will also vary accordingly as all our nerves and all other biological activity happens by electromagnetic waves... Thus, time and space are interconnected... However, can I just say that space gets bent by presence of mass... Because somewhere I read that it is incorrect to say space gets bent because it is the space time that is bent... What is the difference? Because to me adding mass to bend the space is perfectly OK and time is just a side effect of it.

It is possible to imagine "bent space" without "bent time" (note that these expressions refer to the mathematics). If you have the concept that "bent time" is a necessary effect of "bent space" then the result is the same.

 

[Premanand]

It is possible to imagine "bent space" without "bent time" (note that these expressions refer to the mathematics). If you have the concept that "bent time" is a necessary effect of "bent space" then the result is the same.

Thank you... So on conceptual level... I can bend the space further by adding more mass? ... I mean adding mass increases 'the force of attraction' by bending the space 'more' ?

 

[A.T.]

Because somewhere I read that it is incorrect to say space gets bent because it is the space time that is bent...

It's not incorrect: 3D space as a subspace of 4D space-time, is curved too. But it's missing the important part that is causing the main effects of gravity. See here the two aspects visualized:

http://www.physics.ucla.edu/demoweb..._and_general_relativity/curved_spacetime.html

 

[A.T.]

I mean adding mass increases 'the force of attraction' by bending the space 'more' ?

No, the effect of "attraction" is mostly related to the time distortion (gravitational time dilation). The spatial curvature only causes minor effects. See link in my previous post.

 

[Finny]

As you posted, time and space ARE interconnected...here are a few insights.

You can picture the paths of a particle as curves in special relativity as you would curves on a flat graph paper. SR involves relative motion and relative time,and one type of curvature but not gravity. When gravitational curvature is involved, as in general relativity the graph paper itself on which the curved paths are drawn is itself curved. That's not obvious and is why it took an 'Einstein' to figure it out.

In general, when relative motion and mass is involved, both space and time are distorted, that is, 'bent' or 'curved'. And there are lots of types of curvature!

 

[Finny]

Higher the mass, higher will be the bent (if that is the right word)... and since speed of light is constant, the 'experience' will also vary accordingly as all our nerves and all other biological activity happens by electromagnetic waves... Thus, time and space are interconnected..

yes space and time ARE interconnected. But don't think 'experience' is all about electromagnetic waves, or light. If you bump your head on a door in the dark, there are no electromagnetic waves, but you'll not like that experience, right? And what about loud firecrackers...those are SOUND waves [air molecules hitting your eardrum] not electromagnetic waves and you might not like those either; but music, now those are nicer sound waves. And don't forget about taste and smell either..not electromagnetic in nature.

As already noted 'curvature' of space and time often goes together. One way I have found to think about 'distortion' of time is that near big masses, lots of gravity, time is 'distorted' a lot. Time runs slower in higher gravity [potential] when viewed from a distance. Another way to think about curvature or distortion is to remember there are three dimensions of space [up/down,left/right, in/out, or x,y,z]; by analogy just think of time as a fourth component and you have 'spacetime'...and all four components can be 'curved'.

 

[Orodruin]

And don't forget about taste and smell either..not electromagnetic in nature.

(My emphasis)
Careful here, while not electromagnetic waves, all of the phenomena which are mentioned here are mainly electromagnetic in nature, just like all forces we experience on the molecular level. If not for the electromagnetic force, air molecules hitting an eardrum would pass right through it. From when you go away from internuclear forces to when you arrive at scales at which gravity plays a role, all forces are electromagnetic in nature.