no gravity, no space time

[hewj11]

Just a quick thought. If there was no gravity would space time or the "fabric" of space be able to exist? Or would a universe it self not be able to exist?

Thanks

[swansont]

Gravity is a proxy for having mass and energy around. No gravity means no mass, no energy. Probably means no universe.

[hewj11]

So with no mass or energy there can be no space time?

[Radical Edward]

well gravity is curvature of spacetime... so. make of that what you will, you can't disentangle the two.

[michel123456]

So with no mass or energy there can be no space time?

 

Indeed. IMO of course.

 

Space, Time & Gravity are linked together.

If you take in account that all 3 concepts are only the results of 3 different measurements of one & only single Reality, you could even imagine the 3 concepts representing the one and same thing. The problem resides in the passage from imagination to scientific demonstration.

[pioneer]

As a simple experiment, say we moved to a position in empty space, away from a galaxy, where the gravity is approaching zero, is there still space-time? Or does time and space begin to break down?

 

Or, say we moved between two planets where gravity vector cancels or equals zero, is there still space-time? Or do cause and effect in space-time begin to break down at that point? I tend to think space-time will be there even without gravity, but the presence of gravity will impact space-time.

 

If we separated a clump of matter into two and moved it apart, the original gravity will lower relative to the original single clump as well as in the space between the two separating pieces. The space-time in the middle will lower curvature and head toward the base state where there is no matter. This will create a special effect that looks just like expanding space-time. This brings us back to the first two paragraphs; two experiments to see if space-time exists at zero gravity.

[ajb]

As a simple experiment, say we moved to a position in empty space, away from a galaxy, where the gravity is approaching zero, is there still space-time? Or does time and space begin to break down?

 

 

The gravitational field can be identified with the space-time metric. Gravity is then thought of a the local geometry, i.e. the curvature. So, far away from a source the "force of gravity" is nearly zero. This means the curvature is almost zero. On physical grounds we argue that the space-time becomes Minkowski space-time in the infinite limit. Thus, the notion of space-time and gravity is ok.

 

We can think of Minkowski space-time (as a limit far away from a source) as describing "zero gravitational field". Or better put "the field has value zero" (I am being very lose what I mean here).

[michel123456]

The gravitational field can be identified with the space-time metric. Gravity is then thought of a the local geometry, i.e. the curvature. So, far away from a source the "force of gravity" is nearly zero. This means the curvature is almost zero. On physical grounds we argue that the space-time becomes Minkowski space-time in the infinite limit. Thus, the notion of space-time and gravity is ok.

 

We can think of Minkowski space-time (as a limit far away from a source) as describing "zero gravitational field". Or better put "the field has value zero" (I am being very lose what I mean here).

 

Isn't that the same with free fall?

[ajb]

Free fall means no external forces acting on the test particle.

 

Let [math](M,g)[/math] be a (pseudo)Riemannian manifold and consider the Levi-Civita connection. Then at any point [math]p \in M[/math] you can always find coordinates, known as Riemann normal coordinates such that

 

1. The connection coefficients vanish at the origin of Riemann normal coordinates.

2. Covariant derivatives reduce to partial derivatives at the origin of Riemann normal coordinates.

3. The partial derivatives of the components of the connection evaluated at the origin of Riemann normal coordinates equals the components of the curvature tensor.

 

This gives "local flatness to first order". This is the rigours statement that space-time looks like Minkowski space-time on small enough scales.

 

Now, in relation to free fall, we have no external forces. So everything is contained in the Riemannian geometry of space-time. The equivalence principle states that all the non-gravitational physics must reduce to that of special relativity in these coordinates. Now, that is not guaranteed by the existence of such coordinates itself. One needs to insist that the formulation of the non gravitational physics does not contain derivatives of the components of the connection, or equivalently it cannot contain second derivatives of the metric. This places constraints on coupling gravity to other fields.

[Severian]

I concur with ajb. No gravity would just mean no curvature - not no space-time.

[michel123456]

I concur with ajb. No gravity would just mean no curvature - not no space-time.

 

Indeed, no curvature, no external forces.

But acceleration yes.

[Bob_for_short]

All electrodynamics (without gravity) lives well in the flat Minkowski space-time.

[hewj11]

Thanks for all the responses. So whats producing space-time, keeping it "together" or accelerating its growth? i guess alot of what im trying to get at is does quantum gravity play a role in empty space?

[ajb]

guess alot of what im trying to get at is does quantum gravity play a role in empty space?

 

By empty space you mean space-time without classical sources.

 

Lets think about quantum general relativity (as an effective theory). There are quantum fluctuations about the Minkowski metric, so yes I expect quantum gravity to still be important on classically flat space-times.

 

That said, we do not have a well defined theory of quantum gravity. It is likely that the classical notions of space and time need modification. So anything at the moment is rather speculative.

[Bob_for_short]

So what's producing space-time, keeping it "together" or accelerating its growth? i guess a lot of what i'm trying to get at is does quantum gravity play a role in empty space?

 

There is no empty space. There are objects with different coordinates whose motion we observe. It is useless and wrong to speak of some space coordinates without a body at it, in my opinion.

 

Also, gravity is apparently an always classical thing, like an antenna of a powerful radio transmitter. No quantum effects in such conditions are observed and needed, actually. What we need is a better physical theory of classical gravity rather than quantization of GR.

[michel123456]

There is no empty space. There are objects with different coordinates whose motion we observe. It is useless and wrong to speak of some space coordinates without a body at it, in my opinion.

 

Also, gravity is apparently an always classical thing, like an antenna of a powerful radio transmitter. No quantum effects in such conditions are observed and needed, actually. What we need is a better physical theory of classical gravity rather than quantization of GR.

 

Agree.

[pioneer]

Time, according to science is not a thing. It is a reference variable. You can not store time in a bottle, except in the imagination, because it is a mental construct. The relationship, "no gravity no space-time", implies that if we have none of a real thing (gravity), we don't have any mental construct either. This defines the limitation of the construct. My mental construct allowed space-time to exist without gravity. This makes it better since it can do one additional thing.

 

If space-time does not need gravity that implies space-time is always there. When a BB effect appears, localized space-time contracts, but not all the way to infinity, since that would take infinite mass. Again it is just a mental construct since time is not defined as a tangible thing.

[hewj11]

What we need is a better physical theory of classical gravity rather than quantization of GR.[/quote

good point, i agree. But its really hard to get away from the classical GR.

 

pioneer: I see what you mean by time being a construct of the imagination. Leads my mind back to what is empty space then? But empty space doesn't space is constantly occupied?

 

On a side note. As space is expanding, is "new" space being created or is the space getting larger (if that's possible or makes sense to understand).

 

Thanks

Edited March 10, 2010 by hewj11

[michel123456]

Time, according to science is not a thing. It is a reference variable.

 

In your opinion, is distance a thing?

[Severian]

In your opinion, is distance a thing?

 

No more than, say, colour is a thing.

[michel123456]

No more than, say, colour is a thing.

 

This was intended to Pioneer who wrote

"Time, according to science is not a thing. It is a reference variable. You can not store time in a bottle, except in the imagination, because it is a mental construct.

 

I don't think colour or distance are mental constructs. And I stopped considering time as a "mental construct" ....a long time ago.

IMO it cannot be that a "mental construct" appears in almost all physical equations. It must be something more than that.