# Does empty space move? What are the laws that govern its motion?

Soumya_M
Does empty space move? What are the laws that govern its motion?

The big-bang set the universe expanding. But universe did not consist of matter at that time. So, what started expanding! Empty space - I suppose. But do the laws of motion govern the motion of empty space as well? Don't we need a fresh set of laws to understand the motion of empty space?

lugita15
Does empty space move? What are the laws that govern its motion?

The big-bang set the universe expanding. But universe did not consist of matter at that time. So, what started expanding! Empty space - I suppose. But do the laws of motion govern the motion of empty space as well? Don't we need a fresh set of laws to understand the motion of empty space?
Yes, we do need a fresh set of laws to describe "the motion of empty space". The theory that describes this is Einstein's General Theory of Relativity. It's based on the Einstein field equations, which describe how the geometry of spacetime is affected by the presence of mass.

espen180
What does it mean for space to "move"? What would it be moving with respect to? How would an observer measure its motion realtive to him/herself?

nismaratwork
It would be better to say that space stretches, deforms, and apparently it expands at an accelerating rate. All but the latter are described in the Stress-Energy Tensor, and since it's all about GR, there is no reference for an observer of course.

You can have frame dragging, which is about as close as having space "move".

nismaratwork
You can have frame dragging, which is about as close as having space "move".

Rotational or Linear... it seems to qualify, but it only occurs under pretty extreme circumstances, right?

espen180
It occurs whenever something is moving, but it's only significant in extreme environments like near spinning blach holes or around objects travelling at relativistic speeds, as far as I know.

nismaratwork
It occurs whenever something is moving, but it's only significant in extreme environments like near spinning blach holes or around objects travelling at relativistic speeds, as far as I know.

OK, so just as we're subject to minuscule Relativistic effects, this is just another we don't notice at "normal" speeds?

espen180
Yes.

nismaratwork
Yes.

Thank you. *plays it cool... don't sweat... don't stutter!*

Naty1
The big-bang set the universe expanding.

some type..maybe big, maybe cyclic :
http://en.wikipedia.org/wiki/Cyclic_Universe_Theory

But universe did not consist of matter at that time.

ok, but matter is not space.

So, what started expanding! Empty space - I suppose.

Unlikely space existed at the big bang..maybe shortly after. Space is an effect unlikely a cause of expansion.

But do the laws of motion govern the motion of empty space as well? Don't we need a fresh set of laws to understand the motion of empty space?

You may have no space mixed up with empty space. "empty space" is not empty...it has quantum fluctuations dark energy for example.

There is NO detailed theory of the big bang itself, not quantum not general relativity. Neither explains the big bang itself. It seems at the moment of the big bang everything was some type of energy in a very unstable state....that transitioned to a more stable less energetic state and out popped space, time, gravity, some more stable forms of energy (like kinetic and potential and dark ) and the four forces which seem to us to be separate (strong,weak,electromagnetic and gravitational.

The cyclic model of Steinhardt and Turok referenced above provide some insights without infinities and discontinuities....They have a great book THE ENDLESS UNIVERSE....

nismaratwork
some type..maybe big, maybe cyclic :
http://en.wikipedia.org/wiki/Cyclic_Universe_Theory

ok, but matter is not space.

Unlikely space existed at the big bang..maybe shortly after. Space is an effect unlikely a cause of expansion.

You may have no space mixed up with empty space. "empty space" is not empty...it has quantum fluctuations dark energy for example.

There is NO detailed theory of the big bang itself, not quantum not general relativity. Neither explains the big bang itself. It seems at the moment of the big bang everything was some type of energy in a very unstable state....that transitioned to a more stable less energetic state and out popped space, time, gravity, some more stable forms of energy (like kinetic and potential and dark ) and the four forces which seem to us to be separate (strong,weak,electromagnetic and gravitational.

The cyclic model of Steinhardt and Turok referenced above provide some insights without infinities and discontinuities....They have a great book THE ENDLESS UNIVERSE....

I was under the impression that the cyclic model was largely discredited?

espen180
It is true that the universe's expansion is currently accelerating, but we have no means to predict whether it will continue accelerating or eventually slow down and collapse. Current models are based on current data, and cannot tell us about situations from which we don't have any data. We can try, but it will only be speculation.

In fact, I think QLG predicts a cyclic universe.

nismaratwork
It is true that the universe's expansion is currently accelerating, but we have no means to predict whether it will continue accelerating or eventually slow down and collapse. Current models are based on current data, and cannot tell us about situations from which we don't have any data. We can try, but it will only be speculation.

In fact, I think QLG predicts a cyclic universe.

I'm sure you'd know better than me! I know very little about QLG (Loop-Quantum Gravity?) if at all, so I'll take your word on this one. I was simply under the impression that observations give us no reason to believe that this expansion could or should slow. I'm not going all, "big rip" on you, but eventual heat death seems possible.

When theoretical and measured values for cosmological constant differ by dozens of orders of magnitude, I would not take such predictions too seriously. Something isn't right there, and until we know what it is, predicting if universe will collapse is about as useful as random guessing.

nismaratwork
So... it's beyond an academic question then... just for pondering?

Well, the actual models that make predictions are useful, because they also make predictions for what should be going on right now, and these we can test and make conclusions about the models. It's just too early to use these to make any serious conclusions about where it's all heading.

Naty1
I was under the impression that the cyclic model was largely discredited?

You mean not generally accepted.......I don't think it has had enough time to be "discredited" just perhaps largely ignored..... in other words, the latest incarnation from Steinhardt and Turok is more recent than big bang explanations.....

but that's unimportant.....can you name any great theory that was intially accepted by " great experts" in the "scientific community"...senior scientists are not usually any more capable of a radical change in thinking than you and me....and must be cautious as evidenced by alchemy and "cold fusion", for example....

Check out how UNpopular general relativity was when first proposed....even "black holes" which Einstein "proved" couldn't exist....and the sun as the center of our solar system instead of the earth.....and dozens if not hundreds of other theories....

GR theory took a giant leap in acceptance based on Eddington's observations of the curvature of light by gravity....maybe a cyclic universe theory will take a similar jump IF the type of gravitational waves it predicts are observed.....

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nismaratwork
You mean not generally accepted.......I don't think it has had enough time to be "discredited" just perhaps largely ignored..... in other words, the latest incarnation from Steinhardt and Turok is more recent than big bang explanations.....

but that's unimportant.....can you name any great theory that was intially accepted by " greatexperts" in the "scientific community"...senior scientists are not usually any more capable of a radical change in thinking than you and me....and must be cautious as evidenced by alchemy and "cold fusion", for example....

Check out how UNpopular general relativity was when first proposed....even "black holes" which Einstein "proved" couldn't exist....and the sun as the center of our solar system instead of the earth.....and dozens if not hundreds of other theories....

GR theory took a giant leap in acceptance based on Eddington's observations of the curvature of light by gravity....maybe a cyclic universe theory will take a similar jump IF the type of gravitational waves it predicts are observed.....

OK, I was under the impression that unlike a new theory such as the ones you describe, the cyclic model is an old theory which gained popularity, and has since waned in popularity. The result is that it is now largely as you say, unaccepted, if not discredited. Would that be correct?

espen180
Yes, that would be better. For a theory to be discredited, it's predictions must be falsified. Thus, until we observe that the universe is not cyclic (assuming we don't observe that it is), it cannot be discredited, no matter how otherwise successful the theories advocating it.

nismaratwork
Yes, that would be better. For a theory to be discredited, it's predictions must be falsified. Thus, until we observe that the universe is not cyclic (assuming we don't observe that it is), it cannot be discredited, no matter how otherwise successful the theories advocating it.

OK, that makes perfect sense.