What is Space?

[ccwebb]

Is it possible that space is another substance? As in, earth, water, air... Space is just a substance that can be interacted with? As oppose to simply 'distance between two objects'?

[elfmotat]

Not really, no. Spacetime is certainly dynamical, but I don't think classifying it as a "substance" is either meaningful nor accurate.

[ccwebb]

Understood. Thank you for the fast reply.

 

The idea came from, in every example of space-time I've seen is always the ball on a trapoline, or the picture of a grid being 'pushed down' by the planet. So the idea was presented, what if... what if it acutal IS something? That would make some sense of this Dark Matter and Dark Energy thing.

[swansont]

Understood. Thank you for the fast reply.

 

The idea came from, in every example of space-time I've seen is always the ball on a trapoline, or the picture of a grid being 'pushed down' by the planet. So the idea was presented, what if... what if it acutal IS something? That would make some sense of this Dark Matter and Dark Energy thing.

 

That's to show you the geometry. If it's a substance, you should be able to measure the properties of it, and how it interacts with things.

[pantheory]

Is it possible that space is another substance? As in, earth, water, air... Space is just a substance that can be interacted with? As oppose to simply 'distance between two objects'?

I like the definition concept model of both space and time. "Space is the distance between two objects," is probably no inclusive enough. "How about space is the distance between matter, and the volume which encumpases the entire universe?" As its own entity, space might be defined as the absent volumes distinguishing substance from non-substance within the universe.

[ccwebb]

 

That's to show you the geometry. If it's a substance, you should be able to measure the properties of it, and how it interacts with things.

Isn't that what gravity is then? A measurment of spacetime and how it ineracts with things?

[altergnostic]

Isn't that what gravity is then? A measurment of spacetime and how it ineracts with things?

We measure how matter interacts with matter due to gravity, we don't measure how gravity interacts with matter, only how matter interacts with gravity.

And we calculate how spacetime interacts with matter, but the curvature of spacetime can't be measured directly, it's geometry is calculated from the measurements we do on matter. In other words, spacetime geometry mathematically models how massive bodies affect other massive bodies to generate the effects caused by gravitation.

 

Interesting. In this sense, gravity is a bit like the aether. We thought the aether was the cause of some observations, but couldn't detect it. Later on we managed to explain our observations without the concept of an aether. Maybe someday we will have a theory that explains what we observe without spacetime curvature or any field whatsoever.

[michel123456]

The aether again.

IIRC Mendeleev had reserved a place for aether in his periodic table.

 

But so far no aether is needed to explain physics.

On the other hand the question "what is space" is very interesting. But not in the sense that space could be some material thing.

 

Space alone means nothing.

Space AND time means something.

 

Since it has been established that time & space are so tightly linked in such a way that what one observer call Time another observer call Space, it should be obvious that time & space are interchangeable (I call that rotatable).

 

Would you ever imagine that time is a kind of material thing? A kind of Aethertime? I don't think so.

Edited January 11, 2013 by michel123456

[swansont]

Isn't that what gravity is then? A measurment of spacetime and how it ineracts with things?

 

Gravity is the effect of the geometry, not the purported properties of spacetime.

[michel123456]

Just found this old superb thread with historical answer from Martin (where is Martin?):

 

OK, that is a good answer. I was just curious what is bugging you. It will have to go on bugging.

When you ask what is geometry made of, you are at limit of knowledge and you stand around waiting, with the rest of us. Like at a bus stop, except we don't queue up in a line like Brits.

There are very interesting papers recently by Eric Verlinde (Amsterdam) and Thanu Padmanabhan (from Poona, India).

They say that geometry (the grav field) behaves according to thermodynamics and as if geometry had a temperature and entropy. They derive Einstein field equation from the laws of thermo. They say if you can heat something then it must have atoms.

This was Ludwig Boltzmann's great insight. If you can heat something then it must have microscopic degrees of freedom that you can't see---so he realized there were atoms and molecules explaining the behavior of a gas, even before these were seen.

Look up E. Verlinde on arxiv. Look up T. Padmanabhan. It is completely leading edge by so-far reputable people.

Geometry is real. The metric is just our mathematical description of a real thing. (Like our math description of an electron or a photon.)
But there may be deeper microscopic degrees of freedom---"atoms of geometric relationship"---which we cannot yet see, and do not yet know. Atoms of angle, of area, of curvature.
And these can be heated and they can have entropy. (You know that DeSitter space has a temp? That a black hole has a Hawk temp?
that an accelerating frame has an Unruh temp? Geometry is able to have temperature, in principle that you can measure.)

Verlinde has derived Newtonian gravity law from "entropic dynamics". Gravity for him is an "entropic force". Read the January paper. It is all simple math and clear reasoning. This is where respected reputable people say things that sound crazy.

So look up Verlinde and Padma on arxiv. Or don't. Don't think about it. Just wait.

I think you will continue to be bugged. Why? The situation is clear: there must be something deeper than geometry, from which it emerges, but we do not yet know the correct way to imagine it. Personally the situation gives me pleasure:D, it is is the right position for humans to be in---to desire and hope, but not have. But you are equally right to be bugged by the situation.

Verlinde is a former string theorist who now says adamantly that string is not the way to go and his way etc etc. Which adds a slight frisson. A whipped cream topping with chopped nuts, so to speak.

Oddly enough an American, Ted Jacobson, published a proof around 1995 where he derived the Einstein equation (governing geometry) from thermodynamics. Just google "jacobson thermodynamics" and take the first hit.

Padmanabhan is so excited that when he gives slide lectures about it he absolutely refuses to cite the work of Jacobson and Verlinde. He cites only his own papers! This in itself is a bit sensational, a histrionic gesture outside of normal behavior. He just gave a talk yesterday about it at Perimeter Institute, available online video. Such is the extraordinary ferocity that comes out in the good times. Passions are aroused.

Whole thing is fun to watch. Better than if the bus would actually arrive.

[swansont]

Unruh temperature is an effect of the vacuum. I was not aware that people are equating the vacuum with spacetime. That doesn't seem right, but it's also not my field.

[Cassandre]

Is it possible that space is another substance? As in, earth, water, air... Space is just a substance that can be interacted with? As oppose to simply 'distance between two objects'?

The word "space" has slightly different meanings; it quickly gets very philosophical.

 

It can't be a material substance like earth, water or air - that is an "ether" concept that nobody could match with all observations (I think that this has been argued by Lorentz and Michelson). However "empty space" can hardly be truly empty either, if we believe in the validity of field theories.

One discussion by Einstein (a bit dated though, of almost a century ago) can be found here:

https://en.wikisource.org/wiki/Ether_and_the_Theory_of_Relativity

 

Oops I now see that you posted your question in classical physics; however many clues come form relativity (as you can see in the link) and also quantum mechanics. Anyway, the linked paper also mentions a clue from classical mechanics, as forwarded by Newton.

Edited January 16, 2013 by Cassandre

[SSDS]

For this topic it may be useful to read arXiv:1110.0003v2, V2.

 

Cheers

Edited January 18, 2013 by SSDS

[pantheory]

Is it possible that space is another substance? As in, earth, water, air... Space is just a substance that can be interacted with? As oppose to simply 'distance between two objects'?

By far the the simplest definition of space is that it is the volume the matter encumpases, or as you said: the distance between matter.

 

But this is not the present theoretical consensus model of space (unfortunately ) Although space is known to have energy within it, as in zero-point-energy, and hypothetical particles within it like dark matter, or theoretical particles like the Higg's and virtual particles, etc. But if present theory is valid then space, absent matter and energy, also can bend and warp like general relativity proposes, expand like the Big Bang model proposes, and accelerate its expansion like the dark energy hypothesis proposes, then space would need to have very complicated characteristics quite different from a volume of "nothingness."