# Is space fabric a real thing?

• MathJakob
In summary, there are multiple theories and analogies that attempt to explain the nature of space. Some treat space as "empty" and without any physical properties, while others suggest that space is filled with particles and energy. The concept of "curved" space can be difficult to understand, but it is a result of the geometrical relationships between material objects. Ultimately, the true nature of space is still a mystery and there is no confirmed answer.

#### MathJakob

I've tried to google the answer but either I'm not reading it clearly or there is no actual answer, but is space materialistic? I know there are various analogies but is there an actual answer? Is 'empty' space itself an object?

From what I understand stars and galaxies rest on the 'fabric' of space which causes it to dip thus producing gravity, but is space fabric even a real thing? If not then gravity itself doesn't really exist, it's just an effect of warped space and if the universe had no matter in it, there would'nt be any gravity?

Someone please clear this up for me, if there is a common theory or something which is most likely the case but isn't confirmed yet please tell me what that might be.

There is no such thing as "empty space."

I don't understand how you say that gravity doesn't really exist depending upon how it comes into existence. Gravity is definitely very real, regardless of the underlying mechanism behind it.

If the universe had no matter in it, there wouldn't be a universe now would there?

I'm not sure I entirely understand the question or what materialistic is.

There are multiple answers to your question depending upon what you mean.

For example classical physics treats space as having "nothing" in it - like an empty graph, but without the lines.

General Relativity tells us that the graph does have lines, but that they may not be straight - and that the lines may change with time.

Quantum mechanics tells us that "empty space" is filled with virtual particles which are due to "quantum fluctuations".

Actual measurements tell us that space is never empty - that there are photons (cosmic background radiation) in every cubic meter, as well as other particles, especially neutrinos. Only the density of stuff changes.

And these are only the standard theories!

Einstein posited space has 'no ponderable properties'. The 'fabric' of spacetime is an analogy that generally serves to confuse, rather than clarify.

Chronos said:
Einstein posited space has 'no ponderable properties'. The 'fabric' of spacetime is an analogy that generally serves to confuse, rather than clarify.

Then how is it possible that an object with x mass can curve space if there is nothing to curve? How can space curve if space isn't materialistic?

I know this is only an analogy but surely you can appreciate how there needs to be some physical material to warp in the first place?

http://www.messagetoeagle.com/images/spacetimeshockwave.jpg [Broken]

Student100 said:
If the universe had no matter in it, there wouldn't be a universe now would there?

Of course there would be. Take away all the galaxies including light and you have still the universe.

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MathJakob said:
Then how is it possible that an object with x mass can curve space if there is nothing to curve? How can space curve if space isn't materialistic?

Take three small objects, not all located on the same line. They're real solid material objects.

Stretch strings between them. These strings are also real solid material objects, and they form a triangle.

Measure the internal angles of that triangle. Do they add to 180 degrees? If not, our three objects are set in a curved space... And I worked this out without ever having to consider whether the space is a material object.

Nugatory said:
Take three small objects, not all located on the same line. They're real solid material objects.

Stretch strings between them. These strings are also real solid material objects, and they form a triangle.

Measure the internal angles of that triangle. Do they add to 180 degrees? If not, our three objects are set in a curved space... And I worked this out without ever having to consider whether the space is a material object.

And you didn't answer anything relating to my question. I know space is curved... I accept it as fact but my question was: Is space itself materalistic?

I know space is curved, but again I ask, how can something which isn't materalistic curve, bend, warp, stretch, contract whatever you want to call it.

MathJakob said:
And you didn't answer anything relating to my question. I know space is curved... I accept it as fact but my question was: Is space itself materalistic?

I know space is curved, but again I ask, how can something which isn't materalistic curve, bend, warp, stretch, contract whatever you want to call it.

What we know and observe are material objects and the geometrical relationships between them. If the interior angles of the triangle added to 180 degrees, we wouldn't jump to the conclusion that space is a material object that happens to be flat; so if they don't add to 180 degrees we don't have to conclude that space is a material object that happens to be curved.

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MathJakob said:
Then how is it possible that an object with x mass can curve space if there is nothing to curve? How can space curve if space isn't materialistic?

Of course there would be. Take away all the galaxies including light and you have still the universe.

You're stuck on this whole curving of space thing.

Okay, taking away all matter is different than taking away ordinary matter. Taking away all ordinary matter will get you get a de sitter, radiation only type universe.

MathJakob said:
I know space is curved, but again I ask, how can something which isn't materalistic curve, bend, warp, stretch, contract whatever you want to call it.
Ask yourself whether you find it equally confusing that something that isn't materialistic can have distance measured across it.

Bandersnatch said:
Ask yourself whether you find it equally confusing that something that isn't materialistic can have distance measured across it.

Of course not because distance isn't a real physical thing. It's just a method of determining how far away two locations are. It doesn't require a physical medium.

But for space to have characteristics such as being curved and warped it must have some kind of physical presence.

A lot of the answers you guys have provided are similar to the ones I found from my Google search, there doesn't really seem to be a proper scientific answer. Is it just the case that we don't know?

What would we see if we took an electron sized region of 'empty' space and magnified it to Planck scale or further, would some sort of material start to emerge? If space isn't materalistic, then I'm just finding it hard to accept that something which technically is nothing, can have characteristics.

Bandersnatch said:
Ask yourself whether you find it equally confusing that something that isn't materialistic can have distance measured across it.

MathJakob said:
Of course not because distance isn't a real physical thing. It's just a method of determining how far away two locations are. It doesn't require a physical medium.

And curvature is a property of the relationships between these distance measurements, such as whether the distance between two parallel lines is the same everywhere. Where's the real material thing there?

The Michelson-Morley experiment demonstrated that there is no fabric of space. It surprised them, and the rest of the scientific community too.

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MrAnchovy said:
The Michelson-Morley experiment demonstrated that there is no fabric of space. It surprised them, and the rest of the scientific community too.

Thanks. I'll have a read of that in a minute. Still my question stands though, if 'empty' space has no structure, there is nothing to bend.

You are using a very naive picture of curved manifolds based on the terrible analogies of "warping" and of "bending", both of which only make sense if the space-time manifold is embedded in a higher dimensional manifold; in GR we care only about the intrinsic curvature of space-time and this is not something you can picture in the above manner. Nugatory has already given a better reply than I can give so I just want to say that this is hinging on philosophy rather than physics.

MathJakob said:
MrAnchovy said:
The Michelson-Morley experiment demonstrated that there is no fabric of space. It surprised them, and the rest of the scientific community too.
Thanks. I'll have a read of that in a minute. Still my question stands though, if 'empty' space has no structure, there is nothing to bend.

Isaac Newton said:
Look at that apple falling from that tree, doesn't it make you think that there is an attraction between objects with mass?
Thanks. I'll think about that later. Still my question stands though, if there is nothing pushing down on us, why do we stick to the ground?

MrAnchovy said:
Thanks. I'll think about that later. Still my question stands though, if there is nothing pushing down on us, why do we stick to the ground?

You might want to start a new thread in the relativity section for that question... I don't mind taking a shot at answering it (actually, pointing you at some of the answers already out there) but it feels off-topic in a a thread about spacetime and materialism.

I think the clearest way to resolve this is to look at the fact that empty space can carry energy and momentum, in the form of gravitational radiation.

And incidentally, there are a number of empty space-time manifolds which, nevertheless, have curvature.

does space have any effect on matter or energy? Is the warping of space only a presumption?

The paths that rays of light take, that is the effect of spacetime geometry. We see the effect of curvature in gravitational lensing of light (which is energy, if you like).

When you drop something it falls. That is the effect of geometry being curved. The curvature is not "just a presumption".

The paths free particles take are geodesics. Geodesics in a curved geometry (like the great circle routes over the Earth's surface that airplanes take) are the analogs of straight lines in flat space. Planets follow geodesics in the solar system's geometry.

So yes, space (or more generally space-time) geometry DOES have an effect on matter and lightwaves. The effects are highly visible and evident.

You might re-read Nugatory posts #6, 8, and 12. Having curvature does not mean that geometry (whether spatial or 4d) is a MATERIAL SUBSTANCE. We have learned that a bunch of geometric relationships can be physically real (somewhat like the lines of force of a magnet you might have played with as a kid) without being a material substance.

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marcus said:
You might re-read Nugatory posts #6, 8, and 12. Having curvature does not mean that geometry (whether spatial or 4d) is a MATERIAL SUBSTANCE. We have learned that a bunch of geometric relationships can be physically real (somewhat like the lines of force of a magnet you might have played with as a kid) without being a material substance.

Thanks. So I can tell people that space itself is not materalistic and I'll be correct in saying so?

I had similar questions regarding what is space. My conclusion was that every macro phenomenon can be explained by the interactions of fundermental particles. Space itself is only a place where it can all happen:

That's the Newtonian view of space. General relativity doesn't work that way; space isn't just a backdrop on which physics takes place.

keepit said:
does space have any effect on matter or energy? Is the warping of space only a presumption?
Sure. In principle, gravitational radiation can impart energy and momentum to matter. In practice this rarely happens to any significant degree, because gravity is so weak.

MathJakob said:
You might re-read Nugatory posts #6, 8, and 12. Having curvature does not mean that geometry (whether spatial or 4d) is a MATERIAL SUBSTANCE. We have learned that a bunch of geometric relationships can be physically real (somewhat like the lines of force of a magnet you might have played with as a kid) without being a material substance.
Thanks. So I can tell people that space itself is not materalistic and I'll be correct in saying so?

You'll be in line with a considerable body of current opinion. I want to stop short of saying "correct". I think there is room for disagreement. Philosophers from Aristotle to Leibniz have taken the "Relationalist" view that space is not a substance but consists of the geometric relations among material things.
That's also the view developed in Einstein GR. The whole idea of "General Covariance" is relationist. There is no absolute space in GR.

But I'm told that there are also philosophers called "Substantivalists" who maintain the opposite. I guess you could say Newton took that line, because space and time are absolute in Newtonian physics. I understand that Newton and Leibniz had a bitter argument about this. Leibniz took the Relational line, and his view seems to ultimately have been upheld by Einstein and later folks (but Newton's physics temporarily triumphed.)

Everything is provisional in basic science. Nothing is 100% sure, theories are meant to be tested and challenged (not believed): they are only the best, most reliable models of reality that we have so far. And there can be legitimate disagreements about which theory actually does give the simplest best fit to observation and the most reliable predictions.

That said, if you want to tell other people that space is not a material substance (what I think you mean by "materialistic") then HECK YES! I'd say that's correct. You'd have a large body of current opinion backing you up on that.

marcus said:
That said, if you want to tell other people that space is not a material substance (what I think you mean by "materialistic") then HECK YES! I'd say that's correct. You'd have a large body of current opinion backing you up on that.

And of some practical significance... The misunderstandings that result will likely be fewer and less serious than the ones that result from thinking of space as a material substance.

marcus said:
You'll be in line with a considerable body of current opinion. I want to stop short of saying "correct". I think there is room for disagreement. Philosophers from Aristotle to Leibniz have taken the "Relationalist" view that space is not a substance but consists of the geometric relations among material things.
That's also the view developed in Einstein GR. The whole idea of "General Covariance" is relationist. There is no absolute space in GR.

But I'm told that there are also philosophers called "Substantivalists" who maintain the opposite. I guess you could say Newton took that line, because space and time are absolute in Newtonian physics. I understand that Newton and Leibniz had a bitter argument about this. Leibniz took the Relational line, and his view seems to ultimately have been upheld by Einstein and later folks (but Newton's physics temporarily triumphed.)

Everything is provisional in basic science. Nothing is 100% sure, theories are meant to be tested and challenged (not believed): they are only the best, most reliable models of reality that we have so far. And there can be legitimate disagreements about which theory actually does give the simplest best fit to observation and the most reliable predictions.

That said, if you want to tell other people that space is not a material substance (what I think you mean by "materialistic") then HECK YES! I'd say that's correct. You'd have a large body of current opinion backing you up on that.
Strongly, strongly disagree with you there. The existence of gravitational radiation makes that a difficult position to defend.

keepit said:
does space have any effect on matter or energy? Is the warping of space only a presumption?

marcus said:
The paths that rays of light take, that is the effect of spacetime geometry. We see the effect of curvature in gravitational lensing of light (which is energy, if you like).

When you drop something it falls. That is the effect of geometry being curved. The curvature is not "just a presumption".

The paths free particles take are geodesics. Geodesics in a curved geometry (like the great circle routes over the Earth's surface that airplanes take) are the analogs of straight lines in flat space. Planets follow geodesics in the solar system's geometry.

So yes, space (or more generally space-time) geometry DOES have an effect on matter and lightwaves. The effects are highly visible and evident.

You might re-read Nugatory posts #6, 8, and 12. Having curvature does not mean that geometry (whether spatial or 4d) is a MATERIAL SUBSTANCE. We have learned that a bunch of geometric relationships can be physically real (somewhat like the lines of force of a magnet you might have played with as a kid) without being a material substance.

An interesting follow-up question!
are these the effects of spacetime geometry or of gravity?...
Thanks to the person who asked! (You know who you are.) I want to respond here rather than in PM.
This raises an interesting point. In GR there is no difference between gravity and spacetime geometry. You often see it stated explicitly as "gravity=geometry".
In GR the gravitational field IS NOTHING BUT the metric tensor that gives a mathematical description of the spacetime geometry.

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i hope I'm not being annoying here but since i haven't taken enough math, i don't see a mechanism here. I was trained in mechanisms.

keepit said:
i hope I'm not being annoying here but since i haven't taken enough math, i don't see a mechanism here. I was trained in mechanisms.

You are right to see a gap. This gap (an undiscovered mechanism by which geometry can guide matter and matter can, in turn, bend geometry) fascinates and attracts researchers to QG.

One wants to find out how, at their roots, matter and geometry are related, so it will seem natural that they interact.

As an inexpert bystander just watching the ongoing research in quantum geometry/gravity I would say that Einstein 1915 GR is merely DESCRIPTIVE of the manner in which matter and geometry interact. The reason people are drawn into QG research is they want to understand the mechanism of that interaction.

The LHS of 1915 GR equation is purely geometric, a package of numbers describing curvature.
The RHS of the equation is about matter a package of numbers describing energy and momentum.
So it is saying that, at any given point, LHS = RHS, meaning that matter is determining how the geometry is bending/changing.
And by definition the geometry determines the geodesics, the shortest distance paths, along which matter tries to flow.
So it is DESCRIBING the interaction that we see occurring in reality. We see matter and geometry behaving like that. But the equation is not saying why they interact like that.

So the quantum relativists, or quantum gravitists if you prefer, seem to be expecting that if they can delve down into the quantum roots of geometry, they will be able to find a connection between the quantum description of matter and that of geometry. A way to put BOTH into the same Feynman path integral, or if you like both into the same Lagrangian (a mathematical machine that physicists customarily use to describe dynamics). That would be lovely! quantum matter and geometry working together in the same mathematical machine which is cranking out combined TRANSITION AMPLITUDES between initial and final combined quantum states of geometry and matter! (You can see how this would excite and motivate some researchers.)

I don't want to overstate this, but I think you are pointing out an interesting and attractive gap in the current picture that people have.

There is also the thing of people wanting to understand how geometry and matter behave at extremely high energy density---what could have been happening around start of expansion? could there have been a bounce when the energy density was very high?
And I gather some people would like to be able to compute scattering amplitudes for head-on or nearly head-on particle collisions where the energy is not high enough to form a miniature black hole but is nevertheless high enough that ordinary QFT doesn't work because geometry does get involved. So there's a bunch of other motivations, things people are curious about besides what you suggested.

WannabeNewton said:
That's the Newtonian view of space. General relativity doesn't work that way; space isn't just a backdrop on which physics takes place.

I really really wish you hadnt said that. I now have to take my undertanding back to the drawing board so I hope you arent mistaken!