Is spacetime a "substance" or not?

[Kuznetzova]

Is spacetime a "substance", akin to the luminiferous aether?

In the 1890s, light (or electromagnetic radiation) was believed to propagate as a wave upon a medium that permeated all of space. That medium was dubbed the Luminiferous Aether. Special Relativity was a theory that showed this need not be the case. Additionally, experiments that meant to find and confirm the aether all failed. Today the most respected theory of gravity is General Relativity, first published by Einstein in 1915. This theory claims that gravity results from curvature of a 4D manifold dubbed "spacetime". The curvature in most cases is created by mass, but can be created by anything containing energy.

But have we come full circle again? Does spacetime mean that material objects are sloshing around inside a sort of "liquid" that changes density? Worse, is spacetime just another desperate recourse to an aether?

It turns out this question was hotly debated in the 5 years leading up to the publication of General Relativity.

For those who are interested in this debate, there is a large article about the issue at Stanford's repository.
http://plato.stanford.edu/entries/spacetime-holearg/

[tillingborn]

Take a simple event such as a photon passing from one atom to another. If the atoms were stationary relative to you, you would judge that the event happens in the time it takes light to travel the distance between the atoms. If the atoms are moving relative to you, you will judge that the event takes as long as it takes light to travel from the first moving atom to the new position of the second moving atom. As it is further, you will judge that the event takes longer to occur. If you believe that spacetime is a substance, it seems to me that you are committed to believing that this time dilation is because the moving atoms warp spacetime in a way described by relativity.
On the other hand, if you believe that spacetime is just a mathematical tool, you are free to argue that the event takes longer because the distance is greater. I've had a bit of training in philosophy and I know better than to commit myself, but I'm leaning towards the latter; William of Ockham and all that.
Anyway, the onus, as ever, is on the believers to prove it.

[Hjarloprillar]

space is according to Einsteinian math. a 'thing'
A thing that can be bent.. by gravity, and thus a true thing.
[in itself.]

I suggest you spend a few weeks simply running that through your head..It took me far longer, im a 150iq idiot.
A disciple of newton. OLD SCHOOL

Nikos of sparta

[tillingborn]

space is according to Einsteinian math. a 'thing'
A thing that can be bent.. by gravity, and thus a true thing.
[in itself.]

Actually, gravity is the 'thing' being bent by mass.

I suggest you spend a few weeks simply running that through your head..It took me far longer, im a 150iq idiot.
A disciple of newton. OLD SCHOOL

Nikos of sparta

No thanks.

[Hjarloprillar]

"Actually, gravity is the 'thing' being bent by mass."
__________________________________________________
"Actually"?
Do you speak from experience or simply wishfull thinking?

It thus can exist independent of mass? true.
law exists first
that the effects it results in where mass is existent. Can be altered by the mass it is effecting.. false
I suggest gravity is a constant under all conditions

can you SHOW me me ONE example where it is not?

Nikos of Sparta

[tillingborn]

So. Gravity is not a law but a thing.

It is a force that acts between objects that have mass.

It thus can exist independent of mass? true.

Not so says General Relativity, according to which: no mass=no warping of spacetime=no gravity.

that the effects it results in where mass is existent. Can be altered by the mass it is effecting.. false

Gravity is a very complicated interplay of the forces exerted by every atom in the universe on every other one. The fine details of the gravity we experience is overwhelmed by the force exerted by the bloody great rock we're all on.

[Hjarloprillar]

reply to above post.. LAW exists inherent in reality. it IS even if mass is not.It does not just pop into existence when mass comes along.
"the force exerted by the bloody great rock we're all on"
that force is what we talk of. We also experience all the other laws.
SoL. 3rd law of T. planks exct.
We could hardly think about something like a law if we had no experience of it. It is our sacred duty as a species that can imagine and conceptualize to work this
shit out.. Otherwise why not be sheep. ?
Regrets,i've had a few , but then again, too few to mention.

If the atoms were stationary relative to you

then time is removed from equation.
And your argument falls down.

Stationary to you.. What part of you? Your eyes. your consciousness?
your hopes and fears?

someone left the kindergarten gate open. And the beast " sohistry " escaped.

[tillingborn]

reply to above post.. LAW exists inherent in reality. it IS even if mass is not.It does not just pop into existence when mass comes along.

The universe behaves the way it does. Whether it is obeying laws that exist independently of it, or it just does what it does and we find patterns of consistent behaviour that we describe with maths is a moot point. I happen to think the latter. I think things like 2+2=4 are true whether or not there are any objects for them to be true about, whereas the laws of physics are just what our universe happens to do, more or less.

If the atoms were stationary relative to you

then time is removed from equation.
And your argument falls down.

Why would that remove time from the equation?

[Kuznetzova]

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[Kuznetzova]

It is a force that acts between objects that have mass.

Actually this cannot be inferred from the Einstein Field Equations directly. The equation itself only says that the curvature of spacetime tells you how things move.

This picture does not dictate that an object at rest would begin to accelerate. We do , however, see this happen in reality. Two observations.

1) When Newton reigned supreme in physics, gravity was a force only by virtue of its ability to make moving objects curve outside of a straight line. But nothing moves outside of a line unless "acted upon by a force". Thus gravity was a force in the 17th century mindset. Today, Gravity is only considered a "force" because when you take a bowling ball up a flight of stairs you can drop it from the 5th story window. When it impacts the ground, enormous amounts of energy are released. Only by virtue of that happening is gravity a "force".

2) Any pointlike particle at absolute rest would never begin to move, regardless of how warped spacetime may be in its local neighborhood. Remember, objects follow geodesics of spacetime, provided they are already in motion to begin with. Quantum mechanics picks up the slack here. Find a cup, spoon, or table in your room. Note that the particles making up that object are always ever in motion. If a cup, flick the cup with your finger and note the sound caused by vibration of the particles that appear "stationary". Nothing stops moving. Everything is in motion.

Not so says General Relativity, according to which: no mass=no warping of spacetime=no gravity.

But you just referred to spacetime as a substance there.

[tillingborn]

It is a force that acts between objects that have mass.

Actually this cannot be inferred from the Einstein Field Equations directly. The equation itself only says that the curvature of spacetime tells you how things move.

To be honest I wasn't trying to infer it that way.

This picture does not dictate that an object at rest would begin to accelerate.

At rest relative to what? Never mind. I've made the point before that the it doesn't follow that because the picture describes the action that the picture is an accurate one. That is true of Ptolemaic epicycles, it is true of string theory and it is true of general relativity.

We do , however, see this happen in reality.

Indeed, because:

It is a force that acts between objects that have mass.

1) When Newton reigned supreme in physics, gravity was a force only by virtue of its ability to make moving objects curve outside of a straight line. But nothing moves outside of a line unless "acted upon by a force". Thus gravity was a force in the 17th century mindset. Today, Gravity is only considered a "force" because when you take a bowling ball up a flight of stairs you can drop it from the 5th story window. When it impacts the ground, enormous amounts of energy are released. Only by virtue of that happening is gravity a "force".

I've made essentially this point somewhere too. I said something like:Newton describes the dent a lump of uranium will make if it hits you; Einstein tells you how big a city it will flatten if it unravels.

2) Any pointlike particle at absolute rest would never begin to move, regardless of how warped spacetime may be in its local neighborhood.

Yeah. but:

We do , however, see this happen in reality.

Remember, objects follow geodesics of spacetime, provided they are already in motion to begin with.

I think it's more the case that objects do what they do and that can be described by geodesics.

Quantum mechanics picks up the slack here. Find a cup, spoon, or table in your room. Note that the particles making up that object are always ever in motion. If a cup, flick the cup with your finger and note the sound caused by vibration of the particles that appear "stationary". Nothing stops moving. Everything is in motion.

Right. So whither

This picture does not dictate that an object at rest would begin to accelerate.

?

Not so says General Relativity, according to which: no mass=no warping of spacetime=no gravity.

But you just referred to spacetime as a substance there.

Not me. That was GTR. I'm more yer old school material monist; I strongly suspect that the cause of the experiences of a universe that is made of some stuff is some stuff that the universe is made of. I also suspect that it behaves pretty much as the most accurate observations say it does and that careful analysis allows us to describe that behaviour using the mathematical expedient of dimensions. It does not, in my view, follow that space and time, or even spacetime therefore exist.

[Kuznetzova]

Not me. That was GTR. I'm more yer old school material monist; I strongly suspect that the cause of the experiences of a universe that is made of some stuff is some stuff that the universe is made of. I also suspect that it behaves pretty much as the most accurate observations say it does and that careful analysis allows us to describe that behaviour using the mathematical expedient of dimensions. It does not, in my view, follow that space and time, or even spacetime therefore exist.

There is more going on here than mathematical expediency. There is also the nagging fact -- that in every situation in which GR has been tested, it has passed that test with flying colors. From your perspective, it was as if Einstein simply tossed darts and made up random equations, and voila some of them accidentally fit the data and a new theory was born. That's silly. Einstein was reasoning out something about coordinates, and he must have been making commitments to geometry at least somewhere in his thinking. Common sense would dictate that if we begin to reason about something in the world around us, and we commit to the idea that geometry will be faithfully followed, this is an indication that the "Kantian stuff" is actually real. "Real" in the sense of having a predictable extension in space -- for why else would we be reasoning about its shape? Think about it.

As a guide to help you better respond to this post, I use the phrase, "common sense would dictate", as an invitation to your thoughts. "Common sense would dictate" means I have rolled out a nice red carpet for you to write on with your reply. I think a good reply would follow suit something like "Common sense certainly does dictate this, kuznetzova, however in this situation there is a more subtle hook because _________ (fill in the blank)"

I honestly don't think that blank can be filled in with "because I am a solipsist who does not believe in science" - or some variation of that stance. I get my fill of that type of stoner philosophy enough in chat rooms.

Secondarily, I don't think that "I refuse to speculate because metaphysics is hogwash and I choose to remain skeptical" works either. Because if given that response, I would first ask you why you have even logged into a philosophy forum -- or worse, why are you responding to threads in the metaphysics section?

[tillingborn]

Common sense certainly does dictate this, kuznetzova, however in this situation there is a more subtle hook because material monism as I have outlined it is a metaphysical world world view. You mention Kant, it was he who tried to reconcile rationalism and empiricism, but had to concede that there is a phenomenal world and a noumenal world. Reading the introduction to the Critique of Pure Reason I was struck by the commentator (I forget who it was, some Irish feller) referring to Hume's idealism. I was aware of Berkeley's of course and I initially assumed the commentator was an idiot; it took a while for it to sink in, but in fact if pushed, any consistent empiricist has to admit that they can only describe the phenomenal world; it's part of the reason why logical positivists, being largely physicalists, got their knickers in a twist. Physics is an empirical enterprise, as Feynman said(ish), it doesn't matter how smart you are or how beautiful your theory, if it doesn't agree with observation, it's wrong.

There is more going on here than mathematical expediency. There is also the nagging fact -- that in every situation in which GR has been tested, it has passed that test with flying colors.

Absolutely! It's a great little bit of maths, QED is even better and it says the world is something completely different.

From your perspective, it was as if Einstein simply tossed darts and made up random equations, and voila some of them accidentally fit the data and a new theory was born. That's silly.

Well, yes that would be silly. As I understand it though, Einstein came up with the physical model and had an almighty struggle (and lot's of help) generating the maths to support it; he said something along the lines of: coming up with relativity was easy, proving it was next to impossible.

Einstein was reasoning out something about coordinates, and he must have been making commitments to geometry at least somewhere in his thinking.

What makes science different from philosophy is that science actually has to produce results. Us philosophy types can twiddle our thumbs and think how wonderful/awful/meaningless everything is and not have to show anything for it. From a scientific point of view, musing over the nature of space and time is useless if you can't tell where or when anything is, that takes 4 coordinates. The most commonly used are Cartesian x,y and z plus t; 0,0,0,0 is always some point chosen for the purpose, there isn't a real one anywhere.

Common sense would dictate that if we begin to reason about something in the world around us, and we commit to the idea that geometry will be faithfully followed, this is an indication that the "Kantian stuff" is actually real.

I'm not sure what "Kantian stuff" you mean. As far as I can work out you are saying that spacetime and/or geometry is real because it has 'a predictable extension in space'. That has the geometric qualities of a circle. What do you actually mean?

"Real" in the sense of having a predictable extension in space -- for why else would we be reasoning about its shape? Think about it.

I think that's the same wonky logic Anselm used for his ontological argument.

Don't get me wrong (the fact that you have demonstrates more sloppy reasoning, which if I were to engage in I might characterise as 'here's another stoner solipsist who disagrees with me', that's more or less how psychoanalysts make their diagnoses and a big part of the reason it's a load of bobbins), I believe in science utterly, the observations we have made are spectacular, the maths generated to support it and that has allowed us to manipulate the world we find ourselves in is breathtaking, but although you can map events using 4D spacetime, you cannot demonstrate 4D spacetime without events. Think about that.

[Kuznetzova]

I'm not sure what "Kantian stuff" you mean. As far as I can work out you are saying that spacetime and/or geometry is real because it has 'a predictable extension in space'. That has the geometric qualities of a circle. What do you actually mean?

Scientists are not merely speculating. GR has passed every test it has been put to. Are you going to play that off as coincidence? I realize equations are created by human beings and they are "Kantian ideas" in their minds. I understand and appreciate that fact fully. But how is it that these equations so precisely predict what is actually measured? Accident? Coincidence? How can you stand there and say there is zero connection between the geometry of the equations and the actual extended geometry of spacetime (that thing out there in the world) ?

"Because I'm a skeptic and your logic is wonky" is not going to be a persuasive answer to any of those questions. Sorry.

[tillingborn]

How can you stand there and say there is zero connection between the geometry of the equations and the actual extended geometry of spacetime (that thing out there in the world) ?

"Because I'm a skeptic and your logic is wonky" is not going to be a persuasive answer to any of those questions. Sorry.

I honestly don't think that blank can be filled in with "because I am a solipsist who does not believe in science" - or some variation of that stance. I get my fill of that type of stoner philosophy enough in chat rooms.

Secondarily, I don't think that "I refuse to speculate because metaphysics is hogwash and I choose to remain skeptical" works either. Because if given that response, I would first ask you why you have even logged into a philosophy forum -- or worse, why are you responding to threads in the metaphysics section?

From the point of view of science and of philosophy, all of the above are completely inept. What is your evidence that any of it applies to me? Can you construct a logical argument to demonstrate that it must?

I realize equations are created by human beings and they are "Kantian ideas" in their minds. I understand and appreciate that fact fully. But how is it that these equations so precisely predict what is actually measured? Accident? Coincidence?

Because if they didn't, we would move on and try and find equations that do. If anyone wishes to make the metaphysical leap from 'the model works' to 'the model is true', that's up to them. For all I know, they may be right; that doesn't make me a sceptic, a solipsist, a stoner or anything else beginning with s. If you wish to discuss the issue you have raised, well and good. If on the other hand this is going to be an examination of my supposed intellectual weaknesses, I might choose to kick your cerebral butt, more likely you can kiss my philosophical arse goodbye.