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Science and Technology "Somewhere, something incredible is waiting to be known." - Carl Sagan.

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Old January 18 2013, 08:34 AM   #1
Gary7
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Basic Science Question - "fabric" of space

If there are any hard science people out there on TBBS that might browse this sub forum, please take a moment to comment here if you don't mind. I have a very, very basic question about the "fabric of the universe".

When you read about gravitational theories, there is often the postulation that there isn't any gravitational force per se, but instead that space is "curved", relative to the mass of the objects within it. A typical demonstration of this is a stretchy fabric pulled taut over a bucket. You put a heavy marble in the middle and then you observe a symmetrical semi-conical sagging towards the center. Rolling another smaller spherical object on the material causes it to "orbit" the center object. This is supposed to simulate how gravity actually works.

So then there's the "fabric" notion. Well, I find the word mystifying because fabric suggests a tangible substance that can be felt. Clearly, there is no such "visible" fabric. So then is it invisible or "dark matter"? Yet, if that is the case, couldn't we detect it in some way when pointing sensors at the moon? That between the Earth and the moon there must be some measurable amount of "fabric" there?

What always gets me about this concept is this: Space is a vacuum. Emptiness. That between the Earth and the moon, there might be some dust or debris floating about, maybe even a comet or asteroid, but discounting those it's empty space. NOTHING THERE. So where's this "fabric"?

In my mind, in order for there to be any warping, there must be something to be warped. Some detectable mass. But when "dark matter" is mentioned, it's in a very nebulous manner... that it's some "background" constituent in deep space. Still... for there to be sufficient gravitational force to cause objects to swing into orbit around the Earth, certainly that mass should be substantial enough to cause the effect and thus be detectable in some manner akin to how we detect other matter. Right?


Anyway, I haven't yet found any explanation that makes sense to me. Anyone care to shed light? Much appreciated!
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Old January 18 2013, 12:23 PM   #2
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Re: Basic Science Question - "fabric" of space

The "fabric" thing is basically an analogy for convenience of explanation, and shouldn't be taken too literally. Let's see if I can remember enough of my physics texts this early in the morning...

First off, Einstein showed that every observer has to measure the same speed of light, no matter how fast or in what direction the observer is moving relative to the light. That seems weird -- if something's moving toward you at speed X and you move toward it at speed Y, it should seem to move at speed (X + Y) relative to you. But light isn't just any old thing. The speed of light in a vacuum, c, is fundamental to many physics equations; changing it would change the laws of physics. But the same laws have to apply equally to everyone, so every observer has to see the same constants, including the same speed of light.

So that's the why, but what's the how? What ensures that every observer sees the same value of c regardless of their own motion? Well, a measurement of speed includes two components, distance and time (miles per hour, meters per second, that sort of thing). So you can change how someone perceives the speed of something if you change the way they perceive distance and time. Basically, when you're moving or accelerated, your measurements of space and time change in such a way as to cancel out any changes you'd observe in the speed of a beam of light as a result of your motion.

Ditto for acceleration, since acceleration changes how you move. And gravity is basically an acceleration, something that changes your speed and direction of motion. So being in a gravitational field is going to alter your measurements of space and time. But this is not an illusion; in relativistic physics, what you measure is what's real for your particular frame of reference. But an observer in a different frame of reference will see your measurements as distorted relative to theirs; because of the gravitational acceleration, you and they will have different measurements of the distances and directions of motion around a massive body depending on your positions relative to it. What would be a straight, unaccelerated trajectory in flat space (i.e. a geodesic, a fancy word that basically means "the shortest distance between two points") would be a curved, accelerating trajectory around a massive body. So since the massive body changes the geodesics around it, alters the way that objects and even beams of light move around it, it's effectively changing the geometry and topology of space and time themselves.

But that's not an easy concept to get across, so it's simpler to talk about changing the shape of something by analogy with something familiar like a flexible fabric or a rubber sheet. The sheet with the heavy marble distorting its shape is a handy analogy for the geometry of a massive body's gravity well (albeit with one dimension subtracted for easier visualization), but not for its causes.
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Old January 18 2013, 09:54 PM   #3
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Re: Basic Science Question - "fabric" of space

Thanks for the info, Chris.

I do understand about the speed of light being a constant, independent of whether you are moving or not. And the principles of acceleration. But when you're talking about gravitational acceleration... that's where I lack understanding. There again it seems logical to say that "force" comes into play, because gravity is acting upon an object, causing it to accelerate. Is the phrase "force of gravity" obsolete but still said because it's so well worn into science? Upon the surface of the Earth, to move something there must be a force acting on it. I can appreciate that the massive sphere shape of the Earth would be responsible for shaping the effects of gravity, creating a curvature that influences object trajectory upon approach. And so "fabric" is used only as a visual cue but not suggestive of existence in the same corporeal nature. But I have been unable to visualize the actual mechanism of the cause. Have you seen any diagrams or videos that do a good job of visualizing how gravity actually works?
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Old January 18 2013, 09:58 PM   #4
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Re: Basic Science Question - "fabric" of space

The shape of the Earth has nothing to do with the shape of its gravitational field, not from a distance. The mass of a body effectively behaves as if it's all concentrated at the center of mass, because the pulls to either side of it tend to cancel out. The spacetime curvature, as I said, is a result of the gravitational acceleration affecting an observer's measurements of distance, direction, and time. That would be true whether the object generating the gravitational field were shaped like a sphere, a cube, or a giant sousaphone.
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Old January 19 2013, 08:29 AM   #5
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Re: Basic Science Question - "fabric" of space

Gary7 wrote: View Post
Thanks for the info, Chris.

I do understand about the speed of light being a constant, independent of whether you are moving or not. And the principles of acceleration. But when you're talking about gravitational acceleration... that's where I lack understanding. There again it seems logical to say that "force" comes into play, because gravity is acting upon an object, causing it to accelerate. Is the phrase "force of gravity" obsolete but still said because it's so well worn into science? Upon the surface of the Earth, to move something there must be a force acting on it. I can appreciate that the massive sphere shape of the Earth would be responsible for shaping the effects of gravity, creating a curvature that influences object trajectory upon approach. And so "fabric" is used only as a visual cue but not suggestive of existence in the same corporeal nature. But I have been unable to visualize the actual mechanism of the cause. Have you seen any diagrams or videos that do a good job of visualizing how gravity actually works?
Maybe this will help.
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Old January 19 2013, 03:06 PM   #6
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Re: Basic Science Question - "fabric" of space

Here's the best visual representation I was able to find, though I'm sure it's not perfect: http://www.relativitet.se/spacetime1.html

I'll try to sum it up as I understand it (which might be wrong):

1) The presence of mass distorts spacetime (it's very important that it's spacetime, not just space). Why? I don't think we really know, we can just describe the result.

2) Objects naturally constantly move in a straight line through spacetime (the red and green lines in the pictures are straight lines in spacetime's curved geometry, they just don't look like that way to us from our normal flat geometry perspective). However, since spacetime is curved, the projection of that straight line on our 3d space is not a straight line, but a line that appears to be affected by a "force" of gravity.

So when the Earth goes round the Sun, it's just following a straight line through spacetime - but due to the way Sun's mass changes the geometry of spacetime, the projection of that line on just our 3D space is a curved orbit.
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Old January 19 2013, 09:56 PM   #7
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Re: Basic Science Question - "fabric" of space

The reason why it's space-time originates from the fact that in Special Relativity space and time are seen to be inextricably linked through the Minkowski metric with diagonal [-1,1,1,1], which is the signature of flat space-time. The -1 encodes the facts that the invariance of the speed of light and that there are no preferred reference frames result an apparent trade off between the measurements of space and time of a body moving relative to an observer (time stretches and space shrinks).

In General Relativity, the presence of a spherically-symmetric mass in space-time results in other metrics such as the Schwarzchild, Kerr, Kerr-Newman, Reissner–Nordström, and so on, depending on whether you also factor in the rotation and the charge of the body. For the Universe, the Friedmann–Lemaître–Robertson–Walker metric is usally invoked, although its assumptions of homogeneity and spatial isotropy have recently been shown possibly to be incorrect over distances as large as 4Gly. (http://www.space.com/19220-universe-...iscovered.html)

You can think of a metric as a mathematical object that encodes the gravitational effect of the body on space-time and thereby on other bodies. Dropping the time component produces predictions for such things as the deflection of EM radiation by a massive body such as the Sun and the precession of the perihelion of Mercury that do not agree with observation. Adding the time component into the mix gives answers that are in good agreement.

The real problem lies in explaining why you can usually only go in one direction along the time axis (apart from rather outlandish solutions of the field equations) but either way along a spatial axis (unless you've fallen inside the event horizon of a black hole).
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Old January 19 2013, 10:11 PM   #8
Timelord Victorious
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Re: Basic Science Question - "fabric" of space

I always enjoy these threads... until someone who actually knows physics comes in and makes my brain implode.
Shame on you!
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Old January 19 2013, 10:27 PM   #9
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Re: Basic Science Question - "fabric" of space

Well, in the end it's all wibbly-wobbly, timey-wimey, isn't it.

ETA: The real, real problem is that we don't have a theory of quantum gravity. Such a theory would of necessity have to ditch the equivalence principle of General Relativity (that inertial mass and gravitational mass, if not the same thing, are always in the same proportion for no obvious reason). Experiment suggests that the equivalence principle is correct to a very high degree of accuracy, but others have suggested that the observed deceleration of space probes leaving the Solar System, and the anomalous rotation of the outer parts of galaxies result from non-equivalence rather than from dark matter or other "here be dragons" explanations.
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Old January 19 2013, 10:39 PM   #10
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Re: Basic Science Question - "fabric" of space

See? Now THAT I can understand.
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Old January 20 2013, 09:44 PM   #11
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Re: Basic Science Question - "fabric" of space

Asbo Zaprudder wrote: View Post
The reason why it's space-time...
Just to clarify, my "Why?" was supposed to mean "Why does mass curve spacetime?", not "Why is it spacetime instead of just space?". I generally understand the answer to the second question but thanks for a detailed explanation anyway.
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Old January 20 2013, 10:31 PM   #12
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Re: Basic Science Question - "fabric" of space

General Relativity doesn't explain why gravitational mass curves space-time. It just describes how to calculate the effect based on a set of assumptions about how energy-momentum and space-time interact. Energy and momentum are seemingly joined at the hip just like space and time. Background-independent string theory or loop quantum gravity might eventually offer an insight why the Universe is the way it is, but they haven't as yet.
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Old January 26 2013, 06:55 PM   #13
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Re: Basic Science Question - "fabric" of space

The Platonists and Pythagoreans believed that there is a higher reality knowable only through logic and mathematics, and that our world is only an imperfect "shadow" of that "true" world. While math is a very useful tool in science, math itself is not truth.

Physics Has Its Principles

This is a very big topic to address in a single post, but to put it bluntly: many astronomers and particle physicists are neo Platonists and Pythagoreans whose virtual edifices are completely disconnected from reality. You've probably read that the Michelson-Morley Experiment (MMX) "proved" that there is no luminiferous (light-carrying) ether, or failed to find any indication of one. The reality is that something was found, but the numbers were too low for the expected ether. (And there's more—podcast recommended.)

It's okay for theories to be wrong, so long as the scientific method is allowed to correct them. Unfortunately, there are sometimes people or groups of people with a vested interest in a particular model and will go to unscrupulous lengths to protect that interest. Such things did not happen only in the past, but continue to the present day.

As a starter, read Donald Scott's book THE ELECTRIC SKY. It is a relatively short book. You will find it a refreshing break from all the fabrics, black holes, dark matter, dark energy, 'branes and other counterintuitive creations of mainstream astrophysics.

Tom Van Flandern's DARK MATTER, MISSING PLANETS AND NEW COMETS is also a worthy read. Whatever you may think of his "Meta Model," the book will at least give you a better perspective on mainstream astrophysical theories.

I often see people quote Sagan's "extraordinary claims require extraordinary evidence," typically when "extraordinary" is defined as "alternative to the currently accepted wisdom." I can't tell you what is correct and what is not—there are many intelligent people trying to work that out. However, you can't decide what is best until you check out the rest.
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Old January 26 2013, 07:31 PM   #14
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Re: Basic Science Question - "fabric" of space

Metryq wrote: View Post
I often see people quote Sagan's "extraordinary claims require extraordinary evidence," typically when "extraordinary" is defined as "alternative to the currently accepted wisdom."
Anyone who defines it that way is missing the point, by making the mistake of defining science as just another belief system. The reason science works is because it's based on the evidence. The "currently accepted wisdom" is, ideally, based on the evidence we have. If a model is supported by a great deal of evidence, then naturally it's going to take some pretty compelling evidence to prove that the model needs to be changed. It's silly to treat it as some kind of ideological war, because if the facts support the new idea, it will eventually win out against any resistance, because it's simply true. Even if there's a generation of scientists who resist accepting it, eventually enough evidence and experiments will support it that there will be no choice but to accept it. And conversely, if the idea is wrong, then the evidence won't be there to support it and it will fall by the wayside.

Look at quantum physics. Initially, when its ideas started to be formulated in the late 19th and early 20th centuries, there was a lot of resistance in the scientific community to its ideas, since they were very extraordinary claims that went against all conventional wisdom. Even Einstein didn't want to believe in it. But today it's been confirmed by such overwhelming evidence that it's become the foundation of modern physics. Its principles are used and affirmed every day in technologies such as the ones that enable me to type these words into an electronic device that sends them out to other electronic devices around the world. The fact that people didn't initially believe in it didn't matter, because it was true, and reality trumps belief every single time.
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Old January 28 2013, 10:21 PM   #15
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Re: Basic Science Question - "fabric" of space

Christopher wrote: View Post
The "currently accepted wisdom" is, ideally, based on the evidence we have.
Ideally, yes, but dogma is an all too common human pitfall. Sometimes even "extraordinary" evidence will not sway the stubborn. Halton Arp's work, right or wrong, was deemed so threatening that he was denied computer and telescope time, his papers suppressed; blacklisted so brutally that he had to go to another country to continue his work.

There are also cases where utterly unremarkable evidence has "extraordinary" consequences (e.g. two tiny Mars observations led the way to Kepler's Laws).

Quantum physics, like Newton's gravity, is more a description of nature than an explanation. Actuarial data.
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