0 Members and 1 Guest are viewing this topic.

the big bang theory is a good one in all , but how does something just explode and create a universe?what was it that exploded?where did it come from?whats it made of?what is the fabric of space made of?

Is there a 'fabric of space'?Gravity?

So yeah--I also agree that it's a confusing term.

... but gravity is just a metric, shaping that space into whatever forms (geodesics) we find matter to take, and the shape defined by the mass taking them.

JP,This is the best explanation I've seen yet of "spacetime curvature" as just a coordinate system, a sort of conceptual geometry for the math by which GR is a clear improvement over Newton's gravity.Yet every intro to GR refers to mass curving spacetime as if it is an entity that "guides",for instance, planets in their elliptical orbits around the sun.

It seems that the "complex geometries" of the non-Euclidean transition made up "shapes of space" that have no referents in the physical cosmos. (Flat, spherical, parabolic, etc.) If space is 3-D volume, how did it become a malleable medium, except in imagination?How did Euclid's fifth postulate on parallel lines get debunked, for openers? How is it that the new "complex (non-Euclidean) geometry" has parallel lines intersecting? How did the shortest distance between two points become a curved line on the 'curved surface' of "spherical space?" Two points on a sphere can still be connected by a straight line through the sphere without restriction to an imaginary surface.I have studied the transition from Euclidean to non-Euclidean geometry (the model for GR's cosmology) in depth for many years, and these questions just "scratch the surface" of ontological study of "what spacetime is supposed to be."

If I started a thread on that, would it need to be in "New Theories?" It is, after all a question of cosmology... what spacetime actually is.

Well, every theory is a model. Anyone that tells you that a scientific theory tells you what something really is is lying to you. We don't have a theory that describes what everything really is yet, and there's debate over whether such a theory is even possible. What GR does tell you is that as a model, it describes space-time as a 4D geometrical construct, and that:1) energy and momentum curves this 4D structure,2) objects moving through space-time follow paths influenced by this curvature.What is space-time made up of so that this geometrical model works for GR? Science doesn't know.

A lot of non-Euclidean geometry is mathematics, which obviously has no problems describing all sorts of surfaces in any number of dimensions which can display non-Euclidean properties such as parallel lines not existing or shortest paths being curved lines.

But if you're dealing purely mathematically, you can imagine geometries that don't behave like our own familiar world, in which the entire universe consists of a spherical surface, and there is no such thing as "going off the surface." In this universe, how would you draw parallel lines?

Let's not have this thread devolve into another debate about realism. If you wish to debate matters of philosophy, ontology and epistemology - please do it in chat or new theories. The main boards are not the place for metaphysical musings.Thanks

...you can imagine geometries that don't behave like our own familiar world, in which the entire universe consists of a spherical surface, and there is no such thing as "going off the surface." In this universe, how would you draw parallel lines?

If science doesn't know what it is... well, what curves after all... besides the obvious... the paths of masses influenced by gravitational attraction?

Quote from: old guy on 21/11/2012 20:16:48 If science doesn't know what it is... well, what curves after all... besides the obvious... the paths of masses influenced by gravitational attraction?I may have the wrong end of the stick here, do you mean newtonian gravitation? if so, given that Newton never described a mechanism by which his postulated attracting force worked, can you tell us ? Am i still holding the wrong end of the stick?

What space time geometry actually is? This is a fine question - and may well be answered by physicists in the future when we see a convergence between quantum field theory and gravitation; but at present it is either a wild speculation that must go into New Theories, or non-scientific and belongs in Chat or New Theories....I changed my mind on the move and split the recent section into this newly named thread

that matter curves space-time because that's what our mathematical model describes. We do this because as scientists we understand that our model isn't a universal truth, but that it works for some range of problems. So when we say gravity curves space-time, we mean that no matter what space-time is on a deeper level, on some level it acts just like a 4D structure with curvature. If that's confusing or misleading, that's really a problem for the broader education system--there are a lot of people out there who don't understand how science works and don't understand the difference between a scientific model and metaphysical certitude.

At least no model in science so far has yielded absolute truth.

So when we say gravity curves space-time, we mean that no matter what space-time is on a deeper level, on some level it acts just like a 4D structure with curvature.

Let me repeat myself, since no one seems to have heard me the first time. Meters and seconds are defined by the constancy of the speed of light and the wavelength and frequency of cesium atom emissions, regardless of where the cesium atom is. The space-time metric of GR is represented by a 4D grid of hypercubes, each measuring 1 meter cube by 1 second. That grid is what is warped, and it is warped because, in a flat space-time metric the cesium atom emission are not constant. By "flat space-time metric" I mean a metric represented by units of distance and time which, by definition, agree with Euclidean geometry. (I am not introducing new theory. Flat space-time is nothing more than a mathematical transformation of warped Minkowski space-time.)

...Measuring the cesium atom emission relative to those units, you would find that a meter stick at the bottom of a gravity well is shorter than one fleter, and an atomic clock ticking once per second would be ticking slower than one tick per flecond. (Someone please correct me if I got that bass ackwards.) I'm not sure, but it might also be necessary to let the speed of light, in fleters per flecond, to be different in and out of the gravity well. (Wanted: A mathematician to determine whether the speed of light, in fleters per flecond, is constant.)

It has been stated several times, in this thread, that gravity bends light. That is true in a flat metric, but it is not true in GR. In Minkowski space-time, the path of light in a vacuum is the definition of a straight line, so photons can't be bent by gravity. Light follows a straight line thru a gravity well, and comes out in a different direction. That's what a warped metric is all about.

Old Guy, I'm ignoring all your posts that try to shove realism into the discussion. That's why I haven't responded.I think we all know that no matter what your stance is on metaphysics, physics describes very precise properties for space-time. If you're unhappy with that description, New Theories is the place for it.

Quote from: JP on 25/11/2012 22:10:07Old Guy, I'm ignoring all your posts that try to shove realism into the discussion. That's why I haven't responded.I think we all know that no matter what your stance is on metaphysics, physics describes very precise properties for space-time. If you're unhappy with that description, New Theories is the place for it. JP,It is a misrepresentation of my intent to say that I am "trying to shove realism into the discussion." I hope it is not intentional. I am trying to bring realism into the discussion in a reasonable way for those who still understand that Earth does not change diameters with every possible frame from which it might be observed and measured. The same goes for distances between bodies in this solar system and between stars (and galaxies.)Different observations of the same object or distance do not make the object or distance change. That is basic here, not "shoving" realism into the discussion. Earth's diameter is just under 8000 miles... not varying with the speed of a passing observer. Same for all the other examples.

Quote from: old guy on 26/11/2012 07:41:12Quote from: JP on 25/11/2012 22:10:07Old Guy, I'm ignoring all your posts that try to shove realism into the discussion. That's why I haven't responded.I think we all know that no matter what your stance is on metaphysics, physics describes very precise properties for space-time. If you're unhappy with that description, New Theories is the place for it. JP,It is a misrepresentation of my intent to say that I am "trying to shove realism into the discussion." I hope it is not intentional. I am trying to bring realism into the discussion in a reasonable way for those who still understand that Earth does not change diameters with every possible frame from which it might be observed and measured. The same goes for distances between bodies in this solar system and between stars (and galaxies.)Different observations of the same object or distance do not make the object or distance change. That is basic here, not "shoving" realism into the discussion. Earth's diameter is just under 8000 miles... not varying with the speed of a passing observer. Same for all the other examples.Sorry, you're just introducing it gently into the discussion despite being repeatedly asked by the moderators not to do so. Going forward, I will shrink posts on realism in this thread, to keep it on track.