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bill_
2009-Jun-12, 02:25 PM
Hello,
I have read that certain measurements show that the universe has 0 curvature.
I have also read that space near gravity has negative curvature.

Could someone please explain in layman's terms what the measurement of curvature for the universe represents and how it relates or doesn't relate to the negatively curved regions near gravity?

Thanks

Ken G
2009-Jun-12, 03:46 PM
First of all, one must distinguish spacetime curvature, which is an observer-independent quantity (so is "objectively real", or as real as any mental construct gets anyway), and spatial curvature, which depends on how we take a spatial "slice" out of spacetime-- that is, it depends on how we choose to coordinatize things. The universe does not have zero spacetime curvature, that's why we have cosmological redshifts. But there is a particularly useful spatial coordinatization, called "co-moving frame" coordinates, where we take observers who are moving with the general flow of matter in the universe, and piece together their own local concepts of distance and time into a global tapestry. Those are almost always the coordinates being used when people make statements in cosmology, like "zero spatial curvature". The spatial component of that way to slice spacetime seems to come out pretty flat on very large scales, that's the result you are referring to.

On smaller scales, like that of a galaxy or a solar system, the gravity is very different. The spacetime is curved even more, like a sharp dimple on the surface of a gently curving golf ball. But when we coordinatize the space separately from the time, we still tend to use flat spatial coordinates. That is where the effect we call gravity comes from-- using a flat spatial coordinate to describe a curved spacetime. It shows up in those flat spatial coordinates looking like an acceleration toward the source of gravity-- in a more appropriately curved coordinatization, the trajectories would seem perfectly straight (called "geodesics").

This is my basic understanding. I don't actually do calculations in general relativity, so one must take with a grain of salt any description that does not come from actually solving the equations, but I'm going to assume you don't want to do that at this moment, any more than I do! And welcome to the forum bill.

undidly
2009-Jun-14, 02:28 AM
But when we coordinatize the space separately from the time, we still tend to use flat spatial coordinates. That is where the effect we call gravity comes from-- using a flat spatial coordinate to describe a curved spacetime. It shows up in those flat spatial coordinates looking like an acceleration toward the source of gravity-- in a more appropriately curved coordinatization, the trajectories would seem perfectly straight (called "geodesics").

"looking like an acceleration toward the source of gravity"

That is also my understanding of GR.
So why is anyone trying to find GRAVITONS?.
Do they get a big research grant to find gravitons?.

I think that the acceleration toward the source of gravity is the explanation of gravity.Mass resists the acceleration because of its INERTIA.
Gravity is easy,INERTIA is the problem.
I am working on it.

Ken G
2009-Jun-14, 02:41 AM
So why is anyone trying to find GRAVITONS?.
Do they get a big research grant to find gravitons?.It's the attempt to achieve grand unification. If all the other forces are somehow the same thing, perhaps gravity is too, and that would require gravitons. A lot of physicists seem to think there should be gravitons, but I'm not sure why, nor am I sure that it makes any difference what a lot of physicists believe on the matter. At least one, Roger Penrose, thinks unification might happen by interpreting forces more like gravity, rather than gravity more like forces. I'm not sure that unification is possible, though of course it is natural for science to look for it.

cfgauss
2009-Jun-14, 04:41 AM
It's the attempt to achieve grand unification. If all the other forces are somehow the same thing, perhaps gravity is too, and that would require gravitons. A lot of physicists seem to think there should be gravitons, but I'm not sure why, nor am I sure that it makes any difference what a lot of physicists believe on the matter. At least one, Roger Penrose, thinks unification might happen by interpreting forces more like gravity, rather than gravity more like forces. I'm not sure that unification is possible, though of course it is natural for science to look for it.

There have to be gravitons because gravity has to be quantized. If you try to do a semi-classical type thing, where you do quantum mechanics on a curved background, you get very wrong answers. Similar things happen if you do, for example, classical electromagnetic fields in quantum mechanics.

And the thinking of forces geometrically is not new. The idea goes back to at least Kaluza and Klein in the '20s and '30s, and things like string theory share a lot of this geometrical structure.

Ken G
2009-Jun-14, 04:54 AM
There have to be gravitons because gravity has to be quantized. If you try to do a semi-classical type thing, where you do quantum mechanics on a curved background, you get very wrong answers. So because a human theory doesn't work, the universe "has to" quantize its gravity? What kind of scientific logic is that? It certainly suggests gravity might be quantized, perhaps betting men might even put up a stake on it, but no more can be said within the confines of sound science.

cfgauss
2009-Jun-14, 05:01 AM
So because a human theory doesn't work, the universe "has to" quantize its gravity? What kind of scientific logic is that? It certainly suggests gravity might be quantized, but no more can be said-- scientifically.

Nothing about this statement makes any sense at all. In fact, it shows a fundamental misunderstanding about what science is... You seem to be missing the mechanism behind which theories evolve.

Your description of if the universe is "flat" or not is also confused...
I will pawn the proper description off to wikipedia, which seems to be reasonably correct,
http://en.wikipedia.org/wiki/Shape_of_the_Universe

Ken G
2009-Jun-14, 05:11 AM
Nothing about this statement makes any sense at all. In fact, it shows a fundamental misunderstanding about what science is... You seem to be missing the mechanism behind which theories evolve.
Well, I'm afraid we cannot agree on that, but I don't view what science is as controversial-- it defines itself quite clearly. What's more, no part of that definition goes like "scientific theories that have been successful must be correct, therefore if some phenomenon cannot be understood within the confines of that theory, then the phenomenon is not understood properly." Instead, what it goes like is "our theories that are successful are never considered to be the last word, nor can they be certain to be applicable to new phenomena for which the theory was not devised to unify."


Your description of if the universe is "flat" or not is also confused...
I will pawn the proper description off to wikipedia, which seems to be reasonably correct,
http://en.wikipedia.org/wiki/Shape_of_the_UniverseSome specifics would be nice. You seem to be given to making sweeping claims without feeling any need to back them up with an actual argument. Do you think that your points should be considered valid simply because they are your opinions? Is that also your view of how science works?

cfgauss
2009-Jun-14, 05:18 AM
Well, I'm afraid we cannot agree on that

Well, if you're disagreeing with that, you're disagreeing with every single physicist. Absolutely 100% of us think that the graviton is a reasonable thing to talk about in some sense or another.

If you're really confused about why, the only thing I can tell you is to learn the history of physics, learn about model building and logical inference, then learn quantum field theory and GR, and it will be "obvious" to you then, too!


Some specifics would be nice.
Specifically?

Ken G
2009-Jun-14, 05:31 AM
Well, if you're disagreeing with that, you're disagreeing with every single physicist. Absolutely 100% of us think that the graviton is a reasonable thing to talk about in some sense or another.
Actually, that is hardly the statement that I disagreed with. You can see the difference between this statement, and what you said above, can you not?

If you're really confused about why, the only thing I can tell you is to learn the history of physics, learn about model building and logical inference, then learn quantum field theory and GR, and it will be "obvious" to you then, too!
Clearly, I was right-- you do think that sweeping claims substitute for actual logical argument. You'll learn that this forum doesn't work that way, fortunately. Citing theories cannot be used to argue why those theories can be known to apply to phenomena to which they have never been successfully applied, I should think that would be obvious enough to any scientist.

cfgauss
2009-Jun-14, 05:41 AM
I don't think you understand what I'm saying... You need to go and learn why, for example, we can use QED to calculate photon scattering amplitudes off of composite particles and get the right answers, even though QED does not know about QCD.

The entire machinery of modern physics is about knowing when solutions work...

Ken G
2009-Jun-14, 05:56 AM
I don't think you understand what I'm saying... I understood exactly what you said, you said "There have to be gravitons because gravity has to be quantized." None of those words are terribly difficult, I know them all. And I know a patently unscientific claim when I see one.


You need to go and learn why, for example, we can use QED to calculate photon scattering amplitudes off of composite particles and get the right answers, even though QED does not know about QCD.No one has the vaguest idea why that is true, just as no one has the vaguest idea why either QCD or QED work. Science doesn't tell us that, I'm surprised you think it tells you that.


The entire machinery of modern physics is about knowing when solutions work...And those words, in your mouth, mean something different than if Ptolemy said them, or Newton? Interesting perspective, I guess there must something different about "modern physics" than the basic characteristics that have defined science all along.

cfgauss
2009-Jun-14, 06:08 AM
No one has the vaguest idea why that is true


Actually, everyone knows exactly (http://en.wikipedia.org/wiki/Effective_field_theory) why this is true, and it's totally hilarious that you'd claim this.

And don't try to turn this into a crazy philosophical argument. It has nothing to do with philosophy. It has to do with hardcore math and understanding models though measurements and observables.

Let me reiterate: you're not just arguing with one physicist (me) but every physicist. These are totally uncontroversial ideas that have been around for a very long time.

Ken G
2009-Jun-14, 06:20 AM
Actually, everyone knows exactly (http://en.wikipedia.org/wiki/Effective_field_theory) why this is true, and it's totally hilarious that you'd claim this.
What is totally hilarious is that once again you think it actually means something to simply state things. It really seems like you think that anything that comes out of your mouth has to be true, by some self-given authority. Utterly unscientific, that. I also make claims and statements, and they may not always be right, but I always give my argument, I do not apply strictly "argument by assertion".


Let me reiterate: you're not just arguing with one physicist (me) but every physicist. Indeed, you now think you can speak for all physicists. And this is some form of logic, you believe? The standard of logic on this forum is much, much higher than that. Please try to bring up the level.

cfgauss
2009-Jun-14, 06:27 AM
What is totally hilarious is that once again you think it actually means something to simply state things. It really seems like you think that anything that comes out of your mouth has to be true, by some self-given authority. Utterly unscientific, that. I also make claims and statements, and they may not always be right, but I always give my argument, I do not apply strictly "argument by assertion".
Indeed, you now think you can speak for all physicists. And this is some form of logic, you believe? The standard of logic on this forum is much, much higher than that. Please try to bring up the level.

I'll give you a hint--the answer was cleverly hidden in a link you apparently did not bother to read.

Ken G
2009-Jun-14, 06:56 AM
Lame in the extreme. But if I return to the thread, I'll be sure to bring with me some interesting questions around the quantization of gravity. No doubt that is an interesting topic, for any openminded enough to embrace the possibilities. In the process, I will pay special attention to those who actually do bother to present arguments, that start out "we know gravity is quantized because...". I will not be looking for whether these arguments are true or false, I will be looking for where they subtly assume what they claim to be showing, because I already know that we cannot know anything of the sort without specific experimental evidence of same (and of course, even then all we know is that we need to generate a model that quantizes gravity, we don't know that some totally different approach won't work better a few centuries from now). It won't be that hard to find where the assumption is embedded in the argument, people so easily forget the subtle but important difference between a scientific theory and a statement of truth. Ironically, when an argument is shown to assume what it claims to argue, that argument becomes "not even wrong"-- a descriptor that already applies to vacant assertions in the absence of any argument at all.

cfgauss
2009-Jun-14, 07:21 AM
I will be looking for where they subtly assume


I sincerely doubt that you are qualified to make any judgments about "subtle" issues in quantum field theory.


I already know that we cannot know anything of the sort without specific experimental evidence of same


We don't have direct experimental evidence of, really, any of the axioms of any physical theory, and of very few consequences. Read up about the history of GR for example, and see how quickly people believed it was true vs. experimental evidence! The reason we believe the theories has to do with their mathematical structure and basic consistency arguments. If your response is "but that's what netwon thought too!!!!!11111" then go back and read that wikipedia article over and over until you understand why that doesn't make sense.


It won't be that hard to find where the assumption is embedded in the argument


Okay, what assumptions are contained in the construction of path integrals? How about the calculation of scattering amplitudes using Feynman diagrams? What assumptions are made in the proof of the unitarity of the S matrix? Or any of the "no-go" theorems involving GUT groups?

Now, on to GR, what assumptions are made in going from global lorentz transforms to local ones? What are the consequences of writing GR as a hamiltonian? Why can it be zero and include dynamics? What assumptions go behind that?

How about EFTs? What assumptions are made in general when you construct an EFT? Why is E&M in matter an example of an EFT? Is E&M in vacuum? Is field theory in general? What are characterizing features of EFTs? What "assumptions" went into your answers for all of these?

These are basic basic issues that you'd better be able to answer, and understand in detail before you can even think about considering what assumptions go behind gravity being quantized!

tusenfem
2009-Jun-14, 09:46 AM
Okay, Ken G and cfgauss, this is enough, if you want to discuss eachothers failing-of-understanding, do it by email or get together at tea time. This has nothing to do with the OP anymore, which was a simple question about the curvature of space, and not about the existence of gravitons, or QED knowing what QCD is feeling (how a theory can know anything, beats me, it is the human interpretation of it) and whether or not 100% of physicists agree with cfgauss or not.

Next to that: cfgauss we do not make ad homs like you wrote here



I sincerely doubt that you are qualified to make any judgments about "subtle" issues in quantum field theory.


don't do that again, this is an official warning.

Thread closed.