Gravitation v. Magnetism

[EnjoyItClem]

Howdy. This topic should just always be up for discussion, and I'm more than ready to keep it difficult:

Why distinguish gravitation from magnetism?

 

You may wish to contemplate some or all of these helpful concepts/persons while outlining your platform: Occam's Razor; hyperbolic geometry; Viswanath's constant; logic and rules; equinoctial cycles; the explanatory gap and epistemology; David Bohm; Friedrick Nietzsche; spherical close-packing; creativity; the autism spectrum; quantum mechanics; Leonardo da Vinci; phi; linguistics; microbiology; cosmology; Maxwell's equations; and/or that which inspires you.

 

I'm waiting.

[tvp45]

Indeed! I find them both attractive. There is, however, that nasty inverse square issue.

[Klaynos]

Because at the energies of the universe today they are completely different?

[ajb]

Different gauge groups; [math]U(1)[/math] is abelian where [math]Diff(M)[/math] is not.

 

Gravity* and gauge theories** do have a lot in common, especially from a geometry point of view but the actions are very different. Importantly, gauge theories are polynomial and renormalisiable where gravity is non-polynomial.

 

Attempts (viz Kaluza–Klein theories) to unite gravity and gauge theory via higher dimensional space-times geometry have failed apart from string theory. (We can get gauge theories by "stacking" branes and connecting them with open strings). If this is what you were thinking of?

 

 

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*here I mean general relativity.

 

 

**gravity is a gauge theory, but by gauge theory I mean Yang-Mills theory , Chern-Simons and electromagnetism.

[Mr Skeptic]

Because they're completely different. It's like asking, why distinguish white, middle class American Republicans from tootsie rolls?

[ajb]

Because they're completely different. It's like asking, why distinguish white, middle class American Republicans from tootsie rolls?

 

I think that one of the most interesting things is that gravity and gauge theories are not so different. Both are (classically) geometrically very similar. Really both Yang-Mills and general relativity are described in terms of principle (and associated vector) bundles. For Yang-Mills we have an [math]SU(n)[/math] principle bundle and for general relativity the important bundle is the frame bundle.

 

Even more interesting are the relations between them such as the AdS-CFT correspondence, the Kawai-Lewellen-Tye relations and MHV Amplitudes via twistor methods.

 

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Tootsie Rolls are chewy chocolate-flavoured candies that have been manufactured in the United States for more than 100 years. The cylindrical cocoa-flavored candies come individually wrapped, and are an American cultural icon.*

 

*Wiki.

Edited July 1, 2008 by ajb

[tvp45]

Because they're completely different. It's like asking, why distinguish white, middle class American Republicans from tootsie rolls?

 

Indeed!

[swansont]

I've built magnetic shields to significantly reduce the field in a volume. Don't know how to do that with gravity.

[ajb]

Another point is that gravity is always attractive. This means that the graviton spin must be even and in fact we know it must be spin-2 in order to be consistent with general relativity. Spin-0 is ruled out by the bending of light round a star and higher spin is not allowed (Weinberg-Witten theorem).

 

For electromagnetism we know that this can be both attractive and repulsive. This means that the photon cannot be spin-even. This means that photons must be spin-1, as is well known.

 

This mean that gravity and electromagnetism are not trivially identical. However, as I said there are some deep relations between the two.

[foodchain]

Can gravity or magnetism correspond somehow to zero point energy?

[EnjoyItClem]

Bear in mind you ought not to *use* your precepts to *explain* your precepts. The reason historians have to feign ignorance of cause and effect when documenting historical events is not just to skirt bias.

 

Klaynos: Good start, I think. It's hard to be sure when you employ vague terminology. I don't want this discussion to get too philosophical, but there are rules even physicists must follow when it comes to definitional logic.

 

Mr Skeptic: Same goes for you. Only... in a funnier way. I'd love to see you defend the perspective you've taken, because you seem to know the fundamental difference between a Tootsie Roll and a human being, whereas I'm a little uncertain. I know they look, feel, taste, sound and smell different, usually, but beyond sensory input...?

 

ajb: You seem a little on the fence. Cognizance of existing theories is important, but why not just make a forward assertion? If I can offer a tip: outside analogy is traditionally the best way around writer's block. Also, ease off the shorthand, cause for this to be a discussion we need to keep it elaborative. Finally, who said gravity is solely attractive? Or far more importantly: why did he say it, and when?

 

swansont: Your experiment missed the point. No pun intended. Explore how a thing can exist while including free space, or even relatively free space such as "volume" versus "solid mass." Take a look at David Bohm's early contributions to this discussion; I think you'll like what you see.

 

I'm not here to make everyone tear down his/her notion of fact. I'm here to make everyone want to tear down his/her notion of fact. We are getting smart, and dichotomous truths are no longer permissible; there is a vital need for philosophical insight and creative thought as well as scientific confidence. In other words, let's kick some ass, everybody. In a true dialog, opposing ideas work toward a common goal.

[Klaynos]

Scientists tend to like to replace their "facts" but only with better ones, which these days tend to be adding onto old ones... even SR simplifies down to classical mechanics for every day things...

 

And back to the op...

 

I think fundementally with this discussion ajb's posts are the ones to take real note of here, as why they are really different.

 

And we say that gravity is always attractive because there is absolutely no evidence to suggest otherwise which you would expect at least some unless we're in a special part of the universe which we HAVE to assume we aren't else we couldn't do anything.

[EnjoyItClem]

Klaynos: You won't find reports of purple crayons not being purple, because if the crayon is not purple, it's not called a purple crayon.

 

Speaking of which, when was the last time you read Harold And The Purple Crayon? I miss that book. I know I used to have it around here, somewhere...

 

Sorry, anyway, this is getting fun.

[Klaynos]

But if I ran some spectroscopy on a purple crayon and found it not to be purple but only appear purple to a human casual observer, that would be a good thing to publish.

[ajb]

ajb: You seem a little on the fence. Cognizance of existing theories is important, but why not just make a forward assertion? If I can offer a tip: outside analogy is traditionally the best way around writer's block. Also, ease off the shorthand, cause for this to be a discussion we need to keep it elaborative. Finally, who said gravity is solely attractive? Or far more importantly: why did he say it, and when?

 

I would not say I am on the fence. I am aware of the many similarities (I mean mathematically) between general relativity and "more conventional" gauge theories such as electromagnetism. I am not an expert in this field.

 

As I don't know what you already know or what level you are at it is difficult to give a very clear answer to your original post. I don't fully understand or appreciate the links between gravity and gauge theories, the relations are non-trivial. But due to these relations you can think of them as being the same.

 

I will need to clarify this. Take for example the Kawai-Lewellen-Tye relations. They suggest the correspondence (at least in perturbation theory to tree level)

 

[math]gravity = (gauge\hspace{5pt} theory )\times (gauge \hspace{5pt}theory)[/math]

 

These relations are best understood using string theory and as far as I know, they are not fully understood starting from the Einstein-Hilbert action.

 

Anyway, it is possible to use gauge theory to make calculations in (perturbative) quantum gravity. Also, maybe more interestingly is that one can get at gauge theory calculations via quantum gravity/string theory. Today this is a hot topic, (viz MHV amplitudes and twistor string theory). So far most of this is supersymmetric, people are working on getting at non-supersymetric theories like QCD.

 

As for gravity being only attractive, unless you have some exotic matter present then gravity is always attractive. A Klay points out, unless we have some real evidence for repulsion why add it? Also, if a quantum theory of gravity is to reduce to general relativity then the graviton would have to be spin-2.

 

So, technically your question is deeper than I suspect you realised

 

 

 

 

I think fundementally with this discussion ajb's posts are the ones to take real note of here, as why they are really different.

 

Cheers Klay

Edited July 1, 2008 by ajb
multiple post merged

[EnjoyItClem]

I appreciate your doubt, guys, cause it definitely shows that you're considering my angle.

But alright, back to it. [Pops knuckles]

 

1. In producing a persuasive argument, the optimum strategy is to first side with the assumption you wish to debunk. Start, for instance, by assuming gravity and magnetism are indiscernible. Then set up a model using this idea, building up from scratch. You know what the overall picture should look like in the end, so it's not a terribly demanding task. However! Don't just put it in terms that the average student could understand. Put it in terms that are actually enjoyable to read. Creativity keeps you from thinking recklessly. Don't worry about me keeping up. Worry about all the folks you might be turning off to this ancient and fascinating topic.

 

2. Gravity was long ago defined as an attractive force between masses. Thus, any force that is not an attractive force between masses is not considered gravity. Equally important is the opposite possibility, which no one here has really touched on, that perhaps I am not against gravity. Is it true that any interaction between two masses that is not both attractive and repulsive would not be considered magnetism? Is it AS true? Is it less true? Is it more true? Don't pretend you know where I'm at on this, guys. I'm stubbornly open-minded.

 

3. Do look over that list of topics I recommended. I'm concerned that you guys haven't considered the depth of the question, yourselves. When I say open up to "outside analogy," I mean open up to those analogous concepts outside of physics; the more lenses through which you can view a dilemma, the more efficient your understanding will be. Analogy is not just a powerful thinking device, it's thought itself. Don't be afraid to beat yourselves up over this! See if you can come back with an analogy like: "if this were the case, then surgeons wouldn't be able to perform [such and such a task]," or "then philosophy would have to abandon [such and such a notion]." Physics loves company.

 

4. Speaking of which, I can't help but raise an eyebrow at Klaynos' signature. You guys are evidently used to having pseudo-philosophers come in here and make trouble just for the sake of trouble, but hopefully that hasn't kept you from studying philosophy inside and out? Right? Bear in mind, many of the most important minds in science *also* contributed to/borrowed from philosophy! Many call philosophy the metric system of understanding. As I like to put it, philosophy is just the glossary section at the end of the Human Knowledge Anthology; its sole purpose is to define concepts in universal terms and thus pave the way for analogy, kind of like the z-score in statistics. For every physical concept, there is a philosophical implication. Failure to comply with this norm does not occur. I don't condone bickering between philosophy and physics, like some civil war over human understanding, but the two factions do need to engage in constant dialog. (This's not to say I've come here to hurl philosophy at you. Just cause I'm not for you doesn't mean I'm against you. This is a science forum in the summer time, so no heart here is lacking in earnest dorkiness.)

 

5. Seriously, now, no one read Harold and the Purple Crayon?

[ajb]

EnjoyItClem,

 

what is the point of this thread? Why did you start it?

 

I do not particularly like the way you are trying to "guide" us. Some of us are more than qualified to discuss the relations between gravity and gauge theories and frankly we don't need your advice on how to post. (Note you have posted only 4 times so far on here).

 

If you want to talk about the deep relations between gravity and gauge theories I can (attempt to) point you towards some references. If not I think I am done with this thread.

[EnjoyItClem]

Hmm, those are good questions. First off: sorry guys. Please forgive the time it'll take me to figure out how the discussions in this forum really operate. I do intend to stick around, and I don't plan on being anybody's nuisance.

 

I'll arrest this topic for now. Sound good? Sounds good to me.

Edited July 3, 2008 by EnjoyItClem

[iNow]

I'm asking you to know more, ajb. If you're qualified to discuss the relations between gravity and gauge theories, discuss them. I understand the ideas perfectly. And I understand their drawbacks. Do you?

Show us. Don't tell us.

 

Also, quit being a jackass to ajb. You're new here, and his qualifications are readily apparent to just about anyone who has bothered reading his posts.

[swansont]

In a true dialog, opposing ideas work toward a common goal.

 

The common goal here is to describe how nature behaves. Gravity and magnetism do not behave in the same manner.

[EnjoyItClem]

iNow: the best i can do is to apologize and keep my head about it.

[ajb]

The common goal here is to describe how nature behaves. Gravity and magnetism do not behave in the same manner.

 

This is the crazy thing. Gauge theories and gravity seem to have more in common than is first apparent, especially from looking at there respective Lagrangians.

[Kyrisch]

Isn't it somewhat of a current theory that dark matter may have 'negative' gravity?

[insane_alien]

no, darkmatter has normal gravity, otherwise we would need another form of matter with an attractive force.

[Motor Daddy]

no, darkmatter has normal gravity, otherwise we would need another form of matter with an attractive force.

 

Do you think if we find dark matter there will be something even smaller needed to fill the gaps?