# So... is it time to kill off this [P and M] sub-forum yet?

Discussion in 'Site Feedback' started by funkstar, Aug 10, 2013.

1. ### MarkM125Registered Senior Member

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115
An elaboration:

Take a look at the Einstein Hilbert action for the gravitational field

$S = \frac{1}{16\pi G}\int d^{4}x \sqrt{-g}R$

Next, we'll use dimensional analysis on $G$. We see that it can be written as

$8 \pi G = \frac{\hbar c}{M_{p}^{2}}$

Here, $M_{p}^{2}$ is called the Planck mass. It defines for us the energy scale on which quantum corrections to general relativity become noticeable. Thus, we'll consider a perturbative expansion of the metric using the inverse of the Planck mass as a coupling constant. That is,

$g_{\mu \nu } = \eta _{\mu \nu } + \frac{1}{M_{p}}h_{\mu \nu }$

The purpose of the constant is to ensure that the E-H action remains canonically normalized - meaning its kinetic term contains no powers of the Planck mass. Then, we can write out the usual expansion of the action, as is done in QFT.

$S = \int d^{4}x (\partial h)^{2} + \frac{1}{M_{p}}h(\partial h)^{2} + \frac{1}{M_{p}^{2}}h^{2}(\partial h)^{2}+ ....$

So, all terms can usually be dropped, due to the very large Planck mass terms in the denominator. However, such terms, known as irrelevant terms, usuallu blow up once they become non-negligable. And that is exactly the case here. The integral diverges at a two-loop term and requires the introduction of a counter-term. However, once couplings to matter fields are included, divergances occur at first loop approximation - rendering the theory non-renormalizable and useless.

3. ### MarkM125Registered Senior Member

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115
If you choose to consider "down" as "negative". But this is arbitrary - my only goal was to demonstrate the production of a constant acceleration without any reference to a non-zero gradient for the stress-energy tensor.

5. ### przyksquishyValued Senior Member

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No, I meant that your metric doesn't seem to be Lorentzian.

It might shut Farsight up, but otherwise I don't see the point of this. I can give you a metric that implies acceleration with $T_{\mu\nu} = 0$. (Actually, for all I know that's what you've done. I'm too lazy to calculate anything at the moment.)

7. ### CptBorkValued Senior Member

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I think the moral of the story is that bowling balls on rubber sheets can only go so far in accurately describing the situation.

8. ### MarkM125Registered Senior Member

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115
Ah, right. The term $g_{11}$ should contain a minus sign. My mistake, thank you for the correction. Editing.....

9. ### MarkM125Registered Senior Member

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In an attempt to bring the thread back on topic, I'll gave my opinion on the topic, for whatever its worth.

Of course, any legitimate questions about mainstream physics seeking layman's answers are critical to the entire concept of the sub-forum, along with technical discussions about current research.
However, "layman questions" is definitely not a category that includes questions along the lines of "Is the Universe a conscious entity surrounded by a black hole?" Questions like these can lead to enormously long threads in which the author refuses to accept the (correct) answers given, and have no good answer except to tell the thread starter to read a bit of physics before speculating like that. IMO, questions such as that belong in the fringe section, not in the sub-forum designed for intelligent discussion of one of the most complex areas in all of science.

Additionally, threads of the form "I've disproved (insert accepted physical model here) without the use of any mathematics or physics whatsoever, and anyone who disagrees is a closed-minded fool" have absolutely no place in M&P, and shouldn't be there. No rational discussion can possibly emerge from them.

In short, there's a difference between answering questions about physics/discussing the subject and uselessly arguing with cranks who won't ever give up.

Now, this does not mean that thread starters can't make claims - not at all. Just not absurd ones. I think a good rule is the following - if a person starts a thread making an assertion, they should have to reference a peer-reviewed article, not some article from "Quantum woo-woo monthly" or "Psuedoscience periodical".

10. ### UndefinedBannedBanned

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1,695
MarkM125, przyk, CptBork & Farsight.

Hi guys, I just have a naive observation/question to put regarding your recent side-discussion of Farsight's 'box' scenario as put in his post #155.

przyk, in my naive understandings insofar as the effective frame for the gravity field/well of the parent gravitating body IS the gravitational field pattern generated by that parent body.

In my naive reading of Farsight's box scenario, I imagined that somehow the parent body and the field pattern 'frame of reference' was retained BUT the field itself 'gedankenly negated' and leave the erstwhile sterile gravity field 'pattern' as the frame of reference for any considerations of 'motion' thereafter.

And so I consider the motional possibilities in that naive reading Gedanken context, and lo and behold, I observe NO motion of any significance above and beyond any 'chaos theory' or 'quantum uncertainty/fluctuations' randomized? forces which would exist in any 'flat space' anyway.

So, the concept of "down" would be appropriate for any suppose motion in the 'box' that would be construed as being towards the erstwhile centre of the erstwhile planet generating the erstwhile (now gedankenly negated) gravitational field 'pattern' which we gedankenly use for such frames of reference for down/up sense to describe any supposed motion in that 'box' space conditions described by Farsight.

Can you explain naively how the mathematical treatments you posted in reply to Farsight affects my naive observation/notion just described in the context of my 'gedanken' as posed? Thanks. I'll be back tomorrow to see what transpires. G'night.

11. ### Farsight

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I didn't want to embarrass you too much przyk.

I'm afraid this is a fallacy. A real gravitational field is either there or it isn't. If it's there you can't transform it away. And if it isn't there, you can't conjure it up. A "uniform" gravitational field is a contradiction in terms. There's perhaps a terminology issue here in that Einstein referred to a real gravitational field as one of "quite special form". This is from section 20 of Relativity: the Special and General Theory:

"This is by no means true for all gravitational fields, but only for those of quite special form. It is, for instance, impossible to choose a body of reference such that, as judged from it, the gravitational field of the earth (in its entirety) vanishes."

Oh, and have a read of Markus Hanke's General Relativity Primer. It's pretty good. Mark has tried to assist, but sadly the metric tensor is "simply a way to define measurements at each point in space-time". It isn't the stress-energy-momentum tensor, which is what we need to focus on when discussing our pencil in homogeneous isotropic space. The one that isn't going anywhere.

12. ### Farsight

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Sorry about the above Mark. The situation on this forum isn't quite clear cut.

I agree, and I think somebody like Undefined will agree too. That isn't the issue. Let me try and get it across by editing your next comment to insert an example theory.

Again I agree. I find people who say "Einstein was wrong" a bit tiresome myself. But people like me tend to point to something Einstein said when making a case for "your understanding of general relativity is wrong". And then people who consider themselves to have expertise in GR tend to be lumping me in with the "Einstein was wrong" people. Even though actually they disregarded what Einstein said, and effectively put themselves into the "Einstein was wrong" camp.

There definitely is. The trouble is in determining who's the crank and who isn't. There are diamonds in the rough.

I think that's rather limiting. I think they should be able to refer to scientific evidence and make a case even if there isn't a suitable peer-reviewed article to refer to.

13. ### Beer w/StrawTranscendental Ignorance!Valued Senior Member

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But, I thought you did prove relativity wrong when you quoted Einstein as saying one of the postulates was wrong -the measured speed of light being constant I think it was.

14. ### CptBorkValued Senior Member

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Can you make a post asking this question and any others in the physics & math section? You could link back to this thread for reference, I just think it's time we brought this sort of actual scientific discussion back to the appropriate venue.

15. ### przyksquishyValued Senior Member

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I took the Einstein field equation and drew a simple conclusion from it. If you think I did something wrong, you should say specifically what you think I did wrong.

And? Acceleration, like looking at a pencil and seeing whether it starts falling in one direction or another or not, is something that you can readily transform away. This is obvious: you won't see a pencil fall anywhere if you are falling along with it, for example. Acceleration, for this reason, is not normally considered a good indication of whether there's really a gravitational field there or not.

(By the way, when reading anything about general relativity, you should keep in mind that there are at least three related but different quantities that are all sometimes called "the gravitational field": the metric, the Christoffel symbols, and the spacetime curvature.)

No, since it is perfectly possible to have constant but nonzero curvature.

This is one respect in which general relativity is very different from Newtonian gravity. If you have a uniform background density of matter permeating the entire universe, then according to Newton's theory its gravitational field will cancel out and it will make no net contribution to the gravitational field anywhere. This does not carry over to general relativity. This is why there used to be a debate about whether the universe would end in a "big crunch" or expand forever in a "heat death" scenario: the average density of matter in the universe matters, even if it were perfectly uniform and infinite in extent.

Those gravitational fields of "quite special form" are precisely those for which the Riemann curvature tensor is nonzero.

16. ### Motor DaddyValued Senior Member

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I find the bold part of your statement very odd. Can you explain to me how you can be "falling along with it" when you are a distance away from it? If you are "falling" towards an object, and a pencil is also falling towards an object, then it is IMPOSSIBLE that you could be falling at the same rate in the same direction towards that object, since you are a distance away from the pencil. The ONLY way that you can be traveling in the same direction as the pencil towards that object, at the same rate, is if you are zero distance away from the pencil, ie, you are the pencil. Please explain.

17. ### AlphaNumericFully ionizedRegistered Senior Member

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Firstly Einstein wasn't perfect, there's plenty of things he thought about gravity and particle physics we now know to be false, quite a bit of which was known in his day. Secondly part of the issue they, we, have with your approach is you clearly have no understanding of his work in terms of details, all you have is wordy quotes. Due to your being practically innumerate you have absolutely no way of getting a handle on the details of his work, you rely on wordy summaries and analogies which necessarily are not entirely accurate.

And I think it's a bit of a joke you rag on people who 'consider themslve to have expertise in GR' when you deem yourself to have a better understanding of, for instance, electromagnetism than a Nobel Prize winner in said domain. But that also illustrates my point about arguing by analogy, as you have regularly trotted out waffle about magnetic fields and curl and whatnot, only to then be told that the structure of the electric and magnetic fields in electromagnetism can be reformulated so swap much of the mathematical roles of E and B. If you understood Maxwell's equations on a working level, rather than just looking at pictures of vector fields someone else made, you'd have already known that. Time and again you've shown that your inability to do any of the quantitative details of pretty much every domain of science you've ever spoken about is leading you to vapid or even demonstrably wrong statements.

Yes, we can all read between the lines, you're referring to yourself. Too bad that every opportunity you have had to step up and show you aren't a crank you have failed. You claim to know better than a Nobel Prize winner yet you cannot do the most trivial of standard problems. You claim your work is worth multiple Nobel Prizes yet you cannot give a single physical system your work can model quantitatively, all the while whining about string theory supposedly having exactly that problem. You are functionally innumerate, in that your mathematical knowledge is below the ability necessary to even get into a university to study physics, despite you agreeing mathematics is needed within physics. So please tell me by what criteria you are not a crank, given all of this evidence for you being a crank.

Yes, there are plenty of people who do not have science educations beyond high school, who cannot do mathematics beyond high school level, who only discuss science in terms of simplified examples and imprecise analogies but they aren't claiming to understand science better than Nobel Prize winners. How do we identify such people, distinguishing them from cranks? They ask honest questions, not because they want an excuse to throw in their 2 cents but because they honestly want to understand. When they don't know something they don't just make up some word salad superficial waffle, complete with the occasional picture of a knotted straw from McDonalds, they go looking for the information and if it doesn't exist they start open and honest discussions. They listen when they are corrected, when their mistakes are explained to them, and they try not to repeat them again. They realise to understand certain topics requires putting in some time and effort and if they really want to understand said topics then they put in some time and effort rather than saying "Sod it, I know better".

Seriously, please give me your criteria for someone to be or not to be a crank. And then explain why you don't fall on the wrong side of that line.

Not all science is in journals and not all journals publish science, sure. But that is just another reason for someone, when they are really interested in a subject, to put in the time to learn some of the details. I could write 2 papers in particle physics, one bullshit, the other valid, and 99% of people couldn't tell them apart no matter how incoherent the BS one is, provided I throw in enough equations and use enough buzzwords. For someone versed in even an undergraduate degree nonsense equations and abuse of terminology sticks out like a sore thumb. Sure I could write a paper to fool an undergrad but the point is the additional understanding of the details provided to someone via learning means they are able to critique things for themselves. I've said it before, I'm certain I know more problems with theoretical physics domains like supersymmetry, supergravity, string theory, compact dimensions, extensions to the Standard Model etc than any hack here, all because I understand some of the details and therefore I see gaps here and there. Likewise when reading papers, I can evaluate them myself, allowing me to extract interesting ideas/methods and to ignore boring and perhaps even wrong ideas/methods. You, Farsight, rely on others to summarise actual physics for you, as you do not understand it.

This "diamond in the rough" stuff is you wanting to play the plucky underdog destined for vindication and Nobel Prizes. You went (or tried to get into) science for all the wrong reasons and you've reaped exactly as expected, nothing but derision.

18. ### Farsight

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Not me mate. I root for relativity. The locally-measured speed of light is constant, because you use the local speed of light to calibrate your rods and clocks. Which you then use... to measure the local speed of light. But the coordinate speed of light varies in a non-inertial reference frame such as a gravitational field. So when you step back and look at the big picture and look askance at all frames at once, you come to appreciate that the speed of light is not constant. Just like Einstein said.

Aw, I think relativity is the Cinderella of contemporary physics. And she has some ugly sisters.

19. ### Farsight

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The big crunch is not even wrong. A gravitational field affects the motion of light and matter through space. It doesn't suck space in. And space is a something, not a nothing. A gravitational field is "a state of space". And the energy of the gravitational field shall act gravitatively in the same way as any other form of energy. To hell with matter. Space has its vacuum energy. And when this energy density is uniform, pencils don't fall down. And neither do galaxies. But the stress-energy tensor does have its pressure diagonal. Because a gravitational field is just an energy-pressure gradient in space, of space. And if you have no gradient you've still got pressure. In space, of space, and pressure x volume = energy, so at the fundamental level, space and energy are the same thing. So if space is not infinite, then space expands. It has to expand. It can't do anything else. It didn't do anything else when it was small and dense. Because here we are.

Ah, don't you just love cosmology? Shame my Uncle Albert got an F for it. Because his usual confidence in his own theory deserted him, so he fudged a universe on a pencil point. That was his greatest blunder, and people are still blundering around today. Space has its vacuum energy. When that is not homogeneous, it has a gravitational effect. And the energy that does this has a mass equivalence, as does matter. But space, of course, is dark.

Oh, and in case you didn't notice: space must have an edge. If it was infinite, it couldn't expand.

20. ### CptBorkValued Senior Member

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Infinite universes are allowed to expand and contract in GR just like finite universes, it's the cosmic scale factor affecting cosmological distances and redshift that changes, along with the resultant mass/energy density of the universe. przyk is right, if you work it out you'll find that (neglecting non-gravitational forces) the box would act like a miniature Big Bang universe in a state of collapse, and the pencil would gradually drift towards the center along with all of the energy in the box (unless the energy in the box is Lorentz-invariant quantum vacuum energy, which as demonstrated in the link I previously provided must have negative pressure and would cause the pencil to accelerate away from the center). Your misconceptions result from trying to make assertions about what General Relativity predicts/describes based on plain-English analogies that don't describe the full situation, instead of simply crunching through the math- it's not an intuitive theory you can deduce based on simple common sense alone.

21. ### CptBorkValued Senior Member

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That's actually a perfectly good question and an important subtlety in General Relativity, but I don't want to pre-empty przyk so I'll wait and see if he gets back to you.

22. ### Beer w/StrawTranscendental Ignorance!Valued Senior Member

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So, you can only appreciate relativity -root for it- once you realize one of its postulates is wrong.

This is very sciency stuff, Farsight. Thanks.

:EDIT:

I failed at my experiment to test your idea, Farsight. When I went to ask my dead cat whether she moved faster than light -she turned out not to be dead! Should I try your Cinderella experiment?

Last edited: Aug 20, 2013
23. ### przyksquishyValued Senior Member

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Well take that up with the Einstein field equation and the FLRW solution to it. You are arguing based on your own personal prejudices rather than what GR actually has to say.

(By the way, the "energy of the gravitational field" only appears in one heavily frame-dependent expression of the Einstein field equation. There is absolutely no basis for interpreting it as a physical quantity.)

False, because expansion is not defined in terms of a hypothetical edge to the universe.