# 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. ### quantum_waveContemplating the "as yet" unknownValued Senior Member

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Lol, not as bad as a drive by, that's true. And there is no reason that people who don't like his tactics can't ignore him, or anyone else for that matter. But, "in the wrong place at the wrong time"? That is a bit of a chuckle.

3. ### quantum_waveContemplating the "as yet" unknownValued Senior Member

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Well said, and if we would pay close attention to what people actually say, and to the context in which it is said, and not to what others say they say, then many of the concerns you allude to by people in the field would not actually arise in the normal course of forum activity. I can conduct a thread in P&M that is perfectly acceptable, and when the history of my non P&M threads, or portions of threads that I have had or asked to have moved out of P&M are injected into the P&M threads, it is mostly done for the purpose of derailing a legitimate thread out of pent up disdain from my fringe ideas, lol. But I can keep them completely separate, and I know when to do that.

5. ### quantum_waveContemplating the "as yet" unknownValued Senior Member

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I'm not saying that the layman enthusiast should be allowed to post alternative and unfounded speculation in P&M; that should be and generally is hauled out to the fringe. I'm saying that layman science enthusiasts are the majority here and they should and do start threads in P&M based on questions they have or things about consensus views that trouble them. How can that dumb down P&M, since the smart content you might mean is the purpose of P&M is applied when the smart people there address the questions submitted? In fact, I think that the smart people look forward to being able to do that and help the layman better understand physics and math. We have different views on what an effective P&M forum should be about.

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

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Well I think it was a mistake to ban Tach. I'd much prefer concise and rude to long winded and wrong.

8. ### billvonValued Senior Member

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Unfortunate but true. That's an argument for better moderation.

9. ### CptBorkValued Senior Member

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I should correct what I wrote earlier... The quantum vacuum is believed to have a positive energy value, not negative, but Lorentz frame invariance requires that it have a negative anti-gravity pressure. Don't know if that makes any difference to anyone's understanding, but I gotta watch for nitpickers

10. ### quantum_waveContemplating the "as yet" unknownValued Senior Member

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Lol, nitpickers are hell. Sounds like the John Baez pages, and the list of inconsistencies he discusses. Interesting to read, and I pick up concepts, but the details ... I know where to find them if I need them, just like you must, and it is smart for you to set the record straight.

11. ### Farsight

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I'll nitpick that: it's positive pressure. I know people say it's negative pressure, but I disagree. The dimensionality of energy is pressure x volume. Space has a positive volume and a positive energy value, so the pressure can't be negative. If you can explain why it is negative, I'd be glad to hear it.

12. ### CptBorkValued Senior Member

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Firstly it's allowed to have a negative pressure because it's not required to have the same properties as normal fluids, we're talking about assigning physical meaning to an abstract theoretical form of energy filling all of space and time. Secondly, as far as the reasoning as to why Lorentz invariance requires positive vacuum energy to have negative pressure (and negative vacuum energy to have positive pressure), refer to equation (3) here.

13. ### UndefinedBannedBanned

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With respect, it appears you left out certain other permutations, like for example: concise and wrong; and: longwinded and right?

In any case, inerrant he hasn't been; rude and stubborn he has been. As observed/experienced more than once before by mods and members alike who have been at the receiving end of his attitude/tactics. All these objective observables reside in the record for anyone wanting to inform themselves about the ban objectively.

14. ### Farsight

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I don't think it's abstract or theoretical myself, but nevermind.

Thanks for that. I think well of Tamara Davis, and you'll be aware that I'm a "relativity guy". But there's no other way for me to say this so I'll just spit it out. That's wrong.

How to explain this?

Imagine you have the flat rubber sheet with the bowling ball. Now flip it upside down and forget about the bowling ball. You have a flat rubber sheet with a "mountain" of positive "energy-pressure" in it. This causes gravity. A passing light wave will curve towards the hump. Matter (remember the wave nature of matter) will fall towards it. So you think that positive energy-pressure causes things to move together, so you think you need negative pressure to counterbalance it. But it isn't the energy-pressure itself that causes the contraction, it's the energy-pressure gradient that does this. Replace the mountain with a high flat plateau of positive energy-pressure, and your gravity has gone. Light goes straight. And in all scenarios it's all positive energy, it's all positive volume, and it's all positive pressure. Get a stress-ball and squeeze it down in your fist. It's all positive energy-pressure, and there's no gradients in it, so there is no gravity. Now watch it expand.

15. ### CptBorkValued Senior Member

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Well you can believe what you want to believe based on ambiguously vague analogies, and I'll believe what I want to believe based on concise mathematical demonstration. Sound good?

16. ### Farsight

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Yep. I'll explain that positive pressure x positive volume = positive energy, and give you an explanation as to why the stress-energy density gradient causes gravity. I could even point out that we're here, and that our homogeneous* universe didn't collapse under its own gravity when it was small and dense. I could even point out that gravity alters the motion of light through space, and matter, but it doesn't suck space in. Or that there's no gravitational field in a void at the centre of a spherical body. The potential is low and the energy density is high, but it's locally uniform, so you float about. You don't fall down. There is no gravity when the energy density is uniform.

But you believe what you want to believe in. Such as in a "magic" fluid where negative pressure x positive volume = positive energy. Because you've seen a "precise mathematical demonstration" based on in general relativity all forms of energy gravitate. A lot of other people believe in this too. But there again, a lot of other people believe in heaven and hell and sweet baby Jesus.

* homogeneous on the large scale now

17. ### przyksquishyValued Senior Member

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Dimensional analysis doesn't work that way. Just because energy has the same units as pressure times volume doesn't mean that energy is pressure times volume.

Your explanation would necessarily be wrong (or at the very least, flat out contradict GR) since it is the stress-energy tensor, and not any of its gradients, that appears explicitly in the Einstein field equation.

The Einstein field equation (without the cosmological constant) is normally expressed as

$R_{\mu\nu} \,-\, \frac{1}{2} R g_{\mu\nu} \,=\, \kappa T_{\mu\nu}$​

where $R_{\mu\nu}$ is the Ricci tensor, $R \,=\, g^{\mu\nu} R_{\mu\nu}$ is the Ricci scalar, and $T_{\mu\nu}$ is the stress-energy tensor, as you should already know. It is well known, and easy to show, that the EFE can alternatively be written as

$R_{\mu\nu} \,=\, \kappa \bigl( T_{\mu\nu} \,-\, \frac{1}{2} T g_{\mu\nu} \bigr)$​

where $T \,=\, g^{\mu\nu} T_{\mu\nu}$ is the trace of the stress-energy tensor. This tells you what sort of stress-energy density you need in the universe in order to have a gravitational field. If $T_{\mu\nu} \,-\, \frac{1}{2} T g_{\mu\nu} \,\neq\, 0$, then $R_{\mu\nu} \,\neq\, 0$. Consequently you can easily have a nontrivial gravitational field even with a constant (gradient-less) stress-energy density. Furthermore, if the stress-energy tensor has nonzero trace then the Ricci scalar is also necessarily nonzero regardless of the gradient of the stress-energy density.

This all follows from a simple and braindead inspection of the EFE, so if you say that it is a necessary condition for the gradient of the stress-energy density to be nonzero in order to have a gravitational field, you are explicitly contradicting the EFE.

18. ### Farsight

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OK przyk, let's say you're in a box with a constant gradient-less energy density. You have a pencil in your hand. You release it. What happens to it?

19. ### przyksquishyValued Senior Member

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Yes -- if you know how to plug things into an equation.

EDIT: By the way, putting a cosmological constant into the EFE is mathematically equivalent to adding a constant, spatially and temporally homogeneous term to the stress-energy tensor proportional to $- \frac{1}{\kappa} g_{\mu\nu}$. Surely you've heard this actually affects things? It doesn't just cancel out.

Assuming what? The box is locally inertial? The energy is (somehow) the only nonzero component of the stress-energy tensor and you are somehow keeping it constant in time?

In that case I haven't done the calculation, but I'd expect not much except that the pencil might experience some tidal forces.

Last edited: Aug 17, 2013
20. ### Farsight

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The pencil doesn't fall down, now does it?

21. ### przyksquishyValued Senior Member

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Things "falling down" is not a good, coordinate-independent measure of the presence of a gravitational field in GR. A pencil "falling down" simply means you are observing it from the perspective of an accelerating reference frame. This can happen whether or not there is truly a gravitational field present. Similarly, a pencil "not falling down" simply means you are observing it from the perspective of a locally inertial reference frame, again regardless of whether there's a gravitational field present.

I note you elected to ignore most of what I said. In particular, I've shown unambiguously that, according to the EFE, a constant, gradient-less stress-energy density can readily imply nonzero spacetime curvature. You have made no attempt to reconcile this with the story you told about only a stress-energy density gradient creating a gravitational field. You don't get to just ignore the EFE when talking about GR, you know.

22. ### MarkM125Registered Senior Member

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Consider the metric components of the form:

$g_{\mu \nu } = \begin{bmatrix} -e^{-2ax_{1}} & 0 & 0 &0 \\ 0 & 1 & 0 &0 \\ 0& 0& 1& 0\\ 0 & 0 & 0 & e^{-2ax_{1}} \end{bmatrix}$

The only non-zero Christofel symbols are:

$\Gamma _{111} = a$
$\Gamma _{144} = a$
$\Gamma _{441} = a$
$\Gamma _{414} = a$

From there, the equations of motion dictate a downward acceleration in the z-direction.

These metric components can be generated with nothing but a uniform energy-momentum tensor. So the pencil falls.

Last edited: Aug 17, 2013
23. ### przyksquishyValued Senior Member

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Shouldn't there be a negative term in there somewhere?