Merged Relativity+ / Farsight

[edd]

Because I understand general relativity and gravity. because I've read a lot of material, including the Einstein material.

Merely reading material does not guarantee understanding. This is one reason universities set exams.

 

[Farsight]

That's not the point. Purely theoretical constructs can be useful in gaining understanding of a theory and for demonstrating where someone's understanding might be lacking.

Can be. But they can lead you astray, and get in the way of understanding. IMHO there's something of an issue wherein people get all wrapped up in abstraction, and lose touch with reality. A good example of this is spacetime. It's a mathematical "space" in which motion does not occur, but people confuse it with space, and forget that Einstein wrote down the equations of motion through space.

Merely reading material does not guarantee understanding. This is one reason universities set exams.

Agreed. But being taught something parrot-fashion along with "shut up and calculate" doesn't guarantee understanding either. I'm forever saying think for yourself.

It's a *speculation*. Do you think that cosmic strings have been ruled out by anything? If so, what has ruled them out?

I'd have to explain gravity to you to justify that, and then it would be merely "on my authority".

It is always possible that cosmic strings exist, but cannot be detected or else distinguished from other effects.

It is always possible that fairies exist, but cannot be detected or else distinguished from other effects.

Farsight, imagine that you had lived in some past decade or some past century. What would you have dismissed because of "lack of evidence"?
Before 1850: chemical elements predicted by Dmitri Mendeleev
Before 1800: atoms
Before 1800: visible light's wave nature
Before 1650: terrestrial gravity extends to the Moon and beyond
Before 1650: white light contains colors

None of the above. We have different metals, sparks fly, lenses, rocks to throw, and rainbows. Even if my answer was different, I would reject your assertion because one could use it to justify belief in fairies, unicorns, heaven, etc.

 

[W.D.Clinger]

Farsight often fails to read his proof texts.

Clinger: Yes he was, and no it wasn't. And you're defending him? Shame on you. My reference was to the Ray quote, and we were discussing the "uniform gravitational field" here with respect to the principle of equivalence. Not EGR v MGR. And certainly not cosmic strings.

In the proof text you cited, Peter M Brown quoted Ray to illustrate the difference between EGR and MGR. Furthermore, Brown's third example (the one with a cone-shaped spacetime) was a cosmic string.

In the correct rubber-sheet analogy, you can make a sheet shaped like a cone (there is not "inverted" or "right side up" in the more accurate version of the analogy---your use of the word "inverted" is what clued me into the fact that you're thinking of the misleading kiddie version). In this cone, there is no gravitational force except at a singularity in the center. An object left at rest on the "cone" will not accelerate towards the center. There are no geodesics that orbit the center. You are correct that such a cone has no Riemann curvature, but you're incorrect to say that a cone has a "uniform gravitational field". A conical sheet has the same Newtonian gravitational fields as flat sheet does---none at all.

OK, so what sort of object has a gravitational field like this?

A... ummm... cosmic string.

Or perhaps you meant that the gravitational potential energy is a cone, with constant slope and therefore constant gravitational acceleration? Draw the GR version of that and you'll see that it DOES have Riemann curvature. Go ahead, try it.

If it's a cone, it doesn't. See above where even Clinger had to confess that I was right:

Farsight's wrong about that. Although Brown's cone-shaped spacetime (for the cosmic string) has no Riemann curvature, the spacetime ben m described here does have Riemann curvature.

Riemann curvature is "the defining feature" of a gravitational field because without it your plot can't get off the flat and level.

Although this may be "the defining feature" of a gravitational field in MGR, this is most definitely not the defining feature of a gravitational field in EGR.

Huh? Vorpal, it's the crux of it. If there's no Riemann curvature you haven't got a gravitational field. Things don't fall down.

Einstein disagreed with Farsight on this point.

In the proof text cited by Farsight, Peter M Brown quoted JJ Stachel summarizing Einstein's disagreement with Farsight:

...what characterizes the existence of a gravitational field from the empirical standpoint is the non-vanishing of the [components of the affine connection], not the vanishing of the [components of the Riemann tensor].

Peter M Brown quoted Ray as an example of how the modern view (MGR) has made it difficult for some people to understand Einstein's view (EGR):

"It is very important to notice that in a freely falling frame we have not
transformed away the gravitational field since the Riemann tensor
(gravitation ←→ Riemann tensor) will not vanish and we will still measure
relative acceleration ….. The first thing to note about the 1911 version of
the principle of equivalence is that what in 1911 is called a uniform
gravitational field ends up in general relativity not to be a gravitational field
at all – The Riemann tensor is here identically zero. Real gravitational fields
are not uniform since they must fall off as once recedes from gravitating
matter".

And by the way, I take exception to you guys crawling out of the woodwork with smoke and mirrors to make things complicated and attempt to say "Farsight is wrong" when I'm not.

That hasn't happened, at least not recently. Almost everything you've written here has been wrong in some way or another.

In recent days, I found only one opportunity to say you're right about something. I took advantage of that opportunity. You then tried to use my endorsement against ben m, but you were wrong to do so.

Sorry edd, but cosmic strings are a 40-year old hypothesis with no evidential support.

Why, then, have you been citing Brown's example of a cosmic string (with its cone-shaped spacetime) as your primary proof text?

 

[ben m]

Sorry edd, but cosmic strings are a 40-year old hypothesis with no evidential support.

So that answers your question. The only object around which the GR embedding diagram is a cone? that would be a cosmic string. If cosmic strings do not really exist, then the answer to the question "what sort of object has a gravitational field that embeds in 3D as a cone" is "nothing, that's not obtainable in nature.". If cosmic strings exist, then they are the answer.

 

[lpetrich]

A good example of this is spacetime. It's a mathematical "space" in which motion does not occur, but people confuse it with space, and forget that Einstein wrote down the equations of motion through space.

Einstein was a firm believer in space-time and motion through it. 3-space is a set of 3D hypersurfaces in space-time. Farsight, if you find hypersurfaces hard to understand, then you won't make it as an expert on general relativity.

(cosmic strings as ruled out...)

I'd have to explain gravity to you to justify that, and then it would be merely "on my authority".

Shall I explain what a cosmic string is supposed to be?

Farsight, imagine that you had lived in some past decade or some past century. What would you have dismissed because of "lack of evidence"?

Before 1850: chemical elements predicted by Dmitri Mendeleev
Before 1800: atoms
Before 1800: visible light's wave nature
Before 1650: terrestrial gravity extends to the Moon and beyond
Before 1650: white light contains colors

None of the above. We have different metals, sparks fly, lenses, rocks to throw, and rainbows. Even if my answer was different, I would reject your assertion because one could use it to justify belief in fairies, unicorns, heaven, etc.

A big collection of non sequiturs. Farsight, I will show how these "no evidence" arguments would have worked.

"There is no evidence that matter is composed of tiny grains! Can't you see that matter is continuous?" sort of like "You can see space. Look at your hands. You can see motion. Look at your hands move. But you can't see time."

"There is no evidence that the force that makes an apple fall is the force that keeps the Moon orbiting the Earth! Gravity extending to the Moon is pure codswallop!"

"There's no evidence of eka-boron or eka-aluminum or eka-silicon or eka-manganese! The Periodic Table of Elements is balderdash and persiflage!"

...

 

[Vorpal]

... A vanishing tidal tensor does not necessarily imply that the spacetime is flat, though that's not directly relevant here.

Huh? Vorpal, it's the crux of it. If there's no Riemann curvature you haven't got a gravitational field. Things don't fall down.

-- A nonzero tidal tensor implies that there is nonzero Riemann curvature.
-- A vanishing tidal tensor does not imply that the Riemann curvature vanishes.
In the part you've quoted, I was addressing the case where there was a nonzero tidal tensor and therefore nonvanishing Riemann curvature. As an aside, I commented that in GTR there could also be nonzero Riemann curvature despite a vanishing tidal tensor.

Therefore, Ray's position that a vanishing Riemann curvature does not constitute a 'proper' gravitational field is completely irrelevant to that issue. The primary disagreement was about whether the situation described involves a flat spacetime in the first place, not about about the proper interpretation of a flat spacetime as having or not having a gravitational field.

Vorpal, let me spell it out: if the force of gravity does not diminish with distance, there's no radial stretch, no Riemann curvature, and therefore no gravitational field.

You are simply mistaken about that. There would be tidal forces in directions orthogonal to the radial one.

And by the way, I take exception to you guys crawling out of the woodwork with smoke and mirrors to make things complicated and attempt to say "Farsight is wrong" when I'm not.

I take exception to your repetitive substitution of a different question whenever you've made a mistake. In particular, the disagreement was about whether the radial field described previously would be uniform. Instead of addressing this (e.g., Roboramma's explanation of why there would be tidal forces, among posts by others), you simply assume that there is no Riemann curvature.

...

Although this may be "the defining feature" of a gravitational field in MGR, this is most definitely not the defining feature of a gravitational field in EGR.

I'm not convinced that there is any kind of consensus on this in MGR (though you didn't say there was either). There are certainly examples of authors advancing that point of view, but also ones that are simply silent about it. Notably, MTW aren't silent but are willfully ignostic about it instead--their stance is an explicit refusal to give any "defining feature" of what "gravitational field" means in the first place (§16.5).

 

[Perpetual Student]

It is always possible that fairies exist, but cannot be detected or else distinguished from other effects.

Might these be the little fairies that wave their wands as they whirl around on photon loops turning them into electrons? Is this your "pointer"?

 

[Farsight]

Why, then, have you been citing Brown's example of a cosmic string (with its cone-shaped spacetime) as your primary proof text?

Gasp! As I said, I cited the Ray quote. And again: we were discussing the "uniform gravitational field" and the principle of equivalence. Not EGR v MGR or cosmic strings. See the multiverse thread page 3, around post #95. And the point is that the principle of equivalence applies only to an infinitesimal region, a region of no extent, so it doesn't apply at all.

So that answers your question. The only object around which the GR embedding diagram is a cone? that would be a cosmic string. If cosmic strings do not really exist, then the answer to the question "what sort of object has a gravitational field that embeds in 3D as a cone" is "nothing, that's not obtainable in nature.". If cosmic strings exist, then they are the answer.

Like I said to lpetrich, they aren't obtainable in nature, but I'd have to explain gravity to you to justify that, and then it would be merely "on my authority". You know. Even if I included a barrelful of Einstein quotes and hard scientific evidence. You know how it is. It's tough to get a groupie to give up his woo.

All: I've been ill. I spent Sunday in bed, and wasn't too bright yesterday. I feel better now, but you know what I was saying about you guys crawling out of the woodwork? Well with the notable exception of benm, you've made at most one post since 1st March. It's now the 4th. Why are you even here? Is it to ward off any scepticism about any aspect of physics, even Max Tegmark's universe that is quite literally made of mathematics?

 

[Farsight]

Einstein was a firm believer in space-time and motion through it.

No he wasn't! Just because you're blind to the distinction between space and spacetime, don't think Einstein was. When he said space he meant space. When he said spacetime he meant spacetime.

3-space is a set of 3D hypersurfaces in space-time. Farsight, if you find hypersurfaces hard to understand, then you won't make it as an expert on general relativity.

You've failed already because you don't appreciate that spacetime is a mathematical abstraction in whi9ch motin does not occur. Yes, the message in Nasty Little Truth About Spacetime Physics is the right message[/url] even if he gets his list of crackpots wrong.

Shall I explain what a cosmic string is supposed to be?

No. We all know what it's supposed to be. And we can look it up anyway.

A big collection of non sequiturs. Farsight, I will show how these "no evidence" arguments would have worked.

"There is no evidence that matter is composed of tiny grains!

Actually, it isn't. A photon takes many-paths, and you can make an electron and a positron out of it, or our of two photons. An electron is not some tiny grain. Its field is what it is. It's quantum field theory, remember? Not quantum point-particle theory.

"There is no evidence that the force that makes an apple fall is the force that keeps the Moon orbiting the Earth! Gravity extending to the Moon is pure codswallop!"

"There's no evidence of eka-boron or eka-aluminum or eka-silicon or eka-manganese! The Periodic Table of Elements is balderdash and persiflage!"

Sheesh. What are you on about? I can fire a stone cannonball, and I can point to a mountain, and to the moon. There is no substance to your naysaying. But there again, there never is.

 

[Farsight]

...Therefore, Ray's position that a vanishing Riemann curvature does not constitute a 'proper' gravitational field is completely irrelevant to that issue. The primary disagreement was about whether the situation described involves a flat spacetime in the first place, not about about the proper interpretation of a flat spacetime as having or not having a gravitational field.

The primary disagreement concerned the principle of equivalence. Edd said I was in breach of it, and I said that doesn't matter because it was an "enabling" principle only. It got Einstein started. But it only applies to a infinitesimal region, a region of no extent. So it doesn't actually apply. So breaching the principle of equivalence doesn't overturn GR.

You are simply mistaken about that. There would be tidal forces in directions orthogonal to the radial one.

Again that wasn't what Ray was talking about. And since we're talking about the principle of equivalence, there aren't any orthogonal tidal forces when you're accelerating through empty space.

I take exception to your repetitive substitution of a different question whenever you've made a mistake. In particular, the disagreement was about whether the radial field described previously would be uniform. Instead of addressing this (e.g., Roboramma's explanation of why there would be tidal forces, among posts by others), you simply assume that there is no Riemann curvature.

I take exception to people trying to suggest I've made a mistake when I haven't. The quotes by Ray and Synge were perfectly clear.

 

[edd]

The primary disagreement concerned the principle of equivalence. Edd said I was in breach of it, and I said that doesn't matter because it was an "enabling" principle only. It got Einstein started. But it only applies to a infinitesimal region, a region of no extent. So it doesn't actually apply. So breaching the principle of equivalence doesn't overturn GR.

I'd like to invite you to construct a device that measures locally the gravitational potential (that is, without sending signals to a distant location - it is obviously allowed some physical size to operate on). Naturally this can be a typical thought-experiment type construct, so practicalities aren't too important. Just describe a physical process that does it.

 

[dafydd]

Huh? Vorpal, it's the crux of it. If there's no Riemann curvature you haven't got a gravitational field. Things don't fall down.

Have you ever seen sausages hanging up in a butcher's shop?

 

[ctamblyn]

...

I take exception to people trying to suggest I've made a mistake when I haven't.

...

I don't think we're "trying to suggest" anything so much as point it out. Here are three mistakes you have made recently:

1. You said that a purely radial gravitational field of constant non-zero field strength g was a "uniform field"; it obviously isn't, the direction varies throughout space.

2. You further claimed that the radial constant-magnitude field didn't give rise to tidal accelerations, and refused to back up this claim with any sort of calculation when challenged to do so. You also refused to change your mind when some of us explained why tidal forces exist in such a field, and explicitly described them to you.

3. You don't seem to appreciate that something defined in terms of infinitesimal regions can still have falsifiable implications for finite regions. On Earth, hydrogen atoms fall at the same rate as helium atoms under the influence of gravity; if that were different on the moon, it would violate the equivalence principle and, consequently, disprove GR. Similarly, if the electron Thomson scattering cross section (which depends on the fine structure constant) were different on the moon, that too would violate the EP and disprove GR.

 

[edd]

Moving this one here, as I think it belongs here better:

How many times do I have to tell you before you'll get it? The evidence is inflation. The expanding universe. If there wasn't an edge, energy-pressure would be counterbalanced at all locations. The universe could not expand.

Another deep misunderstanding about what the conditions are for a universe to expand.

This talk of counterbalancing energy-pressure is nonsense. All the standard cosmological models have that term as being the same everywhere (as it's an apparently reasonable simplifying approximation to make) and I suspect Farsight is thinking along lines because he believes the pressure has some kind of mechanical action in cosmology - that is to say that the force exerted (the everyday definition of pressure as force/area) is directly involved.

 

[lpetrich]

Like I said to lpetrich, they aren't obtainable in nature, but I'd have to explain gravity to you to justify that, and then it would be merely "on my authority". You know. Even if I included a barrelful of Einstein quotes and hard scientific evidence. You know how it is. It's tough to get a groupie to give up his woo.

Einstein quotes? That's what I mean by arguing like a theologian.

These types of topological defects may emerge from Grand Unified Theory symmetry breaking:

• 1+0D: magnetic monopoles
• 1+1D: cosmic strings
• 1+2D: domain walls

These are closely analogous to crystal-structure defects.

Is it to ward off any scepticism about any aspect of physics, even Max Tegmark's universe that is quite literally made of mathematics?

Do you have any argument other than how silly it looks?

No he wasn't! Just because you're blind to the distinction between space and spacetime, don't think Einstein was. When he said space he meant space. When he said spacetime he meant spacetime.

Einstein-thumping. Arguing like a theologian. You are supposed to argue theories, not treat certain people as inspired prophets.

You've failed already because you don't appreciate that spacetime is a mathematical abstraction in whi9ch motin does not occur. Yes, the message in Nasty Little Truth About Spacetime Physics is the right message[/url] even if he gets his list of crackpots wrong.

Objects have trajectories in space-time, and it's those trajectories that we interpret as motion.

Actually, it isn't. A photon takes many-paths, and you can make an electron and a positron out of it, or our of two photons. An electron is not some tiny grain. Its field is what it is. It's quantum field theory, remember? Not quantum point-particle theory.

Farsight, I was imagining what you might have said about the hypothesis of atoms back in John Dalton's day.

Sheesh. What are you on about? I can fire a stone cannonball, and I can point to a mountain, and to the moon. There is no substance to your naysaying. But there again, there never is.

But that has nothing to do with whether or not the Earth's gravity extends to the Moon.

 

[Reality Check]

Sorry edd, but cosmic strings are a 40-year old hypothesis with no evidential support.

Sorry, Farsight, but you asked for an object with that gravitational field (as in a conical sheet) and the answer actually is a cosmic string.
If you had asked for an observed object with that field then the answer would be that we have observed no such object yet.

 

[Reality Check]

Because I understand general relativity and gravity. because I've read a lot of material, including the Einstein material.

Farsight: What specifically in all of that material that you have read and understood rules out the possibility of cosmic strings?
Where does Einstein say that they are impossible?
Give some citations to the scientific literature not your interpretation of it.

 

[catsmate]

I'm reminded (again) of this. Even if it is nearly seven years old now.

 

[lpetrich]

Because I understand general relativity and gravity. because I've read a lot of material, including the Einstein material.

As if Albert Einstein was an inspired prophet. That's the way that a theologian argues, not a scientist.

They aren't my ideas, drelda.

How convenient.

 

[Farsight]

Sigh. Here comes the woo.